Dave writes -

How do you keep an iPhone running for long periods of time when you are off the grid? You can’t change the battery in the phone, so carrying a spare isn’t an option. Devices like LadyAda’s excellent MintyBoost use AA batteries and can keep you going indefinitely, but unfortunately the MintyBoost and several other similar products don’t work with the iPhone 3GS. And anyway what I really wanted was a device that would take power from the high capacity 12V NiMH rechargeable pack that I use to power my portable ham radio gear. I could have re-purposed a charger meant for use with cars, but many of them use linear regulators and are therefore very inefficient, or aren’t compatible with the 3GS; plus they are overpriced. So instead I built a charger to suit my needs: the MintyBuck.