20 hour battery capacity test done

[Davidasailor26]

Every couple years I try to do a full 20 hour capacity test of my house batteries to see how they’re holding up. The last couple times I’ve used a shunt, Arduinos, and Raspberry Pi to log the discharge data for comparison. I figured maybe the charts and data will be helpful to someone.

The batteries (Deka AGM’s) are 7 years old now, and still reading 100 and 103 amp hours, versus their rating of 105. Can’t complain about that. It is fair to say they live a sheltered life. We do a lot of daysails but that usually only pulls them down to about 80-90% SOC. Maybe a couple times per year we stay out and bring them closer to 50%. Whenever we’re not out we’re on shore power with them charging on a 20A Xantrex Trucharge2.

Here’s the capacity test setup -

The battery powers a cheap inverter. Into the inverter I plug a lamp dimmer (the kind with a regular plug on each end), which dims a work light with an incandescent bulb. I can use the dimmer to keep the current draw tuned to about 0.05C (5.1 amps). The shunt is in the ground wire from the inverter. One Arduino reads battery voltage and one reads (with an amplifier) the voltage across the shunt. They send the data back to a Pi that logs it. The whole shunt/Arduino/Pi setup is usually on the boat in summers so I can keep an eye on the batteries there. By using the inverter to power the lamp I find I don’t need to tune the dimmer too often even as battery voltage changes. Every few hours i double check and just tweak it 0.1A or so if it drifts. The only problem with the inverter is that is has a fan that cycles, varying the amperage by about 0.2A intermittently, but I try to keep the average at 5A, and the Pi logs the exact draw each second so I can use the average over time to calculate true AH.

 

[dLj]

So my question for you: Wouldn't it be better to only do this test, say, halfway? It you only ran it to about 50% discharge, haven't you captured enough data that you could predict the rest? If the slope of the curve it tight enough there, wouldn't that give you confidence your batteries are still fine? Why do you need to actually go the the deep discharge in this test? Doesn't this test in itself damage you batteries?

Just curious and I don't know anything about this kind of testing so I'm really just asking for gaining knowledge.

dj

 

[Whatfiero1]

I do a real life test at dock every spring with my refrigeration and volt gauge 20 hr is about right for a weekend outing

 

[Davidasailor26]

So my question for you: Wouldn't it be better to only do this test, say, halfway? It you only ran it to about 50% discharge, haven't you captured enough data that you could predict the rest? If the slope of the curve it tight enough there, wouldn't that give you confidence your batteries are still fine? Why do you need to actually go the the deep discharge in this test? Doesn't this test in itself damage you batteries?

Just curious and I don't know anything about this kind of testing so I'm really just asking for gaining knowledge.

dj

Good question. I’m not sure the slope at 50% would really be steep enough to distinguish. Looking at the data from the 4 tests it looks like there’s a spread of about 0.02 V at the 50 aH mark (with the highest 50aH reading being the lowest final total, interestingly). That’s about the same amount of variation as between 48.3 and 50.77 percent SOC on any one battery. So if I tried to extrapolate out from there I guess I’d be within about 5% of total capacity, which isn’t bad.

As these batteries age I’ll keep charting the data every couple years. When their SOH gets significantly worse it should be easier to see whether the 10 hour mark really distinguishes between good and bad, or if the curves don’t really diverge until lower SOC’s.

I don’t think doing the test every two years is really doing any measurable damage. One 100% discharge every 2 years is not like a cruising boat going to 50% 300 times per year. I’m also sure to put the batteries on a charger immediately when I finish the test, so they’re never just sitting at that lower SOC. Maine Sail has recommended the 20 hour test procedure in the past and said it’s not a significant harm to perform infrequently.

 

[dLj]

Good question. I’m not sure the slope at 50% would really be steep enough to distinguish. Looking at the data from the 4 tests it looks like there’s a spread of about 0.02 V at the 50 aH mark (with the highest 50aH reading being the lowest final total, interestingly). That’s about the same amount of variation as between 48.3 and 50.77 percent SOC on any one battery. So if I tried to extrapolate out from there I guess I’d be within about 5% of total capacity, which isn’t bad.

As these batteries age I’ll keep charting the data every couple years. When their SOH gets significantly worse it should be easier to see whether the 10 hour mark really distinguishes between good and bad, or if the curves don’t really diverge until lower SOC’s.

I don’t think doing the test every two years is really doing any measurable damage. One 100% discharge every 2 years is not like a cruising boat going to 50% 300 times per year. I’m also sure to put the batteries on a charger immediately when I finish the test, so they’re never just sitting at that lower SOC. Maine Sail has recommended the 20 hour test procedure in the past and said it’s not a significant harm to perform infrequently.

I very much support your testing and reporting out the data!

I also completely agree that what you're doing isn't doing any "significant" harm. But if I were to be able to get essentially (as you say within about 5%) the same data without that, first, it would make the test time much shorter, and second, I'd be happier with that test... Although perhaps as you go along, you'll find that the 50% mark just doesn't cut it... I really just asked as looking at your graphs, it sure seemed like you had almost all the data you needed by about 50% discharge, but I've no knowledge really... Seeing how this data changes as the batteries age will indeed be very interesting.

Great set-up you have by the way!

dj

 

[Davidasailor26]

Following up with a little more data 15 months after the previous capacity test results. At the start of this season the batteries seemed to be acting pretty much the same as they always did. Over the last month we’ve noticed the bank voltage dropping a little more quickly than in the past. After one sail where the voltage seemed the be dropping remarkably quickly I separated the batteries and found one of them completely dead, floating below 12V even fully charged. I just did a capacity test of the other relatively good battery and found it only holding 76 AH.

Here’s the updated plot of capacity tests including this latest run to 76 AH:

 

[dLj]

I'm not understanding the legend well so let me just ask - do you feel that you could stop this test at 50% discharge and know the health of your battery?

Thanks for the follow-up.

dj

 

[Davidasailor26]

I'm not understanding the legend well so let me just ask - do you feel that you could stop this test at 50% discharge and know the health of your battery?

Thanks for the follow-up.

dj

It looks like if you stopped the test at 50AH (50% of the specified capacity) the difference between a “good as new” battery and a “pretty worn out” one is 0.2 V. That’s definitely a measurable difference, so yes I think it’s safe to say you could tell when a battery is past its useful life that way. A battery at 90% would probably be harder to distinguish from a new one. That’s probably much closer to the limits of measurement.

The legend is just indicating which line goes with which test. I have 2 house batteries - one that was originally in the aft cabin and one that was in the engine compartment. So for example 2019_cab is the line for the 2019 test of the battery originally in the aft cabin.

 

[dLj]

Thanks - actually I had a hard time seeing which color in the graph was for which legend entry - on my computer the colors are not well matched between the graph and the legend.

Good to know though - Cuts your test time in half, no?

dj

 

[Scott T-Bird]

Following up with a little more data 15 months after the previous capacity test results. At the start of this season the batteries seemed to be acting pretty much the same as they always did. Over the last month we’ve noticed the bank voltage dropping a little more quickly than in the past. After one sail where the voltage seemed the be dropping remarkably quickly I separated the batteries and found one of them completely dead, floating below 12V even fully charged. I just did a capacity test of the other relatively good battery and found it only holding 76 AH.

So, what are you considering for replacement? I see myself going down this road sometime soon, but not just yet ... I just replaced my auxiliary battery, but my pair of 4D AGM house bank batts are in their 8th season now.

 

[Davidasailor26]

So, what are you considering for replacement? I see myself going down this road sometime soon, but not just yet ... I just replaced my auxiliary battery, but my pair of 4D AGM house bank batts are in their 8th season now.

We just replaced them with the same as we had - Deka Intimidator AGM Group 31’s. I know they’re not the favorite AGM brand, but we figure with 8 years out of the first set they did ok. Might not be a great option for a serious cruiser, but since we’re mostly day sailing (above 75% SOC), with a few nights of anchoring to 50% SOC per year they’re adequate. Our thruster and engine batteries are still AGM and we like the low winter self-discharge rates and lack of maintenance, so we wanted to stick with AGM.