Posted by: Dan Sprague | January 15, 2012

Two Batteries are better than one…More Power Scotty

On my boat I have two batteries wired together to majorly increase my electric power. I use a trolling motor to move my sailboat  away from the dock and back to it. That eliminates gas on the boat and the heavy outboard. The electric motor is a big power hog. Then added to that power need, there are the electronics like the radio, the GPS, the depth finder, the CD player and other navigation instruments and the lighting.

It is a 12 volt system. If you wire two batteries together they should both be the same type of batteries. If they are both 12 volt but different AMP ratings, you can damage one of the two batteries.  You need to wire the two batteries in parallel to keep the voltage at 12 volts. . How you hook the batteries together is important.  If the batteries are both 12 volt batteries and you hook both + (positive) terminals together and hook both – (negative) terminals together, the output between the + side and the – side of the batteries will still be 12 volts, but the amp output doubles.  This is called wired in parallel.

2 baterys wired together in parllel

The batteries wired in parallel

If you wire the two together so that the + on one battery goes to the – of the other batery and then you hook up the things you power to the free + terminal on one battery and the – terminal of the other battery, the output will be 24 volts not 12 and the amps stays the same. This is like two batteries stacked in a flash light. The batteries are said to be wired in series.

Batteries wired in series doubles the voltage.

Two batteries wired together in series doubles the voltage.

A 6 volt battery usually has a higher amp rating then a 12 volt battery of the same size. By wiring two 6 volt batteries in series you can get a 12 volt power supply that has greater amperage then a single 12 volt battery can give. The two batteries do not have to work as hard to do the same job as the one 12 volt battery does. This is why most golf carts use 6 volt batteries.  It is nice to have options like that.

I like the AGM (absorbed-glass mat battery) batteries for use on the boat because they are dual purpose batteries. They work well as a cranking battery for starting a boat motor and they are a deep cycle battery for equipment like a trolling motor. They provide high power with very low internal resistance compared to conventional batteries and they have two times the life of conventional batteries. They are becoming popular because the can handle both starting and loads well. Another reason they are popular is that they are sealed and may be stored and used in any position. The do not have to set level like a wet cell battery. The AGM batteries are the battery of choice in the marine industry today.


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