AGM Charging questions... voltage reading when fully charged.

[78Staff]

So, thinking about putting my Northstar AGM31 back in the boat... it's been sitting in garage for about 6-8 months (not on concrete). it was showing 12.1v still, and I hooked it up to a 440PC and let it charge today. When the light went green, the batt tested at 13.1v - does that sound correct? It's been a while, but I though AGM's would show 13.5-13.8 when fully charged? Or maybe just an effect of sitting for so long, and after a few cycles it will ramp up to expected values?

Anything else I should look at before installing, I don't want to have to pull that sucker back out lol.

[CatFan]

Your battery will measure 12.84V when fully charged. The 13.1V you are seeing is the output of the charger in float mode. Unplug the charger and let the battery sit overnight for an accurate battery measurement.

[78Staff]

Well I guess I am mis-remembering, a bit of internet research finds fully charged AGM's should show 13 - 13.1v, down to 12.8 after resting.

[78Staff]

Your battery will measure 12.84V when fully charged. The 13.1V you are seeing is the output of the charger in float mode. Unplug the charger and let the battery sit overnight for an accurate battery measurement.

Thanks, we must have posted at same time :). Charger was powered off when I tested at 13.1, but I'll check again after letting it sit tonight.

[CatFan]

Well I guess I am mis-remembering, a bit of internet research finds fully charged AGM's should show 13 - 13.1v, down to 12.8 after resting.

There is variation among AGM brands. East Penn AGMs run 12.8. Enersys, 12.84. Optimas 13.1 or even a little higher.

[78Staff]

Setting at 12.7 this am after resting overnight... I hooked it back up let it cycle through again and see if any change.

[yam911]

With blood pressure in people, there are norms and there are variances.

Some times the folks with the norms die early while those with the variances live long lives.

So too it is with batteries I suspect.

[yam911]

Decades ago we guesstimated battery voltage by the relative position of a needle on an unmarked dial. Years ago we measured to the tenth of a volt. Now it seems to the hundredth of a volt. Soon to be to the thousandth of a volt?

Is a battery with too much voltage (when compared to a "norm") to be seen as a negative as it is with blood pressure?

[bobcoy]

More than likely an error in the testing or equipment used. Nothing gets higher” Except Prices”

[CatFan]

With blood pressure in people, there are norms and there are variances.

Some times the folks with the norms die early while those with the variances live long lives.

So too it is with batteries I suspect.

Variation in battery full charge voltage always indicates problems. Voltage largely depends on the strength of the electrolyte and there is very little variation on that in a properly functioning battery, so voltage variation, even tiny amounts, points to sulfation in most cases.

Hundredths of a volt is important in estimating charge. A change of 0.10V is about a 10% change in state of charge, so a hundredth of a volt is about 1%

[78Staff]

I'm waffling on this now... I don't really need the AGM for any capacity/runtime reasons - current setup of 125ah Lithium Starter and 3x50ah for trolling is plenty of juice - I've never come close to running them down - typically I'm still above 80% at the end of the day, maybe 70% on a long day on the water.

I was just thinking I've got this damn $500 battery just sitting in the garage going to waste. So I thought, put it back in, use it for boat/motor power, dedicate the 125ah to electronics only, and also get some weight back in the a$$ end of the boat. And, to abate any nagging concerns about using Lithium as a starter for the SHO, despite the numerous reports of folks using it with no issues.

But, it would put all that weight on the drivers side, which is already "heavy" compared to passenger side despite my best efforts to put as much weight as possible in the port lockers lol. Plus I would need to either add a combiner or switch to a 5 bank charger. Just more cost and complexity that really isn't needed... Plus it is a colossal pita to get it in the bilge, due to Skeeter's small hatch and transom transfer brackets. I can't even lift/maneuver it by myself, and would be paying shop to do it, since I am getting some other stuff done there Friday.

So, I am basically trying to talk myself out of it I guess lol...