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Old 04-25-2011, 08:24 PM   #1
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Cool House battery be gone....

Well - I'm just 'venting' here - pun intended. My coach stays in the driveway plugged in. I drive it 2X a month and check all fluids (including battery water) at least once a month. Last week I noticed some blue sticky paper below the motorhome - must be litter. This week I checkrd the battery water level, Hummm - there are only 2 1/2 batteries. The blue sticky paper sad "Warrenty is Void if this label is removed" ?Hummm again? Only the bottom half of the center battery was remaining. It was blown in half. So, I learned to use sewer mits for battery removal when there isn't a grip point, lots of baking soda is good for more than keeping the frig smelling good, and when you see something on the groung near your coach - investigate closer. Just when you think you are ready to go you had better check things again.
Question: My remaining house battery is 10 months old (12V Marine deep cell) - So what do you knowledgable ones say, do I get two new batteries or test fate and just replace the blow-out one with same make/model?
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Old 04-25-2011, 10:32 PM   #2
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I'd figure out why the battery failed in such a dramatic way first. Whatever did in the one may well have caused the other some grief as well.

If your remaining battery is still in good shape and you've determined the cause of failure and fixed it, then just adding a replacement should do well enough.

There is a lot of warning to keep batteries in a bank exactly matched and that is nice but a bit overdone sometimes IMHO. As long as all the batteries are healthy and are the same chemistry they should work together without any significant deficits.

in your frequent fluid checks, have you found a need to add water to your batteries more than once in 6 months or so? If so, check your charge voltages as that is an indication of excess charging.

Does your RV converter have the smarts for a maintenance mode? Many of these as originally supplied are little more than big float chargers and they can be hard on batteries if left connected for any length of time.
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Old 04-26-2011, 07:49 AM   #3
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As Brian says, first find out why the battery blew. Was it overcharged? Did it run dry? Was there a short?

You talk about having 2 1/2 batteries left, but you ask whether to replace 1 or 2. I assume 2 are house batteries and and it was one of those that blew up. One of the others is the chassis battery, and it's OK.

After you figure out what happened to the old battery, I would get 2 new ones. If an older battery is hooked up with a new one, the older one may drag down the new one, then you'll end up buying 2 more anyway.
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Old 04-26-2011, 09:26 AM   #4
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re: "the older one may drag down the new one" -- The reality is a bit different.

With the caveat that this is about normal aging and not battery failure:

As a battery ages, it sulfates and its internal resistance builds. Old batteries have the same voltages as new ones, they just don't have the capacity. This is how you tell when a battery is old and needs replacement because it no longer has the capacity it used to have, not because its voltages are different.

What that means is that the stronger battery will carry more of the load in proportion to its strength over the other but it won't be 'dragged down' by the weaker battery.

In the extreme case, consider a block of wood representing a battery well past its useful life hooked up parallel to a battery in good condition. The block of wood doesn't 'drag down' the battery, it just doesn't do anything.

If a battery bank is normally being really pushed hard with large currents and cycle depths, then that will indeed cause the stronger battery to 'over-exercise' and perhaps shorten its life. The typical RV situation though is usually nowhere near that stressful on a battery bank and any mismatch is rather minor so the proportioning of the load isn't a significant issue in battery longevity.
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Old 04-26-2011, 10:20 AM   #5
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Quote:
Originally Posted by BryanL View Post
re: "the older one may drag down the new one" -- The reality is a bit different.

With the caveat that this is about normal aging and not battery failure:

As a battery ages, it sulfates and its internal resistance builds. Old batteries have the same voltages as new ones, they just don't have the capacity. This is how you tell when a battery is old and needs replacement because it no longer has the capacity it used to have, not because its voltages are different.

What that means is that the stronger battery will carry more of the load in proportion to its strength over the other but it won't be 'dragged down' by the weaker battery.

In the extreme case, consider a block of wood representing a battery well past its useful life hooked up parallel to a battery in good condition. The block of wood doesn't 'drag down' the battery, it just doesn't do anything.

If a battery bank is normally being really pushed hard with large currents and cycle depths, then that will indeed cause the stronger battery to 'over-exercise' and perhaps shorten its life. The typical RV situation though is usually nowhere near that stressful on a battery bank and any mismatch is rather minor so the proportioning of the load isn't a significant issue in battery longevity.
What you said in the last paragraph is what I was referring to when I said the old battery "may 'drag down' the new one." If the old battery doesn't have as much capacity, it will require more charging which may cause the newer battery to be overcharged and shorten its life. You're correct that it may not be a big problem if you never boondock or otherwise put much stress on the batteries. But since they're marine batteries, not something like T-105s, I personally wouldn't take the chance. I would just replace them both.
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Old 04-26-2011, 12:09 PM   #6
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re: "If the old battery doesn't have as much capacity, it will require more charging which may cause the newer battery to be overcharged and shorten its life."

The charging voltages are the same and the current is set by the internal resistance. In the extreme case (e.g. the wood block analogy), what the charger sees is just one battery and charges just that one.

If you charge an old sulfated battery, you'll see that it follows the same voltage profile but it just acts like a smaller battery. When it is charged in parallel with a larger capacity battery, it just follows along taking only the share of current that it needs to charge what capacity it has. Since the current is determined by the charger voltage and the battery impedance, there is no overcharging.

An older, sulfated battery does not do anything to force any of its companions in a bank to overcharge or undercharge just because it is there. It just doesn't contribute as much to a load and doesn't take as much from a charge as the healthier batteries.

The only risk is one common to all multiple battery banks and that is a failure such as a shorted cell in one of the batteries in the bank.

Any shortened life is due only to the fact that you've got one battery doing more than an equal part of the work - but in an RV context, that usually isn't that big a deal.

This is why most RV's will charge the engine battery in parallel with the house battery when on the road as a usual thing. Any mismatch between the two doesn't degrade either when being charged. The current is split between the house and engine batteries depending upon their state of charge up until they are both fully charged.
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Old 04-26-2011, 02:28 PM   #7
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Well, maybe my faulty thinking comes from the fact that we have two 6V house batteries wired in series. In that case current would not be split. The OP's batteries are apparently 12V wired in parallel so it seems he should have no problem with an old one and a new one. Thanks for setting me straight.
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Old 04-26-2011, 03:58 PM   #8
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now you've got me something to think about! ;-)

I can see that series wiring would be an issue if the batteries were significantly different - that gets into the equalization charge idea, too, I think.

I will definitely need to keep this in mind next time I chip in on dissimilar batteries.
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Old 04-26-2011, 07:20 PM   #9
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Thanks again guys. I'm sure it was just a faulty battery. I used the DC system several times and the remaining battery carried the load just fine. This winter I had noticed that the now destroyed battery would drain faster than the other house battery when disconnected (side by side) from all loads. It also required more water than the other house battery. I suspect that with the warmer weather the water level droped faster, the plates became exposed or a fault within caused a spark and the Hydrogen and Oxygen combo did what nature programed them to do. I just didn't catch on to those warning signs. So, I'll scrub and wire brush and paint the battery rail where the acid spilled (most was on the fiberglass nose cowling). Then get a new battery. Thanks again.
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