Sorry for the less technical inclined is this a good thing.

. I'm trying to understand this.
Just demonstrating these
batteries charge fast and so the chargers draw a lot of power. Charging six
batteries at a time is a bit of a stretch for most people though. Fortunately they only draw that huge current when doing the early stage of charging, the draw drops a lot when the
batteries get closer to full charge. The 25 amps is more of a "shock value" number here, to remind people just how beefy of a
battery you need to be able to charge six Spark packs at the same time.
My
Spark batteries are rated 17Wh. (watt-hours) Applying a little easy math, a 12v
battery rated 18Ah (amp-hours) is (12x18) 216Wh. If you assume the inefficiency in charging about cancels out the "my
batteries aren't totally dead", that means I can charge about 12 (18*12/17)
Spark batteries off my 18AH cell. If I were to instead use an inverter to go to 110vac and then the charge sled to drop back down to
battery level, that would be one more stage of conversion (and inefficiency) and would probably drop my capacity a charge or two. (also, my little inverter starts to overheat when I get several
batteries on charge) So I figure using these direct 12v chargers will get me a few more charges.
So the guy wth the 35Ah
battery can charge around 25 (35*12/17)
Spark batteries. If all you have is a little 7Ah hobby
battery, you won't be able to charge more than 5 (7*12/17)
Spark batteries with it. (it somewhat blows my mind that five "little"
Spark batteries can wipe out a 7Ah
battery, they're pretty powerful for their size)