Batteries are the bane of any off-grid renewable energy system. Whether you use wind or solar to provide the power, you must have a way to store some power for when the wind stops blowing or the sun goes down. You also need surge capacity for powering loads that require a lot of amps for a short time. Aside from the heuristics involved in figuring out exactly how much storage capacity you’re going to need, you’ve got various types of batteries, different levels of quality, etc. The cost of batteries eats up 1/2 of the value of the power I can generate. The alternative is to use a grid connected system, but that doesn’t provide any power when the grid goes down! I use my PV system like a huge uninterruptable power supply. I power some of my equipment with it, and, when the grid goes down, it is my backup power.

Up until now, I’ve been using deep-cycle, flooded lead acid batteries. They are powerful and relatively inexpensive. My current battery bank is about 220Ah, of which I can use about 110Ah without over discharging the batteries and shortening their life. I really need a larger bank to take advantage of the power I can generate with the photovoltaic (PV) panels that I have and to get through days with no sun. My panels and my charge controller are capable of a 30A charge rate (approximately). With flooded lead acid batteries, I should get about an 85% efficiency in charging, and I can bulk charge the batteries at the full rate up to 80%. So, after a long night of a nominal 10A draw, I can bulk charge at 20A (30A PV subtract 10A nominal load) for about 4 hours before the charge rate drops and PV power is wasted as the batteries are charged from 80 to 100% charge. That means my battery bank is too small.

With a larger battery bank, I can bulk charge for longer, and I can increase my nominal load. i.e. I can utilize more energy and make better use of my investment. So, I’m researching my options for a set of batteries with at least a 400Ah capacity. I am looking at 2 options, at the moment …

Option 1:

Flooded lead acid batteries, 6V, 220Ah x 4 with an assumed 85% charge efficiency and a 5 year life span (440Ah @ 12V).

If I replace 200Ah of charge each day @ 20A, that would require 10.5 hours of direct sunlight (@ 85% efficiency), so the bank should be large enough.

(cost $360 or $72/year)

Option 2:

Absorbed Glass Mat (AGM) lead acid batteries, 6V, 190Ah x 4 with an assumed 95% charge efficiency and similar lifespan (380Ah @ 12V).

I can only use about 190Ah from this bank, and the batteries recharge more efficiently, so it would take about 10 hours of full sun to recharge this bank.

(cost $560 or $112/year)

It really looks to me that the AGM batteries are just going to cost me $200 more and provide little additional benefit. A flooded lead acid battery can be bulk charged up to 25% of it’s Ah rating. Option 1 could be charged at 110A, but the most I can throw at it (at the moment) is 30A. The down side is that I lose more power to heat with the flooded cell battery than with the AGM version. That would be a big deal, but for $200, I could add an additional 85W of PV capacity, which would more than make up for the 15% loss and the new PV panel would last for 20 years (at least), which comes to $10/year, whereas the additional battery cost comes to $40/year (assuming the price of batteries doesn’t go up).

So, I guess I made up my mind. I’m going to go with the cheaper, less efficient alternative, unless someone can offer some helpful advice or show me the error in my logic.


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