(may be broke/outdated!)
I have two solar panels that were going to be trashed at work. I want to use them to charge batteries on my RV but i noticed they put out around 20 volts in direct sunlight. So i am wondering:
-isn’t this way too much voltage for a 12 volt system?
-do a need some sort of a regulator connected to keep them from overcharging?
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3 Responses
20 volts is a common open-circuit voltage for nominal 12v panels. The panels put out about 15v at full load. If the panel puts out little enough that it takes 20 hours or more to put a full charge in the battery you can get by without a charge controller – just keep the cells topped up with distilled water. A charge controller will do a more efficient job, though. The most efficient charge controllers use what’s called Maximum Power Point Technology (MPPT) which lets the panels operate at their most efficient point throughout the battery’s charging cycle. Available on E-bay. The charge controller should have the same or higher wattage as the panels it controls – if you’re not sure of wattage, assume that they are the same as similar area panels you find for sale.
Before buying a charge controller, connect the panels through an ammeter to a discharged battery. They may have been trashed because of damage and low output. You need at least an amp for trickle charge, 5 amps for reasonable recharge times.
You can use them without the regulator . Just make sure you use blocking diodes so the the power does not flow back into the panels . You can get those at radio shack of some electronic store or off eBay for a few bucks .
You want the panel that is charging the battery to be higher voltage than the battery. A fully charged 12V battery is over 14V. A 12V solar panel puts out between 17V – 20V, so that’s a good size for your battery. They name them all 12V to help show what works together (like Garanimal clothes).
Take an amp meter and measure across the the plus and minus of the panel WITHOUT A BATTERY. NEVER EVER MEASURE CURRENT ACROSS A BATTERY (pardon the capital letters, but it could kill you). You can measure volts across a battery, just not current. You can see someone measuring current across a panel here, http://www.altestore.com/video/video-Solar-Panel-Testing-v3/.
Once you know how much current the panels are putting out, you can decide how big of a charge controller you need. You want to size it at least 25% bigger than the output. If you have 2 panels, you will be connecting them in parallel, plus to plus and minus to minus. That will keep it at 20V, and add the two currents. So if you have 2 amps from each, in parallel you will output 20V 4A. You’d want a charge controller at least 5A 12V. http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/PWM-Type-Solar-Charge-Controllers/Morningstar-Charge-Controllers-PWM/Morningstar-Sunsaver-Ss-6-6A-12V-Pwm-Charge-Cntrlr/p803/
Larger and smaller ones are available here – http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/c892/