An Off-Grid Energy Storage Option

This winter we had a number of neighbors using off-Grid solar whose battery banks were nearing or at the end of their usable lives. They all were using what are known as “wet cell” lead-acid batteries, not much different than the original 1859 cells designed by Gaston Plante, and improved through the use of paste lead positive plates instead of sheet lead in 1881. These batteries last around 10 years in a properly sized home power system as long as you:

  • Keep the water level above the metal plates by adding distilled water on at least a quarterly basis, but only when the batteries are fully charged (if you add it when they are partially charged they may overflow sulfuric acid, leading to severe corrosion of the battery terminals).
  • Do a monthly “equalizing” overcharge, where voltage is allowed to climb over the normal “full” reading of around 14.4 volts, causing lots of corrosive vapor and explosive hydrogen and oxygen to be vented off. This keeps the specific gravity and resting voltage of each individual 2-volt cell in the batteries at roughly the same state of charge. Hopefully your battery box or battery room is well ventilated and the battery terminals, along with everything in the vapor’s path, are protected from gaseous corrosives.
  • Keep the battery top surfaces clean so power doesn’t leak across them, and make sure you don’t see any corrosion on the terminals, which usually shows up as a blue-green (copper sulfate) or white (lead sulfate) powder.

That’s a lot of maintenance and, speaking as someone who has had to repair the results of missed maintenance in several systems, it often gets neglected. Once the batteries near the end of their life they begin to “gas off” even more than usual during normal charging. They also show symptoms of “sulfation”, where lead sulfate builds up in the bottoms of the battery cases and large, hard, insoluble lead-sulfate crystals build up on the plates, both reducing battery capacity. You’ll know which batteries exhibit this by measuring higher than normal voltages (and heating of the case) on some cells/batteries during charging and lower than normal voltages during discharge. Time to look for replacement batteries!

This fall, all of the batteries I used for replacements were Deka Dominator lead-acid gels. Gel batteries are:

  • completely sealed
  • require no addition of water
  • There are no gasses to vent
  • no equalization is either needed or allowed
  • their gelled sulfuric acid electrolyte is leak-proof even if the case is punctured.
  • their self-discharge rate is only around 1% per month, about 5-10 times lower than other lead-acid cells, and on-par with lithium batteries

And some added phosphoric acid gives them some other unusual traits. They can be completely discharged without damage and they can be recharged at any time (not immediately as wet cells require) without sulfation. And their “cycle-life”, defined as the number of discharge-charge cycles to a given depth of discharge, is roughly three to four times that of a wet cell lead-acid battery, giving them extremely long service lives.

The downsides?

  • they require tighter voltage control when charging. At room temperature this means nothing over 14.1 volts in a 12-volt battery, so you’ll need a charge controller that is either set for gel cells or has adjustable charge voltages.
  • they absolutely cannot be allowed to fall below -22 F or they will freeze and the plates will be damaged.
  • they are slightly more expensive than wet cell or AGM (absorbed glass mat) lead-acids.
  • they are slightly heavier than other lead-acids of the same capacity.

Best of all, these batteries are not some exotic beast requiring special ordering and high shipping costs. Around here our local Fleet Farm store carries them as in-stock items, ordering fresh, new, fully-charged ones from the factory if you need more. They even offered a quantity discount for the larger systems. And they accept the replaced batteries for recycling, probably the hardest part of this process on my back!

We have used these batteries for a little over 4 years now and life off-Grid has certainly been easier. We even used them in our all-electric-converted Porsche 924, but learned quickly about frozen batteries. In the winter of 2013-2014 we had a string of days below -25F and one of the battery heating tapes in the carĀ  malfunctioned, causing three batteries to get damaged. The remaining ones are now in our home power system, adding to our storage capacity, decreasing the depth of each day’s discharge, and adding to the whole pack’s longevity. The Porsche awaits some much lighter, but far more expensive, lithium batteries. Dream on little Porsche!

 

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