newb needs Lipo battery help

[shazbot]

So I am a newb to rc vehicles and I was wondering if some kind soul could give me some direction. I just bought a Speed-t RTR. As a power supply I went with a 8.4v 4600 mah Nimh losi battery, I also picked up a 11.1v 3s 3200 mah Lipo pack also by Losi. As a charger I went with a Losi multipro intelligent balance charger. As I read the back of the lipo battery package I noticed that it said to charge the battery when it is at 11.1v, do not let battery drop below11.1v. So I am wondering if the battery is going to get screwed up by the car when i run it, because the low-voltage cut off in the ESC is 3v per cell for lipo's, which is only 9v total when you do the math. I hate to sound like a dummy, I'd just hate to ruin my battery the first time I run it. If anyone could shed some light on this for me it would be greatly greatly appreciated. Please don't make to much fun of me, i've just read some horror stories about lipo batteries is all. Thanx

[Alan]

Horror stores with lipos would either be people over charging or discharging or cheap crap. Battery voltage is going to drop, 2.7v is the lowest a lipo can usually go before it is stuffed although would not recommend going below 3v per cell. Most set the cut-offs between 3v - 3.3v. What exactly does it say on the battery?

[shazbot]

unfortunately there is no per cell cut-off recomendation on the battery itself. All it says is not to let the voltage of the pack drop below 11.1v on the battery package. I don't believe this xcellorin ESC has a customizable voltage cut-off. I believe the voltage cut-off can only be set at 3 different settings with the Quick Programming Card. The rating is 9v for 3-cell lipo, 6v for 2-cell lipo, and an Nimh setting( no cut-off). Most everything that I have read has said that 3v is fine as a cut-off point for Lipo cells, maybe they are just trying to cover there liability by putting the 11.1v cutoff rating on the pack. I got a little worried when I read the specs for the ESC, and the specs for the Lipo pack, because they seemed to contradict each other. Afterall this Lipo pack is the 3cell pack that is recommended by Losi for the speed-t RTR on there product page, it is also manufacted by Losi( LOSB9860). Does anyone know if the available PC connect kit is usable with the Speed-T RTR electronics, and if so can you change the voltage cut-off settings of the xcellorin ESC with it? thanx

[HeatMiser]

My understanding of Lipo voltages and cutoff settings goes like this...

When disconnected, the standing voltage of that 3s Lipo should always be between 11.1V and 12.6V. At 11.1, it's essentially discharged, and after charging fully, it will be close to 12.6. Now, when you put it in the car and drive, it's voltage will drop while under load. This under-load voltage is what the ESC is looking at in order to judge whether or not to cut off. When that reaches 3V per cell (or 9V for a 3s pack) under a moderate load, that is generally a good time to stop. If you were to run to that point, pull the pack out, let is sit for a few minutes, and then measure it's voltage again, it will probably be close to 11.1, indicating a fully discharged pack. If your ESC has an adjustable cutoff, you can use this to adjust it. For example, if the standing voltage is too low after a run, then raise the cutoff voltage a bit.

See? You will need to pick up a cheap digital voltmeter from Radio Shack or somewhere to do this type of stuff. It's a bit more trouble, but should keep you from blowing up your packs.

[Alan]

This care guide might help you out (will require Adobe) http://www.maxamps.com/pdf/Lipo-Care.pdf