Battery Chargers

[Neal Probert]

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?

[Andy Baker]

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.

[Katie Reynolds]

Originally posted by Andy Baker
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.

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

[Rob Colatutto]

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

[mtaman02]

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!!!!

[rbayer]

Originally posted by Mike522


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.

[Mike Schroeder]

I dont even know what we did with our new charger we just used the old ones and order and extra one

[Andy A.]

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.

[Al Skierkiewicz]

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,

[Greg Ross]

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.

[rbayer]

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.

[sanddrag]

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.

[Al Skierkiewicz]

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

[hot69z]

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.