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Old 04-23-2015, 09:34 PM   #1
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How do I charge the coach batteries using the vehicle alternator?

What do I need to do to charge the coach batteries off the bus alternator? I'd like to be able to charge the battery bank while driving, but isolate the bus starter batteries from the coach batteries when camping.
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Old 04-23-2015, 10:13 PM   #2
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How I do it.

Buy a constant duty solenoid. Run a cable from the positive side of your start battery to the large input lug on the solenoid. Run another cable from the large output lug of the solenoid to the positive on the coach batteries. Run a 12 gauge wire from a power source that is hot when the ignition is on. Use an inline fuse. That way when your ignition is on the batteries will be connected and will charge. When you shut the ignition off the solenoid will disconnect the batteries. If the ignition switch shuts the power source off when it is in the cranking position I run another 12 gauge wire and another inline fuse from the positive on one of the batteries to a toggle switch and then to the small terminal on the solenoid. That way if your start battery is dead you can flip the switch on and give yourself a jump.
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Old 04-23-2015, 10:15 PM   #3
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Sure Power 120 Amp Multi-Battery Isolator - (Part#: 12023A)

One of these would work, too.
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Old 04-23-2015, 11:48 PM   #4
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Both of the above suggestions will work fine. The cheapest way to connect/disconnect is a simple battery disconnect switch but you do have to remember to disconnect the two banks when parked to avoid running down the starting bank.

BWD S5404 - Cut-Off Switch | O'Reilly Auto Parts
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Old 04-24-2015, 08:48 AM   #5
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I put an isolator between my starter batteries and one of my house battery banks, to save money/time/effort I would not do it again, I would just put a battery selector switch and then manually switch between charging one or both batteries/banks.
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Old 04-24-2015, 09:00 AM   #6
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I had some bad experiences with isolators. They never seemed to work very well, but then I suck at messing with electricity.
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Old 04-24-2015, 09:32 AM   #7
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Question: with the simple setup of the big solenoid between the bus batteries and house batteries, say you have 4 big deep cycle Trojan batteries for your house bank. Say you have run these down to empty (12.1 volts or whatever it is) and start your bus, let it run for a minute, then flip the switch to close the solenoid so the house batteries connect to the bus batteries. Is there a way to figure out how many surge amps we are talking about with this? Like will flipping all those on at once cause a huge voltage drop in the system and screw things up? I have a 160 amp alternator and right now, with the inverter hooked to the bus batteries (I only use it while driving) if the 120 volt fridge plugged into the inverter starts up while the engine is idling, the idle will drop for a second while it is starting. Will it do something like this too when you suddenly add the load of a bunch of discharged batteries? Do they self-meter charging current or how does this work?

Thanks.
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Old 04-24-2015, 10:17 AM   #8
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Although a solenoid sounds like the way to go, I wouldn't do it that way. It'd work great if all the batteries were equally sized, equal health, and equal discharge. But when they aren't I've seen bad things happen(burned wires, trashed alternators, boiled batteries, etc). An isolator will allow one alternator to charge/discharge each battery separately at a controlled rate.

To do it right, get an isolator from a good manufacturer.
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Old 04-24-2015, 10:38 AM   #9
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Quote:
Originally Posted by porkchopsandwiches View Post
Question: with the simple setup of the big solenoid between the bus batteries and house batteries, say you have 4 big deep cycle Trojan batteries for your house bank. Say you have run these down to empty (12.1 volts or whatever it is) and start your bus, let it run for a minute, then flip the switch to close the solenoid so the house batteries connect to the bus batteries. Is there a way to figure out how many surge amps we are talking about with this? Like will flipping all those on at once cause a huge voltage drop in the system and screw things up?
Yes, it will cause the voltage for the entire system to drop the second you flip the switch. On an old mechanical engine, might not be that big of a deal, but on your electronic engine it wouldn't be good at all. That is why they have isolators.

An isolator will:
A) Let the chassis voltage remain at a steady voltage
B) Charge the chassis batteries at their own rate
C) Separate the chassis and house batteries, yet use one alternator to charge both.
D) Charge the house batteries at slow rate(which is what you want if they were discharged slowly)

12.1 house volts is discharged but not that low. Imagine if you left a load on all week (It will happen) and voltage was at 6. Dumping that dead of a battery bank on an alternator will certainly peak the alternator out(shortened life), drop the system voltage way too low (bad for the computer), cause the bus batteries to discharge(something you don't want), and it will probably kill the house batteries as well due to the 100+ amp charge rate you just put on them.

None of that is good. Use an isolator.
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Old 04-24-2015, 11:25 AM   #10
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Quote:
Originally Posted by Roadrunner View Post
Buy a constant duty solenoid. Run a cable from the positive side of your start battery to the large input lug on the solenoid. Run another cable from the large output lug of the solenoid to the positive on the coach batteries. Run a 12 gauge wire from a power source that is hot when the ignition is on. Use an inline fuse. That way when your ignition is on the batteries will be connected and will charge. When you shut the ignition off the solenoid will disconnect the batteries. If the ignition switch shuts the power source off when it is in the cranking position I run another 12 gauge wire and another inline fuse from the positive on one of the batteries to a toggle switch and then to the small terminal on the solenoid. That way if your start battery is dead you can flip the switch on and give yourself a jump.
Roadrunner, thank you for this advise, this is a good set up, what if while the bus is in storage would like to charge batteries with a solar panel, but still have the same set up you suggested.

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