Rix wrote:Snowchyld,
I took this from Desire Power web site. I have never charged my RC car lipos at 10c, but Desire Power says its possible depending on time constraints. I never had any thermal breakdown on my lipos charging at 4+C. I have stopped a charging cycle before because they were really warm to the touch. Looks like I am off on the useful voltage range though. However, I got that info from another forum on this site: they quoted "lipo rules, never above 4.3 and never below 2.7". Apparently that info is incorrect which is why I asked here. As you pointed out, there are a many other forums to go to for my inquiry, however the information can be wrong there. Kepler and Hyena have both done lipoly conversions with great success which is why I posted my querry here. I believe this forum will point me in the right direction with sound advice. I can do the math and come up with a more accurate voltage range as long as the info provided is correct. BTW, Watts can be measured in current draw output and also in capacity of a storage devise. Stealth Bikes lists the capacity of their batteries in watts on their website. Bomber's battery capacity is 1.5 KW (probably 72+ volts 20 Ah) and the fighters battery capacity is 1.1 KW (probably 55 peak volts at 20 Ah) not to be confused with the bomber's output up to 4.5 KW (probably 60-65 Ah @ 72+ volts) and the fighter up to 3 KW (probably 50 Ah @ 55 volts). I think your max discharge rate example for a 10 Ah battery being discharged at 8Ah max tells me that you are referring to a battery rated for 1c continues discharge. The lipos I am looking at rate anywhere from 35c continues up 65c with short burst that doubles these numbers. Having said all of this, I am not very knowlegible about Lipos, I use them in my RC car and my RC Helicopter with out any problems. They are 11.1 volts and 7.4 volts respectively. For me it is litteraly plug and play and my smart chargers balence and charge the batteries. Anyways thankyou for taking the time to answer my questions and for providing the information about voltage ranges for lipo cells.
if they're rated for 10c then 4c shouldnt be a problem... though the drastically shortened life suggests that they're not ideally suited to those charge levels. they're obviosly not as concerned with cycle life, and they're not charging up huge packs, so if 1 small pack goes pop its usually not a big danger (relatively), if 10 or 20 of those packs go up at once, thats a different matter, hence the conservative 4.2/4.15 to 3.6 in the ebike world.
and just a small point on terms... you've got the Ah/A and Wh/W's mixed up... Watts is a measure of power, or work done per unit time, Watt Hours is a measure of capacity or storage. Ie when you refer to battery capacity, such as the fighter, its in Kwh (kilo-what hour) ie it can draw 1.1kw for an hour. its calculated from multiplying the nominal volts (ie about 3.7-3.8V for lipo, about 2.9V or something for life, dont quote me on life though) by your cell count, and then by your usable capacity in Ah.
When determining the power output of a battery, you take the net voltage at that time, and multiply it by the amp (A) draw. this will give you the power output, ie 3Kw for the fighter.
If you want to calc the max power output of the batteries you need to know they're discharge C rating. lets say the packs in fighters are rated at 20C. you take this figure, multiply it by the Ah of the pack, and then by its voltage. You can also multiply the capacity of the pack in Kwh by this number to get the same result. in this case, 22Kw.
so a fighter has a peak output of 3kw, (lets say a 60V battery being discharged at 50A). it has a capacity of 1.1kwh (60V multiplied by its capacity in Ah, about 18Ah).
when I stated the 8 ah that is to help calculate the 'safe' usable capacity for lipo if you want it to last... Ie the rated capacity of my 24s 10ah lipo pack is 24 x 3.8 x 10, about 910 Wh, or 0.910 Kwh. If I want it to last, I need to multiply this figure by 0.8, the reason I said multiply your Ah by .8 is it gives you a clear number for Ah used on heads up displays like the cycle analyst. It also removes the inaccuracy that comes from people's varying opinion of 'nominal voltage', which is arguably between 3.6 and 3.8 volts. for me I use 3.8 since about 50% of my capacity comes above that figure. This is where the 8Ah number comes from. It has nothing to do with the rated discharge rate of my packs (which is 20C, or 200A for a 10Ah pack). Personally I usually multiply the 'rated' C rate by 0.5 to get a more realistic value. Which still means my pack should output 100A without breaking a sweat.
Hope that all makes sence/clears things up for you. either that or I've confused you further....
nothing wrong with asking questions mate, I'm meerly pointing out that there's answeres to be found (i'd suggest searching ES via google ie 'lipo questions endless sphere') on ES, as you can imagine lots of people have lots of questions, but rather than people answering each person's individual q's each time, its often easyer/quicker to search up on the basics. personally if you're looking at a stealth, I'd be keeping the stock battery. it doesnt have quite the same wh/weight as lipo, but for the sake of worry free plug and play, I'd keep it stock, atleast until they pack started fading/was out of warranty.
edit
just to be clear, im not suggesting you should shy away from a lipo conversion, meerly pointing out that the simplicity of a stock stealth is quite desireable, and is what I'd choose initially if I could afford one!
Thank god for a strong chain and stealths integrated torque arms!
