I have already had one request for a wiring diagram for dual batteries that can be isolated. Below I have included a wiring diagram that allows you to isolate the batteries, hook them together and charge them, and use the second battery as a jumper if you run the primary battery down. It is all very painless. Whenever you turn on your ignition switch, the batteries will be hooked together. Turn it off, they are isolated. You can run your accessories and drain the secondary battery and not touch the primary. Start your engine and the secondary battery will charge. Just about idiot proof. If your primary battery dies because you left your lights on in the fog, flip the switch and instant jumper cables. Start the engine and it will charge the low battery. The solenoid in the middle of the batteries that connects the two positive posts is a continuous duty 250 amp solenoid. They generally have to be ordered. A relay is used to power the solenoid to take the heavy load off the ignition switch and auxiliary switch. Cable in these photo is 0 gage stranded copper.



The battery connections were made cleanly with aftermarket battery terminals pictured below.



All connections were painted with a product called Electrical Tape sold at hardware stores

 

Excellant thread.

 

really good idea, but in the event of the primary battery dying, the difference of charge between the two batteries when the ignition is turned on is hard on them. the batteries will equalize charge as fast as possible, and the batteries will heat up and potentially start gassing. plus if youre operating accessories without the ignition on, and you dont hit the switch to activate the solenoid the difference of battery usage between each other isnt the best thing

 

Hasn't been a problem so far. The optimas are not a conventional battery and they don't gas. Anyway, to start a vehicle that is in normal repair takes only seconds, and the battery will limit itself to the rate of charge

 

Nice job on the battery hook-ups and wiring diagram..I use to do a lot of custom auto electric stuff myself..My 1986 Astro van had 3 alternators 2 of which were externally regulated at between 16-24 volt..A total of 5 batteries of which 4 were 8 volts wired into a 16 volt configuration..2 stereo amps and 2 CB linear amps..Oy!

 

Thanks, but I must bow to my superiors here.

 

yeah the optimas are good batteries. theyre a glass-mat style battery are they not? i would be more worried about screwing up my warranty, but as long as they dont find out eh..

 

Mine are old enough to be off warranty anyway

 

Not bad. For variations on wiring up a solenoid as an isolator, see the instruction sheet on my website (there are schema of the variations, and you can also mix and match as required.)

NB: I have never liked "liquid electrical tape" for coating connexions - it either goes on too thin (leaving the connexion effectively unprotected) or has too light of a body (allowing it to "wick in" to the wiring and screw up the connexion.) I use double-walled heat-shrink with a co-extruded heat-activated adhesive liner - the double walls ensure solid protection will still allowing a bit of flexibility (triple-wall is rigid...) and the co-extruded adhesive seals the connexion without wicking in to the wiring and screwing things up. Since the adhesive simply needs to bond the HST to the wiring insulation, you don't need much of the stuff - so the risks that would normally be associated with adhesive flow are greatly minimised.

Apart from that, looks like a decent job! I just figured you could use a couple of tips (you can apply them if you like, or not.)

 

This was applied more as protection from touching something grounded. Some of the post is exposed, but I will correct that.

 

Could some one give me some pics of dual batteries installed.

With the CAI I can't hardly figure where to put the second one in the engine compartment.

Thanks
MO

 

Yech. LET (Liquid Electrical Tape) is definitely not enough for something like that, especially with the high potential current you can see there! Double-walled polyolefin HST is the order of the day. However, since you're not likely to be able to get it on with the terminals assembled, you might want to look into getting a roll of "amalgamating" tape (room-temperature "cold shrink" tape) to use to wrap your crimps. The tape, given some time, will eventually form itself into a single bit and effectively insulate the connexion.

That's why I don't use crimp battery clamps - well, part of the reason anyhow. I use ring lugs on all cables, then use battery clamps with binding posts or screw attachements - they're more flexible for the purpose. I'm looking to expand the offering - I've got a supplier that is advertising their new line of "lead-free" battery post clamps, and I'm waiting for datasheets on them (since they have "universal/flat-style," marine-style, and ordanance-style listed, I want to know more. If they're made from what I think they are, I'll probably be adding them to the line fairly soon.)

 

"LET" is more than enough, unfortunately. as long as you have a nice uniform coat it is more than sufficent. voltage determines insulation strength, not current. and for the battery post, as long as you arent a ding-dong and put a wrench from the terminal to any grounded source, youll be okay. this is a 12v system were talkin about, it isnt dangerous. only danger here is igniting fuel.

 

Could just be me then - when you've worked on damned near everything at some point or another (gensets, three-phase standup panels, and that sort of thing as well...) you tend to get fixed opinions about things.

I'll not argue that insulation is rated by "breakdown voltage" (that's why you'll invariably see a voltage rating on wire,) but when the insulation is thin and the current potential is high, it could be a problem. Throw in having potentially sharp(ish) metal objects floating around, and it could get hairy.

Could I be wrong? I'm sure I could. But, I'd sooner err on the side of caution than not....

So, I suppose I'm just cranky. Probably got something to do with the same impulse that has me doing residential electrical work IAW commercial Code...

 

very true, but cmon its 12 volts, all that current is gonna do is spark and bang and scare you i shorted my share of electrical circuits, and auto electric is nothing haha.
you should get into commercial, its way better than residential.