Need alternator & battery isolator advice please

[Green1]

that's exactly how to do it... and yes, the trickiest part is finding a lead that's only on when the engine is running... (btw, I have need for that same connection, so once you track it down please let me know where you found it)
The other suggestion other than the alternator is oil-pressure sensor... but I haven't found either yet (nor have I had the time to really try)

[dpd]

Yes, I'll look for the oil pressure sensor, I think I've seen it while poking around the alternator, a black wire with a plastic connector that plugs in next to the oil pan. I'll test it out and let you know how it goes.

I got some feedback in a PM from Jay TardisDeli (thanks!) confirming that the way to go is the solenoid. I'm convinced now, so if anyone wants a motomaster dual-diode 120A isolator I've got one cheap :) Jay recommends doing it with the key-on switch; I'll do it that way if I can't find an engine-on lead. I guess there's not much difference as long as no one leaves the key in 'on' with the engine off for an extended period; the starting battery with around 50 amp hours would drain into the house batteries with 420 amp hours pretty readily, which I want to avoid by making sure they're only joined with the alternator running.

I just thought of something else though: If I find an engine-on +12V, I'd better make sure that it's OFF while the starter cranks, or else I'm joining the battery banks together every time I start the vehicle! I think this would spell a drastically shortened life for my deep cycle batteries, they're not supposed to be drained at the kind of amps that starting a diesel engine will pull every day.

Does the oil pressure sensor only energize after the engine has turned over and run for a few seconds, or will it light up while starting? If it's on while starting, I could still relay it off from the steering column starting wire to a normal-on relay, but that would complicate it a bit...

[Green1]

Jay recommends doing it with the key-on switch

this is NOT a good way to do it...
If wired in this manner you will have problems starting any time the auxiliary battery is very low because as soon as you turn to "on" for the glow plugs the aux battery will be charging from the starter battery, it will equalize the charge between the two and cause starting headaches (yes, this is the voice of experience talking!)
(no problem will be experienced under normal circumstances, only when you have one of the batteries with a very weak charge, which rather defeats the whole point to isolating in the first place)

[dpd]

I found the oil pressure wire today, it was what I thought, black stiff wire plugging in just rear of the alternator on a plastic attachment.

Multimeter reads 0 volts with key in on position, 12 volts with engine running. The pressure must build while the starter is turning, as the voltage creeps upward over a second while turning over, so using the oil pressure wire on the isolator solenoid will probably trip it while cranking, especially if it's a longer start. That rules out the oil pressure wire alone, as the starter will always pull amps from both battery banks as soon as the pressure/voltage increases enough to trip the solenoid. Apart from damaging my 6v trojans by subjecting them to cranking amps at every start, more advice from Jay warns of fire dangers if the wire is not fused, and the fuse would have to be a big one if cranking amps are going... Jay recommends 300 amps (do you know where I can find a 300 amp fuse?)

If using the solenoid on key-on is ruled out (I agree, although apparently this is the standard way to wire the isolator solenoid in RVs) then I will still use the oil pressure wire, but put a relay to the key-start in the steering column inline with the oil pressure wire to the solenoid.

If I wire the relay to be normal-closed, and open on starter-relay current, then the solenoid should be closed when the key is in 'start' position, and the batteries isolated. As soon as the key is back in 'on' and the engine is running, the banks join. This guarantees that the banks will not be joined when starting, and only when the alternator is supplying current.

I think I've got the best solution here, the only thing that's missing is emergency-start from the house bank. I'll be putting an 80 amp fuse between the solenoid in the battery tray area and the 2/0 gauge wire to the house batteries... so I can't use that for starting anyway, the fuse will blow for sure.

I always have the option of jump starting myself by manually running jumper cables direct from the house bank to the starting battery. This should leave the 80amp fuse from the solenoid alone and I won't have to unplug it, since the solenoid will be disconnected if I get the starting-relay wiring right. Also I could use my 1800w inverter to plug in a small battery charger and wait for a couple of hours to recharge the starter if I have the patience.

I would like to take the 'patient' route when I can, and never subject the trojan 6Vs to starting, but I know situations sometimes call for different. The batteries cost $700 though and I want them to last a few years. There are serious advices out there to not start on these deep cycles, I'm not sure how much to worry about it though for the occasional need.

A whole other way to think, maybe overkill, is to put a cheap spare starter battery in the tray, isolate it full time from the main starter with the surplus diode isolator that I have now so that it's always topped up, and use a short jumper cable from one battery to the other, manually, for emergency starts. This would be crazy dependable but I'll try not to get ahead of myself :)

[TardisDeli]

Hi DPD, this is Christine TardisDeli. You have been pm'ing with Jay TardisDeli.

My girly advice re your worry about spare starter battery is : buy on sale one of those booster pack things from Crappy tire. Need 600 ccamp, says Pedro Delica who uses his, even in Cold weather (ours is 300 amp but wouldnt start the deli the one time I left the lights on all night). I am watching for one, with the tire compressor, Just to use for camping stuff.

Cheers, Christine.

[Green1]

my advice for a "sure start" system is a switch to link the batteries, even through that 80A fuse, just link them, wait a few minutes, then try to start. The waiting step will prevent sucking the starting current through the 80A fuse as the battery charge will have time to equalize between the batteries a bit first (I've done this method in the past with a 30A fused wire, and after a few minutes the vehicle started fine, I didn't dare try it immediately on linking though for fear of blowing the fuse (not to mention that the wire was only 10 gauge (or maybe even 12))

[dpd]

Thanks to green1, Jay, Christine for the valuable advice.

The final set-up is:

- 2 parallel exide purple haze AGM starting batteries
- 2 series 6v Trojan L16s house batteries
- solenoid isolator from starter to house batteries, connected with 2/0 gauge cable and 300 amp fuse
- on-off-on DPDT override switch selects:

on1: solenoid closed (join) with engine on (oil pressure). Opened (isolated) on crank & engine-off
off: solenoid always open
on2: solenoid always closed (emergency starts, battery bank equalization)

The whole thing may be a bit over-engineered... but fairly idiot-proof, as there's no possibility of problems do to the key left in 'on' for too long, and with the override switch I can force whatever mode I want, to fully join the banks in an emergency or fully isolate them while running to charge up the critical starting batteries quickly when everything is low. Also when the banks are joined and both full, I can use my solar panels (90 watts on a 4' by 3' rack on the roof) to keep them both topped up when parked long-term.

The 2 purple haze AGMs are a lot of battery, but the battery the Delica shipped with was pathetic for -35 degree starts in the Edmonton winter. So lots of cranking amps and zero maintenance (and no flammable gases near the fuse & solenoid) will be nice.

I'll be wiring it all up this weekend, thanks again for the advice, it has helped me change my plans 180 degrees and definitely for the better.