On our post about installing a 24 volt battery equalizer, Parker Sharpe asked in the comments section how to wire two 12 volt batteries to make 24 volts.
The Short Answer
Connect the negative terminal of one battery to the positive terminal of the second battery. You will be left with two battery terminals free of the total four. One will be positive on one battery, and one will be negative on the other battery. Take the free positive terminal and connect it to the power lead on your 24 volt device. Connect the negative to the ground connection of your 24 volt device. That's it!
The Longer Answer
It is called wiring the batteries in series, as opposed to in parallel. Here is the difference:
To connect batteries in parallel, connect the negative of each battery to the negative of the next battery. The same goes for the positive terminals - connect the positive terminals together. Here's what you get: the combined voltage of the batteries averages out, but the combined available amperage is additive. For example, two 12 volt batteries connected in parallel gives a combined average voltage of 12 volts. And if one of the batteries is capable of delivering 400 amps and the other can deliver 600 amps, the combined amperage is 1000 amps.
What does the combined average voltage mean? Here is another example. Say you have one 12 volt battery and one 6 volt battery. If you combined them in parallel you would have a combined average voltage of 12+6 / 2 = 9 volts. And most people will be connecting 12 volt automotive batteries together. So the combined average voltage is 12+12 / 2 = 12 volts. Get it?
Why Connect Batteries in Parallel?
In the examples above you keep the voltage the same (you don't normally connect batteries that are not of the same voltage), but you get more available amperage. This translates into the ability to do any of the following:
- run your equipment longer (i.e. car stero)
- run a more powerful piece of equipment (i.e. a large starter)
- run equipment at colder temperatures (say starting your car in the middle of winter).
Why Connect Batteries in Series?
And this leads you to the next question, why connect batteries in series in the first place?
Since the voltage is additive when you connect batteries in series, you will be able to run equipment that is designed with a different voltage. The example that Parker asked about was for his 24 volt troller motor for his boat.
A lot of marine equipment is often 24 volts. And this presents a problem for boats that have a 12 volt starter on the main motor, and the owner has purchased a simple trolling motor that came off-the-shelf for 24 volts. Now you have two systems on your boat - a 12 volt main system for your main motor and boat accessories including the charging system, and a 24 volt system for running the trolling motor.
All of a sudden the whole problem gets a lot more complex. The question is how to connect batteries in series and parallel at the same time? The short answer, is no you can't. The soluton is another article.
But in the short term you can use the brute force method: buy two 12 volt batteries, connect them in series to make 24 volts, and run your troller directly from them. Eventually the troller will drain the batteries, and you'll have to quit, go to shore, hump the batteries back home, connect them to a charger to recharge them, and bring them back to the boat on your next fishing trip. What a hassle, but it would work (make sure you've still got your main motor to get home or else bring a paddle!).