Battery Chargers
 

We (Pioneer High Team #1015 in Ann Arbor, MI) are looking for a good battery charger. Something we can run off of A/C as well as 12VDC (auto).

The one we used with (Huron High Team #830 in Ann Arbor, MI) was a bit slow and took many hours to fully recharge the batteries. After a couple of competitions, plus testing and practice, we were pretty low and wouldn't have survived the final round had we made it.

Does Radio Shack or Batteries Plus have a good charger? What are your recommendations?

Not just any charger
 

FIRST has let us use any battery charger in the past, as long as it is rated no stronger than 4 amp-hours. They have hinted strongly that this same type of charger will be allowed for next year.

I suggest buying extra batteries once we get into the competition season. It's good to have 4-5 good (and legal) batteries when you compete at a Regional or at the Championships.

Andy B.

Re: Not just any charger
 

I agree. Our team has about 7 batteries that we can use during competition ... It's nice to have extras to lend to a team in need if we aren't going to be using one for the remainder of the competition!

As for chargers, I would wait until the build starts to buy these. I don't know if you recall last year's contorversy over that or not ... I would wait and see what FIRST has to say this year!

- Katie

it was a really good thing we had 7 batteries last year, since our robot took an entire battery in 3 minutes, so it had to be changed after every single round. the extra batteries are a big help

i agree with everyone here. its always good to have xtra batteries. u change the battery every 2 rounds out it on charge and take it off when its needed. we had a good 6 batteries from the past 3 years that we used plus close to 4 chargers. we used myabe 2 batteries for practice and charged the rest for competition use.




No matter what anyone says

YOU CAN NEVER HAVE TOO MANY BATTERIES OR CHARGERS


has anyone tried charging a battery with 2 charges at the same time! just wanted to know!!!!

No, and I'm not sure if it would be a good idea if both chargers are of the "float" type FIRST gives us. I'm no electrical engineer, but I think things could get very strange, very fast, and while batteries won't start exploding, it may actually take longer to charge if the chargers are flipping back-and-forth from float mode as they may read the other charger's current as a full battery.

On another note, make sure you have chargers other than the pieces of junk they gave us last year. Given the 17.2Ah batteries we use, those chargers take about 17.2 hours to fully charge a battery as they are just 1amp (or maybe they were 2amp @ 8.5 hours?). The old, heatsink-equipped chargers were 4amp, meaning a full-charge time of just over 4 hours. These numbers are a bit over-simplified as its not quite a direct relationship, but its close enough to get the idea.

I think I converted the charging cable from the 1 amp charger into a nice set of jumper cables for testing. The 1 amp was that worthless.

Anyways, don't use anything over 4 amps. After 4 amps you'll be boiling off eletrolite from the battery, which will eventually degrade it. Thats not to say that a high current charge will kill it out right the first time, but you're batterys won't last nearly as long. Other then that, get a good charger that will shut off when it senses a charged battery. Expect to pay quite a bit.

As for the number of batterys you should have, I can't see a team really being competitive with fewer the 4 good batterys and two chargers. Under that and you'll suffer low currents, espically near the end of the day.

-Andy A.

I am not reading these forums as often as I would like to BUT never and I mean never tie two battery chargers in parallel. There are very limited circumstances when voltage sources can be tied in parallel and this is not one of them! Use devices as intended.
As to draining a battery in a single match, you are killing the battery to draw that much out in a short time. You are almost creating a dead short across the battery which everyone should know by now is a dangerous position to be in. If you were drawing this much energy from the battery and not tripping the main circuit breaker, the breaker was faulty or there was something else seriously wrong. This is one of the reasons that FIRST judges should be checking current draw on a powered robot during inspection. Were you burning motors as well?
Let me know,

Al,

I'm thinking his definition of "drained" and yours may not be the same. He may mean simply that the battery is drained enough that there isn't sufficient juice for another match -- and that I would have no trouble believing.

Honestly, my team has always gotten at LEAST three matches out of a battery before the voltage indicator on the OI starts to even drop a little. Here's some fun math stuff:

The batteries we use are 12Ah @ 1hr discharge rate
Extrapolating since we drain significantly faster than that gives about 10Ah in practice.

10Ah is 600Amin (amp-minutes. I made the unit up myself!) A match is 2mins. Thus, to drain a full battery in one match would require a continuous current draw of 300amps. Given that the main breaker will trip at just 60amps sustained, I don't see this as being even remotely possible. Even draining a battery in two matches would require 150amps, which is 150% of the maximum sustained discharge rate of the battery.

Assuming you could prevent the breaker from tripping, the battery still won't output more than 100amps continuous (450 short-duration). Taking a weighted average, lets figure on 110amps max average draw. At 600Amin, that would be 5.5mins, which is almost three whole matches.

Assuming the max draw is 60 (as limited by the breaker), we get 10mins, albeit that the breaker will sustain higher amperage currents for short periods of time.

The only way I can realistically forsee needing to change a battery after every match is if you drive train is horribly twisted and inefficient or if you are dealing with old batteries. Given that most teams (I think/hope), don't even touch the batteries in the kit of parts until the competition, I don't really see that as a possible explanation either.

While the current the battery can deliver will certainly not drop over a couple matches, the voltage will drop some. That is why some teams charge after every match to get those extra few tenths of a volt the battery has when it's peaked. It really can make a difference if you drive precicely.

Unfortunately strictly intrepreting the battery sheet isn't a good indicator of what actually happens in real life. The reality is pulling enough current out of the battery in a two minute match to make it unusable for a second match means that you have either sucked enough current out of it to reduce the terminal voltage to 8 volts(Controller reset) or so raised the temperature that the internal resistance has gone through the roof. I worked with several teams who were able to trip the breaker. If you would like to see some of those discussions, check the archieves. It is not just the current that trips the breaker but the buildup of internal heat as well. Drawing 100 amps for 15 or 20 seconds will derate the breaker to under 60 amps trip.
Of particular note, if you have the battery data sheet, is the discharge curve vs. terminal voltage on page 2. C in that battery curve relates to the current drawn at the specified amp/hour rating or approx 17 amps. Note that the battery when at only 3 x 17 amps=51Amps draws down to 8 volts in 6 minutes. Now it is true that the battery can deliver 600 amps for a short period of time (and much lower than 8 volts at the terminal due to internal resistance) battery damage results and this is sometimes catastrophic in nature. That is why the 60 amp breaker was chosen. In any event, permanent, irreversable damage to the battery results when high current demands are placed on it.
As to instantaneous current draw, we measured currents of very short duration, in excess of 130 amps per motor on a four motor drive without tripping the breaker or dropping the terminal voltage. We were lucky, many teams we helped were drawing much more than that when we analyzed with our Stangsense current monitor.
Please remember these facts when you design and troubleshoot next season. (Oh My. it's only two months away!) Remember, they are a number of people you can turn to with questions, I am available through e-mail as well as a lot of other good engineers.
Good Luck All

Hey another good idea for trying to use a batt for as long as possible is to put over ride switches on your control pannel. What my team does is bring a volt meter onto the field to see how much juice it had left. We figured that if it had 13.5 volts the batt would stay for another round.

Last year's battery charger was a disaster so I have been looking at better technology. I found an interesting site:

http://www.4unique.com

which advertises chargers with "RediPulse" technology with claims on how it is better ya ya ya. Could some of you double EE types take a look and check it out. (I think they are in Oregon - so maybe one of the teams up there can go on a field trip and maybe get them as a sponsor!

I don't want to spend my teams money on a bozo product,
but I also have a bunch of bad batteries to take to recycling, something I wish to avoid this season!


I also want to know if there is any truth to the rumour that stroing batteries on a concrete floor is "bad" and will ruin them.

leaving a battery on the floor
 

there is alot of truth to a battery going dead from being on the floor. I have been working in body shops for quite some time and have learned 2 important things about automotive style batteries. 1 is commonly known the cold and the second is leaving a battery on the ground not just cement. the battery looses power thru its plastic casing its not the positive charge that it looses its the negitive charge that is why the battery goes bad! I have seen many remidies for it such as cardboard under the batt and putting the batt on a shelf with rubber feet. my team keeps its batteries in a foam containers that one of my fellow team mates made during the 00 season because we had lost our battery the year before due to leaving it on the floor. I hope this helps you out.

Dr. Bot,
After going to the website you listed here is what I surmise. The statement that sulfation is the death of batteries is true. In most cases of liquid lead acid, the sulfate builds up as a normal byproduct of charge/discharge and falls to the bottom of the cells. When the buildup is sufficient enough to touch the bottom of the plates then they short out, killing the battery. The sulfation of the plates themselves is more due to the length of time a battery is charged and the current at which it is charged. By using "pulse" technology, the charge current is fed in high current pulses rather than in low constant current charging. All rapid charging devices use some form of pulse charging since it is able to achieve the electro-chemical reaction needed to charge and it is more easily controlled to prevent battery overheating. I think that their claim to be able to remove plate contamination might be possible if the charger were able to pulse a high current load on the battery between charge pulses thereby forcing the sulfates back into solution in the acid inside each cell.

The battery specs says it can be operated in any position - what about storage? Should it be alway upright? What about charging?
Upright? Not sure if a gel cell is different from an older style.

If you look close, you can operate the battery in any orientation but there is one position you should not charge it in as it blocks the breathers (over pressure valves)in the cells. As for storage, it just seems to make sense to store upright with all batteries facing the same way. This prevents shorting batteries together and/or causing high voltage source. Two batteries connected in parallel but opposite polarity are shorting each other. Batteries in series add so it is easy to get a 60 volt 17amphour battery. if the right terminals touch.
As to the story about batteries on concrete, I have not seen anything on this lately. I do remember reading about this occuring back in the old days when batteries were not made in a plastic case. Batteries were at one time constructed of a variety of insulating materials some of which were permeable to some extent. It would make sense that contaminants could enter the cells from the floor through this material and disrupt the acid solution rendering the battery dead. During the forties and fifties is was a common practice to rebuild you own car battery by opening the cells replacing the seperators and cell plates as needed. You would then replace the plates, seal the cell with parafin or asphalt and refill with fresh sulpheric acid.

O.K.

I am running an experiment - I am going to charge up two batteries and see if the self discharge rate is different on a concrete floor or a wood bench!

Can you e-mail your results, please?
Are you going to just measure terminal voltage or do you have a load tester?
Many Thanks

spare battery sources
 

Having expereinced a weak or damaged battery condition last season, I want to take along the 4 or more extras this season. I had planned to have four last year but somehow the extras got left behind in the packing:(

Battery sources conti.
 

Sorry must have hit a wrong key there:confused: I don't trust any of our old batteries and would like to purchase a set of four or more just for competition. Are local sources an option for batteries? Must it be the Yusika??? brand or can any exact amp*Hr rated battery be accepted? So can I pick up the spares at say Interstate battery shops or must I find a local dealer for the brand supplied in the kit?

You will need to buy the exact model that is provided in the kit by FIRST. You may want to wait until we have received the kit to make sure that we have the same model battery, so you dont waste the money on an illegal battery.



(Psst. There is an edit post button right under your post so that you dont have to make a new one. Or you could use the delete button:D )


Cory

I don't think there will be a change in the battery. It's tried and tested for our competition. The availability is very good from so many sources but as the season wears on the batteries may get hard to come by. This battery is very commonly used in emergency lighting and therefore is replaced on a regular basis by maintenance people all over the country. I know that the battery is available off the shelf from some battery stores, McMaster Carr, Newark, Allied and other electronic houses and industrial suppliers.

Battery chargers
 

I have used the 17Ah batteries in Battlebots and in my airplane. The ones that are supplied for FIRST do not have the high amp capacity like the Hawker Odessey PC680s, but they are the same technology and form factor. The PC680 will deliver enough current to crank my 200HP Lycoming over just fine. The inexpensive 17Ah batteries would melt themselves internally if you tried that.

One thing you don't want to do with these batteries is us a standard car type charger with them. They will continue to climb in voltage until they go into thermal runaway and bulge out. Been there, done that.

The charger I use now is a Deltran On Board Battery Tender 12V 6A unit. Deltran also makes a high frequency version that is capable of 20A. :yikes: A lot of the Battlebots teams use the 24V 20A chargers to recharge their batteries when the time was really short. I don't see any need for us to push the batteries like that. The Hawkers will take it, but I'm not sure the Yuasa (or whatever brand) batteries that FIRST supplies will take it.

I don't remember what the rules were on chargers last year. We brought previous year batteries along, but only as a backup. The older batteries tend to drop off capacity. So we would use the new batteries and recharge them between matches, and keep the previous years batteries ready to go just in case we failed a new battery.

Sources for batteries and chargers: For practice only, you can get these batteries from meci.com. $15.95 each, they are the same form factor. These are cheap, but they may have already dropped some capacity. We tried some of these before we bought Hawkers, and they were real pooches. But they may have been good enough for the FIRST robots. The last Hawkers I bought came from batterymart.com, also the Deltran chargers. There are links to the documentation there also (batterytender.com). Good info about what is going on for voltage rise and such.

A comment about the story of not storing batteries directly on concrete: The only thing that I ever saw that could make sense dealt with the temperature differential between the floor (and thus the internal bottom of the battery) and the air temperature. As the air temperature would cycle, you would have a temperature gradient between the top of the battery and the bottom. This could cause circulation inside the cells, and somehow accelerate the discharge of the battery.

Here is a link for a battery capacity testing rig http://aeroelectric.com/articles/battest.pdf.

Enough for now!

My team has def. done this, we call it the "High Octane" line, and it loks pretty cool. Never had a problem in using it for 3 years and def. charges faster than just one charger. Guess we are playing with fire.....

-Rc

Using two 4 amp chargers results in an 8 amp charge current

Thats a pretty high current, and it is not legal during competition. The rules regarding the battery are pretty strict, and they aren't arbitarry. Its best not to do. Besides, I'd say that in most cases, two mostly charged batterys are better then one quickly fully charged battery.

If you want to use it during the off season or for testing, thats just fine, but use caution.

The same rule about boiling pots fits here, a watched battery never finishes its charge cycle.

-Andy A.

Brian,
Don't do this!!!!!
Just because you haven't had a problem yet doesn't mean that it's not going to blow up in your face the next time. You have been extremely lucky! The specs published by the manufacturer comes from testing in environmental chambers where catastrophic failures can be correctly handled. They test to destruction to prevent your injury in actual usage.

Anyone notice that the list of kit parts says we get a 6 amp charger this year?

I ran an experiment - I couldn't see any difference in discharge rate on the cold concrete floor vs. the cold wood workbench.
This was my unheated San Jose garage - so not much of a thermal gradient - ambient tmep about 60F.

Yes, and it is in fact a 6 amp charger, which means to me that we can use 6 amp chargers, yay!

K ...........here it is from the expert........a former BOEING electrical engineer of 40 years.First ....we have never used a spare battery ever.We installed a power stud near the battery (1/4in) covered with a 35mm film container and added a receptacle on the bot from the mated pair of those red batt cables in the kit.....the other half is terminated on the charger cable.
Then we added an onboard meter RS 12 bux .

In a lead acid battery the state of charge is a function of voltage 12v=full rated capacity.The batt could have a "surface charge" in it so turn some gear on to get a true picture.A more accurate way is to look at charging current.......... when it reaches 3 amps ................you are finished .......so add an ampmeter to the charger if you wish .........it must be switched in and out since a dead battery will look like a short circuit and may wing the ampmeter.
The next step is critical .......take that lil plastic charger you got last year the one without the big heatsink on the back and throw it as far as you can or use it for a doorstop.It is a trickle charger and not a real charger.FIRST agreed it was a mistake.Tag it NOT FOR MEETS.Its ok for keeping batts topped off back at the ranch .

Email me if you have any questions about this topic.
Good luck and may the trons be with you. Jim H :yikes: :yikes:

I hate to quibble on this, Jim, and I don't have a data sheet yet on this year's battery but last year's data sheet claims 13 volts open circuit for a fully charged battery. Typical lead acid battery chargers are of a constant voltage type such that the charger puts out a constant voltage just slightly higher than the terminal voltage of a fully charged battery. As the battery goes through charge, the current begins to drop as the terminal voltage rises. When the terminal voltage is the same as the charger voltage, current is zero. More sophisticated chargers also have a battery health monitor that will periodically switch from charge to discharge and alert you of a battery that is not holding a charge. These types of chargers are also capable of pulsing charge current above manufacturers specs to speed up charging. The charge current specification on last years battery was .25C (where C is the rated amp hour capacity) then it is easy to see why a 4 amp charger had been the charger supplied by FIRST up until last year. (That charger is still available through a variety of sources.) An additional current supplied to the battery beyond full charge is lost as heat to the internal structure of the battery. A sufficient amount of current transferred to heat over a lengthy charge where current is kept high, will just boil off the electrolyte. This happens anyway over years of use. Any team that has a battery that is say four years old will tell you that battery weighs less than one of the new ones. (That is my initial test by the way. Pick it up, if it's light and doesn't hold a charge, it's recyclable.)
I fully agree on last years charger since it was not able to supply 4 amps (actually less than two)it would take overnight to fully charge a battery. That made it well suited for charging in your room but not during competition.

Al hate to quibble on this as well so change my fully charged value to 13 .........sorry bout that .I have never seen a battery draw zero current.These cells are GEL cells and dont contain wet electrolite they are alkali GEL cells comprised of an open weave cell"plates"coated with a GEL and rolled up like a jelly roll and the only time they outgass is when they are abused during the charging cycle.The important issues are ......provide a quick disconnect for charging in the pits .......monitor voltage ......and use a robust charger....... dont think you will get busted if you use a real charger with an meter and tapering feature Al discussed .......they are under 40 bux at most automotive stores.
Jim :yikes:

Jim,
The zero was just part of the illustration. We haven't opened the new charger to check out it's operation yet. It appears to be the "good charger" in a higher current, slick package. I would caution everyone that using a NON-FIRST charger in the pits could get you a DQ. If you test before we do please report your findings.
Thanks

Al We will...........Jim:ahh:

Why not just set the battery on a scale while charging? Everybody knows that a battery with more energy has to have more mass. After it's increased mass by .000001g or whatever the threshold is, you know it's done. Simple as that. That's the solution of the team who's leader is a physics teacher and has no engineering mentors.

An AGM (absorbed glass mat)/sealed gel cell/whatever else you want to refer to the batteries provided by FIRST will not change in mass while charging. A normal, non-sealed lead-acid battery will, but these batteries are designed so they will not offgas. As all the materials inside the battery are the same, and only chemical reactions change, you cannot increase or decrease the mass of the battery. Also, the 'full rated capacity' of batteries is not 12 volts, its 13.5 volts. most chargers will bring batters up to at least that, if not closer to 14 volts, at full charge. Be very careful with different battery chargers, if they don't auto-sense and auto-limit the current going into the battery, sealed-cell batteries are very easy to kill through overcharging.

Tyson

Interesting,
Except what about the dirt, duct tape, marker and paint that the batteries pick up in the course of competition affecting weight?
Back to reality, no battery is perfectly sealed and these batteries are certainly not. There are vents but they are hidden. This battery does out gas but not a lot unless over charged. They will get lighter (with age ed.)because after so many charge/discharge cycles some of the electrolyte does boil off or evaporate. The manufacturer specified fully charged terminal voltage as 13 volts. If you charge to a higher voltage and immediately remove the charger and test open circuit with a voltmeter it will be higher than that, but let it rest and it will come back to 13 volts (or at least "pert near") when new. Although this is listed as a "sealed, lead acid" battery it is distinctly different than the lead acid battery in your car which has a charged open circuit terminal voltage of 13.8 volts or around 2.3 volts per cell. Which is why FIRST does not allow car chargers to be used to charge the battery. They do charge at 14.2 volts or so which results in more than the specified charge current for the battery we use. In the above example using that car charger, it is possible to read 14.2 volts open circuit immediately after charge but by that time you have already reduced the life of the battery, perhaps damaged the internal structure and raised the internal temperature high enough to insure boiling some of the electrolyte or worse. Taking that battery out to the playing field to expect to get more play time or higher power is an empty bag. The higher temperature alone will have increased the internal resistance to the point the battery cannot support the RC's 8 volt minimum. The terminal voltage will rapidly fall back to 13 volts or less after just a slight discharge, and damage to battery and team personal is not worth the risk.

Good input Al
 

I allways tell the students to cycle the bot breaker to get rid of the surface charge so the battery will settle down to the real voltage.
That change in battery wt sounds like a shaggy dog story that you understand but you students may not understand the expression........its like "dude what are you smokin man " No offense to that teacher but i would like to see the data.
If your club is working on DASHBOARD and they include batt voltage its kind of a moot point ........what can you do on the field if you know the batt is at 9 volts...........cry

One lesson we learned in the first bot design is to locate the batt on the perimeter of the bot for easy access and if you like lots of traction locate it over a drive wheel..
We charge (no pun intended) the tool person with connecting and disconnecting the batt and checking voltage.Make someone responsible for this task.
This battery, SLAB (sealed lead acid)was developed by GE and Gates and i served on the MIL spec group that introduced it to aircraft application 20 years ago.GE ,GATES held the original patent they or the current vendor may have a SLAB 101 battery course available ...........have a student search for it .

Jim :ahh: :( :(

Ugly Last years Battery
 

This morning I started to charge a battery from Last year (Had been charged a month ago) With a 6amp charger - It started taking about 2 amps - which is usually normal for a
moderately discharged battery. After about 15
minutes I noticed the battery was making lots of noise and electrolyte was oozing out the top! I never saw and Exide fail in this way before. Any opinions on what could have happened? (Yes I am sure I did not hook it up backwards!):ahh:

Doc,
Was it out of all the vents or just one? It is possible to short a cell and have that condition during charge, but all of the cells would indicate something wrong in the charger. I was checking through the literature on the charger last night, and it appears that the charger switches between charge and load to determine the state of the charge on the battery. The LED on the front tells you when it is charging or switching in the load. When it is mostly green, it is fully charged and on smart trickle.

You are correct
 

The charger went bad. It was an old model - I liked it because it had a meter so you could see the charging rate. I tried another battery and the same thing - I put a voltmeter on it at
zero load it was floating at 12.6 volts - as soon as it tried to charge a bettery it went to over 18! So that charger is on the scrap(er recycle) heap! Too Bad. I think I lost too much electrolyte from the Exide to salvage it for bench use. Fortunatley the acid didn't ruin anything the baterry was sitting on a slab of particle board. Neutralized the spill with baking soda - plop plop fizz fizz oh what a rekief it is.

At least you have the smoking gun. I hate when that happens.

Re: You are correct
 

QUOTE]Originally posted by Dr.Bot
The charger went bad. It was an old model - I liked it because it had a meter so you could see the charging rate. I tried another battery and the same thing - I put a voltmeter on it at
zero load it was floating at 12.6 volts - as soon as it tried to charge a bettery it went to over 18! So that charger is on the scrap(er recycle) heap! Too Bad. I think I lost too much electrolyte from the Exide to salvage it for bench use. Fortunatley the acid didn't ruin anything the baterry was sitting on a slab of particle board. Neutralized the spill with baking soda - plop plop fizz fizz oh what a rekief it is. [/quote]
Dr. Bot,
If I was you, I would go get the charger off the scrap heap and throw the batteries on there instead!
When you put a battery on charge (providing that the charger is working OK) and the voltage goes up way beyond normal, 18 volts in your example, then this is a sure sign that the battery is heavily sulfated and is not in 'prime' shape. The sulfation has the effect of raising the internal resistance of the battery, making the current go high, while keeping the charger output low.
As for the electrolyte leaking out, this is another indication that the battery is toast. These batteries are Valve Regulated Lead Acid and have a gelled electrolyte. In theory, they are sealed to avoid any loss of moisture from the electrolyte as once its gone, it can't be replaced as in a flooded lead acid battery. When a battery is on charge and the voltage rises, the plates begin to give off a mixture of hydrogen and oxygen - in a flooded battery, this can be seen as a stream of bubbles. In a VRLA battery it can't be seen, obviously, but it still takes place. This begins to occur when the battery voltage gets to about 2.3 volts per cell or 13.8 volts for a 12 volt battery. In a flooded battery, this results in 'gassing' and the need for having to add water to the cells periodically. In a VRLA battery, you cannot allow this 'outgassing' or the electrolyte dries up. The dilemna is, if you stop the charge when the battery is at 13.8 volts, then it will not be fully charged and will gradually lose capacity and become sulfated. To get around this problem, the battery is 'Valve Regulated' in that the 'Valves' are set to withstand a certain pressure without opening and allowing the out-gassing of the moisture. Internally, the hydrogen and oxygen gas is re-combined back to H2O and retained in the cell. If an incorrect charger is used on a VRLA cell and does not sense the gassing voltage has been reached and cut back the current, then the pressure will quickly build up inside the cell and the 'valves' will open. Once they open, the battery is doomed as they are not able to close again and the battery will quickly lose the moisture from the electrolyte during charge until it eventually dries out completely. This is why the battery that you mentioned was oozing electrolyte from its valves - the battery voltage was at 18 volts and the plates were gassing a lot. These bubbles of gas cannot easily pass through the gel, so it expands and forces its way out of the open valves.
As I mentioned above, if you stop the battery charge at 13.8 volts, then it will not be fully charged. We get around this problem by using a different charge profile than we would for a flooded battery. A simple flooded profile is a taper with dv/dt where the charger tapers the charge - as the voltage rises the charge current tapers off and when it hits a pre-defined voltage indicating 80% charged then it goes into the dv/dt (Voltage against Time) finish stage and completes the charge. The return of charge factor is about 1.15 which means for example, that for every 100amps taken out of the battery, then you have to put back 115 amps from the charger to get to where you can use the battery to give you 100 amps in output again. With VRLA batteries, the charge factor is a bit more efficient <>1.06 and the charge profile used is more controlled (IUI - constant current/constant voltage/constant current) so that the battery does not gas profusely and lose its moisture. That is why it takes a long time to re-charge a gel battery as the finish current is very low to avoid raising the internal pressure and blowing the valves, but allowing the battery to take a full charge.
Whew, this was intended to be a quick reply - hope that somebody finds it helpful.

Dave,
You certainly were very technical in your discussion. I would like to remind members of FIRST that "wet" cell batteries of the type in you cars are nominally 13.8 volts when fully charged or 2.3 volts per cell. The manufacturers spec on our sealed, lead acid, battery or SLA, is 13 volts or 2.16 volts per cell. Different battery types all have different cell voltage, 1.25 volts for NiCad and nickle metal hydride, 3.6 volts for lithium, 1.5 volts for alkaline, etc. Now should you use a voltmeter to measure the terminal voltage of your car battery while it was charging in your car you may find the voltage regulator is putting out 14.2-14.5 volts which is a little high for our battery but fine for the "wet cell". Therefore any battery charger which puts out 18 volts with a battery connected would, in my opinion, be ready for the circular file. One that puts out both 18 volts and damages batteries has crossed over to junk status and should be treated as such.
I would like to remind everyone that the chargers supplied by FIRST or equivalent are the only ones that are legal for use in competition.
C20 When recharging Kit batteries, you may use the charger provided by FIRST or one
with equivalent charging characteristics.
(I like the new 6 amp smart charger but still use the 4 amp charger First had supplied for years.) Using other chargers, auto battery chargers included, runs the risk of battery leaks, battery damage, and injury. They put team members at risk as well as others that might be close by. If detected in the pits, I would expect a DQ on such use. Don't take safety lightly, especially in the pits.
Here is the official spec sheet on the battery...
http://www2.usfirst.org/2003comp/Specs/Exide.pdf