Cooking 2 cells of my Nimh battery while running my SV27

[jetskier]

I have 2 new 7.4v 4200mah batteries installed and after running it for a while. I notice 2 end cells of one battery is so hot that it melted the plastic shrink wrap. Not sure if the 2 cells are trash now. Anyway, what cause that and will it help to stop running the boat when it start to slow down due to low battery?
Thanks,

[Fluid]

Even with nickel cells you don't want to run the battery down to empty. In your case, the two hot cells were likely a little low on voltage to start or were of lower quality than the rest of the cells in the pack. It is difficult to say if they are ruined or not.

Have you ever balanced the packs? You do this by fully charging the pack then letting it cool down fully. Now hook it up to the charger and charge at a low rate - 1/2 amp - and charge until the pack is done. This low rate of charge will fill all the cells equally and eliminate one reason for the hot cells.



.

[Make-a-Wake]

If those are not matched and zapped packs that will happen, you can either get nimhs of that nature or two 2s lipos for $25 bucks each and have much more punch and power. Matched and zapped packs are more expensive than these lipos.

http://hobbyking.com/hobbyking/store...Lipo_Pack.html

[jetskier]

Thanks for all the replys. The 2 cells seems fine. Both batteries are fully charged again equally. And I will apply low current charge to balance them. Will not run them that low again. It just that I'm new to RC boat and they draw so much more current then RC cars. I'm use to running the cars low till they don't move anymore. Thanks again. And they are match sets Venom 4200mAh

[ray schrauwen]

If you have a test meter and you sharpen the test leads so they are pin sharp you can measure each individual cells voltage at any time by piercing the heat shrink.

Do this only if you feel capable of doing so. It will not hurt round cells to do this.

[jetskier]

If you have a test meter and you sharpen the test leads so they are pin sharp you can measure each individual cells voltage at any time by piercing the heat shrink.

Do this only if you feel capable of doing so. It will not hurt round cells to do this.

all cells are balance. thanks.
I've been monitoring my batteries temp about every 3 minutes of run, I notice when the batteries still have lots of charge in them they are pretty much cold, and about half ways through they become warm, and at their almost end they become hot. What cause them to become hot at the end and not through out the whole run? will running the boat at 1/2-3/4 throttle be better or worse in keeping the batteries cooler? the reason I ask is on the owners manual they don't recommend running 1/2 throttle for extended period of time due to overheating of the ESC and motor, is that true?
anyway.. RC Boating is a blast!!
Thanks again.