dual battery problem

Hi all, man I haven't been around in a long time. Long story as short as possible.

I have a dual battery system hooked up to a battery separator. Starting battery in the cabin area, its a optimum type battery, connected to the separator in the engine compartment where the accessory battery is.

I've been laid off and the jeep has been sitting a lot more than usual.

I took a quick run the other day, started fine, came home started fine for the ride home, had to boost a buddy on the way and I've never had a problem with this before so I did it with the front battery because it's just easier to get at. It boosted buddy fine and I came home.

Next morn I get up to go to post office and dead, completely, no clicking at all!! Never happened before. I tried boosting it to the front battery with of course no luck, that battery is isolated until the rear one powers the solenoid right? So I boosted my rear battery and voila started fine, took it for a short run, 5-10 mins max and came home and parked, checked it and it started right away, good. Checked it in an hour or so and started fine also, good, just an anomaly. Went to pick up my daughter from work tonight and dead again, but this time at least clicking but to dead to turn the starter.

So I'm wondering whats happening? If it was just lack of driving to charge the starting battery why would it have started in the first place? I'm not sure if I should put the charger on the cabin/starting battery overnight or leave it to diagnose it.

Could I have done something the the battery isolator and now the battery is bleeding power because it's locked open? I'm not sure how to diagnose this. It would be nice if it was just sitting too much and it was simple as that.

Please help, thanks in advance.

Voltmeter is your friend. Check voltage on both batteries, not running. 12V or more on both should be found. If, not, charge that battery. Start Jeep and check again. 13V or more should be found. If not, your dual battery system is flawed. Do you have a switch? What solenoid or rectifier are you using for isolation? Check voltage at this point to determine if it is open or closed.

Verify voltage on both batteries to see if one is bad. Otherwise it sounds like a bad ground.

Ok, thanks, will check in morning, bit rusty on this, installed it approx 2 yrs ago, life has been so busy since then. No switch, its basically a solenoid that separates power until the ignition has been switched on and is running...I think. It's an rv isolator unit iirc. Yeah, have to dust off the old digital voltmeter. Keep the ideas coming please, and thanks for the responses. Bad ground wouldn't account for the slow loss of juice tho would it? Had juice when I got home from a quick charging drive, then still had juice an hour later, then 4 hrs enough juice to click the starter but not turn it? Maybe I'm wrong?

Do a current drain test on the main battery, put your multimeter on DC Amp (10 amp) setting and put it in line with your main power cable on your battery, MAKE SURE THAT YOUR IGN KEY IS IN THE OFF POSITION AND THAT ALL ACCESSORIES ARE OFF!!! or you will either pop your fuse in your meter or blow it up if it's unfused, make sure your door is closed too so that you don't read current drain from your domelight, not sure on the exact stock current drain on the xj, but anything more than 0.040 milliamps should be abnormal.

If you are getting high current drain, (like 100-250milliamps) start by disconnecting your solenoid, then check meter again, if there is still a drain, start by unplugging one fuse at a time on your fusebox while checking the meter everytime, once you see the drain drop, it will be easier to troubleshoot the drain by narrowing down the circuit the fuse is giving power from.

Hope this helps, I'm an auto electrician if it matters. =)

p.s. always work back from the last thing that you did or was done with the vehicle, it's always the culprit.

Well, what a day. Narrowed the problem, I thought, to the starting battery first, a bad isolator 2nd. So off I go to have the battery checked and grab an isolator if necessary. No problem right? Well my rad pukes out 5 minutes from home and I'm stranded! Call towtruck, wait 1 1/2 hrs, get home and now a rad's on the menu also.
Get home replace the rad.
On to the battery problem. Battery checked out good at parts store, just needs recharging, so thats good news.
That leaves the isolator, which was acting funny when I was testing.
Remove isolator and check, it checks out fine!
So heres what I'm thinking, like Anarchy Jet said I've decided it must have been a bad ground on the starting battery in the cabin. It's the only one that was draining. That was not allowing the isolator to open up. Only question I have is why all of a sudden? Like I said it started fine one afternoon, I boosted someone on the way home, then the next day...dead. I don't recall messing around in the back that I may have loosened the ground wire, but it was NOT all that tight to begin with. So I cleaned and tightened all grounds and I charged the battery, and fingers crossed. I'll check it tonight. Thanks all, I'll keep you up to date.

Most isolators are what they call " Normally Open " which means at rest, that is with no current applied to the solenoid it keeps both hi current terminals open,

the ign feed wire that you connect energizes the coil to connect your aux battery to the rest of the system, and ground normally is the steel body of the solenoid that grounds out to the chassis.

So, what you suspect cannot happen, if the solenoid is not "opening up" then you would actually have both batteries available for starting all the time, or since both batteries are always connected, the weaker battery will suck the "good" battery down, since they are not a "matched set"

I've had bad experience with optima batteries, don't get me wrong I love optimas, I just haven't had good luck with the redtops.

I would check for current drain, maybe a stuck relay or something simple like that.

I understand Slave, it appears like it was a loose ground wire in the rear battery. The main ground for this battery. If it was loose or faulty it would cause the problems I had right? It could have been loose for a while and slowly draining the battery because it wouldn't be getting charged? Plausible? Either way, all seems fixed now. I'll keep you posted.

tkjeeper said:

I understand Slave, it appears like it was a loose ground wire in the rear battery. The main ground for this battery. If it was loose or faulty it would cause the problems I had right? It could have been loose for a while and slowly draining the battery because it wouldn't be getting charged? Plausible? Either way, all seems fixed now. I'll keep you posted.

Click to expand...

You'd be amazed how many automotive electrical problems can be tracked back to shonky grounds.

I'm sure you and I discussed your dual battery setup before, but what sort of isolator are you using?

Hi Jon, I bow to the king of electrical! Hehe, it's a rv isolator. It has 2 large posts, and the smaller post out front that connects to ignition. Yeah, the main battery ground was never bolted as tight as I would have liked but it was never a problem. I found a new anchor and wire brushed it and its nice and tight now. Bitch of it is that it cost me a rad for this, I was running the jeep after charging the battery and on the phone with a buddy mechanic to diagnose this and lost track of time and it overheated and blew the top rad hose connector off the rad before I could get out to it to turn the electric fan on. Dumb Dumb Dumb. Oh well, live and learn. Now I've noticed that the water pump bearing is suspect, so I'm probably going to have to remove the rad soon anyways! Life is so fun.