Aux battery for audio

I have a 1987 R30 w/454 and TH400. It has the camper special option including a tray for an auxiliary battery. It has the upgraded 94 amp alternator option. I am installing 1100 Watts of audio equipment. I was thinking of installing the auxiliary battery and running the amplifiers off that. But where do I connect to the aux battery? I assume the alternator charges the aux battery?

Check the wiring diagram for your truck. My ‘75 Camper Special has a simple (stock) setup with a relay and charge wire for the 2nd battery that runs to the junction near the brake booster.

- when the truck is running and you have ignition-on power, the relay energizes and the battery positive charge wire is connected to the power junction. This allows the battery to charge from the alternator.
- when you turn the truck off, the relay disconnects/isolates the 2nd battery from the power junction. Anything powered from that 2nd battery when the ignition is off cannot run down the primary battery.

I checked the wiring diagram for my truck on the gmsquarebody web site. It shows a relay. I'm pretty sure I have the stock setup. I will confirm it tonight. Thanks.

Just parallel the 2 batteries nothing else needed. unless you plan on bumpin for long periods of time parked with the engine off, in which case isolate the “house battery” and run the amplifiers load off of it. There’s a current thread on someone doing that on here. But it’s not too complicated. Just pick what kind of isolator combiner you like.

Grit dog said:

Just parallel the 2 batteries nothing else needed. unless you plan on bumpin for long periods of time parked with the engine off, in which case isolate the “house battery” and run the amplifiers load off of it. There’s a current thread on someone doing that on here. But it’s not too complicated. Just pick what kind of isolator combiner you like.

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I think I have an isolator relay that disconnects the aux battery when the ignition is off. I'm going to confirm that tonight. If that's working, I would only have to buy a new aux battery and be done. Then, I hope there is a terminal block for the aux inside the cab (but OK, if not)

^It might still work. But it will likely just be a normal old relay like an old Ford starter relay. And a 37 year old one at that. I’d recommend doing it right. Which may be just paralleling the batteries unless you actually plan on draining the house battery with the truck parked. Or redo your setup with any number of better yet affordable options that are from the 21st century and actually an upgrade from taxing old wiring and components with high amp loads.
JMO.

Grit dog said:

^It might still work. But it will likely just be a normal old relay like an old Ford starter relay. And a 37 year old one at that. I’d recommend doing it right. Which may be just paralleling the batteries unless you actually plan on draining the house battery with the truck parked. Or redo your setup with any number of better yet affordable options that are from the 21st century and actually an upgrade from taxing old wiring and components with high amp loads.
JMO.

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I appreciate your input. I snagged the last relay on Rockauto. It was even 50% off in closeout (woo hoo). It must be a sign. The negative terminal on the main starting battery is connected to the radiator support frame. The negative terminal for the aux battery is also connected to the radiator support frame. So I know I have a good ground. I will check the ground wires to make sure they are not broken inside. There is 14.4 volts up to my old relay. However, I only have 2.5 volts across the aux battery terminals (with the engine running and no aux battery installed). I'll replace the relay and clean the grounds on the radiator support frame. Hopefully, that fixes the problem. I definitely want to go the route of the aux battery for powering my amplifiers. I like the original engineering with the relay disconnecting the aux when the engine is off.

^Sounds like you’re tracking correctly. And since you have a new relay, use that.
My comment (as I’ve done similar to several trucks/boats/campers) was in regards to the type of relay specifically. If I’m repeating what you already know, apologies, but maybe it’s helpful for others too.
There are obviously different types of relays/isolators/combiners and my understanding is as follows.
Simple relay with 12v trigger (typically ignition on signal for something like your application). Signal latches the relay and it then effectively puts the batteries in parallel for both charging and load. Like wiring them parallel with a disconnect in between.
Diode type isolator. Similar to above but the diode blocks reverse current flow. IE power can flow into that battery on the downstream end (charging or upstream battery will equalize down into the downstream while triggered to “on” if the downstream is lower voltage.
Auto sensing relays. No ignition trigger. It senses voltage above nominal full charge battery (12.6-12.8V) and when that is sensed it latches the relay to allow charging and use of both batteries seamlessly. When that voltage goes down but not low enough to kill the battery, it disconnects preventing from draining both batteries from a load that is coming from one or the other.

I prefer the latter. For a couple reasons. It doesn’t match with just the “key on”. It doesn’t require a voltage source to latch the relay. And it allows for equal use/charging/balance of start and house batteries whenever charging voltage is present and even when it’s not, down to a certain voltage (preserving adequate charge in the start battery).

If I’m off on my presumptions I’d Appreciate comments as this is just my shade tree understanding.

Grit dog said:

^Sounds like you’re tracking correctly. And since you have a new relay, use that.
My comment (as I’ve done similar to several trucks/boats/campers) was in regards to the type of relay specifically. If I’m repeating what you already know, apologies, but maybe it’s helpful for others too.
There are obviously different types of relays/isolators/combiners and my understanding is as follows.
Simple relay with 12v trigger (typically ignition on signal for something like your application). Signal latches the relay and it then effectively puts the batteries in parallel for both charging and load. Like wiring them parallel with a disconnect in between.
Diode type isolator. Similar to above but the diode blocks reverse current flow. IE power can flow into that battery on the downstream end (charging or upstream battery will equalize down into the downstream while triggered to “on” if the downstream is lower voltage.
Auto sensing relays. No ignition trigger. It senses voltage above nominal full charge battery (12.6-12.8V) and when that is sensed it latches the relay to allow charging and use of both batteries seamlessly. When that voltage goes down but not low enough to kill the battery, it disconnects preventing from draining both batteries from a load that is coming from one or the other.

I prefer the latter. For a couple reasons. It doesn’t match with just the “key on”. It doesn’t require a voltage source to latch the relay. And it allows for equal use/charging/balance of start and house batteries whenever charging voltage is present and even when it’s not, down to a certain voltage (preserving adequate charge in the start battery).

If I’m off on my presumptions I’d Appreciate comments as this is just my shade tree understanding.

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The diode type isolator sounds great. But I have no idea what to buy. If my cheap-o Rockauto relay fixes the low voltage issue, I will buy an aux battery identical to the starting battery.

I remember that you have to use two of the same sized batteries for it to work.

1987R30 said:

The diode type isolator sounds great. But I have no idea what to buy. If my cheap-o Rockauto relay fixes the low voltage issue, I will buy an aux battery identical to the starting battery.

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What you have will work. Just nuances between the different types.
Again personal preference is auto sensing relays. Have a Blue Sea ACR in my boat that has been trouble free for over 10 years.
77C10 came with a new diode type isolator installed. PO revived his grampas truck camper hauler during Covid. That big 10 spent its life with a 9’ (I think. It was longer than the tailgate. Tailgate is like new because it was never on the truck for 40 years) cab over full featured truck camper in the bed.
It works as intended as well.

85K304SPD said:

I remember that you have to use two of the same sized batteries for it to work.

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You “should” use matched (size and age) batteries to maximize battery life. But in reality, that’s mostly to prevent an old battery dying and taking out the newer one.
Capacity is less of a concern unless there’s a gross difference. But that too is a recommendation as different size batteries are going to act the same as when you drain down one battery and not the other and they try to equalize when linked together.
Also bit more of an issue in a true parallel scenario than isolated.
Primary reason why there are multi bank chargers for those applications where one battery or bank is drawn down more than the other.
Reality is, if you have 2 batteries that are at least somewhat similar and in good condition it’s mostly a non issue.

1987R30 said:

I appreciate your input. I snagged the last relay on Rockauto. It was even 50% off in closeout (woo hoo). It must be a sign. The negative terminal on the main starting battery is connected to the radiator support frame. The negative terminal for the aux battery is also connected to the radiator support frame. So I know I have a good ground. I will check the ground wires to make sure they are not broken inside. There is 14.4 volts up to my old relay. However, I only have 2.5 volts across the aux battery terminals (with the engine running and no aux battery installed). I'll replace the relay and clean the grounds on the radiator support frame. Hopefully, that fixes the problem. I definitely want to go the route of the aux battery for powering my amplifiers. I like the original engineering with the relay disconnecting the aux when the engine is off.

Click to expand...

Soooooo... Is the negative terminal on the cranking battery only attached to the core support?

I know its common for the negative to be attached to the core support with like a 12 Ga wire, however there should be like a 4 Ga (approximate) from the negative terminal to the block or alternator bracket. The core support does need a good ground to supply headlights and horns with their ground but its not a good spot to establish the ground for the rest of the truck's circuits.

Same question for your auxiliary battery, is it only connected to the core support?

The quality/quantity of the grounds for your battery system should be at least as good if not better than any positive leads

CalSgt said:

Soooooo... Is the negative terminal on the cranking battery only attached to the core support?

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No, There is a normal ground that runs to the engine block. This truck has 59K Miles. It's pretty clean.

CalSgt said:

I know its common for the negative to be attached to the core support with like a 12 Ga wire, however there should be like a 4 Ga (approximate) from the negative terminal to the block or alternator bracket. The core support does need a good ground to supply headlights and horns with their ground but its not a good spot to establish the ground for the rest of the truck's circuits.

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This ground on the core support is pretty hefty. I want to say it is more like a 10 gauge wire.

CalSgt said:

Same question for your auxiliary battery, is it only connected to the core support?

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Yes. This connection is pretty hefty too

CalSgt said:

The quality/quantity of the grounds for your battery system should be at least as good if not better than any positive leads

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I concur with this statement.

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