Overheating batteries

This was our first year building battlebots and we need some advice about batteries. We purchased some NiMh batteries (I know....will purchase some LiPo's next year). They are 19.2V, 2700 MAh. The battery was powering two DuraTrax 550 motors. After driving around for a few minutes, the batteries became smoking hot.....literally. Battery acid fumes were spewing out and acid was sizzling with occasional popping. Not good at all. We had to put water on the batteries to cool them off. Pretty sure the batteries are now toast. We had a weapon running on a separate battery pack, which got hot too, but not quite as bad.

Questions I have:

1. Is it common for NiMh batteries to overheat?
2. Are there some battery specs that we should be paying attention to (didn't see a maximum discharge rate)?
3. Can LiPo batteries handle a higher discharge and therefore will stay cooler?

Any help will be greatly appreciated. We are just trying to figure out where we went wrong so that we don't make the same mistake again.

fogdog wrote:

This was our first year building battlebots and we need some advice about batteries. We purchased some NiMh batteries (I know....will purchase some LiPo's next year). They are 19.2V, 2700 MAh. The battery was powering two DuraTrax 550 motors. After driving around for a few minutes, the batteries became smoking hot.....literally. Battery acid fumes were spewing out and acid was sizzling with occasional popping. Not good at all. We had to put water on the batteries to cool them off. Pretty sure the batteries are now toast. We had a weapon running on a separate battery pack, which got hot too, but not quite as bad.

Questions I have:

1. Is it common for NiMh batteries to overheat?
2. Are there some battery specs that we should be paying attention to (didn't see a maximum discharge rate)?
3. Can LiPo batteries handle a higher discharge and therefore will stay cooler?

Any help will be greatly appreciated. We are just trying to figure out where we went wrong so that we don't make the same mistake again.

what gearbox were you using with the duratrax motors they pull alot of amps in starter boxes that they were made for.

nimh batteries will retain heat in the centermost cells of larger brick style packs as they discharge and 2700 mah isnt alot for two 550 motors depending on the amps they are pulling at 19v. It could be that they were not cycled properly before they were used traditionally nimh cells prefer to be stored fully charged and topped off before use and when fully discharged they start to remember thier lowest peak and will retain it until they are cycled about a 2:1 ratio of charge discharge to gain that back.

Also I have seen nihm cells just burst because they were not resistance matched close enough. we burned up about 4 packs in breaker box at robogames because they were not matched close enough and under low amp load they built up excessive heat.

As for the lipos yes they will have a higher discharge if you choose 30c or higher packs and will run alot colder then nimh cells in most applications.

Edit: didnt see alex's post... i second the a123 over the lipo if it is for an educational robot.

Thanks for the good information! I was under the assumption that the duratrax RS-550 was the Banebot RS-550. Oops. I had no idea that the duratrax was a different manufacturer and used much more amps. Thanks for pointing out what is now so obvious.

To give a few more details about our robot:

- It is in the 15# weight class
- We used a 16:1 ratio gearbox for the duratrax motors with 5" diameter wheels
- The battery pack was arranged in what Robotmarketplace called a "brick" shape
- I did cycle them once (discharge/charge) before their first use
- We bought three of these chargers: http://www.robotmarketplace.com/products/0-GPMM3153.html.
- We live in Arizona and currently only have National Robotic League (NRL) competitions. For some reason, the rules only allow Li-Po batteries. I am not really sure why, since everything I have read states that Li-Ion batteries are "safer". I am not sure how close the judges examine the batteries, since I was told by some teams that they were using Li-Fe batteries.


A few more questions for you guys, if you don't mind:
- Will the above charger we have work with A123 batteries? Our charger is for Li-Po batteries, but does it have to be specifically for A123?
- Is there any way to measure motor amperage? All the meters I found only go up to 10 amps. Also, it is probably difficult to measure using actual combat conditions.

The charger won't charge A123s... you'll need an A123 charger like this one: http://www.robotmarketplace.com/products/0-THP610.html

As far as measuring high current goes, the easiest and cheapest way I've found is to use something called a shunt resistor. 1 foot of 10 Ga solid copper wire has a resistance of almost exactly 0.001 ohms. Wire this into your circuit in series, and measure the voltage drop across it. Every mV you read corresponds to an amp going through your circuit. It's approximate, and the accuracy is pretty dependent on your connectors and stuff, but it's not a bad estimate.

While the designations of batteries are a bit confusing. It is definitely Lithium Polymer (LiPo) batteries that the NRL does not allow. These batteries pose a unique fire risk not associated with A123s or other Li-Ion batteries.

You may want to check the RC groups forums on modifying LiPo chargers to work with A123s. I've heard of some people doing this, but I'm not sure of the details.

i've got a onyx 230 it hasn't failed me yet -- although some of the cheap plastic braces inside the unit have broken. caused a bit of rattling till i took it apart to remove em.

With a voltage of running 16:1 with 5 inch wheels may have a killer theoretical top speed, it is gonna put your motors deep into torque/stall range and draw way too much current to be efficient. Additionally, you may want to check how easily those wheels turn-you may have something binding in your drive to cause that sort of current draw. Did your drive motors or ESC's also get hot/smoke, or just the batteries?

If the motors and/or ESCs got hot too, I would say the battery is not your major problem. However, it does seem like you should have cycled them more than once before competition. (We used to run two NiMh 7.2V packs in series, usually around 3000mAh, with a 20C or 30C rating, but it's been a while and I dont recall the details well. We only got hot batteries from the weapon motors.)

On the other hand, my experience with 3s LiPo (11.1V nominal) at 26:1 running 3 inch wheels the motors will get hot, but any lower than that and your bot is too slow to be competitive.

Evan Steeves wrote:

Also I have seen nihm cells just burst because they were not resistance matched close enough. we burned up about 4 packs in breaker box at robogames because they were not matched close enough and under low amp load they built up excessive heat.

I am a little confused when you say you were "under low amp" and still building up excessive heat. I was under the impression that resistance was only a factor with the wiring size. And what do you need to "match" when pairing a motor with a battery? I understand the effect of battery voltage (determines the RPM) and Ahr (duration of battery power), and now the importance of the C rating.

this was happening to us this year when i had the weapon on kv at full speed for 2 or 3 seconds they would poor smoke. the first time i did a test run on on the weapon it actually exploded.

it was the first time it ever happened to us

we used a123's
we found that the problem was that we soldered them our selves and the the braid was not thick enough
we put thicker braided wire in the battery pack and it didnt smoke as much
we are going to get them professionally made this year though so we dont have that problem

fogdog wrote:

Evan Steeves wrote:

Also I have seen nihm cells just burst because they were not resistance matched close enough. we burned up about 4 packs in breaker box at robogames because they were not matched close enough and under low amp load they built up excessive heat.

I am a little confused when you say you were "under low amp" and still building up excessive heat. I was under the impression that resistance was only a factor with the wiring size. And what do you need to "match" when pairing a motor with a battery? I understand the effect of battery voltage (determines the RPM) and Ahr (duration of battery power), and now the importance of the C rating.

matched packs are rated on their internal resistance of each cell, since each cell is made just a little bit different it will discharge and charge at a different rate from one another. When you purchase a high power nimh pack it will almost always be resistance matched and that being the internal resistance of each cell now where things can get all sorts of fubar'ed is when you have a large robot with brick packs that are internally matched, a brick with 20 cells all matched to eachother may be up to 5 amps off of another perfectly matched pack so when you get 2 or more in series or parallel they try to even eachother out.

What i ment under low amp load was breaker box was just driving around pulling a calculated 150 amps out of nearly a 26ah of battery bank and smoked two packs the only just cause for this was two packs were not close enough of a match in discharge and charge rate as the other 4 in the robot so with 6 battery packs total two of them were far enough out of match that they essentially were pulling 4 times their rating by trying to equal out against the other packs.

this is why when 10 years ago in rc car racing when people built their own nimh packs they would only match cells that had an internal resistance matching in the thousands. if a pack matched only in the hundreds then by the end of the race the pack would become so hot it would ruin itself and be trash only the perfectly matched packs ensured that they would last a full race season.

the only way to test this is to discharge and charge each cell by itself so if you buy a premade pack you wont be able to get a very close match, the best thing to do is when you buy a new nihm pack is to cycle it about 5-7 times then it will start to learn its memory, once you have done this match the packs based on how many miliamps they absorb in the course of 15 minutes at their highest safe amp charge. if you do this in a 30 minute absorbtion process you will match packs improperly and will find one or more will not be in the same discharge range as the others.

personally i would run lipo or li ion as they no longer explode but smoke out just as nihm and nicad cells do. also lipo and li ion cells are better suited to running large packs in series and parallel as they will absorb eachothers voltage smoothly unlike nimh and nicad that charge based on pulsed voltage.

hope this helps its a little early for me to rack my brain like this lol

Thanks for taking the time to explain. Yes, it does make sense. Glad this forum exists. Without a place to chat, I am guessing that more bot designers would be pulling their hair out due to frustration.