Topic: Warning for powering Apple iBook on 12v with iGo adapter

[taper420]

So I figured I'd get a decent SLA and a 12v iGo Juice 70 car adapter to power my laptop for extended festival runs. So I tested this on a 7.5ah battery and after about an hour my screen went blank and all the power to my usb and firewire ports were shut down. If I held a light up to the screen I could see the computer was still on and I was able to put it to sleep and wake it again, fixing the problem (after disconnecting the adapter). As far as I can figure, the laptop goes into some low power safety mode instead of reverting to the internal battery, which was at about 95%.
So as far as 12v > 24v conversion, a 7.5ah battery only lasted an hour.
Next using the same battery, I plugged in a DC>AC 70w inverter and powered the laptop off the iGo adapter but this time with the ac power plug instead of the 12v cig. lighter adpater (this is equivilent to using the computers supplied wall power adapter). The battery supplied juice to the inverter, trouble free for an additional half hour. In addition, when the battery started losing juice with the inverter, instead of shutting the computer down to low power mode, it merely reverted to the internal battery. For the next half hour, the power switched back and forth between the inverter and the internal battery, in about 5 second intervals (longer at first). During this time the internal battery began draining, but I'd say at about half the rate as normal. After about a half hour the bursts of inverter power we're down to about 1 second, and the internal battery didn't seem to gain much from this.

So the conclusion is that converting directly from DC to DC in this situation is less efficient and more dangerous... in fact I don't think it's suitable for any recording application, as it immediatly disconnects any external audio device. The inverter powered it for almost twice as long, and it did it worry free, without risk of failure.

Needless to say 2 hours is crap for a 7.5ahr battery. But its actually right on as far as the math's concerned...24v at 1875ma is 12v at 3750ma... for two hours that's exatly 7.5ahr. Its seems the inverter takes nothing away from the equation, and is better suited to operating at lower voltages than the dc converter.