Battery Issue

TheBikeMan

Pedelecer
Mar 23, 2018
58
1
60
Cardff UK
Hi all,
One of my customers has a 48V 20Ah battery. When he plugs it in to the bike nothing happens at all. He assumed this was the BMS so he asked me to put a new one in which I did the old one was in wrong anyway. It had the B- going to the charger/power out and the P- going to the negative on the battery. Anyway, new BMS and still the same problem. The battery accepted the charge and measured 51.5V when it indicated full but on the bike it does nothing.
However, when you attach the charger to the battery when it is on the bike, the bike powers up fine and runs.
So: The battery is measuring the correct voltage but does not power the bike unless the charger is plugged in.
Any thoughts?
Thanks
Karey
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
19,990
8,172
60
West Sx RH
48v 13s should read 54.6v fully charged, 51.5v is too low.

Check the voltage output of the charger should read 54.6v.

A cell group or two may be out of balance and are of a higher or lower voltage then the remainder, if this the case the bms will not switch on to allow the charger to do it's job.
BMS usually will not allow charging if any cell is < 3v or > 4.15v4.18v

Disconnect the bms sense wire jst connector and take 13 voltage readings from the wired block. Start at B- & B1+ for cell 1, B1+ & B2+ for cell 2, B2+ & B3+ for cell 3 and so forth till you get 13 readings. Any abnormal/ widely disparaging results should show up if balance is indeed out.
 
Last edited:

TheBikeMan

Pedelecer
Mar 23, 2018
58
1
60
Cardff UK
Thanks, Yes, one of the cell groups was just about flat. If you don't mind me asking another question. How do you check a BMS to see if it is okay? This battery was still measuring 50V on the battery but only 38V on the output line of the BMS.
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
19,990
8,172
60
West Sx RH
BMS are too complicated to fault find.
Readings can be taken via the sense wires or main supply/discharge wiring to determine if it is working as it should, otherwise nothing the normal layman can do with them.

I would assume the new one you have wired correctly should work but will only do it's job if the parallel cell groups are close voltage wise to start with.
Any low cell groups will need manually charging up to match the other group voltages then the BMS will be able to manage the balancing. Charging and balancing then can/should take place as normal.

The 38v on the BMS out line may be due to the flat /depleted group, depends which cell group was bad.
What was the voltage of said group ?
Did you get 12 other good voltage readings ?

For instance if either group 3 /4 or 9/10 was totally dead then it may not allow voltage through via the series connection so first few or last few groups won't register hence why BMS may only show 38v.

Does the BMS jst read 50v when you probe pin 1 & 14.
 
Last edited:

TheBikeMan

Pedelecer
Mar 23, 2018
58
1
60
Cardff UK
Yes, that explains it. It was cell group 10 and it was reading .5V so it was dropping out other cell groups. The other groups were reading between 3.1V and 3.5V.
Part of my confusion came from the assumption that the charger would only indicate a full charge when the battery reached a certain voltage but I'm now assuming that it has nothing to do with a Voltage reading but depends on the balance of the cell groups.
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
19,990
8,172
60
West Sx RH
Depending on the cells & age the 0.5v may be recoverable, on ES a guy has recovered PF cells that were reading 0v. They charged up and stayed charged giving good use and full ah.

BMS minimum charge cell voltage varies (2.8v is often the min), 0.5v will not allow BMS to switch on for charging only chance is to replace the cell group or try and manually charge the group up to 3.1 at least.

An old 5v mobile phone charger can be used to do this, keeping an eye on the volts as they rise is needed then stop charge at above 3.1 - 3.5v.
Cut the charge end off and solder a small pin to each wire, stick one in jst #9 and the other in #10 and see if the cells respond.

The wrongly wired BMS has obviously led to the current state of affairs.
 

Advertisers