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Old 07-07-2008, 10:06 AM   #1
1 Rivet Member
1986 25' Sovereign
Wichita , Kansas
Join Date: May 2008
Posts: 9
adding battery

I am adding a second 12 volt battery to my 1986 airstream and would like to know if it will affect the charging or any other existing system on my trailer. I plan to take the positive cable from the existing battery and put it on the new one, leave the neg cable on the existing battery and install jumper wires from the smaller threaded posts positive to positive and neg to neg. If this looks like the wrong approach or if there is a better way I would appreciate and info.


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Old 07-07-2008, 10:21 AM   #2
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1978 Argosy Minuet 6.7 Metre
1977 31' Sovereign
1977 Argosy 24
Cincinnati , Ohio
Join Date: Jun 2008
Posts: 64

Your plan sounds OK, however, here are a couple of considerations:

The second battery should be the same type (Lead/Acid, Gel-Cell or AGM) and should have roughly the same capacity as the existing battery.

You didn't indicate where you are putting the second battery. If it is inside the trailer, not in a vented battery compartment, it must be an AGM battery. Normal batteries out gas hydrogen, which is highly flammable. AGM batteries don't. Note that the so called "Gel Cell" batters are NOT AGM batteries.

Finally, please be sure to secure the second battery. If it slides or flips over and shorts out against something metal, you could have a fire on your hands.

Your method of connecting positive to positive and negative to negative is exactly how you should do it.

The additional battery will not add any additional "stress" to the system.


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Old 07-07-2008, 10:46 AM   #3
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1991 34' Excella
Princeton , New Jersey
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Batteries should be purchased in pairs. Don't add a new battery to an old one. This insures they look the same to the charger and will discharge at the same rate.
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1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles

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Old 07-07-2008, 11:42 AM   #4
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1978 28' Ambassador
Morada , California
Join Date: Jan 2006
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You've got the right advice recap:

1. 2nd battery should be the same GROUP size so that the individual cells are the same size as the 1st battery - this is important so both batteries will charge/discharge evenly.

2. The 'age' of both batteries should be close to the same - I'd say if the 1st battery is no older than one year, you are still good to go. As batteries 'age', their internal capacity becomes less over time, and then the older battery would effectively have 'smaller capacity' cells, even though both batteries are the same group size.

3. You have wired your two batteries parallel connections, you want to hook the POS cable from the trailer to one battery....then hook the NEG cable from the trailer to the OTHER battery. This ensures even charging/discharging. Of course you'll also have the POS to POS cable, and the NEG to NEG cable between the batteries to make it a 'Parallel' system (voltage remains the same as each battery, but the Amp Hour capacity is Doubled).

4. Never mix different types of batteries in a parallel system. Enough said...Don't do it!

5. Make sure your 2nd battery is properly secured...even a small amount of 'movement' while on the road could wear a hole in the battery case, causing fluid loss, etc.

6. Use only completely sealed type (AGM) batteries when storing INSIDE the RV. Some Maintenance Free deep cycle batteries these days look like they are 'sealed', but aren't...ask your supplier if you are not sure.

7. Flooded cell batteries (the one's you can add water to and the Maint. free types) must be stored in a compartment thats ventilated to the 'outside' to properly vent the Hydrogen gas that generated when charging AND discharging.
Ray & Pat; Morada, CA
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Old 07-07-2008, 10:42 PM   #5
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1975 29' Ambassador
Reno , Nevada
Join Date: May 2003
Posts: 1,351
There are some factors that don't necessarily step in line with some of the suggestions I see here.

AGM batteries may not vent in normal use and many use them inside in non battery code spaces - but code still says they should be in a proper vented box. If overcharged they can vent and make a mess. (I stashed a couple under the bed when I used them so you can see how I understood the risk)

While it is nice to keep all batteries in a bank as much the same as possible, the only critical issue is that they are the same chemistry. Lead acid wet cell with wet cell, sealed with sealed, AGM with AGM and so on. The inside and outside thing reminds me that both should be located in the same environment because temperature significantly influences the float and charge voltages.

Do not worry about uneven charging or discharging because, with equal chemistries, the voltages determined by the chemistry will control the currents into and out of the batteries and that will properly distribute the current. The only time this is a problem is if you have a bad battery with a shorted cell.

Nearly every RVer mixes batteries in a parallel system all the time. This is why you have that charge line on the tow vehicle. Doesn't hurt anything. Charges the trailer battery just fine. Charges the engine battery properly, too.

So don't fret too much about matching batteries in a bank. Just keep in mind what is most important and what is just nice to have and what really isn't a good idea. Understand the reasons why.

The non-AGM sealed batteries have calcium rather than antimony to help keep the plate material together (I get which is which confused so check yourself if it is important to you). This slight difference changes the self discharge rate but that is the tradeoff in these batteries for reduced gassing.

The wiring can also be a rather important factor depending upon the currents involved. It is best to do like Airstream does and wire both batteries with the same length and gauge to a common spot (the fuse panel in a late seventies, for instance). Differences in wiring can cause sufficient voltage drop to cause an imbalance in how the batteries are exercised and that will influence overall aging.

Know the reason why - there is an awful lot of misinformation about when it comes to batteries.
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