Blue Sea ACR Wiring Question

[dwh]

There is a lot of good information in this thread, I am glad I used the search function. I have a question about setting up my ACR which I didn't see covered. The generalized cartoon Blue Sea uses to show how to set it up looks like this:

View attachment 278291

The geometry of my engine bay is that the alternator is across from the starting battery, but right next to the house battery. I was wondering if there were any issues with wiring the house battery through the ACR directly to the alternator, and also leave the connection between the alternator and starting battery? Like this:

View attachment 278290

That would work. Doesn't matter which end of the alternator->engine battery wire you connect to. Only possible gotcha is that the factory alternator->engine battery wire is almost always undersized. Fine for a quickie recharge of an engine battery that just started the truck, or running the normal vehicle loads. It'll even serve to charge a second battery since the engine battery recharge is usually like 1 amp*hour or less after starting the truck.

But if you had a situation where something (FatAss(tm) inverter?) on the aux side racked up a huge draw, and needed to suck up more than the aux battery + alternator could supply, the extra current to feed the load would have to come from the engine battery - and that factory alternator wire would have to carry it.

So it would work, but (depending on the situation), I'd be looking at (maybe) upsizing the wire between the alternator and the engine battery (but probably wouldn't be needed).

 

[dwh]

Erm...

Too quick on the trigger...

Actually, if you have a winch connected to the engine battery, then that could be a problem if the alternator wire has to carry the load of the alternator + aux battery in order to help feed the winch.

In that case, you should certainly upsize the alternator wire.

 

[James86004]

Thanks Diplostrat and dwh. When I redid the heads last year I traced the alternator wire, which was something like 8 ga and corroded at one end. It went from the alternator at the left front of the engine, over behind the right cylinder head, and down to the starter solenoid on the right side of the engine, where it connected to the fat cable from the starting battery on the right front corner of the engine bay. I threw that alternator wire away, and ran a 52 inch 4 ga cable directly from the alternator to the battery.

The winch is directly connected to the starting battery.

The house battery will be 18 inches of cable away from the alternator. I don't anticipate it running anything more than the refrigerator, lights, and ham radio.

 

[dwh]

Thanks Diplostrat and dwh. When I redid the heads last year I traced the alternator wire, which was something like 8 ga and corroded at one end. It went from the alternator at the left front of the engine, over behind the right cylinder head, and down to the starter solenoid on the right side of the engine, where it connected to the fat cable from the starting battery on the right front corner of the engine bay. I threw that alternator wire away, and ran a 52 inch 4 ga cable directly from the alternator to the battery.

The winch is directly connected to the starting battery.

The house battery will be 18 inches of cable away from the alternator. I don't anticipate it running anything more than the refrigerator, lights, and ham radio.

You're good to go then. No worries, do it the easy way.

 

[DiploStrat]

18 inches? Sigh. Even my starter batteries are further than that!

 

[James86004]

That 18 inches is why I was questioning this, since the alternative is a 60 or 72 inch cable across the front of the engine bay. The cost difference buys me a pretty nice lunch. The batteries will be in opposite front corners.

 

[Tiktaalik]

Bringing this thread back! So I never installed the dual battery system in my Defender but I am getting close to installing it in my Mercedes 300GD with two Odyssey 34-PC1500 batteries. My question to the experts out there is related to the alternator. I just ordered a remanufactured stock alternator rated at 55A. This is less amps than what is common today. Will the 55A alternator be up for the task in properly charging these two batteries?

 

[dwh]

The engine battery is going to be full all the time unless you run it down winching or leaving the lights on or some such. Starting takes almost nothing out of the battery and it's topped off again in a few minutes.

So it's a question of how much of that 55a is being consumed just to run the truck and how much is left over to charge the aux battery.

You have extra lights? Ghetto blaster stereo? Aircon?

 

[DiploStrat]

The issue is probably less one of amps, 55 is probably plenty, and more one of volts. Some MB run at 13.9v and, if that is the case, a deep cycle, lead acid battery is likely to be chronically undercharged. This may mean that you should look at a 20-40A battery to battery charger.

The whole exercise starts with knowing the output voltage of your vehicle's charging system.

 

[Tiktaalik]

I don’t know the answer. The output of the charging system is determined by the alternator, correct? This is the stock alternator: 55A Bosch Alternator. And this is the battery I have: Odyssey 34 PC-1500 Is there a formula to calculate the voltage output of an alternator?

The issue is probably less one of amps, 55 is probably plenty, and more one of volts. Some MB run at 13.9v and, if that is the case, a deep cycle, lead acid battery is likely to be chronically undercharged. This may mean that you should look at a 20-40A battery to battery charger.

The whole exercise starts with knowing the output voltage of your vehicle's charging system.

 

[Tiktaalik]

Great question. I’ve been told that my old G draws next to nothing stock. Here is a rough draw calculation of the accessories I have. The compressor would be intermittent use and I may add a winch at a later time.

• LED Headlights ~2.5A
• LED Driving Lights ~2.5A
• Fridge ~3.3A
• Webasto ~3.8A
• LED Light ~1A
• Air Compressor ~30A
• HAM radio ~?A

The engine battery is going to be full all the time unless you run it down winching or leaving the lights on or some such. Starting takes almost nothing out of the battery and it's topped off again in a few minutes.

So it's a question of how much of that 55a is being consumed just to run the truck and how much is left over to charge the aux battery.

You have extra lights? Ghetto blaster stereo? Aircon?

 

[Tiktaalik]

Crazy question. Is there some sort of load sensing device that I can connect to my battery to get an accurately load history report from?

 

[Tiktaalik]

I've been told that 13.8 V charging voltage is typical for the OEM 55A Bosch alternator.

The issue is probably less one of amps, 55 is probably plenty, and more one of volts. Some MB run at 13.9v and, if that is the case, a deep cycle, lead acid battery is likely to be chronically undercharged. This may mean that you should look at a 20-40A battery to battery charger.

The whole exercise starts with knowing the output voltage of your vehicle's charging system.

 

[DiploStrat]

Many (most?) alternators are internally set at 13.9v. This means that if all else fails, this is what they will put out. And this is not a bad voltage for most standard starter batteries. There was some chatter on MB forums, complaining of short battery life as MB started fitting batteries that really wanted to be taken to 14+v. You will find similar chatter on South African and Australian overland forums about being unable to charge camper/deep cycle batteries with Toyota vehicles that are similarly set to 13.9v. Hence their mad love for battery to battery chargers and their deep faith that "split charge relays can't work."

Many, e.g. Chevrolet, vehicles add an external control of the alternator, via an external voltage regulator and/or computerized engine control system that adjusts the voltage to compensate for temperature, headlights, wipers, etc. In the case of Chevrolet, this results in a Bosch alternator putting out over 15v at freezing temperatures.

The first thing you need to do is put a voltmeter on your battery and monitor the voltages for say, half an hour. What is the voltage immediately after starting the engine (it will probably be your highest) and what is it after running for 20 minutes. How does it change at idle speed and highway speed. If it ramps up over 14 and stays there, then a relay based system is probably fine - check the voltage requirements of your camper battery. If it stays well below 14v, say 13.8v or 13.9v, then you may be in the market for a battery to battery charger to boost the voltage.

The Blue Sea ACR is a great device, but it is only an intelligent switch - it does not boost or cut voltage.

 

[luthj]

Alternators are a VOLTAGE source (approximately constant voltage). With a CURRENT LIMIT. They do not output constant current, instead the loads determine output current, up to the alternators current limit. When you hit the limit the voltage will start to drop.