Charging problems

[Wilkco]

First of all sorry if this is in the wrong thread.
I have a 48v 15ah battery and I’m having problems charging it.
The charger should output 54.6 v but when I test the voltage when not connected to the battery it reads 89v!
When the battery has run flat I connect the charger and it starts charging as it should. But it only charges for around 40 mins.
The voltage reading when the charger turns off and is still connected to the battery reads 54.6v.
So is the charger turning off as it should?
I have opened up the battery when it is supposed to be 54.6 and taken a reading from the BMS and the battery is only showing around 48-49v.
Do you think it’s the charger with its high output voltage that’s the problem or is it more likely the BMS that’s not functioning properly?
Many thanks in advance for any ideas on my problem as I’m not sure where to go from here.
Brian

 

[Deleted member 4366]

If we take your facts at face value, it would mean that your BMS is working and shutting off the charging rather than the charger.

It's not possible to have 49v at the battery and 54v at the terminals when the charger is connected unless the BMS has shut off the charging, but that would mean that the charger is putting out 54v, not 89v.

Before doing anything, you need to check the battery in your meter. When the battery runs down, any measurement results can't be trusted because they go all over the place.

 

[Nealh]

Test a new AA,AAA, PP3 etc,etc if the voltage is weird
put a new battery in your meter.

 

[Wilkco]

Thanks,I have two new multi meters both with new batteries that give the same reading,so I take it it’s more than likely the charger as should it not read 54.6 when on and not connected to the battery instead of 89v?

 

[chris_n]

Open circuit voltage with a digital multimeter is meaningless.

 

[Wilkco]

So I’ve opened up the battery and measured the voltage again. I think I must have missed a cell from the sense wires when reading the last time.
The battery cells read 4,4.1,4.2,4.2,4.1,4,3.9,4.1,4.2,4.2,4.2,4.2,
and 4.1 which gives 53.5.
The charger charges to 54.6 then turns off, which was last night so would it have lost a bit of charge overnight?
So is the battery actually fully charged and it’s my BMS that is shutting down the battery after about 15-20 minutes on the bike?

 

[Nealh]

The battery is out of balance, parallel string #1 & 7 are especially so (out of balance), the bms is doing it's job and the lvc is shutting the battery down.
Charge and ride again, measure the cell voltages before charging to see if any cell strings are crashing or if all are dropping linearly and charge again. Once the charger has indicated charge is complete leave it on for another couple of hrs to see if the cells bleed and balance out.

 

[Wilkco]

Thanks nealh,I will try that now

 

[Nealh]

If after a ride #1 or esp 7 are much lower then the others by more then 0.3v then the cells are either poor or the weld nickel contact may be bad so check carefully both sides.
If a bad nickel weld then one or two cells in a P string won't be charging or discharging correctly, the string will under perform also affecting the overall battery performance.

After discharge/cut off post your 13 voltage readings again.

 

[Wilkco]

So the bike managed around 2 miles up a steep forestry road before the battery cut off.
The battery readings are now: 3.8,4,3.9,4,
4,3.9,3.6,3.8,4,4,3.9,4 and 4.
The output from the battery now reads 50.9v.
So is the battery charging ok and the bms is working ok, it’s actually a low cell voltage that’s switching the battery off.
The bike managed further than it did yesterday,if I keep charging and discharging the battery will it balance it’s self out?

 

[Wilkco]

I’ve checked the nickel contacts and they all look fine,should I charge the battery back up now?
Thanks Brian

 

[Nealh]

String #7 is the main culprit 3.6v is the resting voltage but it will have seen greater voltage sag and caused the lvc to act.

What are the cells used or cell specs and bike details (ie. motor / controller rating).

Cause of unbalance could be caused by a few reasons.
Bad/poor cells, bms faulty, poor cell connectivity.

Manually balancing all cell groups equal to 4v then charge as normal to see if they all max out the same, then leave to stand for a few days to see if the voltage bleeds down.

 

[Nealh]

With charging once a cell hits 4.2v the bms will cease charging and go into balance mode, a varying discrepancy in voltages may be the reason why the battery remains unbalanced.

The cells aren't discharging very linearly with 0.1v - 0.3v throughout, the bms may be a crappy one.
As mentioned cell specs and controller specs are useful to know.

Decent cells with a good bms will behave perfectly.
My 3+ years old 29E 36V celled dolphin is perfect all 10 cell strings reading 3.43v after discharge with a 20a rated controller and charge nicely to 41.4 - 41.8v for a pretty good balanced pack.

 

[Wilkco]

The bike has a 1500 watt motor with a 15ah 48v battery.
I’m sure the controller is a 20 amp.
The bike was bought years ago from Xipi.
I’m just learning all this as I go,so how would I manually charge the cells and is it possible to bypass that cell and just have a reduced capacity battery?

 

[Deleted member 4366]

I had a similar battery that I 2as testing a couple of days ago. When I pealed off the nickel, 50% of the cells showed signs of leaking. Can you see any brown stains on or around the nickel on the positive ends of the cells?

 

[Wilkco]

Possibly a tiny bit of brown stain?
I’m not sure

 

[anotherkiwi]

pouch cells!

 

[Wilkco]

Well that doesn’t sound good

 

[anotherkiwi]

Look particularly at cell string n°7, are they puffed up? Leaking?

 

[Deleted member 4366]

Possibly a tiny bit of brown stain?
I’m not sure

I was assuming cylindrical cells. Pouch cells are different. They puff up instead.