perhaps I screwed it up

In the past year my iBook battery life has fallen dramatically. When we got to NZ it had already eroded, but we could get about 3-4 hours on a charge. A few months ago, that was down to 2 hours. Now, the gauge tells me it has about one hour on a full charge, but when it runs down to zero the computer actually has about 15 minutes left, another new aspect to the battery problem.
I just assumed it was a case of a poor battery combined with poor battery management - we tend to charge it up at any time rather than letting it run down all the way. However, I realized there might be something else to it. We don't have a transformer so we're just letting the 220 volts of power they have everywhere else than the US flow into the battery.

The documentation says the computer can handle the voltage. But I'm not sure it has.