| In 1999 and 2000 the .com era was exploding and there was a rush to outfit data-centers all over the country. Basic infrastructure was set up that included large lead calcium batteries installed as UPS (uninterruptable power supplies). Then came 2001 and the great dot-bomb crash, which we as a country still haven't recovered from. Still sitting in many of these data-center locations are 48 volt strings of large amphour capacity that have never done anything except sit, at float charge, for 5 years. For RE enthusiasts, this has become a potentially cheap supply of batteries for solar and wind electric systems.
It's important that we make the distinction between these batteries and the deep cycle lead acid batteries that have been used in, and become the standard for RE use.
To begin with, Most of the newer telco type batteries are lead calcium, VRLA (valve regulated lead acid), AGM (absorbed glass mat) batteries. They are designed to sit on a float charge for 20 years and still be useable for UPS type applications. There are a few things to keep in mind when contemplating using these in an RE environment.
- Know the recommended (by the factory) set points for bulk charge, float charge and temperature compensation and live by them. Set chargers to abide by those set points and test to make sure they are doing what they're supposed to. If they aren't, readjust them correctly by what's actually happening at the battery. Disable the charger's temperature compensation when adjusting the set points from battery readings.
- Do not equalize them. AGM type batteries will not boil the way wet cells will and giving it an equalizing charge will damage them. Each company has recommendations for equalizing their cells that should be looked into if you feel you need to do so.
- If you want to keep these in service a long time, don't deep cycle them. This applies to "deep cycle" batteries too, by the way. While I've seen studies that show large numbers (thousands) of cycles to total discharge and back again with little to no degredation, I don't know if I trust them. I don't want to discharge more than 85% full. For my 24 volt system, this means I don't want to see an at rest voltage of under 24.5volts. I know I'm being anal about this, and I won't loose sleep if I go below 24.0v once in a great while, as long as I can get it back to a full charge relatively soon.
Only slightly off topic...
3a. What the above section is really aiming at is that it is vitally important in any RE system that you size the battery and the Electrical sources to the load you will actually have. You have to have enough generating capacity to make up for the time it isn't available.
For example, You have a refrigerator that pulls 300 watts when running, and it runs 50% of the time. That means you have 12 hours x 300 watts or 3.6 kilowatt hours per day. To break even, you need to produce a bit over 3.6 kwh/day (there are always losses to make up for, plus it doesn't really draw nothing when it's not running full bore). Say I get 6 hours of decent sunlight during the winter months and I have 1000 watts of solar panels. If I actually got 1kw for six hours I'd be getting 6kwh/day, in which case I'd be in great shape. I can run the fridge and still have enough to charge the batteries what they used overnight.
In reality, that 1000 watts is really more like 750 watts and I'm not going to get that for all 6 hours except on the sunniest of days with a tracker (yes, I do have one). So maybe I really get something like 4kwh/day. This is cutting it close because of the aforementioned losses and because if I have a few overcast days in a row, I only have a few 100 watthours per day to recharge the battery with. If I miss two days of charging, I'm now 7.2kwh short that have been taken from my batteries. With only ~200 watthours/day excess capacity, It would take over a month to regain full charge after two rainy days, assuming the days are bright and sunny for that long. If I have a rainy month, I may never get back up to float charge. |
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