Safe Charging Specs.?

I recently made a  homemade external battery for my smartphone. I know I ask a lot of questions on this website; but I don't have a Yahoo! account and don't like Yahoo! anyways. :P

My question is, what is the safe charging temperatures/voltages for the phone? It's an LG Optimus G. When using a power-hungry app, I've seen the temp go up to 90+ degrees Fahrenheit; so I should be okay in that aspect.

But what should be a good voltage to stay at? I've heard smartphones can handle spikes of up to six volts. It usually is at  four volts. The charger that came with it is rated at five volts. Help!

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