Ammeter with Auxiliary Battery

[rpmerf]

1970 Chevrolet C20 Custom Camper.

Please excuse the rambling.

Factory dual battery truck, but it has been changed a bit over the years. I have had the truck running in single battery mode for the past 8 years or so. I can't remember what I changed when I removed the second battery in the bed, but I know I took out the factory dual battery harness as it is in a box in my garage. I recently installed a second battery tray and got it running in dual battery mode again. I am using a battery isolator rather than the factory relay setup.

As it was, I had one side of the ammeter running to the junction next to the passenger's side battery. The other side of the ammeter is connected to the junction on the driver's side fender. This is fine with the single battery, but with the dual batteries, it creates a link thorough which the 'starting' battery can discharge into the 'accessory' battery, unless there is some disconnect I didn't notice. It seems the equivalent connection would be be in parallel with a 1 foot long 10 gauge wire between the driver's side battery and the driver's side junction. I don't know that would be effective with such a large, short shunt wire.

How was it wired from the factory? Thoughts? Opinions? Thank you in advance.

[jocko]

Not sure if this will help, but here you go:

[dmjlambert]

This is how I think the wiring works:

The original wiring that does not have the aux battery wires: One side of the ammeter is connected near the alternator output, the other side is connected near the battery. The length of 12 gauge wire between those 2 connection points is the shunt for the ammeter, and should be a single piece of 12 gauge wire in order to work correctly. There will be a voltage difference between those 2 connection points. Which point is lower in voltage depends on which direction the electricity is flowing. "Normally" no electricity flows in that shunt wire at all, because the alternator powers stuff when the truck is running, and the battery does not need charging. So the ammeter points straight up.

The electricity running in the fat wire that goes from battery to the starter bypasses the ammeter. Right after starting the truck, the battery should be a little expended by the starter, so the ammeter will lean towards C for charge for a while. If you turn off the truck and run things such as lights and radio, the ammeter will lean towards D for discharge.

Now, add to that the wiring that jocko gave a picture of, where the original wiring is shown with dotted lines. There is a new 10 gauge wire that goes from the terminal near the battery through the relay to the aux battery. The result is, the shunt wire and therefore the ammeter, is responsible for all the electricity that could flow from the ammeter to the batteries. So, the ammeter will show the effect of both batteries charging. Also, any electricity that could flow from either of the batteries that goes along that same shunt wire past the alternator and into the cab of the truck will show on the ammeter as discharging the batteries. Normally discharging will not happen unless you run electric stuff with the truck off.

Now here is where exceptions to measuring charge or discharge with the ammeter happens: If you have any accessories connected to the main battery terminal block on the passenger fender, that connection bypasses the ammeter and therefore the ammeter will not show the discharge happening. If you have any accessories connected to the aux battery terminal, that connection also bypasses the ammeter and the ammeter will not show the discharge. But that is OK, because the whole point of having the aux battery is so you can run electric stuff when the truck is off and not worry about the discharge.

[rpmerf]

jocko:
Thank you. Unfortunately, this diagram does not show the ammeter wiring. Not sure if that means they intend to leave it stock, or if that might be covered elsewhere. I do have a 1970 service manual I could look into.

dmjlambert:
Thank you for the explanation, and I agree with what you've said.

My main concern is being able to drain my passenger's side battery through the ammeter. With the truck off, the passenger's side battery should be 100% disconnected, so I can kill the driver's side battery, and still be able to start the truck. With the ammeter wires into the driver and passenger side junctions, it creates a link between the 2 batteries.

I think I will take the ammeter connection that was going to the passenger's side junction and put it directly on the driver's side battery. That way I can see the difference between the driver's junction and driver's battery, and there is no issues of discharging the passenger's side battery. The shorter wire will throw off the ammeter a bit, I'm not sure if there is a way to adjust it.

[jocko]

Originally Posted by rpmerf

jocko:
Thank you. Unfortunately, this diagram does not show the ammeter wiring. Not sure if that means they intend to leave it stock, or if that might be covered elsewhere. I do have a 1970 service manual I could look into.

dmjlambert:
Thank you for the explanation, and I agree with what you've said.

My main concern is being able to drain my passenger's side battery through the ammeter. With the truck off, the passenger's side battery should be 100% disconnected, so I can kill the driver's side battery, and still be able to start the truck. With the ammeter wires into the driver and passenger side junctions, it creates a link between the 2 batteries.

I think I will take the ammeter connection that was going to the passenger's side junction and put it directly on the driver's side battery. That way I can see the difference between the driver's junction and driver's battery, and there is no issues of discharging the passenger's side battery. The shorter wire will throw off the ammeter a bit, I'm not sure if there is a way to adjust it.

Unless I'm missing something (highly possible), shouldn't the ammeter wiring be the same whether you have an aux batt or not? With ignition on, the batts are connected via the solenoid and ammeter should indicate whether system is charging (both batts). Does your system have the solenoid between the batteries that separates them when the ignition is off (mentioned in paragraph 1 of the pic in my post above)? I'm learning here too, so following along. Good question rpmerf.

[dmjlambert]

No, the ammeter wires should not wire into the main and aux battery junctions. They wire into the alternator and main battery junctions and do not involve the aux battery. Here is the diagram jocko posted, arranged in an easier-to-follow logical or linear way (I hope), and with the regular wiring and ammeter wiring added. When the relay is not energized, the aux battery is completely isolated from the rest of the truck.

[rpmerf]

That diagram is VERY helpful! I was confusing how the factory setup was wired. I had removed the factory aux batt harness about 7 years ago. The second battery was in the bed, and I wasn't crazy about that setup. I wasn't sure how factory it was, and there were some other wires run also that certainly weren't factory.

I've attached an image of how I wired it. I thought I was basically replacing the relay with an isolator, and moving the alternator from the driver's side junction to the isolator, but I was wrong.

A bit of explanation on why I wired it this way. The isolator has 2 diodes, so each battery get charged by the alternator, but they are never directly connected together. I am using the isolator because I've always heard you do not want to directly connect the batteries as it can cause a large amp flow if they are unevenly charged. I've also heard it can under charge / overcharge from the alternator if they are not equal charge and capacity.

I like it wired this way, because if I leave my headlights on (or something on the in cig lighter, or a door not closed all the way, etc.), it will kill the driver's battery, but the passenger's side battery will remain fully charged, and be ready to start the truck.

Hope this all makes sense...

Given this, I think I will move the amp meter between the driver's side junction and the driver's side battery. This way I can see charging from the alt to the driver's side battery, and discharging from the driver's side battery into the fuse block and accessories.