Foldable solar panel

System

Howdy,

I bought this because it was cheap and I wanted a way to recharge a Xantrex 600 powerpack when I'm out camping.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190186780703

I have it out today in Tulsa--very sunny today. When I put a digital multimeter across its terminals (no load) I get about 29 volts and 0.47 amps. What gives? The ebay description says it will charge batteries from 12-16 volts.

I'm wondering if this is actually a 24 volt panel? If so, can I still use it to power the powerpack?

Another quick question--If the multimeter reads positive, does that mean that I have correctly guessed the polarity--they are not marked.

Thanks

66mercury

BB.

Re: Foldable solar panel

Yep, open circuit voltage on a solar panel is going to be above 22 volts. Vmax (for a 12 volt battery) should be around 17.5 volts (or higher). Your panel is roughly 17.5vx0.47a=8.2 watts of useful energy (assuming clear sunny day when you did the measurements).

Regarding the voltage +/-... Yes, you are probably correct. (obviously, I don't know your meter or expertise). To be sure, you should measure the same polarity on the +/- connection to the battery. If you hook the panel up backwards, it will probably cook the protection diodes (if it has any) and possibly the panel itself.

Normally, for such a small panel, a solar charge controller (regulator) is not needed... This is mostly a maintenance charge (float for storage). If the panel does not have a blocking diode, you will need to disconnect the panel when their is no light on it (it will slowly drain the battery--solar chargers also have the blocking diode function internally).

At best, you can expect roughly 12v*.47amp*5 hours per day (summer)=28 watt*hours of useful energy per day from this panel. This is ~14 watts for two hours (equivalent to a "60 watt" / 13-14 watt CFL running for 2 hours per day). Useful for a small radio, charging a cell phone/digital camera, etc... But not a lot more.

Don't expect to get too much useful energy from this panel. Your best bet is to measure your planned loads and see how much energy per day you will want.

-Bill

PS: Typically, the maximum voltage required for charging a 12 volt lead acid battery is around 14.5-15.5 volts, plus ~2 volt drop for the solar charger/blocking diode). Vopen-circuit is no current flowing--a 24 volt battery would require 29-31+ volts for charging--so this panel is not for that.

Also, the voltage of the panel is not too important for charging your battery (once it is above the minimum requirement). Your battery will keep the voltage down around 13-15.5 volts under almost any normal charging conditions (unless the charging current is way too large). You need to watch the voltage (see battery manufacturer spec. sheets) to make sure that you don't cook the battery with high voltage/current for hours/days on end. Some batteries are wet cell and you can refill them. Others are sealed units (your's might be) and if they are over charged--they will be destroyed. This panel is relatively small and for short periods (days of usage), it is unlikely that it will damage your battery from over charging (undercharging, that is a real possibility). Your 600 watt inverter can only run about 60mperh*28w*h/600w=2.8 minutes per day at full 600 watt load with this panel and 5 hours of sun per day (actually less for other reasons, but you get the idea).

System2

Re: Foldable solar panel

2.8 minutes per day? How am I gonna dry my hair?

Actually, I don't have hair.

Thanks for the reply. The portable system would only be used for some very low wattage LED lights(1-2 watts), cell phones, radio, etc. Maybe to run a portable dvd for a movie now and then. My camping trips don't go more than 3-4 days and the xantrex has a 28AH battery. The foldable panel would probably be used to just keep some charge coming in.

I had an old deep cycle trolling battery in the garage (don't have the barrel connector to charge the xantrex yet and I didn't want to disconnect the battery from my home solar setup). Hadn't been charged for quite a while. Read about 10.5 volts. Hooked up the foldable panel and it started charging at 0.25 amp. Has since come up to about 11.5 volts after 2-3 hrs of charging.

Thanks again

BB.

Re: Foldable solar panel

If the old battery reads ~10.5 volts and has been sitting for months+, then it is probably, more or less, dead and ready for recycling...

However, if you are going to try and use it--I recommend that you charge/equalize it a few times first. And load test it to see if it has any useful power before you take the old battery out the field and need it to perform.

Have fun!

-Bill