I've had the phone now over three years and just purchased a new one from a decent seller on this very forum

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I'd like my Rizr as a PAYG backup phone now but have problems when i use the charger it came with. It takes a long time to even begin a charge, sometimes not at all.
Sometimes it also doesn't like charging on my PC either but i tried just a moment ago and right away it worked but again this is a bit hit and miss as to whether it charges or not here also.
Any ideas, do you think it's the actual charging connection inside or just the battery which i could hopefully replace?
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I have no PC cable for the V500 but also have no luck getting that to charge anymore.