[Q] Pls help me for my problem of milestone XT720

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I've got XT720 around 1wks and root it before, when I use it with many task at the same time with around 60% battery, it sudden switch off and can not restart at that moment.
I tried to recharge & bought a new battery for this but it always no response when I press switch button & I can not entry the recovery mode also, I tried many many times for switch on, I found that I just only can switch on with bootloader mode(press upper volume, camera shutter & switch at the same time) & it only show "battery low, cannot program".
First, I used the desktop charger that I'm sure the batteries got fully charge, second, I tried to put out the battery with a moment & it can boot it normally (just few time only and switch button out of order that I need to reboot again....).
Besides, I used rsd lite for flash rom(sometime bootloader without problem) but it was failed at last step (I'll try to update screen cap later).
I just query bootloader with crash that it can not boot normally, can I update bootloader or it was hardware problem??
Pls kindly help me if any solution, I'm come from HK & I'm not in advance in Eng, pls forgive me if any problem. Thanks for help.