Different SG in each of the batteries in my bank

GRWeldon · February 2017

Good day all. I too new to really know the answer to my question, but I'm sure one of y'all will rescue me.

I finally purchased two Deka 9C12 12-volt scrubber batteries, 228AH capacity. I'm wiring them in series for a 24V bank. I've got my Midnight classic 150 set to the charge specs that Deka recommends. I DID NOT check the specific gravity or voltage of either battery before I wired it up.

I checked specific gravity today after 5 days of charging. One battery was 1.235 in all the cells, the other was 1.175 in all the cells. I assumed (possibly incorrectly) that when charged that they eventually reach the same voltage and same specific gravity. I also noticed that when there was very little sun present to my PV panels and the charge current was low (1-2 amps) that the battery voltage dropped quickly, say from 27 volts to 25.9 in under 10 minutes, This is with zero loads attached. The inverter is disconnected from the battery.

I'm thinking that I may possibly have a bad battery. Here's what I intend to do unless advised differently...

I can use my automotive 12V smart charger (10 amps max) to charge each battery individually and then check SG. If all is OK, I can rewire in series and attach to controller and observe what happens.

OR, I can change the voltage on the charge controller and charge each battery separately bringing them up to full, then reconnect them in series and reattach to the charge controller and observe what happens. The problem with this is that we are having partly cloudy days for the next few days and I'd really like to have a steady charge applied.

I need to find out soon if I have a bad battery so I can exchange it for another one. Any advice that is given would be greatly appreciated!

Thanks,
Glen

mcgivor · February 2017

Sounds to me that your array may not be producing enough to fully charge, too small or not enough sun. With new batteries you would need to do a commissioning charge, according to manufactures specifications which may include an equalization charge to bring the SG to required levels, balancing cell voltages. This requires the charge voltage to be in the 30 +V for a 24V bank, so if the array output is insufficient, it may not be able to achieve this. Using an automotive charger to bring then to a fully charged state, may help but keep in mind it depends on the condition of the battery, are they new and of equal age? The automotive charger will generally not be able to equalize.

More details regarding your system would be helpful, array size, specifications panel arrangements, orientation, angle , parallel series etc. to determine if the configuration is correct.

BB. · February 2017

It does sound like you have a failed cell--Especially if this is a new installation. The specific gravity for that one cell is roughly 40% state of charge--You do not want a battery to have most cells at 100% and one at 40% when new--That is not good (low SG cells that sit for days/weeks (and months) become sulfated and ruined.

As you have learned, take voltage and specific gravity readings (logged, temperature corrected) of your batteries before you put them into service.

-Bill

mcgivor · February 2017

Think Bill might have misconstrued what you described, all cells are equally low in both batteries, not one cell, correct?

GRWeldon · February 2017

Thank you everyone for your replies. Reading them, I think the first answer by "mcgivor" is probably correct. Bill, all my cell SGs are low on the one battery, not just one cell.

As for details, for the moment, I have (3) 250W panels but they are leaning against the side of my building as I haven't yet mounted them on the roof. They are angled about 20deg. from vertical and are almost perpendicular to the sun path at this time of year and they are facing due south. They are series wired producing about 8A and 100V in full sun.They do not get full sun all day long and it's been partly cloudy on and off here since I've had the panels up. I actually cut down an 18" dia. pine yesterday to remove some of the shading. I'll be mounting them on the roof this weekend and that will eliminate all but the late-day shading. Eventually, that will be taken care of as well.

I have yet to buy a charger for my generator but it looks like I'll be doing it sooner than expected. I'll proceed with charging the batteries one at a time with the automotive smart-charger I have and I'll check the SG before I put them back on the Midnight Classic 150.

If anybody has anything to add, I'm all ears...er...eyes...

Thanks,
Glen

Estragon · February 2017

I would definitely use the auto charger to charge individually, starting with the low SG battery. At 10A it may take quite a while (like overnight). Once nearly charged I would reconfigure the controller for 12v and do an equalize cycle. It doesn't take a lot of current to do the EQ so light cloud may still work. Run the EQ until SG stops rising. As you check SG also watch water levels and add if needed to ensure plates are covered, but not so full that bubbling will overflow.

While doing the EQ on the low battery, charge the other one with the AC charger.

My guess is the batteries just need a proper charge, but monitor battery temps during the process just in case.

BB. · February 2017

I would still want to know why that one battery (all cells) has such a low state of charge. Did it come that way? Did you use the battery for a while and not recharge it?

If they were simply wired up in series and the one battery is that low of SG--Then either it was old stock or has some other issue.

I would suggest replacing it... But charging it--But charging the one battery with an automotive charger to bring it up to the same state of charge of the other battery would be the next step if return is not possible (or your did discharge the battery prior to placing in series string). Recharging the other battery to full would not be bad either (start with both batteries at the same state of charge--Equalization is basically "over charging" some cells to get charging current to others--When they are 20% or more SoC apart--something is usually not right).

-Bill

Estragon · February 2017

Unfortunately OP said he didn't check voltage or SG when he got the batteries, so we can't know if he got them partly discharged. Seeing how they take to being properly charged individually might offer a few clues?

mvas · February 2017

Did you check the DATE CODES ?
You do not want to connect an old battery in series with a new battery.
Before you connect batteries in series, both batteries should (must) be at the same State-Of-Charge.
I would not have purchased a battery that has been sitting at SG 1.175 = Discharged & Sulfated.

Different SG in each of the batteries in my bank

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