36 V pack not charging

Batteries, Chargers, and Battery Management Systems.
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amaurer   100 µW

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36 V pack not charging

Post by amaurer » Dec 06 2019 1:24pm

Hi, I'm new in this forum so please bear with me if I ask a stupid question.
I have a 36 V battery pack that consists of 40 cells.
The pack was working properly until from one day to the other it was completely discharged without any use.
If I connect it to the charger it immediately disconects itself. I measure the charger voltage and it is around 42 volts but the pack only gives me 15 volts.
I unpacked the battery pack and if I measure voltage from one end of the pack to the other it reads 40 volts so I guess the individual cellas are ok and are properly connected. But the pack at the output keeps giving me 15 volta.

There is a small "controller inside the pack that I guess is damaged.

Any ideas ?

hemo   100 W

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Re: 36 V pack not charging

Post by hemo » Dec 06 2019 7:25pm

40v isn't too good, so sounds like one cell group is low (< 3 or 3.1v) in which case the BMS you can see will not switch to allow the charger to do it's job.
On the BMS you will see the parallel group sense wire connector (usually white) with 11 wires for 36v, you need to unplug it (might be tight fitting) and take ten voltage readings from each of the pin outs and write then down 1 - 10 as xxxx voltage and post them for us to see.
Two ways of measuring, a single cell group reading for all 10 or a cumulative reading as you measure each pin.
For single cell reading place Black probe on the Black small sense wire pin out and the Red to the next one along for a cell reading, then move both probes a long one pinout until you have 10 voltage readings.
For accumulative reading place Black probe on Black sense wire and the Red to the next pin out then after that just move the Red probe along all 10 pinouts to get all voltage readings. Any cell group below 3.2v will need to be manually raised to allow charging which we can advise on once we know your ten cell group voltages.

amaurer   100 µW

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Re: 36 V pack not charging

Post by amaurer » Dec 07 2019 1:43pm

Hello Hemo:
First of all, thank you very much for your help.
I've done what you adviced me to do and here are the results.

There are 11 leads, labeled B-, B1, B2,.....B9 and B+. B- is of course black and B+ red. The rest are all white.

Voltages
B- TO B1 0.6 V
B1 TO B2 4.2 V
B2 TO B3 4.2 V
All the rest are 4.2 V

B- TO B1 0.6 V
B- TO B2 4.8 V
B- TO B3 9 V
B- TO B4 13.2 V
B- TO B5 17.4 V
B- TO B6 21.6 V
B- TO B7 25.9 V
B- TO B8 30.1 V
B- TO B9 34.3 V
B- TO B+ 38.5 V

So I guess everything is OK except for B- to B1. Right ?

I would very much appreciate if you tell me what shall I do to correct this.

Regards

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MAXIMUM_AMPS   10 mW

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Re: 36 V pack not charging

Post by MAXIMUM_AMPS » Dec 07 2019 2:32pm

Sadly you'll have to replace that whole group of cells from B1 to B-, and likely the BMS board. If you know someone with a battery spot-welder, this can be done cheaply, but if not, it's very difficult.

hemo   100 W

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Re: 36 V pack not charging

Post by hemo » Dec 07 2019 7:39pm

Hello Amaurer,
Yes B1 is toast.
The BMS has allowed it to deplete, likely a faulty bleed resistor.
Luckily B1 & B10 are the easiest groups to replace, I would also replace the BMS. Ann battery on Aliexp I have always found to sell good basic BMS, I usually opt for ones with the added switch wiring and implement a switch to the battery packs I build.
With all sense wiring if there is one more the then the series count then B-/B0 is always negative and the others are always positive, mostly B-/B0 is Black the others can be White/Red or whatever colour they choose to use.
I had a couple of LG BMS and the sense wire sequence was like a rainbow.

amaurer   100 µW

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Re: 36 V pack not charging

Post by amaurer » Dec 07 2019 8:32pm

Thank you Hemo.
But how would youweld the new batteries ?

hemo   100 W

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Re: 36 V pack not charging

Post by hemo » Dec 08 2019 6:28am

For welding you have to know a friendly ebiker/battery builder who can help you out or go diy, make one as in various threads on the forum which forumites have done or buy a ready built one.
Malectrics , Kweld & Boss hand held welders all work well, the downside is the expense if only for a one off.

If you haven't yet taken the battery a part you can try charging the low cell group up to see if will maintain a charge but likely it will be the weak link, nothing to lose in giving it a go.
To do it very cheaply you need an old 5v/400- 500ma mobile phone charger and cut off the charge plug, with a meter check the wire polarity and colour them Red/Black with tape or shrink wrap.
Buy, beg, borrow or steal a couple of pins from a pin strip commonly used in computers/pcb boards and solder one to each of the bared charger wire ends, insulate the them with shrink to leave 4 or 5mm of pin showing.
On the sense wire connector to the battery insert the Black pin in to B- and the Red one in to B1, plug in the charger and switch on ( Be careful not to short anything). Alternatively you can attach the wires directly to the battery B- & B1 cells.
With a multimeter check the voltage on B- to B1 every 15 -20 mins to check progress until voltage has reached 4.2v max ( do not leave the charging and do something else). When done leave the BMS unconnected for 24 - 48hrs to see if cells discharge if they don't you might be in luck but you will need to replace the BMS as it has caused the issue in the first instance.

obcd   100 W

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Re: 36 V pack not charging

Post by obcd » Dec 08 2019 8:50am

So the advice is to connect a 5V charger to a group of cells that got discharged to 0.6V ?
First of all I would recommend to "at least" place some resistor in serie to limit possible charge current.
Like 5V - 0.6V / 0.1A = 44 ohms. A 56 ohms should work as well. It will choke maximum 4.4V * 0.1A = 0.44W
Use at least a 1W version or use 2 100 ohm 0.5W resistors in parallel.
Second of all, it means you are charging the battery cell group with 5V.
This means you need to monitor the cell voltage continiously.!!!!!!!!!
Stop charging when it reaches 4.0 - 4.1V
Charging to a voltage above 4.3V can cause a cell thermal runaway. This means it will produce magic smoke and likely some flames.
A charging adapter marked 400 - 500mA means it can deliver that current continously, but you have no idea how such charger will react if you ask more current. Some might simply limit the current to that 400 - 500mA but others might provide more (for a short period) and finally fail.
Our local fire department also states that you should never try this with a LIPO battery. If those got fully discharged and you try to recharge them, even with a limited current and below the maximum allowed voltage, they still might overheat and burst in flames.

You will need to replace 4 cells. Is it worth the risk burning up your battery?

If you try to recharge the group, at least also monitor the temperature during the proces.

amaurer   100 µW

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Re: 36 V pack not charging

Post by amaurer » Dec 08 2019 9:30am

Thanks to everyone for their feedback. I'll try to replace the 4 batteries in my pack.
As for the BMS, which do you recommend ? I've seen that there are some that do "balancing" and other that don't. Of course the latter is cheaper.
What if I put one of those ?

obcd   100 W

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Re: 36 V pack not charging

Post by obcd » Dec 08 2019 11:35am

Balancing ensures that all cell groups (for 36V you have 10 groups in serie A group could be 4 or 5 cells in parallel) remain equally charged
Without a balancer, if one group has a lower voltage, it will stay like that as they all receive the same amount of energy. The group with the lower voltage should get a bit more energy to come to the same charge state as the other groups.
In an ideal world, with a pack with all equal cells, you don't need a balancer as all cells will discharge equally and recharge equally. So the voltage of the groups should also stay equal.
BMS normally need 5V as supply voltage to function. Instead of making that voltage from the battery voltage, some just use the voltage of the 2 lower cell groups to make their 5V The voltage there is between 5V and 8.4V, so it's easier to use. This however also means that the 2 lower groups will become lower in voltage compared to the others as they have to power the bike and the BMS board. The BMS power consumption is low compared to the bike, but after a longer period, it will start to show a difference.
A good BMS is what protects your battery against most mis behavour. It controls maximum charge current and maximum discharge current. It als disconnects the battery output if the voltage becomes to low, and it disconnects the charger port if the cell group voltage has reached it's "charged state" If it does it's job properly, it will minimise the risk of your battery catching fire.
The problem is that some chinese BMS boards don't do their job properly. So buying a cheap BMS board from an unknown source is a risk. Some allow the cell group voltage to raise up to 4.3V. This will seriously reduce the number of charge cycles before your battery will start to degrade. The best BMS boards are those with a bluetooth interface. They allow to monitor all the battery parameters from a pc or a phone. So you can simply see the voltage of every cell group. You can also adjust the parameters like maximum discharge current and low voltage cutoff treshold. As usual, with great power comes great responsability. If you set the low voltage cutoff treshold of a 36V battery to 20V, you will damage your battery, but if you know what you are doing, the option to see all battery parameters is great.
And also very important, get a BMS that also controls your battery temperature. When something goes wrong, the cells will start to heat up. They hopefully don't go in thermal runaway right away if you stop the charging / discharging when a raise in temperature occurs.
So my advice, if you have room for one in your battery case, get a bluetooth BMS with balancing.
(I don't sell BMS's and I don't make them. It's just my opinion that you shouldn't go low cost on safety.)

999zip999   100 GW

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Re: 36 V pack not charging

Post by 999zip999 » Dec 08 2019 1:08pm

Not the safest to be charging a more then dead battery cell. Remember 2.5v is zero in battery talk. Yes zero is 2.5v less then that is dangerous to recharge.
When charging a battery cell or group up parallel cells. With a phone 5v phone is fine if you stop it at 4.2v and not above. It will charge to 4.14v and then it will charge very fast to 4.2v and above do not charge above 4.2v so you must seat and watch and monitor very closely. You could find a vendor to make you a 4 parallel spot-welded string and solder it in not soldering over the top of the cell. Read DrAngle pack builds. Lots of reading there. Do charge the new set to same as others as the BMS is to keep a balance pack balance. Or no don't over work the BMS. Read DrAngle.
.

amaurer   100 µW

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Re: 36 V pack not charging

Post by amaurer » Dec 10 2019 7:15am

Hello again.
I charger the faulty pack as some of you adviced, for about 30 minutes and got it to 3.4 v.
I disconnected it and after about 10 hours it still has 3.4 v.
What is your opinion, shall I keep the pack or replace it ?

As for the BMS, do you think the bluetooth one is worth it

999zip999   100 GW

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Re: 36 V pack not charging

Post by 999zip999 » Dec 10 2019 3:09pm

Now charge the low parallel string to 4.1v and see if it stays don't ever go over 4.2v.. Good luck

obcd   100 W

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Re: 36 V pack not charging

Post by obcd » Dec 10 2019 6:16pm

A bluetooth BMS will be able to tell you if the cell group is still providing current like the other groups. If the voltage of that group drops faster during use compared to the others, some cell damage has occurred. It can also give you some detailed information about your battery state of charge. If your controller display only has the battery symbol with 5 levels, it will provide you a much more accurate reading. It will also tell you how much your pack has degraded compared to it's original power.
It pretty much depends how much you care about your battery.

999zip999   100 GW

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Re: 36 V pack not charging

Post by 999zip999 » Dec 11 2019 11:15am

So we are looking for the group to hold voltage and capacity and its ability to deliver voltage with capacity. And not self discharge or leakage.

999zip999   100 GW

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Re: 36 V pack not charging

Post by 999zip999 » Dec 14 2019 4:46pm

? So what happened ?

amaurer   100 µW

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Re: 36 V pack not charging

Post by amaurer » Dec 14 2019 9:52pm

It seems the pack is holdind voltage, even with the BMS connected.
Do you think I need to change the BMS or can I go long with the one I got ?

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