# Flooded lead acid batteries in series and parallel - voltage drop at negative terminal

This question is a continuation of my previous question : What is the best way to combine multiple batteries (flooded lead acid)?

I have a battery bank of four 150 Ah 12 V flooded lead acid batteries connected in series and then parallel to achieve 24V 300 AH capacity. The batteries are charged by solar panels in the day and used to power connected load of approx 350 Watts at 230 V AC, through a 1.5 KVA 24 V inverter. The batteries are charged to 25.8 V by evening.

The inverter usually trips after about 8 hours of operation due to low battery. My problem is that at the time of tripping, the voltage in the (both) batteries to which the negative terminal of the inverter connected is 8.5V and that of the batteries where positive terminal is connected is 12.5 V. The inverter trip at 10.5 V which is the average of both values. The cables interconnecting batteries are of same size. I tested various combinations to see whether any particular battery is causing the drop but not found any thing abnormal. I Think this is highly abnormal. How can voltage of a 12 V battery can go as low as 8.5V ? The batteries where the positive of inverter is connected are seem to be in full power even as the inverter trips.why no power is drawn from the positive connected batteries ? What may be causing this sharp drop ?. How can I prevent such sharp drop in these batteries?.

• It seems you have a battery balance problem. Get a bulb & hose to measure and record specific gravity of every cell. when cutout and full charge and report back along with battery age. Commented May 9, 2019 at 11:08
• When you have any slight differences in 4 batteries, the weakest cell is aged the fastest by accelerated aging unless you have an auto-balancer Commented May 9, 2019 at 11:10
• Possibility of difference in specific gravity was checked. Specific Gravity is more or less the same in all batteries. More over I have interchanged the connections as well. Also tested parallel then serial combination also. Commented May 9, 2019 at 11:13
• A fast (<10ns) pulse load dump 5W > 25kHz directly across battery during charging works well to restore and prevent sulphation, Commented May 9, 2019 at 11:14
• more or less same as what? > 1.250 <+/- 0.010? Commented May 9, 2019 at 11:15

You have unbalanced batteries. The lowest capacity batteries are running down first. Your batteries may be good, but unbalanced due to charging history. Or one or more of your batteries, or their individual cells may be bad.

Normally, lead acid batteries are balanced by controlled over-charging. Flooded cells gas freely when being over-charged. This is expected, and safe, as long as the gas doesn't collect in any volume (it's explosive), and as long as you top up the cells to the correct level periodically with distilled water (water is lost during overcharging).

25.8v is not enough to overcharge two 12v batteries connected in series. Over time, small normal differences in leakage current will unbalance the batteries if an overcharge never happens.

First, check all your batteries are topped up to the correct level. Fully charge each battery until they have been gassing freely for some time. Now check the specific gravity (S.G.) of the electrolyte. Compare all the readings with each other, and with the expected readings from the battery data sheet. This is where you may learn whether any individual cells are bad.

If all the S.G.s are reasonable, now that you have fully charged the batteries, reconnect them and see what happens again. Ideally, your end of life trip should measure the voltage of each battery, and stop when the first goes below 11v (or 10.5v if you like to live dangerously). Tripping on the total voltage of a series string is not safe.

• 25.8V is the reading after about four hour rest ( after sunset). During charging the voltage go up beyond 28 V. I also regularly took the gravity reading. It is the same in all the cells around 1.25/1.26. The max voltage and Gravity is slightly lower than the prescribe max. But that may not be the reason one set of battery fully discharged and the other set remain at full charge. Commented May 9, 2019 at 11:20
• What is this "end of life trip". the inverter sees only the combined voltage of the series. how can I make the inverter trip on individual voltage lows ?. This what I am looking for. Commented May 9, 2019 at 11:38
• You would need to make your own circuit that monitored the individual batteries, and signalled the inverter. Or buy a better inverter that monitors the batteries individually for you. Or just buy all new batteries, and ensure they get balanced during charging. Commented May 9, 2019 at 12:52
• Any way I plan to use the batteries till the end ( they are already old) as the solar charging is not orderly and it is serving my purpose. But will you answer my key question, that is, why the batteries connected to positive remain nearly fully charged ?. There is only a drop of 0.2 V that is 12.6V , while in the beginning it was 12.8V. My estimate is that at least 2500 watt hrs was consumed n the night and all of it seem to drawn from the batteries connected to the negative terminal. OR is it normal in serially connected batteries ? Commented May 9, 2019 at 16:54
• If you have fully charged the batteries first, and they are purely connected in series, that is, if nothing is sourcing into or sinking current from the mid point terminal, then it's the knackered ones that get drained first, and it was just chance whether you'd connected those to the +ve or -ve terminal. Swap the batteries over to demonstrate this effect to yourself. You may find that doing a capacity test on each battery individually will help to clear your confusion by simplifying what's happening. Do the test, and post the capacities, full charge to 11v, of each individual battery. Commented May 9, 2019 at 18:33