concurrent/independent charging

[ltosolini]

Experts,
I don't know if concurrent or independent charging is the right term for what I am going to described, the fact is that neither serial nor parallel applies....
My new electric scooter will be based on a 20S LiFePo4 battery pack with following cells:
- 8 winston 40ah brand new cells
- 4 sinopoly 40ah brand new cells
- 8 wina cells savaged from an old pack with a measured residual capacity ranging from 39 to 51 ah
So the pack is going to be very dis-aligned from the beginning; so I am aware that no standard BMS will be able to align it.
Therefore I am thinking this:
- forget about BMS, I will constantly monitor individual cell voltage and stop riding when close to low-voltage-cut-off.
- when I need a fast charge, use the 20S 72Volts LiFePo4 charger connected to the serial pack; with a circuitry that will stop the charge as soon as the first cell reaches 3.6Volts
- when I have time for a slow charge, I will use a second charger which is actually a set of 20 independent 1S LiFePo4 chargers, eachone connected to its own cell of the pack; the 20 individual chargers must be of course electrically decoupled. This way I will be able to charge to its full every single cell and I think it will be pretty safe to charge unattended as long as every individual charger is able to disconnect as soon as its own cell is full. So I can safely leave it for an overnight charge.
So the problem is how to build these 20 chargers? They must be small, cheap and simple; from a first scouting I found the following:
- http://www.ebay.it/itm/1000mA-LiFePO4-Akku-Laderegler-Lademodul-mit-Schutz-1S-3-2V-mit-Micro-USBa-Bucahse-/351920139313?hash=item51f0132c31:g:yG0AAOSw4GVYPaVD
This is indeed small, pretty cheap and fully-fledged charger with also LED status, only thing I need an extra 5v power source per unit
- http://www.ebay.it/itm/3-2V-LiFe-LiFePO4-Rechargeable-Battery-Charger-3-65V-1A-DC5-5-2-1mm-EUS-/221876225660?hash=item33a8da867c:g:9MUAAOSwyQtV66nn
This is complete of 220V ac/dc converter, so I just need to buy 20 of them and I am ready to go!

What do you think about slow charging this way?
Any suggestion on the small 1S charger?

Thanks,
Luca.

 

[dogman dan]

You could just charge the pack in 3 sections. After disconnecting each dis similar section, they can be charged, and balanced, by a typical 8s RC lipo charger. Sure,, a bit of unplugging, but not really a problem. 3 chargers will do er, or one if you take the time to charge each section with one charger.


Only trying it will prove if the pack can be bulk charged naked. It might be much more possible than you think. Though the cells may be very different in total capacity, you will likely pull a very similar wh from each type in discharge. So just bulk charging to a set total voltage may work fine. You can always bulk charge to a lower voltage, to give yourself some room at the top for error.


As for monitoring,, a regular bms can still detect a low cell and disconnect the whole pack at the right time. If you want to just watch voltage manually, once you get to know the pack, you will know which type of cells has the lowest real world capacity, and you can merely watch those cells, using an RC type low voltage alarm.

In summary,, I think you can bulk charge ok, monitor the weaker cells easy, and when you need a balance, just one RC charger will do er.