We have an older Class B motorhome with a single group 24 deep cycle coach battery. We have learned how to boondock successfully with a mere 75-80 amp hours and stay within our power limits (see article here). But where we have to be careful is our laptop computers. Since we mostly use them after dark (no solar to replenish the battery), once we run low on laptop battery we must run them off the coach battery. Even using 12 volt adapters (instead of an inverter) the power use is high. We are reluctant to drag down our battery, especially on nights when the furnace might be needed.
About a year ago, we stumbled across a jump starter battery that also claimed to recharge laptop computer batteries as well as USB devices. Although it worked great as a jump starter (and could charge USB devices) it was unable to supply the 20 volts our ThinkPads needed for recharging (despite the advertising).
But recently we found something that really does work. The MaxOak K2 50,000 mAh lithium battery bank – here is a video. At 8 inches x 5.5 inches x 1.25 inches it is not large, but is it heavy for its size. Before you start thinking 50,000 mAh = 50 Ah that is not much less than my 75 Ah coach battery Note this is a 185 watt hour device – the use of mAh can be very misleading, since the voltage it applies to is not stated in specs for many of these multi-voltage devices. (Remember that Watts = Amps x Volts.) This device has a native voltage of 3.7 volts, that is what the 50 amp hours applies to. But 185 watt hours is enough to recharge our ThinkPads at least twice. It has two 1 amp USB ports, two 2.1 amp USB ports, a 12 volt 2.5 amp output and a 20 volt, 3 amp output. It comes with a nice neoprene zipper case that holds the assortment of connectors. There were 9 different laptop power adapters, including the one for our ThinkPads. Also included is 120 volt charger cord. Additionally there was a “gift” of a small USB battery bank that we have found great to carry along for recharging phones or iPods.
We let one of the laptops run down under 50% and hooked it up. The ThinkPad indicated it was connected to shore power and proceed to charge just fine, even while in use. Ah, we have found our solution to late night computer sessions! It wasn't cheap, but it really does the job. Available on Amazon. for $135. Please note, this is for PCs and NOT Apple laptops.
If your electronic use has become a worrisome drag on your coach battery, this is a slick solution. It can be recharged off an inverter while driving (or off inverter via solar on those sunny days after the coach battery is fully charged). And of course you can recharge it while plugged into shore power or while the generator is running. We have been pleased with the performance of the K2.