Stupid question of the day. Parasitic draw.

[Selahdoor]

I know how to perform a parasitic draw test.

Thing is, I've never done one on a truck with two batteries.

This seems simple to me. Seems the answer would be yes. But since I have never done this, I should ask.

You remove BOTH negative cables, right?

And isolate the one that you are not working with.

Also, I can't see that it would make any difference, but... Is one of the batteries preferable to the other, for the test?

 

[YJMike92]

No question is stupid. I would unhook the drives side battery completely then measure parasitic draw on the ground on the passenger side.

 

[The_Josh_Bear]

While you're at it makes sure your batteries are both holding a charge equally! I wasted quite a lot of time chasing a parasitic loss one day when it was actually a bad cell in one of the batteries.

 

[onetonjohn]

A couple questions. Do you have any circuits coming from the driver side (+) battery, or does it go straight to the passenger battery? If it has no circuits, then here's what I would do:
Leave both grounds connected.
Disconnect positive from passenger side.
Set Multi-meter (you do have multi meter correct?) on Amps.
You'll have to move the + lead to the Amps plug in.
connect positive to battery, and negative to the battery connector and see if you have current. You likely would have to set the amp meter on milliamps (mA) to measure the draw.

 

[BeastMaster]

You remove BOTH negative cables, right?

And isolate the one that you are not working with.

Yuppers...Both negatives. Usually, the negatives are much easier to remove as well. Not as much wiring clutter than typically found on the positive hot side.

And a wrench slip to frame is not a disaster when disconnecting negative from frame.

Leave the positive terminals alone. No need to mess with them yet.

Then the headlamp test to see if both batteries are live. Turn the headlamps on. Momentarily reestablish connection at the negative terminal to ground, one battery at a time. See if the headlamps come on, and how bright. Both batteries should give identical performances. If not, bad connection or battery.


Now, test for Josh Bear's condition. Test for current between the two open battery negative poles. Any current is because one battery is trying to charge the other. Bad cell.


If you are really after a solid test, crank using one battery at a time - and your ears. You do not need to actually start the engine, but the starter and glow plug behavior should be similar. I'm not gonna go get a load tester when I've already got one all wired up and ready to go. A puny battery will fail this test right quick.


If that wasn't it, pick either one you want and proceed as before...
Measure one of the Negative battery terminals to ground, with the other one left disconnected.

Now, go ahead and trace your parasitic draw just like you've done before.

 

[Selahdoor]

Disconnected, then completely isolated the passenger side battery cables.

Removed the driver's side negative.

Strange thing...

Between the negative cable, and the negative post, the current varies.

Starts at over 9 volts. Quickly but progressively drops to zero.

Then jumps to over 9 volts again. Then progressively drops to 0.

Over and over again.


Other tests to come. But maybe not all, today.

 

[YJMike92]

Sounds like you have something turning on and off (cycling)
If you're checking parasitic draw you need to be on amps not volts. Start on the 10 amp scale so you don't blow the meter's fuse. If good at 10 amps go to the 2amp scale and then to milliamps. On our trucks, 35 milliamps is probably good.

 

[Selahdoor]

Ok that was just a bit of a mistake. The positive terminal was loose.

Tightened that up, and now the voltage goes to somewhere near battery voltage and pretty much stays there.

Must not be very much amperage at all, though, or the batteries would drain dead in hours at that voltage.

The job for today, though, was to get the electric for the fuel pump installed. I'm going to do that first. Just fooled around a bit with this, out of curiosity.

Batteries are already drained too far down to even attempt to start the truck. So I'll get some circuits done, and then charge the batteries up, and put it on a jumper again from the car.

After that, I'll get started testing the parasitic drain. Priority right now is just to get it started and running again. I really want to get this thing out of the driveway for once...

 

[YJMike92]

The voltage has nothing to do with the parasitic draw. It's current flow you are interested in. Not voltage. I hope you are able to get your pump wired.
My truck has been down so long with a bad motor I forgot what it's like to drive.

 

[Selahdoor]

Ok, when I decided to pack everything up for the day, I decided to do the amps test between the post, and the negative cable.

Started at 6 amps, and steadily went up. When I pulled the leads, it was almost to 9 amps.

Houston, we have a problem.

Going to have to get started on finding that parasitic drain just as soon as I am finished with the fuel pump (All that is left is to run the wires, and put in the toggle switch.)

 

[IDIBRONCO]

Going to have to get started on finding that parasitic drain just as soon as I am finished with the fuel pump

I can easily see my parasitic draw, all I have to do is see how much is taken out of my paycheck!

Batteries are already drained too far down to even attempt to start the truck. So I'll get some circuits done, and then charge the batteries up, and put it on a jumper again from the car.

Unless you have deep cycle batteries, you shouldn't let them get drawn all the way down. I've been told that that can shorten the life of your batteries.

 

[IDIBRONCO]

I can easily see my parasitic draw, all I have to do is see how much is taken out of my paycheck!

I'll bet that probably didn't help much did it?

 

[Selahdoor]

No problems, my friend.

BTW, I said amps. It was milliamps. I had the meter set on 20ma.

 

[The_Josh_Bear]

No problems, my friend.

BTW, I said amps. It was milliamps. I had the meter set on 20ma.

Haha I was like uh... 9A is not parasitic, that's a load! 9mA is a little different

 

[onetonjohn]

9mA actually isn't that bad if you drive the truck fairly regularly. To find the circuit with the leakage current, I normally go to the fuse box with a blown fuse (in my toolbox) and put blown fuse in each slot and put the meter on each tab to measure current on that circuit.