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

[c0m47053]

Hi All,

I was recently given a Cyclamatic Power Plus (I have worked through a few old theads on here, which gave me a lot of info on the bike) with no batteries. 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.

 

[Nealh]

Tell us/show us how you have exactly connected the bms and wire awg used.

Also I found not all contacts with the diy Vruzend were consistent so only needs one of your parallel connections to not work for the load to cause a collapse. I found that 3 or 4 cells weren't contacting properly causing imbalance and poor performance, I now have 3 kits not used in the garage.

It could be the bms or most likely a cell or two not contacting properly via the end caps.

 

[c0m47053]

Nealh, you were right on the money!

I was confident that I had put the pack together well, and that there was good contact between the caps and the cells, and had even used vernier calipers to make sure they were all spaced the same based on a single cell with caps as a reference.

After reading your post, I decided that it was worth making sure there was good contact within the pack, so I got two bits of wood on top and bottom, and stood (!) on the whole pack. Straight away, I got good voltage out of the BMS and the bike lit up!

I wrapped the pack up, and popped it back in the original enclosure as planned, and have taken it out for a few miles and everything is great!

Thanks for sharing your experience, it really helped me out!