agm capacity test

Great stuff Mex

pianotuna wrote:
Hi John,

I read the words but fail to comprehend. Do you mean they should not be cycled to 50% (I had heard 80% was, for some brands, safe)?

When I first read that Mr. Musical Fish, I thought "What is.. " turns out I'm not the only John here. :).

In my post: I assumed he had gotten DEEP CYCLE AGM's. deep cycle batteries are farily common in the larger sizes (Like 8D).

50 percent for Deep cycle.. 75-80 for all the others, that is the rule I follow... Which is one of the reasons I'm shopping for six volt within the next two weeks (in 2 weeks I turn the key again and drive this house..... I may drive it past the battery store rather than lug those puppies myself)

Take the battery readings INDIVIDUALLY

Use a 8 ampere constant load. A 100 watt 12 volt light bulb works perfect for this type of test. 24.00 hour test. Time of connection is important. Try to disconnect to the minute at 24 hours on-time.

Let battery rest for a minimum of 12 hours. Then take a voltage reading.

Are both battery at rest voltage, say with .10 of one another?

This will give you an excellent COMPARATIVE reference point for future use.

Yes, it discharges the battery below 50% capacity. No a test or three will not "hurt" an absorbed glass mat battery.

Don't bother tying to play battery test lab, the protocol is intensely complicated and you'll need to sell your house to finance purchase of instrumentation.

This is a perfect "IS/WAS" comparison.

A pair of 50 watt 12 volt lamps can be substituted for the single 100 watt.

Someone here does 80% DOD on his T1275's routinely. Cant remember how old his bank is though. Trojan says you can do that with reduced cycle life but i think it's more than 200.

pianotuna wrote:
Hi John,

I read the words but fail to comprehend. Do you mean they should not be cycled to 50% (I had heard 80% was, for some brands, safe)?

Sometimes it can be economical to use 80% DOD. A weekend camper could size the bank so that 80% is used on each dry camping outing with recharging delayed until returning.

With a reduced life of 200 cycles, one could camp EVERY weekend for 4 years before reducing the battery capacity enough to call for replacement. That is how I would maximize the R in RVing.

If the goal is to maximize bank life, then use all available space/weight, draw down 5% per day, charge often, and make the bank last 12yrs. :)

HTH;
John

Way back we had threads about whether a load tester (as used for starting batteries) would be a valid test for deep cycle batteries. Some people said no, you had to do the 20 hr rate test to 50% at the 20 hr rate as I have described above.

Mex refers to a load test but wants a "real one" not the garage typical load tester they have.

I don't remember that this ever got resolved, but I am happy with the ten hour load test method, having done it several times and have confidence in it by subsequent results and lots of cross-checking.

BFL13 wrote:
My test is to first make sure the batts are full.

Then I get the 20hour rate by dividing the rated capacity by 20, so EG 300AH would be 15amps. Reduce the est full capacity for really cold temps, but getting the temp correction right is dodgy since some of that is supposed to be internal battery temp, not ambient.

Good idea, wrong EG.. (Should have used 500) He hs 490 amp hours of battery there (Very good Original poster,, nice package) so divided by 20 that is 24.5 amps..

A 500 amp load tester MIGHT be able to do that for 10 hours. but.. Frankly Id reather use light bulbs (About a dozen 1141's and trim from there)

Measure the voltage hourly.. You should hit 50 Percent just about 10 hours into the test.

Good point about testing them individually in case one is much better than the other. I had the same issue with my "previously enjoyed" T-1275s when I got them.

I tested them as above together as a pair, but have not tested them individually, since I use them as a pair, so that is what I want to know.

However, I also have their individual SGs per cell, so I can tell that way that, while one is better than the other, it is not by that much. You can't get SGs with AGMs. OTOH the OP did let them sit after a recharge and they both held their voltages ok.

Also good point about loaded voltages where AGMs hold up under load more than Wets do. You will need to find your own values for that doing the ten hour test, so you can tell if things aren't "right" in the future doing a test. (There is a bit of "art" amongst the "science")

Even if not doing 50-90s but are on solar, you can still be comparing voltages with AH at any SOC and see if it "comes out right." IAW you are always testing for capacity if you don't mind doing a little math on a scrap of paper and use a calculator. (IE without using the bogus % SOC on the monitor just the accumulated AHs and voltage.)

On that, no idea with Victron, but with Trimetric on solar you have to turn off the auto reset of the AH accumulation or else it will reset to zero as soon as amps go negative when it gets dark and that will mess you up.

big buford wrote:
Is it possible to to use say 25a draw until victron registers 245 ah consumed 50%, let batts rest for x amount of time and get voltage reading ? Thank you bfl I will study your post :)

Yes, that will be a valid method to discharge to 50%. You would be better to separate them and run the test on each battery for 10hrs at 12.5A. Per the pdf I linked earlier, you will see that 12.30V represents 50% state of discharge(SOD).

The battery(s) will only need a few minutes, say 5 minutes to rebound, because 12.5A is not going to cause any significant drop during the test. Do the same for each battery. Hopefully, each battery will have the same readings. Paralleling uneven batteries can also lead to a shorter life for the bank.

Now, on the ? re discharging to 50% or 80%, the answer is in expected battery life. Deeper discharge/recharge cycles shorten life. OTOH, which % gives more total Watthours over the life of the bank?

My view is that batteries are consumables. I will deliberately use a smaller bank and work it harder. Does it cost more in the long run if I use 2 x half size banks over the same time as with the larger bank? Others max out available space/weight and shoot for longer life.

HTH;
John

You want cross-checking for any testing, so besides cross-checking voltage against AH in any one test, you can do the recharge test above after you did the "ten hour" load test and see if you get about the same answer.

I find it all works out pretty well. When camping doing 50-90s, I can tell from the "morning voltage" 12.1 that I should be at "50%" and I can compare that with AH down from estimated capacity (temp adjusted) showing on the Trimetric. ( I don't use the Tri's % SOC dial as I believe it is bogus-being based on a capacity that is always changing with temperature.)

Then when I do the 50-90 I can check that for AH and voltages such as when it goes from constant amps and starts tapering as a marker and then when it gets down to 5amps per 110AH worth of capacity being near 90% SOC as a marker.

These all sort of match up and I can tell if anything is off-kilter. If anything doesn't match I can go looking for what's gone wrong--maybe a light left on outside or whatever.