This could either be a bug in the OS, or a cabling issue too. I'd say clean the battery connectors, ensure they are all shiny and connecting properly. If issue persists, upgrade the OS. If even that does not fix it then we have a hardware issue and the device needs to go for in warranty service.
Old battery? Maybe its shot? But I'd lean more towards OS issue. Try backing up > wiping > reload os. Or the latest os from Bell. I'd recommend a hybrid such as Zocster berrylicious v3.
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