HTC Hero uses a Lithium-ion battery.

Answer to your question

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Some extra reading:-

http://en.wikipedia.org/wiki/Lithium-ion_battery

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[/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]Recharging a partially charged lithium-ion does not cause harm because there is no memory[/FONT]
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Google search:-

lithium ion - Google Search

Charge it when you want, for however long you want. It doesn't matter. You will need to replace the battery is a few years depending on use no matter what anyway. Don't cost a lot either.
Case closed
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With Li-Ion batteries, you don't have to worry about the battery's memory near as much as with a Ni-Ca or Ni-H battery, but there are a few things you need to be careful with.

Draining the battery completely every time is generally discouraged because Li-Ion can be ruined if the charge drops too low. Of course there is a circuit in place to prevent this from happening through normal use, but because batteries still drain slowly even when not in use, it can drop below that fail-safe level.

So, the best way to charge a Li-Ion battery? When you first get it, let it drain completely and then charge it completely. After that first charge, you can do it however it's convenient for you, but it's suggested that you drain it completely and charge it completely once every 30 or so cycles.