[BBmeg]

This morning it was too low for radio use (after being on for a good 4 hours with extremely low battery). While commuting it died fully.
Plugged it into my computer at work to charge, and will not turn on if the power button is held.
There is also a solid red light.

Does this mean it is charging?

It has been like this for almost 2 hours.

[Dubdub]

You probably damaged the battery letting it get that low. Lithium batteries do not like to get below 20% and doing so can cause damage.

Try a NEW battery made for the 8520 and see if that fixes the problem.