Balancing two lipos in series.

[Braddock, VC]

I use lipos for my starters, one has a 4 cell unit which gives no problem, the other has two identical 2 cell lipos with a series connector that should provide approximately 14 volts under load. They are each 2600 mAh capacity.

I was running in a new engine so the smaller starter with the two lipos in series took a bit of a hammering and wouldn't turn the engine over.

When i separated the batteries the first battery took about 100 mA to fill, the second battery wouldn't charge as the volts were too low.

I swapped the charge mode to nicad and gave it 0.5 amps for no more than 30 seconds which brought the voltage up then change the mode to lipo and set it up for a balance charge at 0.5 amps as I didn't want to inflate the battery.

After about 20 mins it had taken 70 mA so I upped the charge rate to 1.0 Amps and after a further 230 mA had gone in upped the rate to 1.9 Amps.

3.5 hours later a total of 1877 mA (and rising) have gone in but now the charge rate has automatically fallen to between 0.0 and 0.2Amps with the volts steady at 8.4.

Why is only one battery discharging? The series lead has perfect continuity.

I'm at a loss and understand now why the engine was difficult to start with only about 7 volts to turn it over, hardly enough to get it going.

Any suggestions?

[Braddock, VC]

for info. FWIW, the other battery of the pair turns the starter over fine.

Edited By Braddock, VC on 28/06/2017 23:56:54

Edited By Braddock, VC on 28/06/2017 23:57:31

[Martin Harris - Moderator]

Do you normally charge the batteries as a pack or as 2 separate 2S packs?

If they were charged separately, could you have used the wrong balance leads? Not sure off the top of my head at this time of night whether this could result in one pack charging fully and the other one being sensed as full by reading the balance lead of the already charged one and terminating early but it's all I can think of...

Edited By Martin Harris on 29/06/2017 00:29:01

[MattyB]

Sounds like you have toasted your second pack by overdischargong it. Check it's IR on a charger that can measure that; I would expect it to be very high based on your description. If it seems reasonable do a slow charge / discharge cycle and measure the real world capacity; if it's less than 75-80% of the marked capacity, discharge to 0V over a resistive load (please, none of this salt bath nonsense!) and dispose.

[Braddock, VC]

I charge each battery separately.

Combined input into the goosed battery was around 2000 mA, notional capacity is 2500 mAh.(of each of the pair)

I am faced with the outcome, my query was meant to be what caused the problem, viz the batteries have only been used for starter duties, when not in use they are separated from the load and popped into my tool box yet one battery is drained whilst the other isn't.

So why does one battery, in a series set up, supply power whilst its mate, which is in series with it, does not supply any juice?

My DVM shows 16.08 volts when the series connector is in place and with both the batteries at rest immediately after coming off charge.

FWIW, it's in non-critical use so it'll continue to be used until I can get a 4 cell well used s/h lipo to replace it.

[Nigel R]

"So why does one battery, in a series set up, supply power "

It can't be that the better pack does not discharge in use, it must - basic electronics/physics. But the bad pack will discharge far more if it has a high internal resistance, plus of course it won't accept as much charge to begin with. Result is the bad pack empties out rather much quicker than the good one.

Chain and weakest link, etc.

Worth doing the discharge/charge cycle, I'd say on both packs, to see where they are.

[Braddock, VC]

Last night, after finished charging, battery voltages were as follows:- goosed battery 8.39, other battery, (after a quick spin of the starter to see what the revs were like) 7.71.

This morning voltages were 8.34 and 7.71 respectively.

[Braddock, VC]

Doing a discharge cycle as we speak, goosed battery discharged 1315 mA with a 6V cut off point, The other battery is on its way down as I type.

[Braddock, VC]

Curiouser and curiouser to coin a phrase, the good battery only discharged 322 mA. I'm really hacked off with this. They haven't lead a hard life as they are on my standby starter that only gets used once in a blue moon yet one of them is clearly U/S.

[Dickw]

Posted by Braddock, VC on 29/06/2017 09:45:36:

Last night, after finished charging, battery voltages were as follows:- goosed battery 8.39, other battery, (after a quick spin of the starter to see what the revs were like) 7.71.

This morning voltages were 8.34 and 7.71 respectively.

Looking at these voltages suggests that the "goosed" battery is fully charged, and the other battery is nearly flat.

Your discharge capacity values seem to confirm this.

Looks like the problem might be in the charging - are the balance leads OK on both?

Dick

[Braddock, VC]

Sorry, I forgot to press the "add posting" button this afternoon, I've given up on the batteries, life's too short ( at my age) to faff around so they are binned in the local battery recycling receptacle.

I had made up a connector so I could temporarily use one of my 3 cell 2.5 Ah lipos, this was so successful at starting the 40 surpass that I made a better job of it and now it's permanent.

I may buy a larger capacity 4 cell for winter starting but I'm happy to wait and see.

Many thanks for all the response, I really appreciate them.

[Allan Bennett]

Just out of interest, were the batteries kept fully charged, ready for use? If so, that could account for the demise of one.