How are the stock dual batteries wired up?

[BlueRock Junior Member Find all posts View Profile Close BlueRock]

I'm trying to make sense out of the dual battery setup in my '01 series 90 D-4D, in preparation for a winch setup. They appear to be hooked up in parallel, but not exactly! There's a bit of wizardry going on with the negative terminals, that's not reflected in the schematics. Here is what I've noticed:

1. The positive terminals are connected to one another using a heavy duty cable. Fine.

2. The driver side negative terminal is connected to chassis ground using a fairly thin wire.

3. However, the passenger side negative terminal is NOT connected to chassis ground. There is a thick cable going down, which I suspect is connected to the starter motor.

4. When I measure the resistance between the two negative terminals, I get a reading of 3 or 4 ohms. Which is quite a lot! (For comparison, the reading between the positive terminals is 0.1 ohms, which is probably at the limit of the ohmmeter, so it may be even less.)

What this suggests is that the two batteries might be effectively isolated from one another, for any high-current applications. The resistance might be high enough that no more than a few amps would flow between the two negative terminals. But, where does this resistance come from? The schematics (TSB BE−0025) have been drawn as if there is only one battery! So, they don't give any clue as to what's going on.

Does anyone know how the batteries are really hooked up? Are there any schematics floating around that accurately reflect what's going on? Which battery would be better to hook the winch up to? Does the alternator have overload protection? (In the schematics, it's connected to a 150A fuse, but obviously I wouldn't want to blow that.)

My thinking, at this point, is that if the winch was hooked up to the passenger side battery, and the winch had a good ground to chassis connection, the negative cable would effectively short-circuit the above 3-4 ohm resistance. This would enable a much greater amount of current to flow through the thin negative-to-chassis cable on the driver side battery, and might overload it under heavy winching. So, it might be safer to hook the winch to the driver side battery. Does that make sense?

I was thinking of installing an isolator switch, but given the above resistance, this might not be necessary.

BlueRock

[Schaffer Addicted PP Member Find all posts View Profile Close Schaffer]

My understanding of the factory "dual" battery setup in Prados is only for cold weather climate and is not a "dual" battery system in the sense of providign an isolated battery for auxillary vs a battery to start your car. In the cold weather setup I would think there should be almost 0 ohmns between the two negative terminals but I can imagine this may be distorted by voltage differences if the batteries are holding different charge levels. No expert on this so maybe disregard my comments.

Would be good if you can chase back the heavy cable on the negative of the second battery to where this connects.

I would be wiring the winch to the known primary battery and obviously winch occurs with the motor running to provide charge back into the battery. The "thin" negative dosn't sound right at all.

[BlueRock Junior Member Find all posts View Profile Close BlueRock]

I re-checked the setup, this morning. It turns out my resistance measurements were messed up by the slight voltage difference between the two batteries. So, I disconnected the negative lead on the passenger side, and re-tested: 0.1 ohms between the disconnected negative lead and the negative lead on the driver side.

So, it's a parallel setup, after all! The starter motor ground then must be connected to the chassis ground. That's the only cable going from the passenger side negative terminal.

So, that simplifies some things and complicates others. My main concern, right now, is whether the ground wiring is strong enough to handle the current that might be coming from the 'opposite' battery, under heavy winching. I checked the winch, and the motor ground is isolated from the chassis ground, as is the case with the solenoid box.

So, the scenario of concern is as follows: Winch draws lots of current, under heavy load. This causes a voltage drop in the battery that it's connected to. The second battery tries to compensate. The current along the positive cable is fine. But, on the negative side, it will flow through the starter motor ground to chassis ground, then back up to through the chassis to negative connection on the other side. This tiny cable can probably only handle about 100A, I would think. But, there is a thicker negative cable at the same terminal, going down to other components. Perhaps, that would be where the majority of current would flow through.

The starter motor, presumably, is in the same situation. So, if the ground system can handle the load of the starter motor, it should be able to handle the equivalent current at the winch.

Does anyone know how much current the starter motor can draw?

BlueRock

p.s. I plan to re-wire it at a later date. The two batteries in there, right now, are apparently fairly new. If I keep my winching fairly light, I would think I should be ok, for now.

[BlueRock Junior Member Find all posts View Profile Close BlueRock]

I've upgraded the batteries to Optima Yellow Tops D31A (75+75 Ah). Brackets had to be modified slightly. Configuration is still parallel...

[botz_90 Lurker Find all posts View Profile Close botz_90]

Did you end up installing an isolator switch? I have same setup and would like to know how to go about it.