LIPO - out of balance cells?

[Cardnim]

Hi all,

Im new to lipos and have just received my first charger and 2 batts (one brand new Speed Power, the other an older 2nd hand GM.)

Good practice with NiMH was always to discharge the cells before charging again (to prevent cell capacity memory)

So, plugged in my 2nd hand GM lipo (2 cell, 5400mAH, 45C) and plugged in the balance board that came with my charger.

Set the discharge rate to 1.0A

It went for 73mins but by the end, cell 2 had dropped out of sync with cell 1. Before that, they were much the same.

The charger ended the discharge, with the following spec:
- cell 1 - 3.42V
- cell 2 - 2.80V

I have read that discharging lipos beyond 2.8 is very dangerous, but I am surprised at the difference in voltage - surely the balancer should have prevented this? (or at least stopped discharging cell 2 while continuing to drain cell 1 till it was level)

Can anyone shed any light on this? The Internet is filled with scare mongers who say "my granny once looked at a lipo funny and now she is on fire" - so its hard to filter the fact from fiction.

Thanks as always guys.

[Cardnim]

Argh, this is really confusing and slightly worrying.

Now I have to leave work, and the battery is discharged. More reading has just found some guys talking about how to never leave a Lipo less than 3.4V or it permanently damages it.

I cant really charge it safely till tomorrow as I dont want to leave it unattended.

Is this the right thing to do?

(I hate Lipo's and all they stand for - long live Nickel!!!)

[super__dan]

Get some charge back into it TBH, Lipos are balanced on charge and 99.99% of the time are never run flat so being out of balance at full discharge is not a concern. You need to forget the Nimh things we 'HAD' to do, it's different and all for the better IMHO

[Cardnim]

Thanks Dan,

Ive jsut put the charger on for 5mins before I have to go.
Selected "Lipo BALANCE" at 5 amps

individual cells are now at:
cell 1 - 3.82v
cell 2 - 3.62v

going to have to end it there for tonight.

I hope I havent wrecked the battery.

P.S. if discharge isnt needed on a Lipo, why is it an option on the charger (genuine question, Im not trying to be smart)

[Whiskers]

I think just so that you can discharge it to 7.7v for storage.

[ianhaye]

should never really need to to discharge lipos like that, they are balanced at the end of a charge, touch wood you does loose a cell going that low on 1cell.

[Cardnim]

ianhaye - damn, got me worried.
Is there anything i can do to minimise the risk of this?
Thanks

[Cardnim]

....just thinking, surely this isnt abnormally low.
The cutoff on my ESC (speed passion 2) is set to 2.8v per cell so when it runs out of juice the battery must be at this level.

No?

[bigred5765]

have been running lipos for 2 years now and never discharged a pack ever, theres no need forget what you did with nimh, just charge on balance run but not until flat ??
and just recharge, and use again no worries
ps sp esc should be set to 3.0v per cell

[mark christopher]

Originally Posted by Cardnim

....just thinking, surely this isnt abnormally low.
The cutoff on my ESC (speed passion 2) is set to 2.8v per cell so when it runs out of juice the battery must be at this level.

No?

your speedo should cut off at more than that!! 6 volts is the minimum for a 2s pack

as you have been told on here, do NOT discharg them fully, DO NOT store them fully charged for a long period
charge them, run then charge them is all you need to do, if your racing for 5 mins charge before you last race, then unplug tham and leave them till next time you race.

i doubt you have done damage to your cells

what charger/balance are you using? as far as im aware balancers only cut out at over voltage
most chargers only alow 1 amp of discharge as at a low rate of 1 amp there unlikley to go up in flames

[Cardnim]

Thanks for all your help guys, you always come through.

Im gonna take your advice without question - just using the cells and not worrying about discharging. I'll keep balancing them and not store them fully charged.

However, I would draw everyones attention to the Speed Passion Team Setup (available from the Speed Passion website)
THis is the teams own setup and so I followed it exactly.
As you can see from the Off-Road box in bottom left, their cut-off voltage is 2.8v:
http://www.speedpassion.net/uploadfi...SpESCsetup.jpg

My charger is an Ansmann xBase 2.0

THanks again for all your help, I really do appreciate it.
Happy racing!

[dodgydiy]

really should be 3 volt per cell cutoff, go below 3 volts and you are getting very close to hurting your cells and you wont gain anything by doing it either as the discharge voltage on the lipo's is very constant until they are nearly flat. if you do ever manage to flatten a lipo pack in a race then i would say there is something wrong with your gearing or matching of the pack capacity and c rating to the currant draw of the motor being used