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.