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Old 04-16-2018, 07:44 AM   #1
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Connect my leisure batteries to my second starter battery?

Hello Skoolie-folk! I'm hoping someone here will be able to help with a fairly n00bie electrical question.

I've got a 6.6 L diesel GMC shortbus. It has an 145 amp alternator charging two starter batteries. The second starter batter is located in a fairly accessible location under the bus behind the driver's seat.

Is there any reason to not use the second starter battery terminals to branch in my leisure batteries? I'd use one of the battery isolating systems to avoid running down my starter batteries while the engine is off. There's very little room under the hood for tinkering so I'd prefer to work off the second battery if possible!

I'm a little worried about the voltage drop from the alternator to the leisure batteries making charging inefficient but thought maybe upgrading the gauge of the wire from the 1st to 2nd starter battery might help.
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Old 04-16-2018, 02:56 PM   #2
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A diode based battery isolater will absorb a little of the alternator output and slightly undercharge the house batteries. A three stage charger is best for deep cycle charging but many RVs use the same setup you're considering. Battery life may not be as long but it will work. You could have the best of both worlds and do both by charging with a three stage charger when shore power is available and using the alternator when you can't plug in. That's how our bus is set up.

The cable gauge needed will depend on the distance between the starting battery and the house bank. There are plenty of DC wire gauge charts on the net. Too big is better than too small. Keeping the cable length as short as possible is also a plus.
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Old 04-16-2018, 04:55 PM   #3
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Originally Posted by roach711 View Post
A diode based battery isolater will absorb a little of the alternator output and slightly undercharge the house batteries. A three stage charger is best for deep cycle charging but many RVs use the same setup you're considering. Battery life may not be as long but it will work. You could have the best of both worlds and do both by charging with a three stage charger when shore power is available and using the alternator when you can't plug in. That's how our bus is set up.

The cable gauge needed will depend on the distance between the starting battery and the house bank. There are plenty of DC wire gauge charts on the net. Too big is better than too small. Keeping the cable length as short as possible is also a plus.
I'm going to expand on this, not to disagree with you but to vent a few pet peeves.

What the OP is asking is technically possible, but to do it correctly is not easy and quite expensive.

First peeve ... RV manufacturers don't often do it properly, and they know they are taking shortcuts that hurt the buyer.

To charge deep-cycle batteries from a vehicle alternator is not a smart idea for a number of reasons. The first is that few standard alternators have the required output, so a big upgrade or second alternator will be needed.

Unless that alternator is feeding a DC to DC three-stage charger, it will not undercharge them a bit, it probably won't get them over 80% state of charge. It just doesn't have the voltage to charge them fully, something the DC to DC charger takes care of, for about four hundred bucks.

So RV makers are selling you expensive batteries (if you are lucky), then hooking them up to an inadequate charger that will never deliver full potential, and will wreck them trying. Even the converter/chargers they sell with new units, for charging from shore-power, are often inadequate. They are regulated to 14.4 volts, and that is not enough to fully charge deep-cycle flooded lead/acid.

There are better ways.

You can go down the route of beefing up the alternator and adding a proper charger. Make sure the cables are sufficient.

You can add some solar that will charge while driving, and while not driving.

You can charge via an inverter/charger from shore-power, or a small generator, a very flexible system as the genny can add power for when you need peak-load supplementing.

Trojan T105 RE batteries should comfortably last 8 to 10 years. They are guaranteed for five. Yet all too often we are expected to be happy if RV batteries last five years, or less, while never delivering the power the owners were led to expect.
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Old 04-16-2018, 05:50 PM   #4
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Thanks for the expansion. I've seen the battery-to-battery 3-stage chargers and I haven't ruled them out. That part will depend on how this year's budget shakes out! My primary question was whether it would be a big mistake to daisy chain my leisure system off of the secondary starter battery instead of off of my under-the-hood starter battery.

Because we're going to be mainly road-tripping in this vehicle, and mainly boondocking at that, it doesn't make a lot of sense for us to have a versatile power input system (solar, inverter, etc.) even if the idea has a certain appeal!

If I spring for two Trojan 105RE at $200 USD a pop and they provide 4 years of life (instead of 10) because of being inappropriately charged off of my alternator... That's an annual cost of $100/year over 4 years. If you wanted 10 years of life it'd cost you roughly $125/year over 10 years?

Adding in a $400 DC to DC charger and getting 10 years of life (and significantly better battery performance) works out to $80/year over 10 years.

It seems to me like the second option is clearly preferable if you're pretty sure you're going to keep RV-ing for quite some time. But if you're not sure then the first option isn't crazy? As for beefing up the alternator I haven't finished tallying up my proposed new electrical system's loads, but I'm fairly certain sticking with the stock 145 amp alternator will be okay. We're mainly powering some small interior lights and keeping phones/gadgets charged up.
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Old 04-16-2018, 06:28 PM   #5
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Okay, but bear this in mind.

That 145 amp alternator is quite small for a bus, where 160 or even 180 amp could be expected.

When an alternator that small is fitted, it's probably something to do with cost and fitting an alternator big enough to run the bus, charge the large batteries the bus already has, and provide power for accessories.

So you need to find out how much of that 145 amps the bus needs to run what it already has on board. Bear in mind that the bus batteries may need quite a bit, etc. Powering the bus electrics and electronics will consume quite a bit too.

I'd try starting the bus with the batteries at half-charge, then turning on everything electrical and use a clamp-on ammeter on the alternator cable. That would give you a reading. The difference between that reading and the 145 amps is what you have available, but remember that alternators really do not like running at full output for long.

It's a judgement call, and it's going to be hard to make an accurate call without real numbers.
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Old 04-17-2018, 07:31 AM   #6
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I'd try starting the bus with the batteries at half-charge, then turning on everything electrical and use a clamp-on ammeter on the alternator cable. That would give you a reading. The difference between that reading and the 145 amps is what you have available, but remember that alternators really do not like running at full output for long.

It's a judgement call, and it's going to be hard to make an accurate call without real numbers.
That sounds like a good idea! I know it's all fairly ball-parky but it would be nice to have a number or two to anchor the guesswork!

Bear in mind that this isn't *truly* a bus. It's a Chevy 3500 flatbed truck with a bus body on it. We've removed the electric stop-sign arm, a bunch of the exterior (flashing) lights, the interior lights, the heater, etc. There's no AC, and the fans are pretty much useless/unused. The only thing remaining that's still "bus" in our setup is the electric door opener (which is a big thing, admittedly!). We likely won't even be using the stock stereo.

I did see that there are some DC-to-DC units available that accept a solar input as well:

https://redarcelectronics.com/produc...attery_charger

Other people are saying you're better off keeping solar and alternator-to-leisure inputs separate (and programmable) so that you can make sure, yourself, that the multi-stage charging with simultaneous inputs is happening correctly, and that you shouldn't trust an all-in-one gizmo.

A dual unit (for about the same price as a straight up DC-to-DC charger) is pretty tempting as that leaves the door open for building out the system later. Letting solar take care of the absorption/float phases while the alternator does the bulk phase sounds pretty sweet.
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Old 04-17-2018, 10:27 AM   #7
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If you're going to be weekenders, with that small load you probably won't need to charge until you get home. We are weekend campers and haven't used the alt to charge our two 6v house batteries (225amps) yet. We almost never draw down below 12.4 volts even in the spring/fall when we're briefly using the furnace. Lights, particularly LEDs and chargers don't draw much.
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Old 04-17-2018, 01:07 PM   #8
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If you're going to be weekenders, with that small load you probably won't need to charge until you get home.
We do a little weekending, but mainly big family outings are what we have planned. Last year was two weeks in Quebec and this summer the plan is a round trip from Quebec to Vancouver.

We'll definitely have lots of opportunity to charge off the alternator! We just have to be careful (as the other poster pointed out) that we don't overload it and to have enough battery capacity to last us overnight. I'm a bit worried that a ceiling fun might be necessary to avoid roastin' at bedtime which would be fairly high load.
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Old 04-17-2018, 01:46 PM   #9
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Forgive me if I missed it but I don't see mention of what loads you will be running from battery.

That is a key piece in determining what size battery bank you install.

I would suggest that you figure out what your daily consumption will be and choose battery and charging solutions to fit your needs.

There are a number of online worksheets / calculators that will guide you through the process.
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Old 04-17-2018, 02:01 PM   #10
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To clarify, this wasn't really a "how much battery power do I need?" post so much as a a "is there a big downside to connecting my leisure batteries to my secondary start battery that I haven't considered?" thread.

As for how much power we'll need, the loads are still a work in progress. In last year's outings we basically got by without a leisure battery at all using small LED tap lights, rechargeable flashlights, etc. We charged gizmos (phones, kindle) off of the 12V outlet while driving. We supplemented this by descending on cafes and reststops etc. every now and again with a big power bar and all our rechargeable stuff xD

This year we'd like to add some interior lighting, a back-up camera, and USB charging ports in the living space. We *may* opt for a ceiling fan and a 12V cooler as well. So our exact loads (and the question of whether our alternator can handle them) are still up in the air.
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