Do I have a bad battery?

Hi folks,

My solar system is acting really weird. I got 8 seal lead-acid 12v 260ah batteries (2 groups of 4 for a 48v system) in order to have a good safety buffer for cloudy days. It turns out the performance of these batteries is like 10% of what is advertised. I'm not sure if one of them might be faulty and is harming the whole system.

I tried measuring each one with a multimeter, but the only difference in voltage I get in a particular battery is only of 0.05 - 0.1V. Is this significant? All others have only a 0.01V difference (more or less). Of course this is with load and everything still connected together.

Is it worth to unplug everything and measure each one disconnected?

If not, any tips of what my problem might be? The batteries have only 2 months of use and they were brand new... In sunny days I get them all charged up, but then next morning, without barely any load at night (1kwh total max) they go down to like 60%... :(

Nuno Donato wrote:

Is it worth to unplug everything and measure each one disconnected?

That is the best way to test a battery and know for sure. I don't have solar, but interestingly in the last five years I have had three lead acid batteries with a bad cell. Two of them were a small sealed kind for an electric fence, and one was a car battery with ports on the top to add water to. I personally would not be surprised of one of those batteries has a bad cell, and it will indeed affect the function of all of them as a group wired in series.

You should do a specific gravity measurement of each cell if you can access the cells individually. That is how we would diagnose once and for all a bad cell. Otherwise disconnect each 12 volt battery from each other, apply a 12 volt load to each of roughly 120 watts for 5 minutes or so and take a voltage reading. the One with a bad cell will immediately drop by 2 volts or more...
Best I can offer

I prefer a refractometer to a specific gravity meter but yes everything mentioned in the video is spot on. If you purchase a specific gravity meter get the dial type as shown in the video not the floating glass ball type which are more common but not as accurate. I mentioned the load testing because usually the 12 volt batteries have a cap over the cells and are not easy to access the cell liquid. Most of the time the cap does pop off though but might get damaged in the process.

ah, forgot to mention that I have SEALED lead-acid... so no checking levels or adding water :)

Then a load test of each battery is the way to go. Check your absorb voltage on the charge controller carefully. Too high an absorb voltage will damage a sealed lead acid battery...
Cheers,  David

Nuno,

What is it that you leave on all night ?

Just to check : 1kwh = 1000 watts used in one hour, or 100 watts used for 10 hours. Is that what you meant to say?

Irene Kightley wrote:Nuno,

What is it that you leave on all night ?

Just to check : 1kwh = 1000 watts used in one hour, or 100 watts used for 10 hours. Is that what you meant to say?

yes.
Basically, fridge (about 35w), UV light for water purification (25w), a few standby things and of course the inverter standby consumption (which I cant see but from my research I estimate to be around 40w). So around 100watts per hour for 10 hours

David Baillie wrote:Then a load test of each battery is the way to go.

I have a bad battery on a lawn mower and when I used an multimeter it showed 12 volts. I then tested the battery with a load test and saw it drop to 9 volts. So I got a new battery. Are you keeping your batteries protected from the weather and temperature changes?

Nuno,
Have you determined the cause of below capacity batteries?? What test did you use? .
 Yes, very low temperatures can reduce the capacity but can increase battery life. High temperatures can increase capacity and reduce battery life. Thanks and hope you didn’t have a bad two month old battery.