Considering Adding a Second 12-V Battery in Parallel

[Sportsdad60]

Our 2000 was enough generator for simple things like lights, coffee maker (big draw on generator) and furnace fan so we just ran our generator all night when it was 35 deg one night at Moab. Granted, it was our older rig, 30 amp set up. Being out where no one is near you, quiet time wasnt an issue for a generator running all night to keep the furnace fan going....

[boyscout]

Let me check for my own understanding here by way of a sketch; I think what you are describing is captured in the top sketch and what is commonly done (what I would have done) is in the bottom. Am I correct?
Attachment 31702

The top sketch is the better way to do it, yes, keeping the same-length cables as short as possible.

I've seen people advising to connect parallel batteries like the image below - in fact the image below is taking from a page of advice about how to connect batteries! This is what I was advising against in my previous message.



Here's a good discussion at the link below, with explanation of the issues involved. Method 1 in the document is the really bad one in the image above. Method 2 in the document is "your" method, the lower one in your diagram. Method 3 is the one I've been advising you about for your two batteries. Method 4 would be the one to consider if you were adding more than two batteries together.

http://www.smartgauge.co.uk/batt_con.html

Good luck with your planning and installation.

[grumpypapa]

I have done what you are showing in the bottom sketch,but,the batteries are right now beside each other,I want to move them about six feet apart because
there is so much weight over the one front left leg,or I am just worrying to much about nothing.

[SmallBoxMaker]

I have done what you are showing in the bottom sketch,but,the batteries are right now beside each other,I want to move them about six feet apart because
there is so much weight over the one front left leg,or I am just worrying to much about nothing.

Suggestion - move both to the other side to help balance the weight. That's what we did.

[MJMCCAND]

Thanks for the input and guidance. Always a wealth of knowledge to be obtained in this forum.

As I see it, I have two installation options and am interested in your thoughts on them:

Option 1) Mount the inverter in the cubby hole right below my TV, where I stow the DVD player, and tap into the 12-volt system behind the panel that feeds the stock radio. This would result in a long run of 12-gauge (I think) wire between the battery(ies) and the inverter, but I could easily plug the TV and DVD player into the inverter.

Option 2) Mount the inverter near the battery(ies) using 10 or even 8-gauge wire to the inverter, then run an extension cord from the inverter to that same cubby hole location under the TV. This results in a shorter run between the battery(ies) and inverter, but a much longer run from the inverter to the load (TV/DVD player). It would however offer ease of access to AC power outside the unit should I need it and not be able to run the generator...not sure when I might need this though.

Option 1 would be easiest, but from a voltage drop would it make much of a difference?

Thanks.

[Najataagihe]

Easiest of all:

Option 3) Purchase a 12VDC TV and tap into the 12VDC for the radio.

[LaMancha]

Another bit of advice about installing your additional battery "in parallel" with the original one. Some people literally install them "in parallel", with the second battery connected to the first battery, positive to positive and negative to negative, with the load connected to the first battery. It will work that way but the batteries will suffer for it, especially the first battery.

It's better to purchase high-quality heavy battery cable and terminal lugs and make two sets of cables that are the SAME LENGTH to connect each of the batteries individually to the load. These cables are connected to the battery on one end, and then the two positive and two negative are connected to the load on the other end. Your batteries will last longer if connected this way.

Good luck with your decisions.

I am wondering where you got this from? I am a 35 year electronics tech and have worked on everything from micro electronics to the Space Shuttle and I have never heard that or can think of a rationale for it.

[Najataagihe]

I am wondering where you got this from? I am a 35 year electronics tech and have worked on everything from micro electronics to the Space Shuttle and I have never heard that or can think of a rationale for it.

It stems from the fact that the first battery in the bank always takes more of the surge from the start of a heavy load (Sudden switch to battery power from commercial power while under load, for example) when wired as in the diagram in Post 12 .

Because of the starting surge of heavy loads, it is stressed more than subsequent batteries in the bank, resulting in relatively shorter life.


Equal length wires from each battery to the load is an inelegant (and expensive!) way to equalize the start surge among the bank.

A better way (and the one we use for telecommunications back-up power), is to treat the positive and negative poles of the batteries as separate power buses.


You connect the positive cable from the bank to the load on one end of the entire battery bank and the negative cable from the bank to the load from the other side of the bank.


This causes a "manifold" effect in the load sent to the battery bank and all of them take that initial load "hit".

This method is good for any number of batteries and you add capacity simply by putting more batteries in the middle of the bank.

(Trivia for the day: after WWI the telephone companies used decommissioned submarine batteries to power the telephone system.)


Of course, you are not going to notice a huge difference in battery life due to starting surge until you are pulling a LOT more power than an RV uses!


But, you asked from whence this idea came.

Now, you know "the rest of the story...."

[boyscout]

It stems from the fact that the first battery in the bank always takes more of the surge from the start of a heavy load (Sudden switch to battery power from commercial power while under load, for example) when wired as in the diagram in Post 12 .
...
Of course, you are not going to notice a huge difference in battery life due to starting surge until you are pulling a LOT more power than an RV uses!

Thanks for answering @LaMancha.

Here's an article about battery connections:

http://www.smartgauge.co.uk/batt_con.html

I'll quibble with your suggestion that the issue is insignificant in the RV environment. While not an expert myself I've worked with experts in emergency management and radio UPS systems and solar systems, large and not-large, and all of the several of them have emphasized the importance of making connections between batteries as transparent as possible with good cables of equal length where possible and attention to where the load is attached.

[Najataagihe]

It all depends on what you consider insignificant!

Since the battery bank is smaller in an RV, its response to surge load is proportional.


The only surge loads in an RV are the A/C and furnace fan - maybe an electric fireplace or electric recliners.

The furnace fan is the only one, typically, that is hooked to the batteries.


You might get battery failure in month or two, if an RV system gets hammered every day, but you can actually blow up that first battery, immediately, if you have a full mainframe switching installation jump off commercial power and slam the batteries while running full out.


The manifold approach really keeps one battery from getting slammed worse than another.