backup battery help PLEASE

[greenbuttskunk]

I want to hook up a backup battery source. I already have the A/B switch. (1, 2, ALL, OFF)
I took off the switch cover and, as expected there is a spot to connect a second battery cable.
SO, so I just connect the #2 connection on the switch plate back to a cable, then the other end connects to the positive terminal of the new backup #2 battery. Am I missing something. I am assuming this will allow going from primary to secondary, and when I want to charge both under power I'll switch to "ALL":shrug:
Inquiring minds want to know.
Thanks!
GBS

[dixiefisher]

GBS,
That is how mine is setup..there is also a neg. to neg. connection on the batteries. My understanding in the "all" is when you don't have enough juice in either battery to start the main. I typically start and run the main motor with #1.....when trolling or on anchor I switch to #2....

[greenbuttskunk]

well, that's how I did it. Neg from #2 went to Neg terminal of #1. I should be good to go. Hopefullt.:pray:

[Gundog]

That is all correct a couple notes though don't switch the batteries with the engine running.

If you have one dead battery and one fully charged and put it on both to charge the batteries it will equalize both batteries meaning one battery will feed into the other until they equal out. You might end up draining the good battery significantly before they both start charging and the engines charging system would take a while to build them both up. They will also only charge to a level of the worse batteries capability to take a charge. The best bet is to install a battery isolator that will allow both batteries to charge and not put them on both to do that. Use the both for when you need to start and your battery is dead.

Hopefully Chuck will chime in and add to this he is the expert.
Mike

[chucks electric]

hi there, thanks for the vote of confidence. you are on the right track and if its in your budget add a ground buss and bring all of your grounds to one location. when using a battery switch you will only switch the positive side of the battery(s). the best way to add two batteries is to start off with two new batteries the same manufacture, size and vintage. I'm not suggesting that you buy a 4d or larger but if you can fit a group 31 then do it. remember your batteries are your life line also to get the most life out of batteries is to use them so alternate from one battery to the other. gundog has a good point in that using one battery until its down then switching back to the other or both it will tax the alternator most alternators will not tolerate extended full output. time on a dedicated marine charger is the best way to recharge.
your batteries will need to stay at 14.5 to 14.7 volts for approx 2 to 6 hours this is the recovery stage of battery charging then slowly drop down to a float voltage of 13.1 to 13.5 volts. suffice it to say most alternators will not do this and this will shorten battery life for the sole reason that the battery never fully recharged.
don't be fooled because a battery had enough in it to start an engine that its recharged that was just a surface charge and its reserve capacity is greatly diminished. there is so much more to batteries, charging systems, and wiring so hopefully by thinking about this you'll have more questions. i had one more thought about isolators, because they use diodes they have about .7 to .9 voltage drop this is enough to prevent proper recharging of deep cycle batteries also cause alt reference voltage to be off or too high. so instead of 14.4 for example it will be somewhere aroung 15.3 volts or more which is not good for the alt.