I have started a new venture, producing electric scooters, karts and trikes by converting salvaged materials and second hand bikes etc. These are all individual and build to order at this (very early) stage. So far I have mainly relied upon using existing motors, batteries and controllers and adapting them to suit various applications. I have experimented with rewinding a few motors to run on appropriate power sources with some success but I really want to start building decent batteries. Due mainly to cost, I have been using 5 no. 12 AH lead acid batteries but these are obviously less than ideal due to size and weight, not to mention poorer performance than lithium iron, for example, in terms of range.  

I purchased a job lot of budget lithium iron batteries for tools, disassembled them and reconfigured them to produce a 60 volt pack.
The original product is 20/40 volt, 5.0/2.5 AH, comprising 10 no. Samsung 1865 cells and the necessary circuitry. the output is determined by the tool you connect it to (either 2 banks of 5 in parallel or 10 in series).
I have taken 9 units, soldered the terminals of each to provide 9 no. 20 volt, 5 AH units. Then I arranged 3 groups of 3 units (connected in series to provide 60 volts 5 AH each) and then connected the 3 no. 60 volt arrangements together in parallel to provide 60 volts 15 AH.
This seems to work a treat, it's a fraction of the weight of the lead acid and provides far better range, more than the 20% AH increase which I put down to the fact I'm not hauling the weight of the batteries themselves!!
Nothing seems to overheat or produce any signs of trouble but I'm not entirely confident that I'm not putting them under excessive load when I start the motor from standstill. I reckon I'm probably pulling 37 amps maximum so if anyone can advise on this, I'd be grateful.

My main question is regarding the charging of this battery pack. So far I have been using the specified battery tool charger to charge each unit separately but this is obviously a pain because it means messing around with all the connections and it takes around 35 minutes per unit. The charger's output is 21 volts at 4.0 amps.
I want to charge the whole pack at once so I have been experimenting with rewinding an old torroidial transformer from an amplifier. The plan is to step down the voltage and then rectify it but I know nothing about BMSs and I've read that these are crucial for charging lithium iron batteries. I have stripped off the existing secondary winding and figured out the turns per volt, as it was previously. I have a fairly good understanding of these things but I have been troubled by various conflicting advice on the internet. I know I can achieve the required voltage but I dont want to blast too much current into the thing, obviously. So I figured I would reduce the size of the wire in the secondary winding but I have also read things about weighing the wire and this is where my understanding expires!! If I reduce the conductor size, will I cause problems? Will it overheat the transformer? Can I wind the of the appropriate weight and limit the current delivered to the rectifier?? Any advice or relevant calculation methods much appreciated.

But if anyone is more concerned with editing my spelling or grammar, please, dont waste your time! Thanks
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