Battery Voltage Discrepancies ??

KendallP wrote:
since they're AGMs, I'm not sure specific gravity comes much into play.

Of course specific gravity comes in to play! But because they are sealed and the acid is absorbed inside the batteries, the SG cannot be (easily) measured. IMO, this is a significant downside of AGM batteries.

BFL13 wrote:
Can you relate which batt was where in the arrangement to the different voltages? Do you shuffle batteries in the bank from time to time to "even them out over time" from any minor balancing issues?

No I have never "shuffled" the batteries.. it's an interesting idea though!
They are located in the basement of my TC directly under the battery disconnect switches and inverter/charger (they are only separated by the thickness of the floor). I am a big fan of keeping power wiring simple and as balanced as possible, so the cable lengths are all within an inch or two of one another and even at max current** the voltage loss calculation is less than 1% (yes, I know there are aditional losses at each interconnection). The temperatures should be very consistent, although I only have temp' sensors (three of them!) mounted to one of the batteries.

The positive leads from each battery are connected to one terminal of a high-current (300A con't) disconnect switch with a short (<30") and ~equal length of 2/0 marine-grade cable. Again, each battery has its own switch. The 2nd terminal of the each of the disconnect switches is fastened directly to a common (1/4" thick, 6" long) solid copper bus-bar. There is a 16" length of 4/0 cable between the bus-bar and the inverter/charger. I'll add that there is a 225A "Mega fuse" mounted (with a short solid copper strap) to the positive terminal of each battery.

The wiring from the negative terminals is also a <30" length of 2/0 cable, these are fastened to a common binding post and from the binding post to the inverter/charger is a short length of 4/0.

Are there differences in the resistance of the wiring + connectors? Sure there are! But the resistance value is extremely low... far lower than I can measure with my Fluke 88.

** The theoretical max current IF I were to use the inverter at its rated power output of 2000W is ~180A... so that's 60A from each battery. The maximum current I have actually seen is 155A discharging and 110A charging.

On self-discharge, loss of surface charge, are the battery tops equally clean?

Yes, clean as a whistle and absolutely dry!

pianotuna wrote:
Are they wired as method #3 on the smart gauge page?

Yes, just like #3! Except that at the point where the positive leads come together there are three disconnect switches connected together with the copper bus-bar

I would separate them and charge each one fully--then retest those voltages. It is a small voltage difference--one cell could be slightly weaker than the others.

Sure, that's a simple enough test. Will give it a shot.
In terms of "weaker"... I suspect that this is related to differences in SG, so I guess low specific gravity could be described as weakness.

Have you done an equalization in the last three years?

No. I am charging exactly according to Odyssey's charge protocol. They do not recommend equalization.

full_mosey wrote:
You're feeling guilty for not having anything else to worry about like when you had wets.

Ha-ha-ha! I do like the fact that I never "worry" about my batteries the way I did with my 6V FLA's.
Like I said earlier, I'm a nerd and I'm just trying to get educated about what's going on

Parting shot: If I had a good constant current discharge load I could do a test of the batteries separately and log the results using the Trimetric's data stream... Oh goodness, I don't think I have time for that project :B

Thanks all!!
-Mark