Many readers may have picked up on hints dropped here and there on the blog that our power supply needs improvement. I think that’s true, although I confess that I’m not entirely sure. I’m not speaking here of our recent wind generator trouble (see Tilting Windmills) but of our battery bank.
We installed 8 lead-acid Trojan 6 Volt “golf cart batteries” tied in series and parallel for 900 Ah (amp hours) capacity In June, 2007. They have operated for more than 7 years now.
Lead-acid batteries like ours generally last about 5 to 7 years. We expect to get more use than that because we installed two Pulse-Tech power pulse units on them from the start. These produce a pulse in the battery that helps prevent corrosion from building up on the internal plates, extending battery life. We also monitor usage carefully to avoid dropping below 50%. Once we installed the Trimetric monitor, which gave us far more accurate information about our battery bank, we became even more conservative.
Even so, years of constant use inevitably take their toll.
The new bank replaced the batteries that had been here when we arrived. Housed in the outhouse, about half of them had frozen, and had died. We didn’t need a lot of electrical knowledge to figure out this would not do.
This time around, we have nothing quite so dramatic to point to as evidence that our batteries are wearing down. In fact, I find very few descriptions of how this process plays out, or what might indicate that it’s happening. Essentially, I learned that battery capacity drops precipitately when the batteries start to go bad. Also, that measuring specific gravity provides the best indication.
I don’t know if you’ve ever measured specific gravity in a battery bank, but it’s a thankless process. It requires that the batteries remain at rest for at least 24 hours to get a truly accurate reading. Imagine how often our batteries get a full day’s rest, with no charging or discharging. Not very often. We do use them, after all.
We grew concerned when the low battery alert on our Trimetric panel began to blink more often than before. This started last summer.
The Trimetric allows us to set the low battery level at which we want to be notified. I set ours at 50% initial draw. We rarely saw this alert, because we’d restricted use to the top 25% of capacity. If our Trimetric shows 75%, even in the initial draw, we shut down all loads. In essence, we have a 225 Ah battery bank. We consider the remaining 675 Ah emergency reserve.
Even so, after longer periods of draw without incoming charge, we occasionally saw the low battery alert flash. Always, it would turn off as soon as we stopped the heavier draw. After a period of rest, the battery charge level creeps up to higher percentages, and all seems well.
When we started seeing the low battery alert more often, the initial discharge level dropped to as low as 40% capacity with alarming frequency. Once we shut down the draw, it bounced back up to acceptable levels, often even 100%. When we considered the age of the batteries, we realized we needed to plan replacing the bank.
With lead acid batteries, we need to accept the possibility of periodic bank replacement, including the cost of new batteries and the work of hauling in, arranging, and connecting the new bank, disconnecting and hauling out the old one. We need to be ready to do this every 5 to 7 years, or if we’re very lucky, every 10 years. But, we may have a better plan . . . .