How long would it take to charge a car battery using solar panels?

Question

I have 11 mini solar panels I harvested from walkway lights. All together they are pulling 45volts 23mA. My question is how long would it take to charge a car battery so I could make a "solar generator" using a 400 watt inverter. I know a deep cycle battery would be better but I'm trying to keep this cheap since I already have a battery to work with.

Answer 1

Typically, you want to charge a car battery, or any lead-acid battery, at about 5% of capacity (C) at least.  So if your car battery is rated at 100 Amp-hours, you would want to put at least 2 amps of current into it for slow charge.  With the 23 mA you have now, it is unlikely that you could even keep up with the self-discharge rate of a car battery, and the battery would slowly lose energy, although at a much slower rate than if the battery were just standing alone.

What you could do is charge 10 NiCd batteries, maybe 11, then put those in series to power your inverter for a short time.  By a short time, I mean perhaps 5 minutes before it goes off.  NiMH batteries would not work as well, as they need pulse charging.