I'm so glad I read this thread. Having forgotten to charge my power tank for a few months earlier this year - you've guessed it - it wouldn't charge up and was almost flat. Having read this thread last night I went and connected the power tank to the car battery charger which has a deep cycle trickle charge. It's now fully charged and working great again. Thank goodness - thought I was going to have to blow my wife's bike tyres up the old fashioned way. I've now put a weekly reminder on our