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Old 02-09-2016, 09:40 PM   #1
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A better 12 volt electrical system?

When I first got my 2002 Travel Supreme 40DS RV it had 4 6 volt series/parallel house batteries (wet cell golf cart) and 2 Alliance 12 volt starting batteries in parallel. Because I installed 500 watts of solar panels I bridged these battery banks together to form the deepest "well" that I could both for storing energy from the sun and having it available to operate the 12 volt systems aboard. Sorry, former boater - that just slipped out. Anyway, in mid-January I took the plunge and gutted the battery compartment including the rusting slide out tray leaving only the hanging wires. I had a steel floor cut at a local iron works and primed it, then bedded it in silicone and bolted it in place using 1/4" bolts and nylock nuts. This was to support the weight of the 2 8D AGM Universal batteries that I purchased - these two 250 amp hour batteries were connected with 2/0 cables to a marine 1/2/both/off switch with equal length cables to each battery (important for efficiency and wearing the batteries equally). The output post has the engine start 3/0 cable and a 2/0 cable to a 400 ampere fuse. The output side of the fuse has the inverter positive cable and a 2/0 cable to a 350 amp rated busbar where the rest of the positive wiring was run. Now there is an easy 100% shutoff, and all of the downstream circuitry is protected from a short upstream. You will not find any true yachts without fused protection at each battery bank. The inverter/charger had a high amperage fuse from the factory, but they put it adjacent to the inverter, leaving the 6' of 2/0 cable from the battery unprotected from a short circuit. Not well thought out gentlemen.

As I was bringing everything back on line there was an issue with the 12v power relay - I decided to bypass and abandon it. My fuse panel, converted to a marine grade circuit breaker panel, now controls each individual branch and the battery switch can kill everything if necessary (with solar panels if you're parked outside you no longer have to worry about batteries going dead in storage). That relay - while I didn't do the research - undoubtedly drew at least an ampere 24/7 using a whole lot of battery capacity with no real gain. I harken back to keeping things simple where possible.

Anyway, this was the last of my project list and I'm ready to enjoy solid state maintenance free battery power well matched to my solar array (if 2 panels is indeed an array). Movie night anyone? No generator required.
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Old 02-10-2016, 05:17 AM   #2
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PeterFTH-

Just to clarify: You replaced four 6V house batteries (nominal capacity 450 AH) and two 12V chassis batteries with two 12V batteries (nominal capacity 500 AH)? There are no more batteries dedicated to house or chassis (that is, they are permanently tied together through the switch)?
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Old 02-10-2016, 05:56 AM   #3
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Old 02-10-2016, 08:19 AM   #4
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I wouldn't want my house and chassis batteries tied together. I can live with the lights going out, but I'd like to still be able to start the coach.
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Old 02-10-2016, 08:50 AM   #5
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Great work but...no backup shown.

If something fails and it will yiu could be dead in the water.

Assuming you have a generator.
Our older unit has 3 battery plants.

Chassis, cabin and generator/leveler.

It was done this way from factory as chassis and cabin batteries at rear and genny leveler at front and cheaper to just install a battery than to cable it.

If all rear batteries go dead genny can be started and converter will charge cabin batteries that can start unit via boost switch.

Most generators only need small garden tractor size batteries so a wheelchair AGM can be installed near the generator with same skill you used for rest to give you a redundant power source.
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Old 02-10-2016, 08:59 AM   #6
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I think you are wrong about the amp draw of a battery isolation solenoid. Most are either a latching type, or require almost zero amps once the relay closes. There are probably a dozen things in the coach drawing a similar amount of power.

Isolating the house and chassis system is generally a Good Thing, but if you aren't worried about drawing down too far to start the engine, then your solution is OK. I carry a short jumper cable I can use to bridge the two systems if I need to, but my coach also has a momentary solenoid that can do the same thing. I also have a battery charge controller that cross-charges the two systems and keeps both at 100% from any power source. Granted that is more complex than yours, though.
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Old 02-10-2016, 02:22 PM   #7
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FWIW I get what you did but I would not go there myself. Separate start and house batteries are the safer setup. Also they are the better setup because of the different current draws.
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Old 02-11-2016, 08:59 AM   #8
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Originally Posted by l1v3fr33ord1 View Post
PeterFTH-

Just to clarify: You replaced four 6V house batteries (nominal capacity 450 AH) and two 12V chassis batteries with two 12V batteries (nominal capacity 500 AH)? There are no more batteries dedicated to house or chassis (that is, they are permanently tied together through the switch)?
That is correct. I have never understood the engineering principle of two small 12v starting batteries in parallel ... I would perefer one larger format battery with larger plates and no parallel discharge (there is always a weaker cell somewhere). As to the idea of a dead pair of batteries and the inabiity to start the engine, after years of living on a boat with this arrangement I don't think it's likely - plus, with the solar, by 11 am on a sunny day they are putting out over 20 amps. And there is always the roasdside assistance plan I pay for but have never used. I would love to have a separate generator battery as a fail safe, but with no alternator on the genset it would take more complex wiring to locate and keep another battery charged.
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Old 02-11-2016, 09:47 AM   #9
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PeterFTH-

Thanks for the clarifications.

From this layman's point of view, it seems as if the boat and RV worlds have agreed to disagree on this aspect of 12V system design. There are many components and ideas in common in both worlds, and I admit to having cribbed from the marine side on occasion for both design and components for my coach(es).

That said, there are reasons why the RV folks have settled on two different battery banks. Some have already been stated. One left unstated (except by "nothermark"), but frequently cited, is the different construction between starting and deep-cycle batteries. Conventional wisdom says the former are preferred for chassis and the latter for house batteries.

You've made your decision and seem to be pleased with it. As the saying goes, "More power to you!"
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Old 02-11-2016, 10:08 AM   #10
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I believe the OP is making an opinion of the way boaters handle battery management.

Most boats designed with the A - B switch came with recomendations to run in the both position and then switch to A or B for non running situations. That preserves the other battery for startup.

Modern cruising boats have evolved to seperate start and house banks with a hands off operation and or a seperate battery for an on-board generator.
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Old 02-11-2016, 10:19 AM   #11
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I too just rebuilt my battery box, taking out slide tray in order to install additional batteries for solar. I did however choose to keep two chassis batteries separate. Sadly TS gave us a very small battery compartment.
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Old 02-11-2016, 12:40 PM   #12
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The system used in most RVs is "best" for oblivious owners, of which there are many. Maybe the boating world has fewer of those?

And larger boats do typically have two battery banks with an A-B switch that can provide isolation. Heck, even my 17 ft bass boat has that. A cautious person can get by without it, but many of us enjoy having an assist in case we get careless. Think of it as an insurance policy.
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Old 02-11-2016, 03:38 PM   #13
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Quote:
Originally Posted by l1v3fr33ord1 View Post
PeterFTH-

Just to clarify: You replaced four 6V house batteries (nominal capacity 450 AH) and two 12V chassis batteries with two 12V batteries (nominal capacity 500 AH)? There are no more batteries dedicated to house or chassis (that is, they are permanently tied together through the switch)?


Check your figures - 6 volt batteries rated amp hours stays the same when put in series with another 6 volt battery to form a 12 volt bank. I would guess that the 4 golf cart batteries I removed wouldn't exceed 200 amp hours at 12 volts. I also have the ability to both shut off all electrical supply OR isolate one battery from the other with the 1/2/both switch.
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Old 02-11-2016, 03:44 PM   #14
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PeterFTH-

Thanks for the clarifications.

From this layman's point of view, it seems as if the boat and RV worlds have agreed to disagree on this aspect of 12V system design. There are many components and ideas in common in both worlds, and I admit to having cribbed from the marine side on occasion for both design and components for my coach(es).

That said, there are reasons why the RV folks have settled on two different battery banks. Some have already been stated. One left unstated (except by "nothermark"), but frequently cited, is the different construction between starting and deep-cycle batteries. Conventional wisdom says the former are preferred for chassis and the latter for house batteries.

You've made your decision and seem to be pleased with it. As the saying goes, "More power to you!"
To dedicate 1/3 of the physical space for two batteries that are used for a few seconds to start the motor now and again to me is wasteful. The 8D has no problem cranking my Cummins 8.8 Litre 400 hp diesel - cranking sounds no different than in the original configuration. The additional capacity has yet to be tested but I am confident this arrangement will suit me far better. If I wake up to dead batteries 500 watts of solar will soon fix that sufficient to start the engine, or on a cloudy day I'll get something out of my hitherto unused roadside assistance policy.
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