agm capacity test

Yes 490 ah total

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 :)

25a is not necessarily the 20 hr rate, so my test method would not work unless it is the 20hr rate. I missed whether your bank is 245 or 490. If 490, the 20 hr rate is 24.5 so yes 25a then would be right. (say eight 3a Rv lights on as your load being close enough to start with)

Another test is to get it to 50%, leave it rest to confirm 50 by voltage, take your AH reading on the monitor, then recharge till the batteries will not take any more amps (being full) now take the AH.

Tricky bit is how many AH to allow for heat loss? Trimetric uses 4% so that much is already counted in. Your monitor may use a different amount or none. Somewhere around 5% or even 10 could be it--no idea with AGMs. Anyway, allow for heat loss and that gives you your AH for the recharge, then double that to get the capacity.

Yes, neg off one batt pos off the other. Equal length 1 ga parallel jumpers. I believe my fluke only goes to 10a.

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)?

full_mosey wrote:
IMHO, all AGMs are hybrids. You're not going to get that true deep cycle ratio as with deep cycle wets.

HTH;
John

Hi buford,

Agm's are pretty good on shelf life, so I'm told. Some literature says charge once every six months. I doubt that there is much capacity loss as it sounds as if they were never cycled at all in the 18 months.

25 amp draw at 25C (77f) is what is used for reserve capacity rating in minutes. It is actually a better measurement to use than amp-hours at the 20 hour rate--but rv'ers are none to flexible about changing how they measure things. (me, too lol).

The victron gauge does do a peukert calculation to adjust its readings. If there is a way to disable that it may be a good idea for the purposes of testing.

Do you have access to an amp meter that can measure up to 25 amps?

Are they wired in a balanced manner?

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 :)

I guess it would have been best for me to give all the info on batts. They were 18 mo old when i purchased them. 12.40 resting voltage. 150ea + tax so 160ea. Charged up right away and they hold 12.85 for days. Installed about a month ago, used about 10 days dry camping so far 180-230ah a day.

They were new unused batts, but due to their rough start in life I would like to test them and get a baseline of what I've got.

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.

Now I need a 15amp load to run for 10 hours to get the batts down to 50%. This is not so easy but you can get "close enough"

A. turn on a bunch of lights till the monitor says 15a. As battery voltage falls lights dim and amps fall, so keep turning on more lights to keep amps up near 15.

B. using an inverter with enough 120v load to make it draw 15a. now amps will slowly rise as battery voltage drops. So start with say 14a and let it get to 16a by the ten hour point to average 15.

When to stop? You might hit 50% at nine hours, indicating your batts are not at rated capacity, but at 90% of that. Now you need that monitor to show what AH you are down.

If you go to half the rated AH and stop you might be at 40% if the batts are not up to rated AH, so you don't want to overshoot. (with Wets you can spot check the SG to see 50% that way too) so you have to stop "early" and wait a bit to see what the voltage does. it will come back up but you don't want it to come back over 12.2 or whatever it is for AGMs at 50%. If it does, start up again and try later, till you get it right.

If you do this test a few times you get a good "feel" for when you are close to 50% by how fast the voltage comes back up after stopping. also you can tell by the loaded voltage as it falls. eg, at the 20 hr rate , when voltage falls to about 11.5 you are there with Wets--it might be higher with AGMs.

Once you are at 50% (estimated) then take your AH amount that you are down, times two, and that's your capacity. Compare that with the time it took wrt the ten hours for a cross-check.

pianotuna wrote:
Hi,

Nice battery bank. Are they wired in a balanced manner? How many watts of solar do you keep them fed with?

Capacity increases until 25 to 50 cycles happen. I've never been able to find out if those are deep cycles or shallow ones.

Here's a SWAG method to compare batteries to find the one with the best deep cycle-ness. Divide CCA by 20hr AH.

Here are some examples in order of worst to best for deep cycle service:
Optima grp34/78 - 750/55 = 13.6
Sears P-4 grp 34/78 - 850/68 = 12.5
Johnson Controls grp 31 - 825/100 = 8.3 <- not bad for $180
Deka 8a24m - 525/79 = 6.6
Deka 8a8d - 1450/245 = 5.9
Lifeline ????

IMHO, all AGMs are hybrids. You're not going to get that true deep cycle ratio as with deep cycle wets.

HTH;
John

Yes balanced, 710w 60a xantrex mppt. Thanks john