Weak AGM batteries - what next?

[s/v Jedi]

I agree with Wotname. The capacity is at a 20hr rate. For example, if you have a 100Ah battery then you should discharge it at a 5A load so that it takes 20 hrs to take 100Ah out.

If you discharge at a higher rate then it will provide less total capacity. Some brand also specify a 10hr capacity but if it doesn’t say anything then it’s the default 20hr rate.

[torrmundi]

I got a CarQuest CPE30370 load tester rated for up to 1000 CCA. It's basically a resistive coil and a volt meter. It specifies a 10sec test, which draws about 40 amps as measured by my shunt.

With batteries charged via solar to 14.28V and victron mppt chargers in float mode, then disconnected, I tested for 10 seconds, recorded the minimum voltage during the test, then a few seconds after the test I recorded the recovery voltage. I did two such tests, then recharged the batteries for about 30 minutes and repeated it all.

Battery #1
No Load Test: 1 2
12.86 11.6 / 12.76 11.55 / 12.72
No Load Test: 3 4
12.90 11.49 / 12.72 11.50 / 12.67

Battery #2
No Load Test: 1 2
12.77 - / 12.78 10.88 / 12.60
No Load Test: 3 4
12.90 10.84 / 12.65 10.79 / 12.67

Battery #3
No Load Test: 1 2
12.43 11.13 / 12.39 11.61 / 12.42
No Load Test: 3 4
12.50 11.13 / 12.47 11.41 / 12.47

[s/v Jedi]

Originally Posted by torrmundi

I got a CarQuest CPE30370 load tester rated for up to 1000 CCA. It's basically a resistive coil and a volt meter. It specifies a 10sec test, which draws about 40 amps as measured by my shunt.

With batteries charged via solar to 14.28V and victron mppt chargers in float mode, then disconnected, I tested for 10 seconds, recorded the minimum voltage during the test, then a few seconds after the test I recorded the recovery voltage. I did two such tests, then recharged the batteries for about 30 minutes and repeated it all.

Battery #1
No Load Test: 1 2
12.86 11.6 / 12.76 11.55 / 12.72
No Load Test: 3 4
12.90 11.49 / 12.72 11.50 / 12.67

Battery #2
No Load Test: 1 2
12.77 - / 12.78 10.88 / 12.60
No Load Test: 3 4
12.90 10.84 / 12.65 10.79 / 12.67

Battery #3
No Load Test: 1 2
12.43 11.13 / 12.39 11.61 / 12.42
No Load Test: 3 4
12.50 11.13 / 12.47 11.41 / 12.47

You’re doing tests for start batteries while you have deep cycle batteries. We did explain how to test, you ignore it, now there isn’t much more to say.

[torrmundi]

Jedi, I haven’t got nice programmable loads, or car headlights. I have to jury rig a test setup. Patience, please. I work with what I have, while running a boat. The Carquest testing has shown a potential issue with battery #2.

[tomtriad]

Call Lifeline and talk to tech support - they know their stuff!

I had a pair of Lifeline AGM's (105 AH, group 31 12 v.), 4 years old, worked great year 'round on a solar charging system. I left the boat moored for a month and a fuse holder from the controller failed, letting the small fridge draw the batteries down flat (10.5 volts). They would not accept more than 80% charge thereafter, even on shore power and with a smart charger. I called tech support at Lifeline and they explained to me that they would never get to full charge again and recommended replacement. I suspect yours are in the same situation.

[sinnerman]

I personally would not tie all three AGM (lead acid) House batteries together into a "House Bank" (if that is what you are doing).

IMHO all three should have their own on/off switch or a selector switch to select which what battery is being used while isolating the others..

Having all three connected together (as already stated above) places your "bank" at risk of being taken down by a single bad battery.

Additionally the three batteries want to charge at their own individual charge rates, and tying them together will most likely result in the alternator output/charging voltage being dominated by the healthiest (fastest charging) battery.

If you spend any time hooked up to shore power (i.e. marina), then it would be worth it to get a good 120vac battery charger which can charge three independent batteries at the same time. The batteries will be "conditioned" by the battery charger.

I have this one in my boat (See pic)...


My two cents


cheers

[torrmundi]

Thanks for all the thoughtful responses. I suspect that the multiple discharges to very low voltages have made a permanent effect.

I did recover some capacity by performing the recommmended conditioning: charging at 15.5V for 8 hours. After that, the overnight loads of fridge/freezer and lights brought me down to 11.8V, rather than 10.8V.

I still haven’t run a capacity test on each battery due to the demands of sailing and business. The admiral must have internet to do business, which means Starlink.

[Jeff Pfeil]

7years old? No question, replace them. I had similar problems after five years and replaced mine, then tested them to understand what was wrong. One battery was shot, couldn't hold a charge with no load. The other was still holding a charge but was likely on borrowed time.

[torrmundi]

I finally got the parts to be able to test the batteries. I was able to run a 300W bulb on an inverter for about 25 minutes (27A load) on a single battery before battery voltage dropped too low for the inverter to operate continuously (somewhere between 10.5 and 11.0 Vdc).

So this generally coincides with the house bank battery monitor that showed overnight cumulative Ah consumed of 40 and a 11.3vdc for all 3 batteries.

I was able to replace the batteries temporarily with 3 used Mastervolt 160Ah cells from Sailor's Exchange. That will get me through my next few months until I go to LiFePo4.

Thanks for all the comments!