[BrianG]

40A @ 13v is 520w. Even with a charge efficiency of ~80%, that's still 416w - enough to charge a 10s pack at almost 10A. Many of those "real" units have an adjustment to tweak the voltage. You could get more out of it (and increase efficiency) by upping it to around 14.5v, which is very close to a running car. I tweaked my Astron 25A unit to 14.75v like this.