DIARY ENTRY FOR 14 NOVEMBER, 2009
Battery Discharge Test
What a strange way to maintain one's battery bank: Running them flat! Here they are running a series of auto headlights and a heating element made from a clothes dryer element.
This is exactly what I did. It served two purposes: One was to check that I am still able to store a reasonable amount of energy in the batteries, and the other was to try to even out the differences in the different cells. It's not actually about improving their life, or re-filling, cleaning, or any of the other things that actually do make batteries last longer. I'm just curious about their total capacity.
Ever since discovering this set of old battries, I've lived with the fact that some cells are much weaker than others. It's only a few percent, but if they go too long and get too far apart, the weaker cells pull down the better ones. If the battery isn't regularly kept at float charge voltage, the weak ones develop sulphate build-up. The end result is reduced capacity, and it's obvious that they were disposed of for this very reason.
If these batteries were of the flooded lead-acid type, which you can over-charge to "equalize", this still wouldn't be a problem. Equalizing causes the cells to vent some gas and vapour so you can top them up with distilled water afterward. Unfortunately these batteries are valve-regulated sealed batteries, which means that once they vent gas, it's gone forever. There is no port through which I can pour in distilled water, and even if I tried, I'd have no guarantee that I could properly seal them back up again.
The alternative, then, is to drain the batteries to near flat. The current and the heat may have the effect of slightly de-sulfating the cells. I do not know this for sure. The only way to realy know would be to repeatedly discharge the battery under strictly controlled conditions and compare the successive results. I'm not going to go that far. Since I also want to know if there is much capacity in this bank of batteries as they are, I'll content myself with one test, in the environment they find themselves in.
The next morning, it was warmer, and quickly got warmer still. Then I discovered that the charge in the batteries was increasing. Of course, any battery's capacity is penalized when it is cold, and I was confronted with the fact that as it got warmer, my test would never end!
Here is a comparison of each of the cell voltages. There are some obviously weaker cells, such as #6, 8, 10, 14, 15, 16, and 20. They even get worse as the test goes on. The starting voltages for each cell is above 2.0 volts, when corrected to 25 degrees Centigrade (it was actually about 5 degrees below freezing).
Once I started discharging them, at about 30 Amperes, the voltage dropped rapidly to 1.90 volts per cell, but after that, it took a VERY long time to go much lower! The batteries maintained a nearly constant voltage, constantly pumping out 30 Amps, for over 14 hours on Saturday! I gave up and went to bed, and picked up again in the morning.
I contented myself with having drawn nearly 75% of the batteries' capacity, according to the data plate. By pulling an average 34 amperes for nearly 20 hours, I obtained 675 Amp-hours from the batteries, when under ideal conditions they could be expected to deliver about 920 Amp-hours. From a 24V battery, that is equivalent to 16 kWhr. Since the turbine only gathers about 1 kWhr or so, on a windy day, it will take a long time to fill up again!