Dual Batteries

[madmanvillain]

Im planning on either doing a second battery, just havent decided whether its going to be in my trailer or behind the back seat. My Idea for behind the back seat is to just use the 12v stud on the fuse box and using a fuse and a isolator switch. I would only have the second battery as a back up, and planning ahead for lots of fog lights and possibly a winch in the future. The battery would only be on once a week or so to keep the charge up. Also thinking about having it on my camper trailer im building this winter (build will be documented). What do you guys think? i know a few of you have charging systems built in and all that but i dont need anything that fancy.

[Diacom]

Unless you have the Big 3 I wouldn't consider using the 12v stud on the rear fuse box. Even then I dunno if I would use it. Might be better to run a separate cable back specifically for your second battery of the appropriate size for winch use.

[The Roadie]

I put mine behind the driver's seat, with a 100A switch to connect it to the rear fuse block stud. I won't count on that sending current forward to the front Optima for winching, because it goes by way of the 125A megafuse, and winch loads can be 300-350 A. When I winch, I disconnect the rear battery, and depend on the front battery and alternator to deal with the load.

However, I did check out the charging current sent to the rear battery by way of the OEM wiring, and it's just right as designed. If it was any heavier, it might steal too much from the front, and blow the megafuse. With a totally discharged rear battery, I measured 45A peak current for the first few minutes, decaying down to 10-15 A for the next hour or so. The resistance of the wiring limits the inrush current and I couldn't have picked a better peak current if I designed it myself.

I have a deep-cycle battery in my non-overland trailer (Trailmanor 3023) and the long series resistance of that connection limits the inrush current to 20A peak, which is just under what the 12V constant trailer power circuit is fused for. Self-limiting is cool.

[fishsticks]

Bill, your description of your setup just answered a question that's been bugging me for a long time. "Will charging a flat rear battery overload the system?"

Now I can look at starting my own setup with confidence. Thanks!

[madmanvillain]

what is the big 3? and roadie...little simpler terms?

[Zero]

The big 3 r what we call the 3 most important ground lines under the hood.

1. battery to frame
2. alternator ground
3. engine block to frame


Find those wires, and replace them with a high quality larger gauge for an improved ground through out your vehicles electrical system.

If your gonna put a stereo in, make all the big 3 wires 0 gauge.

[The Roadie]

... roadie...little simpler terms?

It's my nature to push technical terms like "inrush current" into a discussion where they're the most accurate way to describe what's happening. It's my hope (since I wear a technical writer and trainer hat at work) that the gentle reader will Google those phrases and use it as a leaping off point for more reading.

If you've got another phrase that Googling doesn't help with, just post up and I can explain further. Electrical issues are usually well-explained by water analogies - such that a discharged battery is like an empty bucket. The alternator is a spigot on the side of your house, the main battery is a full bucket under the spigot. And the wire from the megafuse to the rear fuse block and rear battery is like a hose. The thinner the hose, the more resistance to water flow (current) there is, but a higher pressure from the spigot (voltage) can force more water through a smaller hose, even if it stresses the hose. Too much flow or too much voltage can damage the hose.

Is it possible to escape high school nowadays and not learn fundamentals of electricity? I'm not being sarcastic - it seems folks just aren't learning this nowadays compared to when I went to highschool in the late 60's.

[fishsticks]

Is it possible to escape high school nowadays and not learn fundamentals of electricity? I'm not being sarcastic - it seems folks just aren't learning this nowadays compared to when I went to highschool in the late 60's.

I graduated in '95. It was still there if you took AP Physics... and paid attention.

[Zero]

I graduated in 03' and you were lucky to find schools with an auto shop anymore, most have phased out wood shop too.

[madmanvillain]

depends on what you look for. i actually took a very technical avenue through high school, but electricity was not my strong suite. so, i guess my question now is "if i upgrade the 'big 3' and put a fuse inline with my isolator switch, will i be ok? and what size fuse should i use?" electrical systems not being my strong point, i am asking others who are good at it (cough roadie cough)

[Diacom]

I put mine behind the driver's seat, with a 100A switch to connect it to the rear fuse block stud. I won't count on that sending current forward to the front Optima for winching, because it goes by way of the 125A megafuse, and winch loads can be 300-350 A. When I winch, I disconnect the rear battery, and depend on the front battery and alternator to deal with the load.

However, I did check out the charging current sent to the rear battery by way of the OEM wiring, and it's just right as designed. If it was any heavier, it might steal too much from the front, and blow the megafuse. With a totally discharged rear battery, I measured 45A peak current for the first few minutes, decaying down to 10-15 A for the next hour or so. The resistance of the wiring limits the inrush current and I couldn't have picked a better peak current if I designed it myself.

I have a deep-cycle battery in my non-overland trailer (Trailmanor 3023) and the long series resistance of that connection limits the inrush current to 20A peak, which is just under what the 12V constant trailer power circuit is fused for. Self-limiting is cool.

Your answer is right here and you read it once already asking Roadie to put it in simple terms.