Recommend a BMS

[Proboscis]

I'm now the proud owner of a slightly ratty but serviceable 36v China e-bike for a reasonably good price.
There's only one snag... it doesn't have a battery or charger.

Now I could just buy them, but... in my shed I just happen to have 4KWh of 20Ah 3.2v LiFePO4 pouch cells from a PHEV conversion kit (doesn't everyone?). So 'all' I need is a charger and BMS. The bike itself just has a two-pole power input - I think it's not doing anything fancier than voltage sensing.

Looking around, I saw the China BMSes which just look like a string of comparators - not terribly impressive. I would have expected a BMS to have at least a microcontroller to decide when to turn things on and off and maybe a connector for data or a display.

Is that true, or do I malign the $10 China BMS boards? Some of the TI chips look quite good, so maybe they are hiding one out of shot of the ebay/etc photos. If the China BMS has one of those and there's an accessible serial/I2C port then that might suffice.

So would anyone recommend what kind of slightly-less-dumb BMS to consider?

If I search for 'open source BMS' I find a variety of projects in various states of abandonment. As it happens I also have (but not in my shed) cheap PCB fab, a pick and place machine and PCB reflow oven, so building one of these isn't out of the question - if there's anything suitable out there. I suspect it's probably not worth the time investment though.

Am I right in thinking that once I have a suitable BMS board, I can use a generic constant voltage / constant current power supply as charger, and the BMS will do the job of turning the load on and off as necessary? From what I see of 'power brick' chargers listed on ebay, there's doesn't appear to be signalling from the battery to the charger.

Suggestions?

(it turns out 'BMS' is something the forum doesn't like me searching for, but I tried my best with Google. Do tell me to RTFM if there's a good description somewhere)

 

[Deleted member 4366]

You need a special BMS for LiFePO4. You can find them on Ebay or Aliexpress. Obviously, they have LiFePO4 in the description. Any 12S one should work. You don't need to worry about what's on the PCB, but the better ones have separate charge and discharge points, and a header for an on/off switch is very handy.

You can use any CC-CV charger with a charge voltage of 43.8v.

It's going to be a very heavy battery, which will make the bike's handling a bit awkward, especially when you're off the bike. If you're going to put it on a rack, it needs to be right up against the seat-post and as low as possible, which means that you need to customise the rack somehow or make your own.

 

[Nealh]

The specs for a lifepo4 BMS will have max 3.65v per cell charge voltage unlike 4.2v for li-on/li-po.

 

[fusion.wind]

So would anyone recommend what kind of slightly-less-dumb BMS to consider?

I struggled recently to try and source a UK based supplier for a recommended BMS, ended up just taking a gamble on random BMS supplier of the moment from somewhere in the world as my battery was very simple anyway. Seems to do the job.

The next stage, consider how far you wish to go. Passive or active cell balancing? Maybe, one with bluetooth that you can programme in battery voltages and cutoffs from your phone. Perhaps, you wish to have one which estimates the capacity by counting coulombs. The TI chips, they are all only surface mount so hope you have steady hands and stable flow to connect to the already worked I2C bus pin

Price goes up with each feature, but my simple BMS cost ~£15 delivered.

Or, next time maybe I'll get one of these:
https://www.speedict.com/tech-specs

 

[Deleted member 4366]

There's always the Speedict Neptune 15, which can do everything. It should be a BMS for life:
https://www.speedict.com/tech-specs

 

[Nealh]

If you require more then 15a output just look for one.
https://www.aliexpress.com/item/36V-BMS-for-12-series-of-battery-assembly/1196501778.html?
Ann battery a reputable outlet on Ali have used several times with no problems.
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