Hypothetical ideal world...

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CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Off Grid Solar & Battery Systems #1
Let's take a look at an off grid system in a perfect world:

Ideal climate with an average annual isolation of 2190kwh/m2 or 6 sun hours per day. In this blissful climate the sun shines nearly every day with only the occasional cloudy stretch that never lasts more than a day or two before predominantly sunny weather returns.

The load on the battery is fairly constant without much fluctuation throughout the day and is 20% of the PV's peak output (a constant 200w load on a 1000w array). For the purposes of this argument the load is in DC watts at the battery.

In this scenario I suspect that on average the PV output will begin to exceed the load at around 9am and fall short of supporting the load after 3pm. This means that the although the system is balanced the batteries are spending 18 hours in a state of discharging and 6 hours in a state of charging. More concerning though is the length of time they are spending at 100% SOC, probably only an hour or so. Can any lead acid battery live like this for long?

Just asking, because in my mind this is always how I pictured an off grid system operating more or less...
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #2
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    Re: Hypothetical ideal world...

    Let's see how this works out: 200W x 24 hours = 4800Wh/day and say 2 days of autonomy and 50% DoD, so you're looking at a 4800 * 2 /0.5 = 19200Wh rated battery. Let's assume by 3pm on the first day the battery is charged, so from 3pm to 9am it will discharge 18 x 200w = 3600Wh, which is about 20% DoD every day. That's pretty good and you could expect a good long life out of the battery. The time spent at 100% SoC doesn't matter so much, as long as it reaches 100% SoC once a week or so and it's kept above 70% SoC most of the week it should live a long and happy life.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #3
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    Re: Hypothetical ideal world...

    Thanks for the tip Stephendv, despite having read a ton of advice on the correct way to use lead acid batteries in off grid systems there is always something that is said somewhere that causes anxiety on how importing charging is.

    For example charging current is stressed a lot, as well as time spent in the absorption (and even float) stages. It can make you over eager with the generator, grid etc when you see the slightest wisp of cloud appearing in the sky!

    Finding useful info on the time it takes for permanent sulfation to form is hard, the most helpful bit I've found is that "it can take a week in cold climates and as little as a day in hot climates". My climate is somewhere in between with the temperature in the battery room being around 26°C in summer and 17°C in winter. Fortunately in summer the batteries spend more time closer to 100%soc than in winter.

    I cycle them harder during sunny stretches than in cloudy stretches. In very gloomy weather I will hardly use the batteries at all, using only grid power and let whatever solar there is keep the batteries on float charge.
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