Here’s a followup on several products I’ve tried.
Pawtec Macbook High-Speed Car Charger
The idea is that direct DC to DC current should be more efficient than a power inverter that converts DC to AC, that is, a 12V DC power inverter to AC to Apple power brick. Maybe.
I was initially thrilled with the Pawtek adapter, as it powered my late 2013 15-inch MacBook Pro Retina with no fuss keeping it nicely at 100% charge.
But when the battery charge was about 75% and I then plugged the laptop into the Pawtec, the Pawtec adapter started making high pitched screeching noises, and the adapter plug became too hot to touch. Things that get too hot to touch worry me. I unplugged it and let it cool. Perhaps it’s normal or perhaps it’s risky—I don’t know but I’d say that the Pawtec is good for steady usage but caution is advised for charging a depleted battery. The MBP Retina power brick is a 90W unit, so maybe that’s too much for the Pawtec.
So far this BatteryBox unit has worked great. The BatteryBox unit is 60Wh; a MacBook Pro Retina has a ~100 Wh internal battery, so 60 Wh is in theory about a 60% gain in battery time.
Tests at home under maximum load (all CPUs busy via MemoryTester) with the BatteryBox connected kept the MBP at 100% charge for about 35 minutes. Since normal usage uses far less power, I’d estimate that the BatteryBox will yield a runtime extension commensurate with its watt hours in relation to the built-in battery (3 hours is about all I get out of the MBP using Photoshop and such, so maybe I'd get 5 hours, total). Further field testing will prove this out. Its a nicely made unit and I’ve suggested to the manufacturer to offer a double or triple size battery. But of course it’s possible to carry more than one battery.
The BatteryBox also charges a USB device via its USB port, but a pending firmware update is needed for full compatibility (some of my devices would charge, some would not).
One limitation is that the BatteryBox takes USB charging via an AC wall-wart, so back to the same old power inverter for recharging (no direct DC charging).
Wagan Tech 200W pure sine-wave power inverter
This is now my meat and potatoes power source for when I’m traveling in my vehicle in the mountains; it charges up everything.
I replaced an older and cheaper square wave inverter with a Wagan EL2600 Elite Pro 200W Pure Sine Inverter. The older unit did not seem efficient; it became relatively hot and ran its fan a lot. It cost a lot less, and I guess you get what you pay for.
But the Wagan EL2600 Elite Pro 200W Pure Sine Inverter runs much cooler and with less fan noise and no fuss. Its pure sine wave AC output is more efficient for devices and hence the cooler operation (lower power draw on the unit itself, which is claimed to itself be 90% efficient).
I charged 3 camera batteries at once as well as the iPhone via USB, and it powers the MacBook Pro with no fuss at all. 200W is plenty for all that stuff, even simultaneously.
The Wagan EL2601 Elite 400W and 1000W Pro Pure Sine Inverter alternatives may be of use to some, but 200W will already blow a vehicle fuse in many vehicles (12V X 16.7 amps ~= 200W, my vehicle is fused at 20 amps but some are fused at 15 amps). The 400W or 1000W units can deliver a lot more power, but only if connected directly to a suitable battery directly with the alligator clamps (not the cigarette lighter socket).