DIY E-bike battery, BMS issue?

[quakerj]

Wondering if anyone can help out with this. In the process of converting my son's 20" BMX bike to electric (FWD hub motor). The bike is done, but I'm running into issues with my battery pack.

I built a 13S3P pack (all new Molicel P42S 21700 cells, three in parallel, then the parallel groupings in series 13 times to achieve 48V). I wired in a BMS that I purchased from Amazon and that's when things seem to have gone sour. The pack is solid, measures 45.5V (each cell was delivered at 3.5V) but the BMS doesn't seem to be functioning correctly, I only measure about 35V from its output. It won't take a charge either.





This is all the instructions included with it, but I believe I followed them to a T. The B- connection @ BMS takes the load from the (-) end of the pack, so should be heavy enough wire to support 30A, checked that box. The C- terminal is the negative output to the load (motor) or input from charger, so the BMS can control input/output from the battery and shut things down if needed (over/under volt, overcurrent, etc.) The P-, according to documentation is only for split units that support an independent charge / output plug, which this unit does not have.

On the balancing ribbon cable, B0 (only black wire) goes to the negative end of the pack (much like B-), then I progressively wire B1-B13 to each positive terminal (of the parallel groups) starting at the negative end of the pack. That used up all 13 wires like it should. Then B+ (positive end of pack) I have going straight to the load (or charger). I don't think I've done anything wrong, but it's not functioning with the BMS installed.

Possible faulty unit? I double, triple checked everything I've done and I can't see anything amiss. Did I miss something? FWIW, the pack works fine if I were to bypass the BMS.

Here's the Amazon listing: Amazon.com: 13S 48V 50A Battery Balanced Protection Board BMS PCB Circuit for Li-ion Lithium Battery : Electronics. They seem fairly popular, available on Amazon, Aliexpress and from other vendors.

 

[gromike]

I would assume that the P- bms port should be connected to the load.

 

[MxusMick]

Your voltage is low, and may have triggered the BMS LVC low volt cutout, I don't know anything more technical....... good luck

 

[docw009]

Some BMS will only recognize a charger between B+ and C-. Load has to be between B+ and P-.

 

[quakerj]

Some BMS will only recognize a charger between B+ and C-. Load has to be between B+ and P-.

View attachment 385372

I think something must be screwy with this BMS. I have the XT60 connector (supposed to serve as output for load or charging input) connected to C- (negative) and B+ (positive straight to pack) as per their diagram. It will not charge at all wired this way.

The unit I have is not a split load / charge model, hence why they show nothing connected to P-. Those models do exist in 60A+ configurations, they just use the same PCB for both, though mine is not used.

I tried using P- for load (like your altered diagram shows), it behaves the same, I get some neutered output voltage (38V or so) and no output current to speak of (motor controller won't even turn on).

Oddly enough, if I place a jumper wire between the C- and P- terminals, the battery will happily charge, but I still get the neutered output which won't support a load. If I bypass the BMS by hooking the negative of the motor controller straight to B-, it's a very lively bike and works fine.

I've ordered a replacement in the event I simply got a faulty unit. That would be best case scenario so I don't have to do a lot of rewiring. If the replacement doesn't pan out, it wasn't meant to be and I will go with a more reputable BMS.

 

[fechter]

I recently got a BMS from Amazon for a small project. I didn't work. After spending time troubleshooting, it turns out the unit was made for LiFePO4 cells and not Li-ion cells. Reading the numbers off the chip on the board allowed me to look up what the voltage settings are (this is a non-programmable unit). The listing showed Li-ion, so the seller had no clue what they were talking about or the boards got mixed up somewhere.

 

[E-HP]

Wondering if anyone can help out with this. In the process of converting my son's 20" BMX bike to electric (FWD hub motor). The bike is done, but I'm running into issues with my battery pack.

I built a 13S3P pack (all new Molicel P42S 21700 cells, three in parallel, then the parallel groupings in series 13 times to achieve 48V). I wired in a BMS that I purchased from Amazon and that's when things seem to have gone sour. The pack is solid, measures 45.5V (each cell was delivered at 3.5V) but the BMS doesn't seem to be functioning correctly, I only measure about 35V from its output. It won't take a charge either.


View attachment 385365


This is all the instructions included with it, but I believe I followed them to a T. The B- connection @ BMS takes the load from the (-) end of the pack, so should be heavy enough wire to support 30A, checked that box. The C- terminal is the negative output to the load (motor) or input from charger, so the BMS can control input/output from the battery and shut things down if needed (over/under volt, overcurrent, etc.) The P-, according to documentation is only for split units that support an independent charge / output plug, which this unit does not have.

On the balancing ribbon cable, B0 (only black wire) goes to the negative end of the pack (much like B-), then I progressively wire B1-B13 to each positive terminal (of the parallel groups) starting at the negative end of the pack. That used up all 13 wires like it should. Then B+ (positive end of pack) I have going straight to the load (or charger). I don't think I've done anything wrong, but it's not functioning with the BMS installed.

Possible faulty unit? I double, triple checked everything I've done and I can't see anything amiss. Did I miss something? FWIW, the pack works fine if I were to bypass the BMS.

Here's the Amazon listing: Amazon.com: 13S 48V 50A Battery Balanced Protection Board BMS PCB Circuit for Li-ion Lithium Battery : Electronics. They seem fairly popular, available on Amazon, Aliexpress and from other vendors.

The description in the listing makes it fairly clear that this is a common port BMS, although for some reason they have the additional connection for a separate port BMS.
For e-bikes, separate port is seen more since it offers more protection. Since common port charges and discharges from the same points, there isn’t a separate overcurrent limit for charging vs discharging, so per the description, that would be 40A, so your charger output is the limit (Charging the pack at 40A might make it burst into flame before it even reaches a full charge).
Both types of BMSs can be used safely, but you should know the limitations when it comes to protection when using common port.

 

[docw009]

You must have bought this one on amazon? It says "balance" in the title, but there are no specs. I always look at the BMS specs to see if they list the balance current. If the specs don't show it, it's not likely to be a balance BMS. If there are no specs at all, it's a pig in a poke, without the poke.

I haven't bought a BMS from amazon in a long time. When I was looking, very few of their BMS balanced even though the listings often said they did, and the reviews verified no balance.

I do have this Jabaida model sold on amazon, but I bought it from another vendor (sriko). Seen good rebviews on it on this forum, but I have yet to wire mine up. Bought two, It's bluetooth.

 

[Nix Liou]

The description in the listing makes it fairly clear that this is a common port BMS, although for some reason they have the additional connection for a separate port BMS.
For e-bikes, separate port is seen more since it offers more protection. Since common port charges and discharges from the same points, there isn’t a separate overcurrent limit for charging vs discharging, so per the description, that would be 40A, so your charger output is the limit (Charging the pack at 40A might make it burst into flame before it even reaches a full charge).
Both types of BMSs can be used safely, but you should know the limitations when it comes to protection when using common port.
View attachment 385374

In my experience, every common port bms worth buying has separate charge and discharge limits, it's just sharing the same mosfets, thus allowing the charge limit to be up to as much as the discharge limit.
Smart BMSes like models from ANT and JBD are definitely worth it and allow for setting these cutoffs, as well as some diagnostics via bluetooth.
I like JBD bms options, I have used a few of them ranging between 14s 30A and 20s 200A with no issues from the BMS. Their SP004 model is nice and compact and comes in a 7-14s 30a variation which sounds like it would be a good fit for this pack.

 

[E-HP]

In my experience, every common port bms worth buying has separate charge and discharge limits, it's just sharing the same mosfets, thus allowing the charge limit to be up to as much as the discharge limit.
Smart BMSes like models from ANT and JBD are definitely worth it and allow for setting these cutoffs, as well as some diagnostics via bluetooth.
I like JBD bms options, I have used a few of them ranging between 14s 30A and 20s 200A with no issues from the BMS. Their SP004 model is nice and compact and comes in a 7-14s 30a variation which sounds like it would be a good fit for this pack.

I would say based on the specs, I wouldn’t buy this one. I like common port BMSs for my purposes though.

 

[molin]

A 13S pack measuring ~45.5V sounds normal if the cells were around 3.5V when assembled (3.5 × 13 ≈ 45.5V). So the pack itself is probably fine.
If you're only seeing ~35V at the BMS output and it won't accept charge, a few things could be happening:
1. BMS protection triggered
Some BMS units ship in protection mode if one of the cell groups isn't detected correctly or if the balance leads were connected in the wrong sequence.
2. Balance lead wiring issue
Double-check the balance connector order (B-, B1, B2 … B13). If even one wire is misplaced, the BMS may block discharge or charging.
3. Cell group voltage mismatch
Measure each parallel group directly. If one group is significantly lower, the BMS could be preventing output.
4. Incorrect BMS type
Some cheaper BMS boards are designed with separate charge/discharge ports, and using the wrong port can result in no charge or reduced output voltage.
5. Faulty BMS
Unfortunately some low-cost BMS units from Amazon can fail right out of the box. If all cell groups are balanced and wiring is correct, this might be the case.
If possible, measure the voltage:
directly at the pack (B+ to B-)
at the BMS output (P+ to P-)
across each series group
That will usually make the problem obvious pretty quickly.
We see similar issues quite often when testing DIY packs in the lab — most of the time it's either the balance lead order or a protection trigger.

 

[Nix Liou]

I would say based on the specs, I wouldn’t buy this one. I like common port BMSs for my purposes though.

Yeah this particular model is a bit suspect...

 

[quakerj]

Thank you all for your help with this. The problem was a faulty BMS. The replacement saved the day, took the old one out, replaced with new one with no wiring changes (common charge/load @ C- terminal according to instructions).

It charges fine, bike works fine. Over time I'll verify how the balancing is working by measuring each 3P group.

The charger stops the pack around 54V (4.15V/cell), the bike starts to really lose energy around 3.1V/cell. On a 20" bike this battery arrangement gets an easy 2-3 hours of run time so far, everything stays cool to the touch, but I have more testing/observing to do.

I'm skilled with batteries/electronics but this is my first foray into DIY E-bike stuff, this is just a simple 20" BMX bike conversion for my 10 year old son. He's thrilled with it and I have a sore butt from riding it-- it's a fun little unit.