When making a battery bank for solar panels out of car batteries, do they all have to be the same?

Hey Dominic, if the batteries are all 12 volts, there is no problem charging them together, unless the age of the batteries varies dramatically. I've never heard the comment Skoda John posted about charging only to the capacity of the smallest one, I know plenty of truckers and boaters, that makes no sense.

We have been running our home on wind and sun for 16 years now, my original battery bank lasted 11 years, on the second one now.

The problem with age comes in during the final charge stage, when your batteries are near full. The voltage they charge at the last 10 % is different for older versus newer batteries. So what happens the old one steals most of the charge current, and might even take some from the newer battery while the array current tapers off towards sunset. At night, the old one will continue to slowly drain the newer one, but during the day buy taking most of the charge current, the old one heats up excessively at final charge, and the new one never reaches full charge, which doesn't help it out either. After a year or so, the old one is shot, and if you replace just that one, it becomes the new one, and the other one is now the old one, so the process reverses. Many people with yachts have this problem, they keep replacing one battery, then just a year or two later, the next one is shot, and so on.

Don't take my word on this though, or anyone elses advice on open forums like this. Go to the local library, look for a book by Richard Perez called, "The Complete Battery Book," I believe. He is the founding editor of Home Power Magazine, also a great source for renewable energy stuff. They have been running their home and business for 3 decades on sun and wind. Just focus on the chapters pertaining to lead acid batteries, like yours.

To be honest, car batteries are not really designed for solar applications. They have thinner, more porous cells than deep cycle batteries, better for providing high amounts of amperage for just a few seconds, then being immediately recharged, like starting a car. If you are building a system from the ground up, the best battery for the buck is the Trojan T-105 golf cart battery. They are made to be almost totally discharged each day, then fully recharged each night, like a solar home might do. We used that type for our first bank, and currently use it for our second one. You can get them any place that services golf carts, try the yellow pages. They are only made in 6 volt models, two wired in series (positive to negative) will give you 220 amp hours (AH), way more than any single marine battery.

We charge our lawn tractor battery, boat batteries, quad battery and spare battery along with our two T-105's with a 120 watt panel all winter here, but there is a diode in front of each battery, which prevents them from draining each other at night, or over taxing the array in the day. A charge controller keeps the charge voltage correct all day. You can do this too, but you can't use the power stored in several car batteries simultaneously to run stuff at night with the diodes in place, it is only to keep all these seasonal batteries charged in the winter with a solar panel. Your car batteries will work for a while, but if the age of them differs more than a year or so, or you discharge them more than about 30% regularly, they will not hold up too long. It's okay if you have a bunch of them you don't need for anything else, later you can get a good pair of golf cart batteries if your solar array is working as well as you hoped. Take care Dominic, Rudydoo

As long as they are the same voltage (6 volt, 12 volt, 24 volt etc) The amps can differ and so can the size. You can even parallel two 6 volts to make a 12 volt and gang those 2 in with other 12 volts

Added suggestion, you should youtube homemade or hillbilly vertical axis wind generator. For added power at a very inexpensive price.

Yes it would cause issues.

The bank will only charge to the capacity of the smallest battery. Ask any trucker about this.

Use a charge controller. I hope your panels make enough current to actually charge them.