Battery failure puzzle: voltage good, little power

[profdant139]

I have (or had!) two five year old group 31 NAPA deep cycle marine batteries, rated at 110 amp/hours each. They were not hooked up in parallel or series -- I use one at a time, swapping them out as needed, using solar to keep them topped up whenever possible. (I know that linking the batteries takes advantage of the Peukert effect -- but I like the redundancy of having a completely fresh spare battery in the event of trouble.)

Both of them were as well-maintained as I could manage -- never drawn down below 12.1 volts, always on a Battery Minder Plus at home, never low on water. I even played soothing music for them while in storage. (Not true -- just want to see if you are still reading.). And I check the specific gravity of each cell religiously. (I must admit that battery maintenance is an odd religion.)

They have been great batteries. After 5 years of frequent use, no problems at all, till last week. We were camped in Kings Canyon in cold weather (just above freezing every night). One dark night, we were running the furnace, and the fridge (of course), and the water pump (DW was taking a shower), and some of the LED lights.

Suddenly, the lights started to flicker. The "check" light came on, on the fridge display. The pump got sluggish.

DW quickly finished her shower. I hopped out of the trailer and swapped out the batteries. The new "good" battery read 12.7 volts, and the old "bad" battery read 12.1 volts. Which is not that low. But I knew that it would give me an accurate reading the next morning, after resting.

The next morning, I took a reading on the "bad" battery, and it was at 12.3 volts! How could it have been out of juice?

But wait, there's more. That afternoon, I discovered that the "good" battery, which had been at 12.7 when I swapped them out, was down to 12.1 after a fairly low-load night of use. That is an unusually steep drop. We had no trouble with the battery, but clearly it was on its last legs.

So I finally had to crank up my Honda generator and jam two hours of juice into the "good" battery. (I almost never use my generator, except for routine maintenance.) Since the next day was the last one of the trip, we made it home in good shape.

But here is the mystery: if my "bad" battery was at 12.3 after resting, how come it could not cope with the maximum load due to the furnace and the pump? How could the battery indicate decent voltage while it actually holds very little useful power??

Thanks in advance for your diagnostic wisdom!!

[MrWizard]

RJ
For load use you are correct
But not useful life/age
For aging gracefully with a longer lifetime, the thicker plates in the 6v batteries will last longer in terms of years of use

But as pointed out many times, 12v batteries will out preform 6v when powering heavy loads like a MW via inverter

[RJsfishin]

Quote:
. If the batteries are bad I would defiantly go with 2 6 v GC batteries, they will last much longer than your 12 v batteries.
-------------------------------------------------------------
Not true !! An amp hr is an amp hr no matter if its in a 6v or 12v.
2 12v w/ a total capacity of 220 amp hrs will last just as long as 2 6v w/ the same total capacity.

[BFL13]

Taking one battery away to recharge it in the sun while the other still runs the rig in the shade is an advantage for sure.

A variation of that is if you have a gen in the truck or you have alternator charging in the back you can recharge the one battery some while driving around doing tourist things or whatever away from the rig.

For long term off-grid and it is time to run a full recharge and then an equalize on solar, a split bank like that lets you do that too. You can't get that done while there is a draw on the bank as there would be with both batts working.

[Gjac]

The only thing I would add to this thread is the difference between voltage and amperage. Just because you have adequate voltage doesn't mean you have the right amount of amps getting trough. You refer not working leads me to believe that may be an issue. Usually caused by corrosion somewhere in the system usually at the battery posts or broken wire strands due to chafing, or loose or corroded wire butt splices. But since you change batteries frequently I would suspect a bad ground somewhere or one off the other conditions I mentioned. If your batteries pass a load test I would try to equalize them first and check and clean all connections. If the batteries are bad I would defiantly go with 2 6 v GC batteries, they will last much longer than your 12 v batteries. I like your idea of two battery banks and was considering adding a separate bank of 2 more 6v GC batteries but after many years of dry camping have never needed the extra power.

[gkainz]

Unless you don't have the physical room for both batteries, why not install a simple battery switch with A, B, A+B, Off positions and then just "twist the switch"?
Murphy (at least the one that lives with me) would say I would be switching batteries at 2am ... always!

[profdant139]

Naturist's point about lithium is well-taken. The only problem is that we frequently camp in sub-freezing weather, and lithium batteries can't be charged below 32 degrees.

I assume that rule applies to solar charging, and not just to generator charging?

And I know that it seems a little extreme to camp in sub-freezing weather. But we really like snowshoeing in the Sierra. And it usually does not get much below 20 degrees at night. (If the forecast is for really cold weather, we try to avoid that.)

[2oldman]

profdant139 wrote:
It's very easy to swap out the batteries. I hit the battery disconnect button. I open the box containing the exhausted batteries and remove the wing nuts holding the wires to the posts.

Used to do that myself. No problem.

[ifd22]

Check your ground wire between the batteries and the frame

[naturist]

May I suggest springing for one of These instead of two of the batteries you have been buying? It will have almost the same usable capacity as the pair and not need replacing for more like ten years, making it actually cheaper than the lead acid batteries.

[RJsfishin]

Exactly why I have a digital readout voltmeter mounted in the television panel, staring me in the face anytime I watching tv. This way I know what the voltage is under the load of the tv/inverter lights, water pump. That's when you learn that 11.9 volts will still go a long while.

[profdant139]

It's very easy to swap out the batteries. I hit the battery disconnect button. I open the box containing the exhausted batteries and remove the wing nuts holding the wires to the posts.

I lift out the dead or tired battery and stick in the other one. Spin the wing nuts, pull the battery disconnect button, and it's done. Takes me about 90 seconds. I could have a switch do the job -- that would be more elegant than my current setup, which is pretty primitive.

The other advantage of having a spare battery is that when we park the trailer in the shade (which is often), I can take the depleted spare into a nearby patch of sunshine with my portable solar panel. We often do exactly that.

And this little snafu with my batteries reinforces my desire to have "one and a spare." If the two batteries were always together, and one dies catastrophically, the remaining good battery would be quickly drained.

I'm a very risk-averse person. Belt and suspenders. 😉

[time2roll]

theoldwizard1 wrote:
You have a rather un-orthodox procedure of swapping out a weak battery in the middle of a camping trip, something very few people would do.

I would recommend a pair of 6V golf cart batteries. 200+ Ah. If you buy them at Sam's Club or Costco, I am sure they will cost less than those NAPA Group 31 batteries.

Always plenty of talk about a single 6v battery failure and you are sunk. So they want two 12v batteries. However a bad battery can drag both down so I see this as perfectly logical and should be common if the single bad battery is a concern. If I did this I think I would have a switch to select battery 1, battery 2, or both.

[theoldwizard1]

profdant139 wrote:
Mystery solved!

And the bottom line here is that although these same types of batteries have usually lasted me five years (three sets in fifteen years), I will probably schedule replacement for every four years.

You have a rather un-orthodox procedure of swapping out a weak battery in the middle of a camping trip, something very few people would do.

I would recommend a pair of 6V golf cart batteries. 200+ Ah. If you buy them at Sam's Club or Costco, I am sure they will cost less than those NAPA Group 31 batteries.

If you camp with a generator, you should not have to carry a spare set, especially if you install a battery monitor.

[theoldwizard1]

A load tester is a good tool. A better that you can use all of the time is a battery monitor, like a Victron Victron BMV-700.

Second, depending on how many cycles those batteries have been through, it just may be their "time".