No Volts Under Load on New Battery Build - BMS at Fault?

Hi All,

I was recently given a Cyclamatic Power Plus. The cycle parts on the bike seem good (apart from BB bearings, replaced now), so I decided to build a new battery pack for it.

I picked up 30 Panasonic NCR18650PFs and a Vruzend V2 kit. I also bought a cheap 10s BMS from ebay (https://www.ebay.co.uk/itm/262605636328) and a cheap 42V 2A charger.

Pack built, all looks good when not connected to anything. I got 35.2V when first built, all cells were individually tested at 3.52V, and each bank of 3 cells were looking good. As soon as I connect it to any load, voltage drops to near zero, but the actual value does seem to vary (up to 3V some times). The load can be the controller on its own, without the motor or throttle connected, but also got the same result when hooking it up to a fan heater.

I have double checked the wiring, but this is my first build so I am not excluding some newbie error here. I have checked the balance connectors using the points on the back of the PCB and between the battery -ve and ech wire I get the expected 3.52V, 7.04V etc. If I connect the same load directly to the battery terminals, everything seems fine. I also get working output if I connect the charger to the charge leads, and connect the load via the BMS.

I have charged the battery up to 4.0V and the same behavior exists. I am currently charging it up to full to see if that makes any difference.

I have raised a request with the eBay seller to suggest anything before I file a return, but am looking for any suggestions on what else could be wrong.

Are you still getting 40-42 volts on the cells directly (between B* and B- on the BMS) when hooked to the bike under load? Can you run the bike connected directly to the cells?

If not, then I would walk the voltage probe up the balance terminals under load. Check for a loose contact in the Vruzend array. You get one bad contact and under load it trips the BMS.

Hey, thanks for the response.

I get full voltage under load from the pack when measured directly from the terminals. I added a second discharge connector to the terminals and hooked it up to the bike, and it runs really well.

So it seems the BMS isn't working?

docw009 said:

So it seems the BMS isn't working?

Click to expand...

That's what I was thinking, but turned out not to be the case.

I asked same question over at the pedelecs.co.uk forum, and a member there mentioned that they had issues getting the caps to properly connect even though it looked good when they put it together.

This inspired me to do something slightly stupid. I took two lengths of wood, put them at the top and bottom of the pack, and stood on the whole arrangement. This did the trick though, and there must have been some bad connections in the pack somewhere. Proper voltage was restored, and I have been able to get the pack back into the factory battery enclosure and take it out for a few miles of test riding.

Thanks for the suggestions though, not sure why I couldn't pick up the bad connections with a multimeter. Just happy that I could get it to work.