Is this normal....lipo - RC-Monster Forums

speed demon · **Is this normal....lipo -** 04.08.2008, 12:39 PM

I bought a lipo awhile back and have been running it for awhile. Its a 3s 18c 4000mah. For the first time I ran it until the speed control shutdown because of low volts. I went to charge it will my Imax b5 charger and it put 4600+mah into it. I charge some of my buddies 4600nimh bats and I can only put 3800mah into them when they are dead. Is the count on the charger accurate? Do lipos normally hold more then their ratings? Thanks.

BrianG · 04.08.2008, 12:45 PM

What voltage did it start at? It's possible you might have overdischarged it a little. It could also be getting old (you didn't define how long "a while back" is).

Also, did you balance at the same time? If so, the balancer does bleed some capacity away while doing its job, but an extra ~600mAh sounds a bit high unless the balancer was on for the majority of the charge.

From the sounds of it, those NiMHs are working too hard or are "optimistically" rated. I usually put right around 3100-3200 mAh back into my GP3300 cells (non-matched or zapped). If NiMHs heat up too much or drain too hard during running, it could reduce their capacity.

speed demon · 04.08.2008, 01:07 PM

The volts were 3.1ish per cell when I plugged it it. Yes I balanced charged for the whole time(70min). I've got about 40cycles on the lipo. My buddies nimh have around 100+cylces and he has gotten them hot a few times...160 pack but 185 on the hump cell. Thanks for the quick reply.

BrianG · 04.08.2008, 02:22 PM

3.1v/cell with no load is too low. Once disconnected from the ESC for about 20 minutes, the cell voltage should drift back up to around 3.5v/cell. If you can, increase the LVC by to 3.1-3.2v/cell. Assuming the LVC is adjustable of course. The LVC will trip sooner, but the cells will rebound to a more healthy level. You won't get much reduction in runtime (might not be even noticeable) since lipo voltage generally drops off quickly near the LVC point.

Balancers only bleed off cells that have a higher voltage than the others. So, unless cell(s) were being actively balanced the whole time, a balancer won't eat up any of the charge capacity.

In case I wasn't clear before; let's say a balancer discharges high cells at 200mA. If you are charging at, say, 4A, the high cells will only get 3.8A (4A-200mA), while the low cells get the full 4A. So, if the balancer was actively balancing during the whole charge cycle, the high cells will only get 4400mAh out of the 4600mAh the charge display shows. If cells were actively balancing for half the charge cycle, they would get 4500mAh out of the displayed 4600mAh. Again, this is assuming a 200mA balancer discharge rate - if it's higher (like 400mA), these numbers will be different. I hope that all made sense.

NiMH cells don't like excessive heat, and 160-185*F is too hot IMO and probably reduced their life. It doesn't help that they have quite a bit of cycles on them.

speed demon · 04.08.2008, 02:36 PM

Yes that makes since to me. I get my lipos from milehighrc and from what I gather he rates his lipos on the low side to cover his butt.

One more thing are all chargers equal? My friend chargers his nimh with a cheap tower hobbies charger and gets his bats to about 140 degrees and says there full. I take that bat after it has sit for about 15mins and charge it with my b5 and my charger puts another 500-600mah into it.

BrianG · 04.08.2008, 03:03 PM

Most NiMH chargers are constant-current with delta-peak voltage detection. Some are "pulse-chargers". Both types will charge fine, but the algorithm for full charge detection may be more sensitive or different than others. If you let that bat sit for 15 min, you could probably put it right back on his charger and get that same 500-600mAh into it. It's just the way NiMHs work.

Lipo chargers commonly use a constant-current charge until the cells reach 4.2v. After that, the voltage is constant and current steadily falls until the end of the charge; typically 1/10 of the charge rate. I've heard some lipo chargers using a pulse-charge system as well, but like the CC/CV version better.

E-Revonut · 04.08.2008, 03:14 PM

I came across this same situation last night. I have a couple MaxAmp 2s 6Ah packs that are about a week old and have 3 cycles on them. Last night was the first time I hit the lvc and it was in my new Rustler vxl, when I threw it on my Hyperion charger it started with 6.8 volts but put 6300+Mah into the pack. Now I know why, my charger is the eosi6060 and it was balancing the whole charge cycle. Either way the pack performed great! Wheelied right till the cut off and 30+min runtime.

speed demon · 04.12.2008, 12:13 AM

Hope you guys don't mind but I have another question. I have put on another 15 cycles on the lipo and noticed something strange. I ran the lipo all the way down again and when I plugged it into the charger the cell volts were 3.31, 3.23 and 2.99 Is this within tolerance or the cells suppose to be closer then that? It's still giving great power and run time. I was thinking about getting another since my friend built a flat oval dirt track for us to race on. What would you guys suggest? Looking for 20-25c 4000-5000mah 3S. Also looking to spend around 160. I like what I see about kokam but not sure. thanks

nativepaul · 08:27 PM

Chargers and batterys are not 100% efficient, it is normal for the charger to try to put in more MAH than the pack has capacity for by up to 20% even for NiCad and NiMh that don't need balancing, your buddies battery will probably only give out 3500mah if that, his pack is buggered, do either of your chargers have a discharge mode? if you charge it up then discharge it the discharger will tell you how many MAH the pack will actually give.

.32V yeah that's a LONG way out, I'd be worried about the cell that was .1V out if the other one wasn't even further out, is this within 1 run of being balanced?

speed demon · 01:07 AM

I think my charger can discharge but honestly never really looked into it. I just(10 min ago) totalled the front end of my rustler so I will have some spare time to look into it.

As for my lipo it's doing great. I run it 3 times a day but only for a short time and the volts drop to 11.7v each time and now we have been staying over after work and then I run the bat until the speed control kicks me into 50%. They volts are all very very close after I run it for the short times. The last two times I ran it all the way down the volts were only off .03. Not sure whats going on with it. Thanks for all the advice.

Forgot yes I balance every time I charge.