Re: ot battery question


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Posted by APB in Frigid NY on Monday, December 03, 2007 at 13:00:29 :

In Reply to: ot battery question posted by mclark on Monday, December 03, 2007 at 09:32:28 :

First, we need to know why you think that two batteries will be better than one.

Do you have trouble starting the engine, or do you have lots of accessories that require high amperage? It is just being prudent, so that you don't get stranded when working and playing far afield?

Putting two batteries in series gets you 24 volts, which is not what you want. Putting two batteries in parallel gets you 12 volts, with double the cranking power.

Series: (+) 12v batt(-)---cable---(+)12v batt(-)= 24 volts

Parallel: (+) 12v batt (-)
(+) 12v batt (-)

(wire both (+) together, and both (-) together)

If the purpose is to ensure that you always have power to start, even if the main battery is run down, then the best way is to use a battery isolator, a momentary switch, and a Ford starter relay. Don't be frightened by the concept of the isolator, it's only two humongous diodes inside a case with cooling fins. They are not expensive. They have three posts, which are the input wire from the alternator, and the two output posts to the batteries' positive posts.

In normal operation, the main battery supplies all the load. The other battery stays charged until you need it. When you need more power, press the momentary switch, which puts the two batteries in parallel as long as you operate the switch. This essentially is the same as putting a pair of jumper cables from another truck on your main battery.

Here are the disadvantages of using the marine switch: Using just an "1 2 BOTH" marine switch without an isolator will eventually (it might take years) mess up your batteries, because they will not be charging evenly. You also need to remember which battery is in use, and to remember to keep one battery disconnected by the switch, in case it is needed. Using the marine switch with an isolator is a good method, but you still need to remember to switch things around.

The momentary button method is simplicity itself. Battery dead? Just push the button, and turn the key. Voila!

Battery isolators use diodes, which have what is known as a "forward voltage drop" of 0.7 volts, but this slight reduction in charging voltage should not affect the efficient charging of your batteries, assuming the alternator is putting out the requisite 13.8 to 14.2 volts when the engine is running. The isolator can be thought of as being similar to a mixing valve in the plumbing world. The electricity comes from the alternator, and is split by the isolator into two paths, one for each battery, but the batteries cannot backfeed each other.

With isolator/momentary switch/relay method, the two batteries' negative terminals are grounded, and the two positive leads go to the isolator. All the truck's load goes to battery A. Cables from the two large posts on the relay go to the two positive battery terminals. When the switch is activated, the relay shunts the two positive posts of battery A and battery B together, allowing both batteries to feed the truck.



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