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Topic: Battery Selector Switch?

Posted By: dlklose on 02/06/13 08:43pm

Heading off to Alaska this summer for a 90 day trip. I want to add some additional battery capacity so I am planning to add a second set of Trojan T-105 6-volt batteries to our FW. Having been a sailboater in my former life we always had a battery selector switch which allowed us to isolate the two battery banks from each other. Seems like it would be a good feature to have for the FW as well. Any thoughts or opinions?

2011 Keystone Mountaineer 295RKD 5th Wheel w/Trailaire Pin Box
2011 Ford F350 SRW Lariat Diesel with 50-Gal Auxiliary Fuel Tank

Posted By: CA Traveler on 02/06/13 09:13pm

Always good to have a fresh battery to start the boat engine. But I don't see a compelling reason for RV batteries, but some add the switch. Parallel batteries should ideally be wired in a balanced manner. Adding a switch will likely add more wiring length if balanced.

2009 Holiday Rambler 42' Scepter with ISL 400 Cummins
750 Watts Solar Morningstar MPPT 60 Controller
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Posted By: mena661 on 02/06/13 09:33pm

Wire the batteries together in series/parallel, you'll get the benefit of more capacity (Peukert) that way.

2009 Newmar Canyon Star 3205, Ford F53 V10
Trojan L16 6V's 740 Amp-hours

Posted By: btd35 on 02/06/13 09:40pm

Agree with the 2 previous posts. My Boat has 2 12 volt batteries on a battery switch so that I can have a good one to start the engine in the am, then switch to the other to charge it. No need for this in a FW, and as said the balance between them is off.

Tom & Beth
05,Grand Junction 35TMS
99, Dodge 3500 Dually.

Posted By: pianotuna on 02/06/13 09:42pm


How old are the existing batteries?

If may be wise to consider isolating the old from the new for storage purposes.

I would use a couple of heavy duty switches--one for each bank connected to a buss bar where all loads and all charging was done.

battery cutoff switch

Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp hours of AGM in two battery banks 12 volt batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

Posted By: time2roll on 02/06/13 09:48pm

For better or worse I would just run them all together.

2001 F150 SuperCrew
2006 Keystone Springdale 249FWBHLS
675w Solar pictures back up

Posted By: HiTech on 02/07/13 06:11am

When you store your RV does it have a source of power to charge the batteries? If not, what is the longest they go without being charged?


Posted By: msiminoff on 02/07/13 08:12am

I have three batteries.... and three switches.... and balanced wiring.
It can be done [emoticon]

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Posted By: ktmrfs on 02/07/13 09:22am

mena661 wrote:

Wire the batteries together in series/parallel, you'll get the benefit of more capacity (Peukert) that way.

x2. as long as both banks are equal capacity, you'll get more useable capacity in parallel than using a switch. Only reasons I'd go with seperate banks is for example on is a bank of 12V and one is bank of 6V. Or if the existing bank is quite old, then maybe using two banks.

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2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!

Posted By: HiTech on 02/07/13 09:33am

I think it came down to what tuna asked. If in storage for long periods without charging there can be good benefit to avoiding the high self discharge batteries from draining the low self discharge units.


Posted By: dlklose on 02/07/13 09:45am

Thanks to all for the responses. The existing two T-105 6-volt batteries are two years old an hold their charge really well. Also, the FW is stored at our house when not in use and I plug it in at least once a week for a day or for an extended period when temps drop into the teens or less.

Based on the above and the feedback in the responses I will probably forego the battery switch and just hook them up in parallel. Thanks again for the great input.

Posted By: mena661 on 02/07/13 10:24am

If you want to know for sure, get a good glass hydrometer and take reading on your batteries. If above 1.275, then adding new one's won't hurt any. My batts are two years old and I wouldn't hesitate to add two more to them.

Posted By: pianotuna on 02/07/13 10:37am

mena661 wrote:

If you want to know for sure, get a good glass hydrometer and take reading on your batteries.

X2 readings need to be nearly the same if not identical.

Equalize the old batteries before adding the new units. It might be an idea to move one old battery to the new, so that there are two sets of old/new, rather than old/old, and new/new.

Be sure to wire the four in a balanced manner. Consider adding a modest solar system (40 watts would do) to take care of possible parasitic energy transfers from one set to the other.

Posted By: RoyB on 02/07/13 10:46am

This is my setup for up to four 12VDC batteries in parallel showing how I use the four position BLUE SEA HD selector switches...


The best trick for us was to have a good BATTERY SYSTEM Monitor Panel. This is what I made up to use.
[image] [image]

This is a floor plan diagram of my OFF-ROAD Camper (minus the tent bed ends) showing where alot of my battery system items are located. Disregard my two batteries shown in this photo - this is future planning use.. I am currently running three 85AH Interstate batteries as shown in the diagram above.

My OFF-ROAD Camping game plan was to figure out what we wanted to use in the 120VAC Appliances using Inverters and 12VDC items direct connected to the battery banks and beef-up the batteries to handle this. I also beefed up the battery system to include smart-mode converter/charging and did alot of "going green" with lights and other power Hungary items. We will run around 20AMPS drain on the battery banks from 8PM to 11PM everynight running the toys and then that drops back to a 1-2AMP power drain the rest of the night until around 8AM the morning. Our Battery Monitor panel is now showing around 12.0VDC which is telling us we are down to the approximate 50% charge state on the batteries. This is when we connect our 30AMP trailer shore power cable directly to the 2KW generator 120VAC receptacle using a RV30A-15A "Dogbone type" 18-inch long adapter (WALMART) or I sometimes use my RV30A-15A "DUAL HONDA" adapter which is not available anymore. This allows us to re-charge our battery bank back up to the 90% charge state in a short three hour generator run time.

We have been doing this method for a few years now and are very successful about camping off the power grid doing it. We can cycle our battery bank down to the 50% charge state for 10 or so days without having to re-charge to a full 100% charge state which takes around 12-hours of generator run time to do this... We start losing battery performance if we don't do this this and I'm sure if no done it will do damage to the battery bank.

Roy Ken

My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS

Posted By: Matt_Colie on 02/07/13 11:18am


Your thoughts are well placed. Paralleling batteries without workable isolation is generally a poor idea. That is REGARDLESS of age. Unless you can isolate them for an occasional equalization charge, you will never get capacity X2 and you may only get X1.5. I deal with this all the time with the cruising sailors that I do my specialty work. So, if you can charge each bank to completion, you will get both better service and better life.

This will take not thinking on your part, and not knowing your particular drive/charge/shore plan, I am afraid that I can't offer you more insight into your particular situation.

Don't get an hydrometer, get a refractometer. They don't need temperature correction.


Matt & Mary Colie
A sailor, his bride and their black dogs going to see some dry places that have Geocaches in a coach made the year we married.

Posted By: RoyB on 02/07/13 11:19am

DLKLOSE - this is how you would connect up your TROJAN T105 6VDC batteries for 12VDC operation.

I noticed you mentioned you would just connect them in PARALLEL above..

Connecting two 6VDC batteries in series to give you 12VDC.

Connecting four 6VDC batteries in series/parallel to give you 12VDC.
sample drawings from google images.

Since your Trojan T105 produce 225AH each the top diagram will give you only 12VDC at 225AH capacity.

By adding the second bank of two Trojan T105 batteries you will get 12VDC at 450AH capacity

Roy Ken

Posted By: RoyB on 02/07/13 11:42am

If I was hooking up two groups of T105s batteries it might look something like this


This is another drawing I had in the mix for hooking up two each 6VDC Trojan type batteries.


You really should have some sort of QUICK DISCONNECT switch in place to keep from running down your batteries when not is use.

Roy Ken

Posted By: sum1 on 02/08/13 12:29am

msiminoff wrote:

I have three batteries.... and three switches.... and balanced wiring.
It can be done [emoticon]
Nicely done!

Posted By: sum1 on 02/08/13 12:47am

RoyB, Good stuff. Thanks.

Some thoughts to OP:

Having boats and MH's for some decades, I have learned that selective isolation is a good thing. One potential problem with permanently ganged batteries is the possibility of one bad battery dragging on, draining, and ruining the good ones. This is not theory, this has happened to me.

Although some of my boats displace drinking water, my greater experience is in
the crusty and corrosive salt environment. Perko Switches have served me well so I use them on land, as well.

Posted By: 69 Avion on 02/08/13 07:06am

I use a battery switch for my Camper/Trailer. It has 4 postions, Off, Battery 1, Battery 2, and Both.



I like the option of isolating the batteries and if one goes bad I can charge and run off of one of them without draining the good one down.

Ford F-350 4x4 Diesel
1988 Avion Triple Axle Trailer
1969 Avion C-11 Camper

Posted By: mena661 on 02/08/13 07:43am

sum1 wrote:

One potential problem with permanently ganged batteries is the possibility of one bad battery dragging on, draining, and ruining the good ones. This is not theory, this has happened to me.
I'd rather have the increased capacity due to Peukert than worry about whether one bad battery will ruin the good one. I bought a MH last July, one of the two grp 24's was bad but I didn't know about it till 4 months later. I removed them to install my batts from my old 5er but kept the 24's in parallel in my garage. Interestingly, charging them didn't reveal any problems. It wasn't until I separated them to equalize them and clean them up (was giving to a friend) that I realized one wasn't holding a charge worth a damn. The other was like brand new, 12.8V and SG at 1.280. No effect from the bad battery on it at all.

69 Avion wrote:

I use a battery switch for my Camper/Trailer. It has 4 postions, Off, Battery 1, Battery 2, and Both.
VERY nice setup there!

Posted By: westend on 02/08/13 11:12am

Another reason to use isolating switches might be the use of different types of batteries in the separate banks, i.e. FLA in one and AGM in the other. This is just my situation. Since the AGM's will charge faster and maintain voltage longer without discharge, switching seems practical.

I used Cole-Hersee M-750 master disconnect switches, rated to 315A continuous and have a sealed backplate with O-ring. The switches are servicable.

'03 F-250 4x4 CC
'71 Starcraft Wanderstar -- The Cowboy/Hilton

Posted By: ktmrfs on 02/08/13 08:49pm

If your not running high loads, like an inverter, having a switch IMHO isn't a bad idea, although not completely necessary if the battery banks are the same and the same age. Has advantages and disadvantages. If your running high inverter loads (100+A), then IMHO you want a big bank not seperate banks, AND very short connections between the banks. A bank selector switch works against you in this case. the extra wire length can add up in a hurry and result in unwanted extra resistance.

I have 4GC wired with very short #4/0 wire in a "balanced" configuration, no switch since I also run a 1000W inverter. #4/0 for connection to the inverter and charger. Since GC aren't the best choice for high current inverters anyway, I wanted to limit the current in each bank. Since the inverter is only occasional use and I do lots of dry camping, it works well for us.

I figure IF I have a battery failure, A quick spin of a wrench and I can seperate the bank if necessary.

I do have a perko ON/OFF switch to disconnect the battery bank when in storage.

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