Measure present Full Battery Capacity (as it declines)

[conachair]

For monitoring a Raspberry Pi and ADC1105 is just fantastic for spotting trends and anomalies hidden in the numbers which you just wouldn't spot without plotting it.
Pretty easy to set up in Openplotter and no need for a monitor. Well worth the effort IMHO. I plot it all in node -red and now have about a years worth of battery voltage and a load of other data recorded once a minute.
More exciting should it actually work is another project on the go with another little board - INA219
Already set up and measuring/plotting accurate currents of various decives down to the mA - what will be just wonderful should it work is to link it to the battery monitor shunt so amps in/out can be monitored and recorded as well as volts. Sounds possible, just parallel the INA input with the shunt, it will measure microvolts across the sense resistors. Probably need a bit of calibration and might knock the battery monitor out a bit if the INA 0.1Ohm sense resister is left.
Then maybe a PWM output from the Pi ( I actually use an ESP8266 to read all the sensors then wifi the data to the Pi) could control some FET transistors to a load to do some sort of a load test but including all the other loads , so no need to turn everything off to do a constant current capacity test of some description and record/plot all the data. Probably be worth adding a DS18b20 thermometer to record the battery temperature as well.
In over a decade of constant cruising I've yet to meet anyone who does more than a "leave the lights on all night" capacity test, even then few get that far.

Just might work........

Below voltage on solar, easy to see the fridge cycling and when I turned off the laptop in the evening

[svlamorocha]

Originally Posted by a64pilot

Lifeline publishes an excellent chart of SOC and discharge rate, plus of course the open circuit resting voltage. However for whatever reason my bank is much higher voltage than the chart says it should be at a particular SOC, so it’s not real applicable for me, for some reason.
.

Do tou let the battery rest disconnected for 4 hours before measuring the voltage?

By the way, your method that ends the test at about 50% (10 hours at 20-hr rate) works great for me; I measure actual SOC 4 hours after the test using that table.

[Paul Elliott]

Originally Posted by svlamorocha

Do tou let the battery rest disconnected for 4 hours before measuring the voltage?

By the way, your method that ends the test at about 50% (10 hours at 20-hr rate) works great for me; I measure actual SOC 4 hours after the test using that table.

That lifeline chart includes values for battery voltage vs %charge vs load current, so you don't have to disconnect and wait before getting a charge level measurement. At very low load currents the voltage is quite close to the resting no-load current anyway.

But the chart is for these particular Lifeline AGM batteries. Fortunately, that's the type I have. I do the annual "turn on all the lights plus other gear to get to around a C/50 discharge rate" test to make sure the batteries are still holding up. My AGMs are seventeen years old and still have better than 80% of their original capacity. But I don't cycle them daily as many cruisers do, my usage is much more intermittent.

[svlamorocha]

Originally Posted by Paul Elliott

That lifeline chart includes values for battery voltage vs %charge vs load current, so you don't have to disconnect and wait before getting a charge level measurement. At very low load currents the voltage is quite close to the resting no-load current anyway.

But the chart is for these particular Lifeline AGM batteries. Fortunately, that's the type I have. I do the annual "turn on all the lights plus other gear to get to around a C/50 discharge rate" test to make sure the batteries are still holding up. My AGMs are seventeen years old and still have better than 80% of their original capacity. But I don't cycle them daily as many cruisers do, my usage is much more intermittent.

Good point. Just to clarify, we are talking about two different charts in same page appendix C of the Lifeline technical manual. The one you are talking about (top of the page) is very useful as you point out but it loses reliability as the batteries get older. In particular, when you get a boat back from Moorings with AGM batteries that have been freshky equalized to death to make them work "for just a bit longer", they will appear to have 90% of nameplate capacity when using the top chart but this overstates real capacity left. I have learned this the hard way

[john61ct]

Note that chart only holds not only for a specific model, but also only for a new bank.

The relationship changes as the bank ages.

[rgleason]

Lots of good ideas/techniques here. Paul do you discharge your agm to 50% or 10.5v? If you go to 50% how do you actually determine 50% and how is it possible to calculate the actual full battery capacity?

Ps I will check soc by measuring resting voltage after 72hr with no loads applied and no charging during that time.

[Paul Elliott]

I'm using the chart "Battery Load Voltage vs. DOD" on page 32 of this manual: http://2cw8eb1vmmgg3g5i7jzt6upo.wpen...cal-Manual.pdf.
I try for a C/50 discharge rate so I can bring the batteries down to 50% in 24 hours. I am watching the voltage, current and AH on my Link 2000-R battery monitor, and try to stop the test when the batteries reach around 12.1V. According to the chart, this is roughly a 50% discharge.

To be honest, I'm not trying for precision, and I won't get it with my technique. I'm just trying to make sure that the batteries are in reasonable health. When the batteries can no longer deliver over 12V at load for a reasonable duration I will know it's time to replace them.

Most of the time the batteries are being float-charged from my solar panels, with occasional day-sails, and once every year or so I will embark on a one or two month voyage. I've got almost 1000Ah of batteries (4x8D) and other than my shorepower-connected charger (which is seldom plugged in), neither my solar or alternator charging sources come anywhere near the high-current that everyone is recommending. 17 years is pretty good, so I'm not too concerned.

[rgleason]

I wonder if this Lifeline chart might apply to Trojan batts too, but I'd have to find the comparable chart for flooded lead acid.

[a64pilot]

Originally Posted by john61ct

Note that chart only holds not only for a specific model, but also only for a new bank.

The relationship changes as the bank ages.

No, the chart specifically states it’s for “aged” batteries.
For whatever reason the chart doesn’t work for my bank, my voltages are always significantly higher than the charts. I hope that means they have sufficiently “aged” enough to be on the chart, and not that they have over aged
Pretty sure they are healthy though.

[john61ct]

Lead batteries have a much lower capacity when brand new.

The commissioning process should help accelerate it, but few people bother, so reaching peak capacity can often take a hundred cycles or more.

During which "breaking in" period, amp rates both in and out should be kept gentle, just like a new ICE.

I suspect that's what is meant by aged?

Of course one that peak is reached, it's all downhill from there, capacity (State if Health) declines, less steeply at first, much faster after 90%, very steeply after 80%, 70% is the latest EOL I recommend.

My point was that the SoC-to-voltage curve is continuously changing during that process.

A major reason why voltage is a very rough guide to SoC.

Along with batt model differences and the recent-current in or out issue vs resting.

Same goes for specific gravity, just less dramatically.

[john61ct]

Originally Posted by rgleason

I wonder if this Lifeline chart might apply to Trojan batts too, but I'd have to find the comparable chart for flooded lead acid.

Every maker's varies, even models within their product lines.

If a group purchased a SmartGauge, they could rotate borrowing it and make up their own tables.

But they would need to be updated each year.

[rgleason]

As a battery declines in capacity, I assume the specific gravity also changes, so that a new battery at 100%SOC with 225ah and SP=1.277
over time changes to be 100%SOC (actually 90%) with 203ah and SP=1.258?

[john61ct]

When the capacity has dropped to 203AH, that is now the new 100%.

On an AH counting SoC meter, that is the number that needs regular updating to keep the SoC guesstimates as accurate as possible.

[rgleason]

Ok, but does the

[rgleason]

"...over time changes to be 100%SOC (actually 90%) with 203ah and SP=1.258?"

Ok, does SP go down to 1.258?