Hello battery experts and fellow nerds :CI have noticed small voltage differences between the three Odyssey PC2150 batteries that make up my "bank". This is after resting with zero load for 24 hours following a 14.7V charge (in parallel) to 100% SOC (charge rate dropped to .001C).The battery voltages were: a) 12.88V b) 12.92V and c) 12.96V. All measured within seconds of one another with the same calibrated digital volt meter.All of the batteries are the same temperature (~70ºF), same age (mfr'd in Aug 2010), and have the same use/abuse history. Wiring is 2/0 throughout and is closely balanced. The three batteries are almost always charged/discharged together in parallel (not individually or in pairs). When tested separately, I have not been able to measure any other differences in any one battery's performance compared with the other two (ie. all three batteries appear to have similar capacity & voltage sag under load). Unfortunately, I never observed/noted the voltages when they were new.So my questions are: What is the cause of this voltage difference? Is this a "normal" production tolerance? Is it caused by variations in specific gravity during production or from aging? Is it indicative of the batteries' "health"? Should I continue observe this over time or just ignore it?I'm definitely not concerned about the voltage differences... I don't plan to change or replace anything in the electrical system, and if a problem ever arose I can isolate any one (or two) with the flip of a switch. Mostly I just want to better understand what's going on electro-chemically.Thoughts?Thanx in advance (for humoring me), -Mark

My vote: NormalAnd since they're AGMs, I'm not sure specific gravity comes much into play.

I would think that between three batteries an 80mV difference would be within production tolerances. That difference could even be attributed to the difference in resistances related to the wiring that connects the batteries. If you assume the the 12.92V battery is the norm the other two are only off by 40mV. I would be more concerned at how they measure after being discharged for a day or so. That would give you a better idea if there is any real difference between the three.

Balancing three batts in parallel is tricky. 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?On self-discharge, loss of surface charge, are the battery tops equally clean?

Hi Mark,Are they wired as method #3 on the smart gauge page?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.If charging separately doesn't do the deed then you might consider a mild equalization or "recovery" process. Have you done an equalization in the last three years?

Hi Mark;You're feeling guilty for not having anything else to worry about like when you had wets. You will get through this rough patch in time. Try using AAAAA-GEEEEEE-EMMMMMM as a mantra you can get past those anxiety symptoms of having nothing to worry about. :)HTH;John

Battery Voltage Discrepancies ?? | Good Sam Community

33 Replies

msiminoff
Explorer II
Oct 23, 2013
bigfootford wrote:
Over the years of testing batteries and converters I have encountered 2 bad battery cutoff switches...

Yes, this is a good point and an interesting topic!
It may be the case that some battery disconnects are so poorly built that they cannot provide a low current connection, but I suspect that there are also a lot of 'em out there that have been damaged due to the arcing (and resulting pitting & carbon build-up) that occurs when the contacts are initially closed. This would be especially true for people (like me) who have large inverters with capacitors that get charged every time the power disconnect switch is turned on.

To prevent unnecessary wear on my battery disconnect switches and on my inverter too, I installed "pre-charge" button that allows the cap's in the inverter to charge slowly and (theoretically) eliminates arcing of the switch contacts.

-Mark
mena661
Explorer
Oct 23, 2013
BFL13 wrote:
Thanks for the complete diagram. Always something to learn from how others do theirs.
X2
BFL13
Explorer II
Oct 22, 2013
Thanks for the complete diagram. Always something to learn from how others do theirs.
bigfootford
Nomad III
Oct 22, 2013
Over the years of testing batteries and converters I have encountered 2 bad battery cutoff switches... Very subtle.
I was measuring converter to battery evaluating the overall resistance. wire/shunt/switch/fuse etc
I found a switch creating a +.015 ohm drop. I was shooting for less than a .020 ohm drop for everything... (dc wire run round trip of 24ft.)

Switch was causing the drop.

Although I do not feel this small of a drop could cause your issue a bigger drop just might.

Jim
MEXICOWANDERER
Explorer
Oct 22, 2013
If I "point her into the wind", she'll shrivel.

Lucky for me the kids live on a promontory. Cell phones are down. I am 900 miles to the northwest of Las Peñas, the sky is blue and I feel guilty. I have GOT to get this medical stuff done and go home.
mena661
Explorer
Oct 22, 2013
Lol @ KP
KendallP
Explorer
Oct 22, 2013
MEXICOWANDERER wrote:
...Hurricane Raymundo is now hammering my little pueblo...

POINT 'ER INTO THE WIND, GRAMPS!!!

HURRY!!!

POINT 'ER INTO THE WIND!!!
MEXICOWANDERER
Explorer
Oct 22, 2013
I guess when I get home I should take some images of my "Umbilical" setup. Hurricane Raymundo is now hammering my little pueblo. Good thing the inverter is stuffed inside a 5mm plastic sack and double zip tied. The generator has a pair of HF silver tarps over it and a ratchet type semi nylon web strap cinched down around the base as a belt.

The 2-volt cells could care less. The steel cases were hot dipped galvanized before the cells were placed in them.

Anyway, nice images. Always a pleasure to see things done correctly. Thank you.
msiminoff
Explorer II
Oct 22, 2013
Updated schematic with solar...
KendallP
Explorer
Oct 02, 2013
KendallP wrote:
The only problem that I see is the one that arises when you do successive 50-90% charges without topping.

msiminoff wrote:
Like I said, they get a daily charge to something close to 100% SOC. In daily cycling I'm typically in the 90-100% range, at least according to the TriMetric. Alas, I have not "topped" them above 14.7 (at least not until today).

Well if you have enough sunshine all year long or run the generator for several hours when you don't... and you never, ever fail to charge to 100% after each and every draw... and you never will... then the information I provided is of no value to you.

That's a nice looking setup you have there. :)