Battery question

[sal_park]

Hi,

I've got a 36v 10Ah Li-ion battery, and I've had it for a year or so and probably used it for 200 charges or so. Anyway, now it seems to be way down on range - it will cut out after a mile or so. If I turn the battery off/on it will always start again but would cut out (LVC) if I draw too much current (eg steep hill) - I guess this is voltage sag due to weak cell(s).



Upon reading around it seems that these types of battery don't have balance circuits built in and given that it still outputs 38v when I checked it last night (since it's last charge it has been used on a couple of short trips and then not charged) I don't think it's totally dead, but clearly in need of some TLC

So I was thinking about charging the battery, taking the cells out and discharging high voltage cells with a power resistor (say 4ohms) to get the cell voltages all the same.

What do you think ? Crazy ? Don't do it ?

Also, am I safe to do this without disconnecting the cells from one another ? I think so, but best to ask rather than be burnt


sal

 

[crea2k]

The problem is if some of the cells have more internal resistance than others the rate of charge per cell is going to be different anyway, so even if you get them all at the same level as soon as you start charging them, some will charge quicker than others. You would be better off using a balance charger and charging them like that, either that or just connecting each cell individually to a charger and charging it as 1s to 4.20 volts.

 

[sal_park]

Hi crea2k,

I agree it would be better to use a balance charger, but right now I need to spend as little as possible to eek out some more use of this battery. In the medium term I'm going to replace it with some RC packs from hobby king which will be balance charged.


sp

 

[crea2k]

You can get a balance charger off hobbyking for like £15-£20

 

[d8veh]

The first thing to do is charge it up until the charger goes green. Then, take out the cell-pack, expose the BMS and measure the cell voltages on the multi-pin connector.

Nearly all batteries have BMSs that balance the cells, so the chance that manual balancing might fix it is not high, but worth a try.

When you open it, show us what cells are in the cell-pack because different ones have different failure modes and need different repairs.

 

[sal_park]

When you open it, show us what cells are in the cell-pack because different ones have different failure modes and need different repairs.

Thanks d8veh. Will update this thread when I've done that.


sal

 

[dnmun]

you can open it up and pull out the BMS and the pack too first. then take pictures of both and post here. measure the output voltage of the charger with your DVM and keep charging the pack until the light goes green and then measure the cell voltages on the sense wire plug or on the cells them selves if you pull the battery out of the case.

measure while charging. do not take the pack off the charger and let the cell voltages drop, measure while charging.

 

[sal_park]

So I opened it up, and sure enough it's got a full in BMS in there

I measured the cell voltages before charging it (it was at around 50% charge state at this point):
(lowest 3.666, highest 3.775)
3.775, 3.775, 3.770, 3.770, 3.774, 3.769, 3.774, 3.666, 3.773, 3.770

I then charged on my li-ion charger: It started out around 1amp and gradually went down to around 200mA, and the battery was still taking this when the charging light went green.... :?

Here are the voltages after the charge, with the charger disconnected:
(lowest 3.855, highest 3.968)
3.966, 3.966, 3.964, 3.966, 3.967, 3.966, 3.966, 3.855, 3.968, 3.966

And here are shots of the BMS:
View attachment 1

 

[wesnewell]

It's just not getting fully charged. Leave it on the charger and all the cells should eventually charge up to at least 4.1V, assuming your charger is putting out 42V. Check voltages after a few hours more of charging. If they continue to increase, keep charging until they don't.

 

[d8veh]

They should all be higher than that if the charger went green. You should have at least one over 4.1v. You need to measure the voltage coming out of your charger. It must be down by a couple of volts for some reason.

 

[sal_park]

They should all be higher than that if the charger went green. You should have at least one over 4.1v. You need to measure the voltage coming out of your charger. It must be down by a couple of volts for some reason.

Yea, I tried to measure the voltage from the charge lead, but didn't get anthing - I suspect the resistance of the DVM was too high so the charger would not turn on. I then plugged the charger into the battery and the DVM into the output plug from the battery.

With the charger off: 39.something
With the charger on: 39.something + 10mV

I left it on charge for 5 minutes: the voltage went up by 1mV.

I cycled the charger on/off a few times and it would consistently increase the voltage by 10mV with the charger on.

Given this I think the charger is broken - it has traveled about 5000 miles in my panier so it has had quite a hard life.

I've just order this from ebay, so will report back when I get it.

d8veh and others - thanks for you help !

sp

 

[crea2k]

Are you sure its not outputting any because it thinks its fully charged.

 

[wesnewell]

A 2A 42V charger will take at least 4-6 hours to charge a depleted 10ah 36V pack to full. Did you leave the charger hooked up close to this long. If not, do so and then check voltage. Fully charged it should be close to 42V for the pack.

 

[dogman dan]

Quite possible the charger is no longer putting out full voltage. I have seen them do that, drift lower voltage after many cycles use.

3.9 or even 4v is not full.

There may still be weak, low capacity cells in there anyway. Batteries age out even if not used.

 

[dnmun]

you have to measure while charging. i explained that at the beginning.

if you wanna know if the battery is charging then put an ammeter in the negative charging lead. you can see the balancing current flow then.

adjust the charger voltage up if you need to but until we have accurate measurements it is not possible to tell you which is causing it to hit the HVC.

 

[sal_park]

Hi,

you have to measure while charging. i explained that at the beginning.
if you wanna know if the battery is charging then put an ammeter in the negative charging lead. you can see the balancing current flow then.
adjust the charger voltage up if you need to but until we have accurate measurements it is not possible to tell you which is causing it to hit the HVC.

I did do that, but didn't put the results in the post, I guess I forgot. When I measured the voltage of the first cell at various point in the charge cycle the voltage slowly rose to around 3.9something. I also measured the current which was a bit all over the place but the max I saw was 800mA. Towards the end of the charge I seem to recall it was around the 200mA mark.

 

[dnmun]

if you can measure the charging current and it measured 200mA then that means it is still charging.

your battery is 10S from looking at the BMS so your charger should show 42V on the charging plug.

you should continue charging and when the current stops in your ammeter then you wanna know which of the cells is at the HVC so you have to be measuring them at the point in time when the charging current stops.

when you know which one is the highest then you can drain charge off of it with a resistor and that will restart the charger unless it hits HVC and does not reset until you discharge the high cell down to about 4.05V which is the standard reset voltage for a lipo BMS.

in that case you would drain some charge off of the high cell down to the same level of voltage as the other cells and then discharge the entire pack low enuff to reset the BMS.

 

[sal_park]

Looks like my old charger was toast:

my new charger has just charged the battery to 41.87v.

 

[Ykick]

Thanks for the update and solution to your issue.