i want to buy a solar panel to put on the top of my golf cart to help keep the batteries charged. the batteries are pretty old and someday i will replace them.
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Hey Jessie, this is a great project you’re on to. Does your cart have a roof or canopy already? Anyway, I’m guessing your cart has 6 batteries in it, probably Trojan T-105 or Exide golf cart batteries, is this correct? If so, they will be 220 amp hour, six volt batteries, wired in series for 36 volts. The only problem with your plan is you will not be able to do it with one panel, unless you can find one wired for 36 volt charging, which would be very rare. What you’ll need is 3 panels, all the same size and output. Let’s say you select a 50 watt panel, you have 3 of them, they are each designed to charge a 12 volt battery. Wiring them in series will allow them to charge a 36 volt battery. If the panels you buy have bypass diodes inside the junction box already (most of them do today), then all you need to do is mount them on the roof, wire them together in series, positive from the first panel to negative of the second, and so on, then the last two connections will be positive from the last panel, which you connect to positive on the first battery, and negative from the first panel, which will go to negative on the last battery.
Most solar panels are wired for 18 volts open circuit voltage, which is the voltage with nothing connected to them while they are in the sun. This voltage is necessary to charge a 12 volt battery. Don’t make the mistake of thinking you can take 2 of them and make 36 volts, they will not charge the batteries at that voltage, you’ll need at least 42 volts, preferrably more. Your panels must all be the same output, you can’t use 2 40 watt panels and 1 70 watt panel. That would work if you were hooking them in parallel and charging a 12 volt battery, but in series, they won’t get along.
Also, if you don’t use panels any larger than 70 watts each, you will not need a charge controller. A rule of thumb is any charging source less than 2% of the battery amp hour capacity is considered a trickle charge, and it will be impossible to over charge the battery. One word of caution, if it’s warm where you live, make sure to water the batteries at least once a month, they will go through lots of water during charging in warm temps. Just keep a couple gallons of distilled water around.
If you end up putting in a charge controller, then the diodes are optional. A diode is nothing more than an electrical check valve, allowing current to flow from the panel to the battery during the daytime, but not allowing it to go the other way at night. A charge controller will do the same thing. Also it’s important to note that your regular charger, which plugs into the homes electrical system, will get along fine with the panel. You can park outside in the sun, solar charge, and plug in charge at the same time if you like, the plug in unit should shut down once the batteries are up high enough for the panels to take over. There’s a great book at your library worth reading before you proceed, it’s called, "The Complete Battery Book," by Richard Perez.
In case you’re wondering, the reason we know all of this is we live in a solar powered home. Making your own electricity is like growing your own tomatoes, you end up learning all about bugs, soil ph, watering and sunlight whether you want to or not, it’s a curse we have. Good luck, and take care, Rudydoo
Rudydoo gave a great answer, but I want to point out a product that might make it a little easier. Solar Converters makes a charge controller designed to charge a 36V battery bank with a single 12V solar panel. It boosts the voltage up x3 (by reducing the current / 3). This will work with a single 12V panel up to 170W. They also make a bigger one for larger panels. http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Solar-Converters-Charge-Controllers/Solar-Converters-CV1236-4PV-4A-Charger/p1244/
If you do use 3 panels, you really should use a charge controller, especially since the cart will be used intermittently and you can’t be sure how often the power will be used, so you run a risk of overcharging. Here’s a 36V in/out one for up to 5A. http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Solar-Converters-Charge-Controllers/Solar-Converters-PT-36-5A-36V-MPPT-Charge-Controller/p1380/ You could use it with three 65W panels wired in series for 196W. http://www.altestore.com/store/Solar-Panels/51-to-99-Watt-Solar-Panels/Kyocera-KC65T-65W-12V-Solar-Panel-with-J-Box/p724/.