Capacity test battery bank

[sailingharry]

First, let me apologize for flooding the forum. This is like my 5th question this month. But busy times!


I have an 780Ah bank of high quality AGM (Northstar) of unknown age and condition. I'd like to get a handle on capacity, so I want to run them down to near zero, and see how many Ah I get. Should be easy.


My intention is plug in a nice big 110V load on the inverter, set an appropriate cutoff voltage, and go away for a day. The next day, I'll have the Ah consumed.


Ideally, I should hit a 20 hour rate (that's how they are spec'd). So, for me, that's around 40A, or 500W +/-. I have numerous resistive loads I could use, but none come close. The vast majority (space heater, portable burner (stove-like thing), George Foreman grill, coffee pot -- all consume 1000+W, which is crazy high. The space heaters (I have two different ones) do about 750 on low, which is still too much. Electric blankets, fans, cabin lights, etc all have nearly trivial loads.


I could bank a bunch of old incandescent bulbs, if I could even find enough to total to 500W. That's like 5-10 bulbs!


Anyone have any suggestions? I'd prefer to have an un-controlled device -- grills and such probably cycle which is not good.





Oh, thoughts on a low cutoff? Too high, and I risk underestimating my capacity. Too low, and I risk hurting them more than I want. I'm thinking a cutoff of 10.5V. That's theoretically below zero capacity, but that allows for internal resistance. Drawing this much current, the voltage dips pretty significantly. I suspect that when the inverter trips off at 10.5, it will recover into the mid-11V range which is around 5% capacity.

[rslifkin]

Check the Northstar datasheet for your batteries. There's a good chance it specifies end of discharge voltage at various loads, so that would tell you where you should set the cutoff.

[tkeithlu]

Your batteries are marked with CCA or other capacity. You can separate the batteries from each other. A $30 battery tester like this one

https://www.amazon.com/KONNWEI-100-2...s%2C343&sr=8-4

will tell you in a minute whether the battery will actually do that. You punch in the CCA or other number, and it does the rest.

[rslifkin]

Originally Posted by tkeithlu

Your batteries are marked with CCA or other capacity. You can separate the batteries from each other. A $30 battery tester like this one

https://www.amazon.com/KONNWEI-100-2...s%2C343&sr=8-4

will tell you in a minute whether the battery will actually do that. You punch in the CCA or other number, and it does the rest.

A short, high load test will tell you if the batteries are failing, but it doesn't accurately assess capacity loss due to age and use. You have to do a full discharge test at a known rate to determine that (by seeing how much power you actually get out of the batteries vs what they're supposed to deliver).

[PaulCrawhorn]

Low and slow is the best SoH test for a House bank.

0.1C or ten hour rate is OK, no need for twenty hour.

10.5V is the standard lowest point 0%, so very stressful only occasionally DO NOT allow it to slip lower, DO NOT let it sit there be ready to start recharging right away ideally at a high rate.

To be really accurate a CC dummy load is best but if seat of the pants reality check the way you say is OK, trusting a coulomb counting wattmeter to give your total.

[KompetentKrew]

Phone around local battery specialists, and ask if they can test them.

I agree with the previous reply that a CCA test is inadequate and that the only way to properly test batteries is measure the voltage over several hours whilst discharging with a known load. I guess this may in the past of only been done in the lab, but there is now equipment on the market that is accessible to small shops.

A couple of years ago Plymouth Battery Centre did this for me, and didn't even charge me for the service.

The tester they used appears to be made by Emrol.

[Simi 60]

Originally Posted by sailingharry

!


I have an 780Ah bank of high quality AGM (Northstar) of unknown age and condition. I'd like to get a handle on capacity, so I want to run them down to near zero, and see how many Ah I get. Should be easy.

You mean run them down to near 50%?
That is AGM zero.

[rslifkin]

Originally Posted by Simi 60

You mean run them down to near 50%?
That is AGM zero.

No. To do a proper capacity test, you have to run them down to truly dead (according to the manufacturer's end of discharge curve). You wouldn't want to do it regularly, but there's no completely accurate way to determine where 50% is while actually discharging, so you have to drain them all the way to test capacity.

[tkeithlu]

My understanding is that pocket battery testers measure internal resistance - they certainly do not produce the heat you got from an old fashioned load tester. If you REALLY want to know capacity, then I gather there are bunches of parameters to control in your testing, as in:

https://www.dv-power.com/battery-cap...d-experiences/

I'm not a battery specialist, so let me ask you a question. Is this a matter of just how much you need to know in maintaining your battery bank, or judging whether it is adequate for the task ahead?

[ScottRhodes13]

A couple heat lamps could get you 500W.

[Simi 60]

Originally Posted by rslifkin

No. To do a proper capacity test, you have to run them down to truly dead (according to the manufacturer's end of discharge curve). You wouldn't want to do it regularly, but there's no completely accurate way to determine where 50% is while actually discharging, so you have to drain them all the way to test capacity.

I considered our bank out of capacity when alarms went off @ 24.2v under load and, according to this that's around 50%

I don't know why anyone would want to run them lower, unless on a mission to kill off the bank
https://marinehowto.com/under-load-b...oltage-vs-soc/

[ikanode]

I'd take a middle approach compared to the info provided so far. Run them down with about a C20 load to the voltage where you'd disconnect them during normal operation. That will give you the routine use capacity. Isn't that what you want to know?

Somewhere around 12V ± a few tenths. Let them rest for a few hours and then check the voltage again.

[patfarrell1949]

For what its worth...I have a Balmar SG200 battery monitor which tells me state of charge and state of health. If I understand correctly state of health is a measure of current battery capacity versus design capacity. I read about 91% state of health which seems about right for 5 or 6 year old Lifeline AGMs. If so might this be an alternative means of measuring aged capacity?

[sailingharry]

Well, the test is off and running. I found an old Halogen work light that is allegedly 500W. It's only pulling 30A from the battery, so it's more like 400W, but it's still close enough to the rated 20hr discharge rate to be meaningful.


The problem with 50% is, as mentioned, there is no way to assess the results. I've had it down to around 50%, even more like 40% on a few occasions, and, well, who knows? Was I more like 20%? You can't compare a "resting voltage" that's only been resting for a few minutes to any reference table, and you can't let it rest for a few hours when you are actively cruising (and, actually, when I'm at 50 or 40%, I'm usually minutes away from getting underway!).


Krew, a professional test is surely tempting, but I hate to think what each of my 200Ah batteries weighs. North of 100lb? I'm not sure what criteria they used to determine that your battery was 133% of rated capacity. They only ran a test for 4 hours, so if they pulled it to zero, they were running 5 times the specified current (40A out of that battery to zero it in 4 hours!). If they pulled a lower current, what was the criteria for calling it done, and compared to what standard?


Really, the only way to compare the current battery capacity to the factory original is to draw it at the rated current (20 hr rate) from full to zero, and count Ah. The remaining question is the accuracy of the equipment -- indeed, pulling it by a calibrated CC test device would be ideal! But the Link2000 is certainly a reasonably accurate coulomb counter, and my 30A is reasonably close to the 37A that I should be using.


That link to Maine's page alludes to doing just this test, although he was doing a SOC vs In-use voltage article. Of course, he has the equipment to do a much more accurate test, which is why he can say that the battery in the article was originally 105Ah, but has been tested to a current capacity of 95.69 Ah, or 91% of new capacity -- exactly the information I'm looking for!


A key part of this is determining WHERE that 50% is. Currently I'm trying to stay above 50% of mfg rated Ah (nominally 50% of 800 is 400). But, if the bank is now really a 600Ah bank, I should stay above 300Ah of discharge. This, along with how far I am from a LFP upgrade, is what I'm trying to find out.



While running AGM (or even VLA) to 0 is considered undesirable, mfg data sheets show they can take a hundred or more of these without signifcant damage. I certainly don't intend to make a habit of it!


On a related note, there is an article on the internet that compared DoD to cycles and determined that the damage is overrated. The gist was that if I cycle my batteries to 25%, I get "X" cycles. If I cycle them to 50%, I get 1/2*X. If I cycle them to 75%, I get 1/3*X. So it's more a lifetime Ah delivered -- if I run them 3 times longer between recharge, I only get 1/3 the number of cycles -- but I only NEED 1/3 the number of cycles! It's not quite flat, but close.

[Scubaseas]

Originally Posted by tkeithlu

Your batteries are marked with CCA or other capacity. You can separate the batteries from each other. A $30 battery tester like this one

https://www.amazon.com/KONNWEI-100-2...s%2C343&sr=8-4

will tell you in a minute whether the battery will actually do that. You punch in the CCA or other number, and it does the rest.

Yeah, no. Especially with AGMs. Even the expensive, good quality capacitive testers ( and i owned two of them when in the auto industry) will not tell you bank capacity. Starting CCAs sort of but even then the $30 testers do not work well with AGMs even in checking crank amps. You need to do a 20a load test properly.