Voltage Driving Me NUTS!

[Unforgiven]

I need voltage help from anyone who can explain this.

Today I finally got the truck registered & decided to go for a drive on some dirt roads. I flipped the lights on so that I could have blinking turn signals.

Later at home, a neighbor stopped to tell me that my lights were on. I thought that was odd since I installed kill switches on the batteries. When I shut off the truck I turned the kill switches.

It turns out that my kill switches only kill 12v of the 24v batteries, leaving 12v going through the headlights (that I forgot to shut off)

For the life of me I can't figure out why the kill switches only cut the 24v down to 12v. It's driving me nuts.

Here is what I have :

One 1-wire 12V alternator
One 1-wire 24V alternator
Two 12V batteries wired in series for 24V
Two 12V batteries wired in parallel for 12V

All 4 batteries are connected to a common ground. The negative from all 4 run through the kill switch before going to the frame.

When the truck is running & the kill switches are on everything is fine. 24V is 24V. 12V is 12V. But when I turn the kill switch off the 12V goes to Zero, good. But the 24V only goes to 12V. There is still 12 V of potential difference from the 24V terminal to the truck frame.

Why? My thinking is not right on the batteries. Shouldn't killing a common ground kill the voltage for both the 12 & 24 volt systems?

Why is there still 12 volts to ground on the 24 volt system when the kill switch is off? And how in the world are the headlights forming a complete 12 volt circuit when the negatives of all 4 batteries are severed from the frame via the kill switch? The switches are fine, I tested them.

Am I going to have to rewire the kill switches to be on the + side of each of the 12 & 24 volts?

My original post is here:

http://www.steelsoldiers.com/deuce-...off-switch-fuse-box-wiper-install-w-pics.html

Any help would be appreciated. I'm going bonkers trying to figure out why the headlights are seeing a completed 12 V circuit.

 

[AN/ARC186]

The 12 V is probably coming from a backfeed through the one wire 12V alternator, I've had it happen before on some customs that people brought in with electrical issues.

If you kill the pos through the switch rather than the neg that should correct it as well.

 

[frodobaggins]

Never seen a kill switch on the negative side before, but AN/ARC186 sounds like he's on the money, it's got to be a ground somewhere else.

 

[Carl_in_NH]

You need at minimum two kill switches to isolate both the 12V battery and the 24V battery string. Neg or Pos should not matter for the kill switch - as long as you don't tie the Neg of the two seperate sources together, as you've done. What's likely happening is something is back-biased - like a diode in the 12V alternator, allowing your rather interesting collection of batteries to become a string that looks like 24 - 12 == 12V at the 24V point when the ground is isolated from the chassis.

Use two kill switches - one for 12V, and one for 24V - Pos or Neg side will not matter at that point - just that you open both kill switches to isolate these battery banks.

Hope that helps.

 

[Boatcarpenter]

My wild guess without seeing a wiring diagram is that the 12 and 24 volt grounds being tied together, even though disconnected from the frame with the kill switches, is creating a complete 12 volt circuit somehow in the 24 volt side of the wiring. My first move would be to disconnect the 12 volt grounds from the tied together ground lugs and see if that cures the problem.
With the lights being on the 24 volt side of things, it seems that is how the 12 volt could get into that harness.
After that, well, just start probing with the trusty multimeter in a logical manner and see where the bypass in the system is.
Of course, it makes a big difference whether the light switch was on or off when this occured. If off, then look for a common connection between the 12 and 24 volt harnesses where a backfeed/crossover could occur after the light switch. If on, and you can turn them off with the switch, then the bypass/crossover is before the light switch.
BC

 

[Unforgiven]

Thanks guys,

I think you are right. The 24 V batteries are going back through one of the self-actuating alternators, making a complete 12v circuit.

I'll rewire it this week for the kill switches to be on the + side & use one switch for 12v, the other for 24v. I should be okay to have a common ground on the frame, right?

My worst fear is being stuck out in the middle of nowhere camping & having 4 completely dead batteries. There's no way I could push start this thing by myself in sand or gravel.

As it is now, the batteries are staying charged. I was having a battery drain problem before. Still, I would like the kill switches to kill all the batteries entirely.

Luckily I have large gauge wire going to the kill switches. Switching them over to the + side shouldn't be a big deal.

 

[Carl_in_NH]

Common ground and kill switches on the positive sides of the 24V and 12V battery banks will be just fine.

 

[datsunaholic]

Actually it's not the alternators that it's backfeeding through. It's feeding back through the 12V 12v (parallel) batteries, the 12V alternator, and any 12V units that are always on. It's actually driving the battery backwards. Though all it really did was cancel out the 12V and half the 24V.

Putting the cutout on both (+) sides is the best thing, it keeps from having unintended loops. We had that problem with our old race boat, with a 12V bilge pump but 24V battery. If you shut off the master but left the bilge pump on it would backfeed 12V through the oil pressure warning light.

 

[tiger422]

With a 12 volt and 24 volt system using 4 batts. you should only have 3 negitive wires to ground.
1 for each batt in 12v system 1 for 24v system.

Jim

 

[Unforgiven]

yes and yes

I only have 3 negatives. I shouldn't have said 4. I have two batteries in series making for 1 ground. I have 2 batteries in parallel making for an RV type 12 volt system with a decent amp-hour reservoir. I guess you could say that is 2 grounds. I linked the 12v together before running to the kill switch so I really only have two wires from the ground of the batteries, one from 24v & one from 12v.

I still don't understand how the backfeed 12v works, but I believe you. If all of the negatives are cut off, I can't see how it can still make a complete circuit. But I'll change it over to a positive-side cutoff tomorrow.

If the 24 v sees 12 v to the frame with the switches off, that means the frame is at 12v relative to true zero. But the 12 volt system is fine. The frame is at zero potential.

My head hurts.

 

[datsunaholic]

The reason it backfeeds in that scenario is because your 2 ground leads (the one from the 24v and the one from the 12v) combine before the switch. That causes the 12V to actually be hooked up in series with the 24v batteries ONLY when the switch is off. When it's on, they're paralleled up. Because the 12V set is reverse from the 24v set, the 12v cancels one of the batteries in the 24v set, making a 12v loop. Instead of cutting the system off, you altered the circuit. But it still had a complete loop.

This was a scenario that we learned, and practiced with, in Electonics Engineering- hooking up a voltage source backwards. In THEORY it cancels. In practice, you're lucky a battery didn't explode. The fact that 4 batteries were involved may have helped avoid that because the system might have been somewhat balanced (one side had half the voltage but twice the current capacity, so the overall power was equal).

You could make it work just fine with 2 switches on the negative side. As long as the switches are before the 2 circuit combine.

 

[Unforgiven]

Thanks, I'll test that theory later when the wind dies down. It's too windy to do anything at the moment.

Right now here's what I'm thinking:

The 24 V headlights (with the light switch accidentally on & battery kills off) is completing a circuit to the truck frame.

From the truck frame it is going "backwards" through the 12 volt 1-wire alternator, eventually reaching the +12V terminal of the 12V batteries.

As far as the 24 volts is concerned, it sees the 12 volt positive terminal as ground. The potential difference would be 12 volts, which is exactly what I'm measuring.

You are correct. Had I not caught this it would have drained the 24V batteries & it would have over-charged the 12V batteries, possibly making them explode or at minimum ruining them. Theoretically, they would have all reached a common voltage of 18 volts. I think the 12 volt batteries would have fried before that.

Good thing this only lasted for a few minutes before being caught. Better now, in the driveway, than later in the summer in the mountains.

NOTE TO SELF: ALWAYS PUT CUTOFFS ON POSITIVE SIDE WITH MIXED 24 & 12 V SYSTEMS.