Series or parralel?

[Slinger]

I want to hook up another battery with my present one for more juice when starting, allways affraid the battery will give out after multiple starts and leave me stuck out on the water. My electronics days are way behind me but I remember somthing about batteries in series and parrelel (my spelling ain`t so good anymore either), one gives amps and one gives voltage. For starting I assume you would want more amps? What is the proper way to increase starting power, + to + or +to -? Double the amps or double the voltage?
Slinger

[Slamdance]

You will probably fry something if you try to double amps or volts. The best way to do it is to have a 2-battery set up with a switch that allows you to use one battery or the other. You need to make sure the batteries have enough cranking cranking amps to turn your motor over, but too many will fry your powerpack, starter and/or alternator. With the standard two-battery set up, if you drain one battery, you can switch to the other and hopefully have enough juice to get you home. Most marine electronics are on a 12-volt system. Doubling the volts will damage the electronics.

To answer your questions, Series - Negative to positive. This doubles the voltage and keeps the amps the same. If two 6 volts golf cart batteries are connected this way the voltage doubles to 12 volts and the amps remains the same. Again, One Trojan T-105 has about 200 amps of electricity and is 6 volts. If you series it with another, then you use a jumper to go from the negative terminal to the positive of the other. This now makes the set 12 volts but keeps the amperage rating the same at 200 amps.

Parallel Circuit - Positive to positive or negative to negative. Voltage remains constant, but the amps double. Using our Trojan T-105 example, if you get another set of T-105's, a total of four you now have two 12 volt batteries. These two sets need to be paralleled to double the amps but keep the voltages at 12 volts. This is done by connecting the positive of one battery set to the positive of the other and the negative of one set to the negative of another.

[bass3six]

Operation/theory on a simple series circuit...
http://www.thetech.org/exhibits/online/topics/12h_flash.html

[anut]

If you only need charge for emergency start, that is not running more electronics on boat. Just get a portable jump starter. I have two batteries (2 engines) and a portable jump starter. The portable has saved my bacon a few times when at a long anchor and too much music has driven the batteries down.

Other wise you will want to install a 2 way battery switch with the second battery. This will allow you to use either or both (more amps) batteries to fire up your engine.