Battery capacity test

[tfox2069]

So I've been scouring the boards here, MaineSail's site, and taking on board advice from a number of you regarding rewiring my "factory" battery and charging system. I'm learning a lot, and greatly appreciate everyone's willingness to educate others. This weekend I did a **rough** test to determine the usable Ah capacity of the batteries on my new-to-me boat of 9 mos ago. The results follow, and I'd be very interested in your thoughts.

Boat: 2009 Hunter 36 purchased in June 2018.
Batteries: 2 x 198Ah East Penn (West Marine branded) 4D AGM batteries of unknown age wired in manufacturer/dealer standard configuration (wired separately to 1, 2, Both, Off switch). Batteries in cockpit locker separated from engine. Since my purchase boat kept in slip w/shore power and batteries kept fully charged except for weekend overnights when engine charging used to replenish. Largest discharge event 69Ah in 2018 sailing season. Batteries run in “Both” configuration for duration of this test.
Monitor: Victron BMV-712 battery monitor wired through shunt.

Notes: Load varied between 3 and 10 Ah during test. Load overnight from 2308-0816 varied between 2-7 Ah, mostly on the lower end of that range. 3YM30 29hp engine is original with 398 hours on it, and the alternator appears to be stock and original. Belts changed and tensioned before test. Engine started for first time since winter layup and started on first crank from low battery charge state. Engine and alternator charging run for 10 minutes and shut down, at which time shore power ProMariner ProTech 1230iPlus 30 amp charger in AGM 2 setting engaged and run for ~50 min for final reading. Shore power charging continued after end of test to regain full charge and then maintain float charge.

ProMarine ProTech manual extract of charging profiles:

Observations:
- Batteries appear to have ~50% of original capacity available (400Ah bank; 200Ah usable capacity; 106Ah actual capacity to 50% SOC)
- Alternator appears to be putting out ~2/3 its rated capacity even when cold

Questions:
- Did I really run the batteries down to 50% SOC? If not, what should the ending voltage reading be at 50% SOC? (MaineSail writes in his www.marinehowto.com site of using a 12.1 @5.2 amps as cutoff for one of his tests to discharge a battery down to 50%)
- Is the observed alternator output normal for a cold 60 amp alternator (temp at engine start 50 deg F)?

 

[Davidasailor26]

- Did I really run the batteries down to 50% SOC? If not, what should the ending voltage reading be at 50% SOC? (MaineSail writes in his www.marinehowto.com site of using a 12.1 @5.2 amps as cutoff for one of his tests to discharge a battery down to 50%)

Interesting question. I don't know the answer, but I do have another data point. I did a full 20 hour capacity test of one of my Deka AGM's this weekend. The battery is rated at 105 aH and tested at 100aH. Looking back at my data, at the half way point I was only at 11.93V. Not sure why yours and mine were so much lower than Maine's at 50% DOD. My test was at the 20hr discharge rate of 5.25A. Note also that although you were measuring at about the same amperage your bank is much larger than the single battery Maine is testing, so really your voltage should have been higher.

 

[Maine Sail]

If the discharge test is not done at 70-80F, and at the 20 hour discharge rate, and discharged continuously until the bank hits 10.5V, the data will not be very accurate.

That said, I can't really seem to figure out what your data really means? Once you start a discharge test, stopping it or pausing can thrown the data off by quite a bit. To get an accurate sense of where you are in terms of SOH, compared to "as new", your batteries would be tested individually at 9.9A constant loads until the battery hits 10.5V. Always be sure to re-calibrate/zero your Ah counter at the top of charge then run the discharge test with no other loads or charging at the rated Ah / 20 as your load....

In other words if you don't run a 20 hour test, the way the batteries were rated, the data is only a hip shoot at best.

As batteries age they way they hold voltage can vary and AGM's from different manufacturers also vary slightly in how they perform under load. For optimal battery health stopping your discharge at 12.1V (under your average house bank load) is going to lead to longer life than stopping the discharge based on Ah's removed.

 

[tfox2069]

All fair points, particularly on a rough field test. The "rest" at the 12-hour point was to see what the unloaded battery voltage recovered to, just to make sure I didn't need to stop the test there. But point taken on that throwing off the data.

I just discovered your excellent post on testing battery capacity here, and will likely conduct a re-test with these instructions:
https://forums.sailboatowners.com/i...use-batteries.190209/&highlight=gel batteries

 

[tfox2069]

So I finally got around to conducting MaineSail’s 2 hour test. Disconnected the fully charged batteries for 24 hrs, reconnected, ran 20 amp load for 2 hours, then disconnected and let rest for 24 hrs. Temp varied between 63 and 75, so not perfect. Results surprised me:

400 ah AGM battery bank (2x200Ah)
14.06 on float at 75 degrees
12.81 after 24 hr resting w/no load
12.37 w/20 amp load at start
12.35 w/20 amp load after 2 hrs
12.54 w/no load after 2 hrs
12.70 w/no load after 8 hrs
12.69 at 15 hrs at 65 degrees
12.69 at 22 hrs at 63 degrees
12.69 at 24 hrs at 63 degrees

East Penn resting voltage table:
100% 12.80+
95% 12.76
90% 12.72
85% 12.68
80% 12.64
75% 12.60

Published table shows 75% and 100% so I calculated the other charge levels. The 2 hr test seems to indicate that the batteries are roughly ~5% lower than rated capacity when fully charged. I’m pleasantly surprised by that. I’ll be reprogramming my VictronConnect to reflect the slightly reduced capacity, but in any case, will adhere to the 12.2v rule that MaineSail advised (and recharging fully at every opportunity).

 

[kappykaplan]

So what happened to the results?

 

[tfox2069]

So what happened to the results?

Sorry, hit “Post” accidentally mid-post! See edited complete test results.

 

[kappykaplan]

Sorry, hit “Post” accidentally mid-post! See edited complete test results.

Tks!