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!!

[profdant139]

Thanks for all of the education! I see now that voltage and capacity under load are two different things. Fortunately, the vacation was not ruined at all -- we were able to patch things together long enough to last the whole week.

And this was by no means our first camping trip in cold weather -- we camp several times a year in sub-freezing weather (down to about 20 degrees or so). 34 degree nights are usually no problem at all.

time2roll, I suspect that you are right that if I had measured voltage during the "collapse" (while it was under load), it would have been at 10 or so. But by the time I measured it with my multi-meter the next morning (around 12 hours after the collapse), the weak battery's voltage had rebounded.

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.

Google Calendar has made this a much easier task than it used to be -- in the pre-Internet days, it was difficult to give yourself a reminder for a date several years in the future. The only reliable way was to put a sticker on the item in question -- "Replace at the end of 2024."

And then hope that the sticker would not fall off! 😉

[wa8yxm]

Also measure voltage at two points
The battert (Terminal to terminal) and somewhere "Down stream" like the back side of a light switch... you may see a dim light (won't be a great light at the 2nd spot)

[time2roll]

With the sudden drop in voltage cutting out the fridge I would have suspected you to measure 10.5 or less volts as I believe the fridge should operate down to this level. I would keep an eye out for a loose connection by measuring voltage at the fuse panel or at the fridge itself. Nothing should complain at 11 volts let alone 12.1+

[MEXICOWANDERER]

How I grumble about using derivatives for analysis (frown) . Connect 8 D cell flashlight batteries and Tah-Dah you have 12 volts. For dainty hands, Harbor Freight has a true adjustable carbon pile load tester. The 2nd best way to accurately test a lead acid battery. Parts store cheese grater load testers are as useful as a treadmill with a recliner chair.

I've seen batteries that will exhibit 12 volts
Put a 20 amp load on them and they slump to 0.00

Load test batteries at 50% of their cold cranking amperage rating.

Welcome to the world of a ruined vacation 😞

[Fisherman]

You say 12.1 isn't that low, I would say it's not capable of electrocuting a dead mouse.

[wopachop]

Rare to camp in that temp?
I think it's just more voltage drop due to the cold weather. Of course age as well like people mentioned.

[BFL13]

The missing number is the "loaded voltage" with the furnace and the water pump both running. 12.1v is the voltage after the "bounce back" after the load is removed.

12.1v resting is 50% of capacity. Any capacity. 50% of what? is the question. A battery that has aged out will show good voltage until a load is put on it, when it "collapses".

You could run a little test for that. A voltmeter in the RV would also show that while camping. I like the little ones that stick in a 12v socket, that also have slots for USB charging.

https://www.amazon.com/Palumma-Charger-Adapter-Display-Battery/dp/B0773BYS6P/ref=sr_1_4?crid=2V53IA1...

[3_tons]

Batteries are likely showing signs of aging (chemical soup sluggishness)...

Though a bit anecdotal, I recently had a very similar issue on Safe electronic combination keypad with 9v battery - Voltage showed 9.7 but no matter what, safe would not dare open... After a few days of nagging frustrations a competent Locksmith told me that the voltage means nothing, and to simply replace the battery... I did this and safe then opened immediately!

Best to think of voltage as static characteristic and current as dynamic...

3 tons

[2oldman]

2 things immediately come to mind: age, and temperature. I rarely got more than 4 years out of 6v Trojans, and they would act like yours when failing.

Cold batteries, if that's the case, don't do well either. Voltage indicates the *potential* to perform, but not the endurance.