New battery for ups. Different voltages.

vicens

100 W
Joined
Aug 27, 2015
Messages
108
Location
Barcelona. Spain.
I have bought two new batteries for my ups.

The batteries are 12V 7 Ah lead technology agm. The batteries are connected in series. The batteries are from the same date of manufacture.

The batteries came charged at 12.82V and 12.85V. I have charged the batteries separately with a three stage charger. The second stage at 14.4V. The charger gives the option of charging at 14.7V.

Once the batteries are charged, I have mounted them on the ups. When connecting the batteries to the ups, the voltages are 14.4V and 13.7V for each battery. After two hours the voltage of each battery drops to 13.7V and 13.5V.

Should I worry about the imbalance of the two batteries?

Will the imbalance of the two batteries disappear in the next few days?
 
Depends on the batteries and why they are imbalanced. Each one has six 2v cells in it, so if any of those is not working correctly / fully for any reason, you'll see a difference in that battery vs another that has all six working right.

With SLA you don't have the option to open up and balance individual cells, so once they start to unbalance, that's just how they're going to be. (even if you did have the option, there is already a problem with a cell, so it will just get worse over time just like any other kind of cell).


If the entire battery is balanced, but just not fully charged (takes longer for "absorption stage" than the other one), then it might finish charging if it's left on longer.

If you connect the two batteries in parallel and put them on that 3 stage charger, they'll be at the same voltage the whole time; this doesn't mean they will remain that way when done, but it is something to try. If they change once separated, then one of them is internally physically different from the other, for whatever reason. (manufacturing, aging, handling, environment, etc).


The imbalance just means that one of them has lower capacity than the other, by at least the difference in voltage on their discharge curve chart from the manufacturer (if they provide one). Like the way this one works for a random 18650 cell:
Vapcell%20INR18650%202900mAh%20K29%20(Purple)%202021-Capacity[1].png
where a cell that starts at 4v vs one that starts at 4.15v has half an amphour less of capacity even at the same current draw.
 
I have turned off the ups, I have disconnected the battery, I have let the battery rest for an hour.

The two batteries have measured 13.33V 13.32V.

I have connected the batteries to the ups, I have plugged in the ups and the batteries have measured 13.7V 14.4V. The battery with a higher voltage is the battery that a few days ago had the lowest voltage. At the moment the battery voltage of 14.4V has started to drop.

Thirty minutes later I have measured 14.05V 14.03V

Two hours later it has measured 13.66V 13.56V

It seems that the batteries have been balanced.
 
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