I have a few volkswagen solar panels, those that they use to keep the batteries charged.
I´m planing to use them in combination with a car battery and a small power inverter with a timer to pump water only a couple of minutes a day to water my bonsais,
I have the panels, the battery, the timer, the power inverter and the pump,
What else do I need to make it work?
A friend told me that I have to prevent the energy from going back or something to shut off the timer when the batteries are charging. Thanks!
It sounds like you are using a truck to carry a pencil across the street! You can buy a simple hose-bibb garden timer to turn on a hose daily. If it must be solar, use a solar charger for the AA or AAA cells. If you must use a car battery, try using a 12V DC pump like a bilge pump for a boat. Drawing current while the battery is charging is not a problem. You need a regulating controller so the solar panels don’t overcharge the battery. Unless you have hundreds of Bonsai trees, this is overkill!
A diode, will allow current to flow in only one direction.. One should be placed in series with the photocells and battery. This will prevent reverse current flow (discharging the battery) through the solar cells at night.
If you can use a 12vdc pump you can save power consumption by not having to use a battery.
Also the type of timer you use will also efffect power consumption. A microcontroller based timer which "goes to sleep" and wakes its self periodically to count and check for day light will consume a fraction of power that an electro mechanical timer will.
That’s a tricky question.
You are going to need one or more diodes connected between the PV panels and the battery, to prevent it from discharging through the panels when the lights are out.
A single car battery typically has a charging current of about 0.8-1 amp, and needs a charging bias of about 2.3-2.5 volts over the top of the battery’s voltage. I assume that since the originally intended purpose of your PV panels is for charging car batteries, that they are sized correctly in terms of voltage and current output.
The diode(s) is another matter. Here are the specifications you are going to want. I have added here an approximate fudge factor.
forward voltage : ~0.8v
reverse breakdown voltage: 30v
leakage current: micro-amp range
forward current: 2 amps. If you can’t find diodes this large(unlikely), you can just connect enough in parallel to get the current capacity you want.
Another problem, is the fact that the inverter will draw a small to moderate amount of current whenever it is switched on, even if the pump is not on. I am not sure how to solve this problem besides just physically turning the inverter off when it is not being used. This kind of defeats the purpose of having an automatic system though….
Mark’s idea of using a 12v dc pump and ignoring the battery and inverter altogether, is a good one.
You might want to talk to a bona fide electrical engineer in RL, and ask him what his opinion is. Electrical engineering is not my specialty……
Good luck with your project
~Donkey Hotei.