FWIW I went looking for an economical but effective larger scale
battery charge system, and here's what I came up with:
https://www.amazon.com/gp/product/B075K37YT5
(they got taken apart for modification, and I found they were designed much better than I was anticipating, they're microprocessor-controlled and adequately rated, though they could use better cooling)
I bought three of these, $18 ea. So for $54 I can charge six
Spark batteries at a time. (plus a controller and phone/tablet) They run off 12v, so I hardwired them to a single 12v harness (powerpole connector actually) to an 18AH SLA
battery. You can get the 7AH SLA's all over the place and charge them with any cheap low powered
battery charger or maintainer, but an 18 will hold a lot more power - those are often referred to as "fire alarm
batteries" and can be found where commercial fire products are sold such as fire extinguishers. (or just lug a car
battery, cheap from the local auto salvage) Take that out to the park and you can charge lots of
batteries. How much you can recharge is only limited by the size of the 12V
battery you bring. (car
battery or in-car = a lot more) Spark rates their
batteries at under 17WH. If we want to guess 85% efficiency, an 18AH
battery (@12v) contains 216 WH (12*18), 85% of that is 183WH, so we can top off
at least 10 (completely dead) 17WH
Spark batteries using that. Since we don't run them flat usually, forget the 85% and we can just call it an even dozen charges?
I'm expecting this "single conversion" method to be significantly more efficient than inverting 12 up to 120 and then back down using say the flymore charging sled. (each conversion wastes some power) It also costs a lot less if all bought separately - $10 less for the charger, save $15-20 more not having to buy a small inverter, all while charging twice as many
batteries at a time. I'm guessing a typical car
battery (~500WH) would charge around 30
Spark batteries. (or essentially unlimited in a periodically-running vehicle)
Does anyone spot any errors in my math, or see any grossly bad or wild assumptions?