Running Free

NAPA PHI54140

Troth

you would deffinetly not want to drain the optima battery at all let alone constantly. they are great batteries but are not recomended to be depleted at all each time it is depleted the battery life is cut in half and theyre not cheap

5-90

Battery "competition," as you put it (and as good a word as any!) is a larger problem at low charge rates - like a bank of storage batteries being fed by a solar panel or something like that. Also, it's a problem when the batteries are in a "discharge" state - read: being used. In cases like that, you use a set of identical batteries of the same age. Replace one? Replace the entire bank! The problem is akin to what you have when you have a 'dead cell' in a single battery. Automotive batteries actually have six cells - and one cell going "dead" will discharge the other five.

I've set some vehicles up like that - one starting battery, and two or three auxiliary deep cycle batteries in a secondary battery bank.

Your case is already different:
1) There's only one auxiliary battery.
2) The auxiliary battery is of a different type than the starting battery.
3) The auxiliary battery is of a different age than the starting battery.
4) The two batteries will be separated by an isolator when not being charged.

Points #1 and #4 take care of the "bank" problem. The high rate of charge offered by the alternator (typically about max rated output less 40A or so for engine management) takes care of the other two. (Bear in mind, the typical PV backup array runs a 2-5A charge rate, tops, unless you get stupid with the PV cells! Solar is notoriously inefficient at generating electricity - I think the current SoTA output rate is something like 12-15% of the energy falling on the panel being converted. This may be dated - I haven't checked recently. I'm fairly sure it hasn't topped 20% total conversion efficiency, tho.)

So, say you have a 90A max rated output alternator (meaning 95.5-99A possible max output.) Drop 40A from that (engine management systems and operating overhead) and assume that all other accessories are off, and you've got 55.5-59A available for keeping your batteries topped up. No ache - you're good. (This doesn't obviate the need to monitor system voltage accurately - if you see a sudden surge in voltage, figure you've lost a battery somewhere, and the alternator has swung to full output to try to charge it off. Cut the failed battery out, or you're going to kill your alternator - most likely to rectifier diodes will fail due to heat.) Of course, charge rate may be improved by an alternator swap - you have options of a 117A (should fit neatly,) a 136A (should fit with some mods) or a 160A (likewise) - assuming 1991-up XJ. If you have a 1990 or earlier with a Delco, you should have no trouble getting "upwound" at a local shop (or you can check through the shop I use.)

And, if you're thinking about "going deep discharge" on your secondary, don't bother with the Yellow Top. Get the Blue Top - as a deep cycle battery, it's designed for relatively low discharge rates over a long period of time, and will last through many more charge/discharge cycles than the Yellow Top (and far more than the Red Top!) Construction of the starting and deep cycle batteries do differ significantly, although I don't recall how at the moment.

As far as solenoid rating? I seriously doubt you're going to use the full supply potential of the battery when running accessories - so don't go by CCA ratings (besides, that's a high-output/short-term rating, and 750A over a minute or so is going to cause you some trouble. Anyhow, 0000AWG cable is spendy...) Accessories should be wired directly to the battery, so that's not a factor. Go by expected charge rate, assuming the alternator's output not going into the starting battery and running the engine has to go into the aux battery (use the Max-40A rule I gave you above.) Size your solenoid using that as the continuous rating (the 85A becomes sufficient for most applications, using that rule.) Finding a solid-state isolator becomes difficult once you get up that high - that's what's delayed my finding something effective to carry.

There are several ratings for an automotive storage battery:
CCA - Cold Cranking Amps. This is the "instantaneous" current it can provide at (I believe) 0*F for about thirty to sixty seconds.

HCA - Hot Cranking Amps. This is what it can throw out at (again, I believe) 100*F.

Reserve - Listed in Ampere-hours (Ah,) this is the current that it can provide on a continuous basis over time. This is the rating you're interested in for auxiliary batteries.

Let's say you have a battery with a "Reserve" (or "Reserve Capacity") rating of 100Ah (just to keep the maths simple.) This means that any combination of amperes and hours that multiplies out to 100 Ampere-hours can be expected over the useful service life of the battery. Thus:
* 100A for 1 hour = 100Ah.
* 200A for 1/2 hour = 100Ah
* 50A for 2 hours = 100Ah
* 10A for 10 hours = 100Ah

You should get the idea. CCA and HCA aren't ratings you want to worry about for auxiliary batteries - since they don't apply.

Do you have any more specific questions? I've started to wander, so I'll shut up now...

alpine.adrenaline

I'm grateful for the extensive answer! I never thought about the discharge rate you mentioned at the end of your post, it makes complete sense though. I'll go with a blue top then... (thought I'd check CF before I drive down to 4wd Parts, glad I did!) How does a normal wet-cell battery compare to those optimas in terms of frequent discharge? Would the blue top still be ideal or is there another type I should go with? I've love my yellow top; I frequently park my XJ for two-three days at 12,000 ft and it fires up with ease every time. It almost sounds like the risk of discharging a yellow top is worth the secondary battery alone. Just so I understand it, the batteries will be connected while the engine is spinning the alt, and disconnected when the engine is off. Will the charging of the alternator engage the solenoid or do I just wire that to the ignition?

4.3L XJ

I have had some experience with Optima batteries now and have had two go bad on me. They are great batteries and have lots of reserve power. But there is one thing you just can't do to them. You cannot discharge them completely and leave them that way for a couple of weeks or months. That is what happened to mine. As far as deep cycle discharges, the yellow top is rated for 185 complete discharges.

alpine.adrenaline

I've never had to jump start my optima battery, so it has never been completely discharged, and I plan to avoid doing so. I just want to be able to leave the stereo and lights on for two hours and still be able to fire up my rig.

Troth

Ive seen where guys have a battery and switch in the back of XJ for that so they flip the switch for AUX power. Id use just a regular battery for that though

5-90

The solenoid would be wired to an ignition lead, but there are a couple of options you can do as well.

1) Have a switch in the ignition lead to the soleniod, so you can force it off if you need to (say, you've drained the backup, and you want to start the engine easily. You can cut the secondary in once the engine is running.)

2) Have a switch from +12V to the solenoid trigger post, to force it on if you need to (self-jump-start, usually, without having to dig out your cables.) The surge rating of the solenoid and the wiring proper (use at least 4AWG if you want this to be an option) will handle the starter motor load - I've calculated 4AWG to be good for a good 140A or so constantly, and it has no trouble passing the 160-170A that the start draws for the few seconds it does.

I'm perfectly willing to help you design and build the system backchannel - you can see my site for information and parts.

projectmayhem

dude youre your thinkin way to hard about this, im a technician and for what your wanting to do this is bs. but im a bit lost on what ur gonna do that is gonna completely drain a freakin optima battery i have had system in my jeep for a while now and i have never even completely drained advanced auto battery KOEO, not saying it wouldnt but thats just me, relocate ur battery to the cab somewhere ur pleased with and then attach bat no. 2 positive to positive and neg to neg. i have this setup on my plow truck and its been goin for 5 yrs now

5-90

Somehow, I'm thinking he's not just running a "system" on his rig.

He's mentioned an inverter, and I'm sure he's going to have some other appliances as well - and probably "site floods." I'm willing to bet he wants this because he goes camping - and doesn't want to have to buy an RV

In which case, this is perfectly valid and not a bad idea.

I've also done wiring for SAR rigs - they're designed to run some huge inverters, and to run the winch frequently. The secondary battery is the supply for the inverters (usually tool power,) and the buffer for the winch, which leaves the starting battery alone and allows that side of the electrical system to remain reliable.

Not to disparage your experience at all - but bear in mind that everyone's needs are different, and there are different operating environments for vehicles. A lot of my work has been industrial and on emergency vehicles, which requires different thinking from conventional automotive mechanics. Industrial requires a somewhat higher safety factor, because DOT, OSHA, and other organisations get involved (part of what I did industrially was inspections on medium and heavy trucks to get DoT and CalDoT numbers.) On emergency vehicles, you are working for absolute reliability within the limitations of the parts available. I often ended up making parts, because I wasn't satisfied with what I could buy.

If I was designing a rig for a camper, I'd have it set up to add a secondary deep-cycle battery - all you would have to do is select the battery, get the cables out from under the back seat, and mount the thing & wire it up.

If you're going to use the appliances KOEO, you're going to want to have them wired to a secondary cell (bank) and have that isolated to preserve the charge on your starting battery. Might not be a bad idea to hardwire a solar "battery maintainer" panel to the setup as well - which would allow the battery to be topped off during the day (when you're probably not using it anyhow...)

Believe me, overkill in design isn't always a misplaced concept!

BlueXJ

Jon you are always a storehouse of knowledge of many things and the undisputed king of all things electrical. At least in my book.

alpine.adrenaline

Naysay as you wish, projectmayhem, but there's more to this complex system than its necessity. I'd rather get the gauge of wires, heavy-load parts, etc right the first time so that I can be confident in my system. I could have just rigged something up but I want it reliable. I used to be on a SAR team near Black Hawk, CO and my XJ was invaluable in the various weather, but I'm not going to jeapordize that reliability. The setup I want is rather complicated, at least for me, but I'm learning a lot in the process as well. Getting close to having all the parts and when I do a writeup hopefully I can help other people out too.

5-90

I do what I can. What's the point in all of the learning of the last thirty years if I don't try to help other people with it?

@alpine - if you want more detailed help, feel free to hit me backchannel and we can kick things around some more. As I said, I've designed setups like this, and it seems that the only way they get screwed up is when someone manages to do something stupid with one of the wires (like drop it in a cooling fan or something.) Reliability is a critical concern for me as well - I do every job like I'm doing it for my wife's vehicle - if something breaks down on me, I can fix it. If something starts coming apart on her, I want her to be able to make it home so I can fix it...