Time to Start Building my Battery.

[Storapa]

Hi Folks,

My BBS02 kit (48v 750W) turned up and is now fitted to my bike. It was a simple enough job. Tidying the wiring took the most time. I'll post up pics in the morning. Anyway.......

It's time to start building the battery. I'm starting with some 26650 LiFePO4 cells that I was given. I'm a mechanical engieer so a lot of this is black magic as far as I am concerned. Here goes.

The cells were in packs until about 3 months ago. I broke them up and checked them, then put them away. Today I checked the voltage and all cells were between 3.31 and 3.32 v. A nice surprise. The cells are 3 years old and have only been used briefly. I intend to build two packs. A 9Ah pack (3p - 16s) and a reserve 3Ah pack (16s).

Here goes with the questions.

Is it ok to solder the cells? I don't have a spot welder. I have heard that you can build one out of microwave transformers.

Can I use brass strip if I solder (readily available) to bridge the cells?

The big pack will use a BMS from somewhere like Eclipse. Same for the charger. I will have two chargers, one at home and one at work.

The reserve pack will use 2 low voltage alarms (1 per 8 cells) to save on the cost of a second BMS. Can I use the same charger to charge both packs or will I need a dedicated balance charger for the reserve pack?

Feel free to recomend chargers or BMS modules.

Can somebody recomend a suitable plug / connector from the motor to the battery box? I want to fit one end of the plug to the battery box structure.

Right, I'll leave it there for now. More questions are sure to follow. I'll take pictures as I progress.

Cheers.

 

[Deleted member 4366]

Are you sure that those cells can provide the current? LiFePo4 is often 1C, so 9aH will give you 9A. The 750w BBS02 needs 25A.

 

[Storapa]

Hi d8veh,

Thanks for the reply.

I have the cells down as 3C max continuous and 5C max. They came from an EV pack (1200kg car) originally.

Should I build a 12aH pack to be on the safe side?

 

[Deleted member 4366]

You have to balance the three parameters of weight, capacity and discharge rate. LiFePO4 is relatively heavy.

 

[Storapa]

Hi d8veh,

Hmm, not sure what to do now.

I think that I might take the same route as Kinninvie then and use the Hobbyking Lipos with low voltage alarms. This will at least get me on the road quickly.

I need to find a balance charger now.

 

[Deleted member 4366]

If you're going to get the batteries from Hobbyking, you could get the charger from there as well. At least you'll get a genuine one. Don't forget that most Lipo chargers require a 12v supply. A pair of those 6S 16000maH lipos will be very easy to deal with. Personally, I prefer the BC168 charger (Not available from Hobbyking) because it's basically 6 single-cell chargers, so its useful for charging and balancing any group of cells (up to 6). It's essential equipment if you want to build batteries.

 

[Storapa]

The Hobbyking cells are amazing value. I'll buy a charger from them to keep at work and a BC168 to keep at home.

I might build a smaller capacity pack though. The 4s and 3s 5.2aH packs look interesting.

It's chucking it down here so I've got some time to do some research.

 

[Deleted member 4366]

If you use smaller 4S packs, the wiring and charging is much more complicated.

 

[Storapa]

Good point about the wiring.

I'll build up a pack using 2 Hobbyking cells. Just need to decide on the capacity now. My commute is 10.5 miles and is reasonably flat. I also want to be able to conceal the packs.

I'll go and grab a photo of the bike in a minute.

 

[Storapa]

A pic of what the kit was finally put onto. I'll build up a more suitable bike over winter. The wheels might be a bit delicate.

 

[Geebee]

If I am carrying a second battery I just put it in my pannier bags and have a wire coming out to the connector, invisible and keeps the weight low and doesn't matter what the batteries look like or size.
If you went with the 16ah ones you could put one in each pannier for balance, just make sure they have something harder around them for safety, even corflute works and is light. This assumes you want panniers
If you want lighter the zippy 5 ah 20c 6 cell packs are pretty good value and less wiring than the 4 cells.

 

[Storapa]

Hi GeeBee,
Thanks for your thoughts.
I'll probably build my reserve pack first (5.2mAh - 6s) as the cells are in stock.
I've got some plug questions, but I'll put them on a new thread.