What is the minimum amperage needed to charge a netbook battery?

Hello. I'm trying to find out the current necessary to start to charge a netbook (Acer Aspire One) battery. The situation is that I'm trying to charge a netbook (switched off) at as slow a rate as possible using a solar panel. Unfortunately, the computer's charging circuitry expects a certain amperage before the "charging" light comes on. I'm trying to find out what that amperage is because it dictates what solar panel I buy. Even if someone can give me a rough idea, it would help. I'm trying to use the smallest solar panel possible as I'm intending to carry it in a bag with me. I don't care how long it takes to charge the netbook battery with the netbook switched off. I would just like to be able to charge it during the day for use for a while in the evening. In addition to a solar panel, I'll be using a voltage regulator (output 12V) and a car cigarette lighter-type laptop DC-DC converter. My laptop is rated at 1.58A at 19V but I know the charging circuitry will work at less than that. I just don't know how much less than that and I don't have equipment that can vary amperage for me to be able to test.