dimcar

Here is my little story. I have an X5 Performance, and about a month ago, the LCD display broke. I was lucky because i found a non-working X5 400MHz through ebay for $40. I bought it and replaced my broken LCD display with the one from the non working Axim. Everything went fine... But after that, i have a problem...
I fully charge both batteries and starting to use the Axim. Well, when the battery power reaches about 80% for the first (the one with the non working Axim) and 70% for the second (the one i already had), it shuts down. When i turn it on again, it says "Battery power lost. " and something else.. But after that, it may continue work for 5-15 minutes or may not and shut down again.... Any help..?? I want to buy a new 2000mAh battery, but i have to be sure that the problem is caused by the batteries. The one i had, is about 1,5 years old and was out of current for 20 days, until i fixed my Axim