Adding a second battery

[MilCop4523]

Can anyone post a response with directions on how to add a second battery to a TT? Some photos of it hooked up would be gr8 as well.
We are thinking of adding a second battery for support to the stock one if we are out where there is no hookup.

I started with the battery box and battery, now I need to figure out what else I need for parts and how to hook them up.

Thanks

[Herk7769]

This is how I did it in my 5th wheel.

[MilCop4523]

Do the batteries need to be the same size - mine are by luck because I didn't know what I had OEM when I went to the RV dealer for the box and 2nd battery.
Awesome attached article as well. Thanks

[Triguy]

Herk covered it, but since I was writing it anyway, here goes ...

I assume you have an existing 12 volt battery and want to wire a second 12V battery to increase capacity (but not voltage). Here's a simple diagram of two batteries connected in parallel.

So, if you had two 12V batteries of the same type, age and size (say 100Ah each) wiring this way gives you 200Ah capacity.

Use the largest cable available to you to minimize voltage drop. You also might consider a battery disconnect that can switch between the two batteries, allow them both to supply power or disconnect them both, as needed.

Words of warning. Don't use two different types such as one flooded and one AGM. Be careful using two batteries of different age. If so, you might want to have the battery disconnect so that only one battery is connected at a time. If cost is not an issue, then consider going with two 6-volt batteries in series or buying two new big 12 volt batteries.

Here is a common tutorial on batteries ... The 12volt side of life.

This one entitled understanding and maintaining your RVs 12v system is also very good.

[Triguy]

Originally Posted by MilCop4523

Do the batteries need to be the same size - mine are by luck because I didn't know what I had OEM when I went to the RV dealer for the box and 2nd battery.
Awesome attached article as well. Thanks

And "yes" the batteries should be the same size otherwise the smaller battery will discharge below the recommended 50% level quicker and will charge more quickly too which may cause it to overcharge and boil away.

[MilCop4523]

Thanks guys The battery on the 2012 Salem has been in use Sept. 2011 for less than 3 weeks and stored indoors on a trickle charge for the winter. The same for the second battery as for trickle charging.

[Herk7769]

I just spent 30 minutes writing a reply with links and lost the whole darn thing. ARRRGGGGGG

Short answer:
Current flow must be as close to identical as possible between the batteries to make sure they age the same and provide the capacity you think you have. Like 2 resistors piggybacked in a circuit; current flow depends on the total resistance from the point of entry to the point of exit. If the reistance is equal, each resistor carries an equal load. If the resistance is not, one "works harder" than the other.

1) External resitance can be managed by measuring cabling and making sure each "side" is excatly the same length.
2) So, the internal resistance of the battery is the key.
3) Internal resistance changes with manufacturer; battery type; capacity and use (number of charge/discharge cycles).
4) Use causes "sulphating" of the plates over time. The deeper the discharge per cycle, the more the plates get crystalized, the more the internal resistance of the battery goes up. Eventually the plates are so crusted over with sulphates that they will no longer hold a charge and the battery is "dead" and must be replaced. There are "De-Sulphaters" that will breath life into an old battery but they are never the same.

Rules of Thumb
Always change batteries in sets.
Always start out with two identical batteries.
Always recharge before battery capacity drops below 80%
High Current pulls reduce available capacity, (see attached charts)

[gsac]

I have a 1 year old interstate battery, We have never used it to power anything.It has been installed in my 5th wheel since we brought it home.Ijust bought a identical one to install as we will be going to a race where there is no power just a generator. will this be ok? the old battery has been well taken care of (indoors for the winter and took a full charge in about a minute.)I belive this will be fine.

[Herk7769]

Originally Posted by gsac

I belive this will be fine.

Then it will be ...

[Triguy]

Its not the worst thing but its not the best either.

The general idea is to balance two 12-volt batteries in parallel as closely as possible. Different ages cause an imbalance.

The good news is that it requires a little more time and effort but you can mix batteries of two different ages if you want. It requires a disconnect.

The one year old Interstate battery is potentially weaker than the new battery. Batteries age and each battery only has so many discharge/charge cycles so even if you didn't use it, its aged.

In reality, that means that it will draw off the newer better battery during discharge and the better battery will not become fully charged during charging.

The worst case scenario is each battery will have a lessened life span.

An easy way around this is to simply add a battery disconnect that allows you to separate the two 12-volts when discharging and charging allowing both batteries to be used to their fullest extent.

[win]

Hey Herk, were did you buy that dual battery box? if you don't mind me asking

[Herk7769]

Originally Posted by win

Hey Herk, were did you buy that dual battery box? if you don't mind me asking

Two bus trays. One on top and one on the bottom.

Bus Tray & Utility Box 7-5/8-Gallon 21-1/2w x 17-1/8d x 7h Gray

use a hole saw to make the vent holes. I used 1/2 a gray plastic barb union to join to the vent hose.

[win]

Herk, Thats awsome!!!! thanks alot, what a great idea.

[Herk7769]

I have 2 24 series batteries in there and am using a foam "noodle" to fill up the spare space. I plan on putting 2 group 27 batteries in when these go to the recyclers.

[MilCop4523]

Any idea where one can get this disconnect? and can you post some pictures of what we may be looking at?

The other way to do it I suppose would be to take a wrench with you, have both batteries charged and when one is almost dead switch cables and start the generator to recharge the one previously used.

[Triguy]

Originally Posted by MilCop4523

Any idea where one can get this disconnect? and can you post some pictures of what we may be looking at?

The other way to do it I suppose would be to take a wrench with you, have both batteries charged and when one is almost dead switch cables and start the generator to recharge the one previously used.

Hi Milcop,
This thread provides information on the disconnect and how to install.

Question about adding 2nd battery and switch

[MilCop4523]

Originally Posted by Triguy

An easy way around this is to simply add a battery disconnect that allows you to separate the two 12-volts when discharging and charging allowing both batteries to be used to their fullest extent.

thanks for the link ... should be useful.
Where are they available Marine only?

[Triguy]

There are a few places but Blue Sea brand is primarily marine.

Amazon is a good place to buy these.

Blue Sea Systems 9001e e-Series Battery Switch Selector