Sealed Battery Testing

[Paul_NJ]

I have 5 solar installations in remote villages of Malawi, Africa, with a combined total of 25 sealed AGM 12v batteries. All the batteries are the same. I need a means to test their condition and SOC when I go back each year for several weeks. Obviously a hydrometer is not an option. I’d appreciate any suggestions of an instrument I can purchase to give me some indication of battery health.

My solar supplier suggested a few: Global Energy Innovations EC2000, Alber CRT-400, Midtronics Celltron Ultra, but $5000+ for a test instrument is not in the budget for humanitarian venture.

Suggestions are most welcome!

Thanks

[Mike90250]

a simple voltmeter, test an idle battery. Or maybe the charge controller has logging you can query with a computer?

[Paul_NJ]

a simple voltmeter, test an idle battery. Or maybe the charge controller has logging you can query with a computer?

Each center has identical controls, and 3-4 batteries wired in parallel. So the battery bank voltage can be monitored by the local operator. But some batteries have been in service for 3-4 years, and I'm getting reports that in some cases there seems to be shortened operations at night. We actually ship batteries to Africa, as quality there is a problem. So getting some data on individual battery health is important as we have to plan well in advance.

I didn't think I could get useful battery health insight with just a voltmeter . . .

[Mike90250]

The only other way is a dumb load test, Fully charge the battery, and apply a heavy load, to draw it down. Each battery has a different load/voltage drop curve, but the Mfg's seldom share them. Compare the battery performance to the curve, and it either passes, or fails.

Sealed batteries in heavy cycle use are only good 2 or 3 years, just replace as reports of "dud's" come in.

[littleharbor]

As you cycle in new batteries maybe you can go with a pair of series wired 6 volt AGM L-16's instead. You would get similar capacity as the 4 12 volt batteries in parallel without the uneven charging problems. You ought to get longer life from your bank with this configuration.

[Paul_NJ]

The only other way is a dumb load test, Fully charge the battery, and apply a heavy load, to draw it down. Each battery has a different load/voltage drop curve, but the Mfg's seldom share them. Compare the battery performance to the curve, and it either passes, or fails.

Sealed batteries in heavy cycle use are only good 2 or 3 years, just replace as reports of "dud's" come in.

OK, so I need to characterize batteries I have in the field. They are all (C/100) C = 116 a-h sealed 12v AGMs. I could rig up a dummy load @ C/5 of 275 watts and record measured volts vs time, but that could take 6 hours to drop the voltage to 10.5 volts, and I'm not in-country long enough, with tons of other projects to run, to do that for 25 batteries. Let's say for each battery I just record voltage vs time for an hour. Would comparing 1 hour plots give me sufficient insight to gauge battery condition? I can drag a datalogger and laptop with me to automate plots . . . I just can't justify spending fundraised 2500 US$ for a tester. Not sure what the load/voltage drop curve you're referring to is . . . same as voltage vs time @ constant load?

Would appreciate some practical ideas - I've got some time to cobble together a test apparatus before I sit on a plane for 18 hours.

[Sunking]

I think I can help you out, but first let's clear up a few things.

If all you want to know is the SOC is easy. The battery just needs to be open circuit and rested. Measure the voltage and use a table to convert. Easy peasy. Although using voltage is not as accurate as a hydrometer, it still gives you the exact same information if done correctly. A hydrometer is just more accurate and can be done in teal time on an operating system. But let's be clear the SOC wether by voltage or hydrometer only tells you the SOC and has nothing to do with capacity or battery health. Only an indicator of the SOC and nothing more. I can give you a battery that measure specific gravity and voltages perfectly balanced and indicates 100% and is a POS dead arse battery. Put a load on it will die within minutes.

To determine the real state of health and capacity require two more test. One is a Capacity Discharge test where you measure the actual Amp Hour capacity. As you already know takes time, anywhere from 2 to 20 hours and some special equipment to do accurately and manufactures conversion charts which you likely do not have access to or knowledge to use. The other test is measuring the Internal Resistance and is about the only test you can do with sealed batteries other than capacity. Drawback is you need a baseline Ri when the battery was new to compare it too.

OK having said. that there is a simple test you can do to get an idea if the battery is good or not. A simple LOAD TEST. That is where you put a significant load on the battery for a short period of time and watch the battery voltage. If they are 12 volt batteries, just buy a battery load test meter at any good auto parts store. They are nothing more than a heating element with a volt meter. Ideally with an AGM you want to draw a C/4 load. So on a 100 AH battery would be 25 amps. You measure the open circuit voltage, then apply the load for 30 seconds and observe the voltage.

Assuming the battery is fully charged and still has good capacity, the voltage should drop no more than 3% initially and at the end of 30 seconds should not drop more than an additional 1%. So if at the start you measure 12.6 volts open circuit, you apply the load it should not drop below 12.2 volts, and after 30 seconds should not drop below 12.1 volts. Release th eload, wait 5 minutes and measure the open circuit voltage again and it should be roughly what you started out with of 12.6 volts.

If the voltage crashes say below 12 volts when you start, and drops even lower after 30 seconds, then you know it is gone. After it rest the voltage will still recover to what you started with. Why you ask? Goood question. Because the battery Ri has significantly increased with age and is shot. It means the plates and grid have accumulated lead sulfate crystals, or the same plates and grids have eroded and internal resistance has greatly increased.

Pb batteries have 2 failure modes. Lead Sulfate accounts for 95% of all batteries (chronic under charging and age), and corrosion from over charging and letting water levels get low make up the other 5%. One exception is car batteries. Most SLI batteries die from corrosion because alternator voltage regulators run a bit high.

IMO and other professionals like the manufactures prefer you to run on the corrosion side of the knife edge. Reason is simple batteries last longer if slightly over charged vs under charged. It is impossible to stay balanced on a knife edge for any meaningful time. You have to choose which side to error on. Pros choose the longer life side of slightly over charged.

[Paul_NJ]

Sunking, thanks so much for your insight and for writing that up! That is the sort of practical approach I was hoping to learn, so I can find out where I stand in the systems we have operating.

[Mike90250]

I think I can help you out, but first let's clear up a few things.

If all you want to know is the SOC is easy. The battery just needs to be open circuit and rested. Measure the voltage and use a table to convert. Easy peasy. ........

Thank you Sunking, better said than I could do.