Battery Bank Questions

Ok - so I have read enough forum posts and blog entries to know that the general consensus is: Don't mix new batteries with old batteries. If batteries are 6 years old - it's time to replace. Don't mix group 27dc with group 29dc. The best is a grouping of 6volt golf cart batteries.

BUT.

My RV: 84 Pace Arrow. Came with two kirkland group 27dc 12v coach batteries that are steadily reading 12.36(ish) when I multi-meter. (not fantastic but good enough) I was not intending to change these out quit yet as I am already a couple grand over my initial budget and they seemed to be working just fine for us. I had a couple hundred dollars extra left over in my "power budget" after purchasing my solar panel set up (two 158w panels - 30amp controller, all the wiring, brackets and such) and as we are going fulltime and planning on boondocking and BLMing it a majority of the time - I wanted to beef up my battery bank so we purchased two new deep cycle group 29dc batteries from walmart.

Now - when I purchased these I thought the batteries we had in there already were group 29 and newer than they actually are. After getting them home is when I realized they are actually 6 years old and group 27.

My question is: I think most of you would say not to mix them and to go with just the two new ones but why? Can I absolutely not add these two with my older two? When I hear that mixing new/old effects the life of the new ones....are you speaking over the course of their lifespan or would I see less storage from them now? I don't have much (any) money left to be building up battery banks - and I want as much juice storage as possible. It seems silly to swap out the two old with the two new and just toss these other two that technically still have life in them...if I hadn't gone to beef up the system - wouldn't have touched them and would have lived with the way they are.

It seems to me the two new...backed up with the addition of the two old would give me more power storage than JUST the two new right?

Looking forward to advice and opinions.

You will get many different comments and views on this subject.  My opinion is that you can install the two new batteries along with the existing two old batteries.

I would install them in such a manner that I could (using switches) parallel them or separate them into two distinct banks, for charging or use, as I choose.

I currently have two banks of 6V GC-2 batteries that I parallel (most of the time), that are two years apart in age.  The effect of (new-on-old) and (old-on-new) is insignificant in my usage requirements.  YMMV

AmeDeBoheme said:

My question is: I think most of you would say not to mix them and to go with just the two new ones but why? Can I absolutely not add these two with my older two?

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My humble opinion is that mixing old and new batteries is over-glorified. I see no issue with mixing them up. Yep, maybe the new ones might not last as long but I doubt it. Throwing out the used ones seems silly to me.

My 2 cents: If you set them up as a bank of four the charger will never fully charge them due to the two older batteries only charging to 12.36 volts.  Additonally they will try and equalize when the charger is not working so you will drain power to the old batteries until they equalize.  Neither is a catastrophic situation, just not the most advantageous. I installed 2 - 6v batteries in my M/H along with the existing single Group 24 12V battery.  I also added a marine style battery switch that allows me to use them all as one bank, use the 6V batteries only or the 12V battery only. It also allows me to disconnect entirely. If i set the switch to "All" the volttage is limited by the lowest common denominator, which in this case is the older 12V battery.

Battery Selector Switch
http://www.boatstore.com/142303-battery-selector-switch-4way.html?utm_source=googlepepla&utm_medium=adwords&id=22209859648&utm_content=pla

I can tell you that my golf cart with six 8 volt batteries keeps charging and charging and charging trying to get the total bank up to 2.2 volts per cell. The result is that the good batteries get too much charge and as soon as I remove the charger they drop to sum of the batteries and over time to the lowest common denominator Sloop mentioned.


IMHO separating the new and old into two banks is your best bet.

Setting them up as two separate banks is intriguing and sounds like a solid idea....a few questions:

besides the switch linked above, what else would I need for this kind of set up? Is there a way to get the charge to both bank sets if they are not connected? Slash, if so, how? Also - with an inverter for things like my laptops/cellphones etc....would I have to chose which bank I am connecting this to? It could not be set up to work off whichever bank is selected via the switch correct?

Does anyone have a link to a guide/instructions on how to set something like this up?

With a couple of hundred dollars to spend I would get two new batteries with the highest ratings possible to replace your six year old ones. With the voltage already dropping they aren't long for this world if you start heavy use.

Jeff said:

With a couple of hundred dollars to spend I would get two new batteries with the highest ratings possible to replace your six year old ones. With the voltage already dropping they aren't long for this world if you start heavy use.

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I agree.  That's the best use of that money, and now is the best time to do it.  You can add all the switching later if you find it necessary..

This drawing depicts what I did when I added my additonal 6V batteries.  Everything feeds to the converter.

http://img.photobucket.com/albums/v358/jwergers/M-HBatteryAdditon_zps000d04fc.jpg

Sorry, tried to post the picture with no luck. If anyone knows how to make it work . . .
Edit: Fixed link.

... tried to post the picture with no luck.

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Inline images are disabled here, but images as attachments work well. Meanwhile, I went ahead and activated your link so folks can click on it to see the picture.

Thanks Tom. I was not aware inline images were disabled. Have to practice with the attachment function.

I had two 6 volt golf cart batteries. After a year, I added two more that were identical, except for age. I hooked up each series pair with one old battery and one new battery. That way, the two parallel legs should both be the same.

Joel

I agree with the others that you can mix battery types and ages, IF you know what you are doing. The reason for the "don't mix" rule of thumb is that most people don't know enough about batteries to safely do so.

I would NOT recommend that you put two new ones with your old batteries. Not because they are old, but because you stated they are only producing 12.3dcv. That indicates they are only capable of about 70% charge at best, which means they will suck power from the new ones and also cause the charger to run continuously trying to bring the set of batteries up to 100%. Bad plan to mix batteries when the state-of-charge differs that much. So you should bite the bullet and get two new ones now, and save up for more later.

As for mixing sizes, it's not a big deal unless there is a wide disparity in amp-hour capacity. A 27 vs a 29 is only about 10-15% difference and that's not enough to worry about if both are in good condition (new or near new).

It seems silly to swap out the two old with the two new and just toss these other two that technically still have life in them..

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I disagree that your existing batteries still have life in them. They are on their last legs and would let you down severely if you tried to run on them without shore power. My guess is that you would replace them anyway after your first attempt to boondock for a weekend.

Sorry for the confusion. I had already purchased two new thinking "great now I'll have four - what a great bank I shall have!" Then realized how old and how different my orig two are. But still -  wouldn't four heads - even with two on their last legs - be stronger than two?

Tomorrow I am going to be installing the solar system (who says only gods can create solar systems lol) - installing the two new batteries - my inverter - and the like. This switch that Sloop speaks of is something I am looking more into and I feel I should probably do that while I am knee deep in installations and re-wirings no? Does anyone think I can just add the four (two old and two new) together or is this across the board a definite mistake on my part to do that?

I had already purchased two new thinking "great now I'll have four - what a great bank I shall have!" Then realized how old and how different my orig two are. But still -  wouldn't four heads - even with two on their last legs - be stronger than two?

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Not necessarily, and, at 12.3v,  your existing batteries are on the slippery slope toward death. The weak batteries drag down the voltage on the new ones, and sap the charging energy from the charger as well. The voltage in your entire system sags towards what the weakest battery can produce, impeding every light and appliance with low voltage.  And I'm betting those weak old batteries will probably drop well under 12.0v with any sort of load.

Under what circumstances did you measure the 12.36v? If that's with no load and the charger running, I would toss those batteries in the trash right now, cause they are totally dead. If at rest with no load and no charging underway (disconnected form the RV), they are merely very weak.