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Let's suppose we have a new very healthy fully charged AGM battery that is ready to put into service and it is rated at 12V 100Ah (at a 12 hour drain rate, therefore it has 600 Wh of usable energy per charge). Also suppose during its lifetime, it can be charged and discharged 500 times each, down to 50% SoC, therefore having a lifetime capacity of 500 * 0.6 KWh = 300 KWh.

Case 1: we put a 50 watt load on it for 12 hours a day, which is 600 Wh of drain, then charge it back to 100% SoC in the next 12 hours and repeat that cycle 500 times.

Case 2: we leave the 50 watt load on 24/7, however, during the daytime, it can be assumed that the battery will be charged full within 12 hours of daylight using a solar panel (assume 150 watts average over the 12 hour daylight period, enough to compensate for inefficiencies of AGM batteries and solar charge controller inefficiencies). So the battery will drain down to about 50% SoC overnight and then slowly go back to 100% SoC during the 12 daylight hours.

So how will case 2 affect the total usable lifetime capacity? Is having 100 watts of charge power and a 50 watt load simultaneously similar to no load and a 50 watt charger? Will the battery have the same # of charge discharge cycles in these 2 cases? Does a battery behave differently having a 50W load and 100W of charge power vs no load and only 50W of charge power, or can those 2 cases be considered pretty much identical?

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    \$\begingroup\$ "Is having 100 watts of charge power and a 50 watt load simultaneously similar to no load and a 50 watt charger?" Should be as far as the battery is concerned. \$\endgroup\$ – DKNguyen Feb 20 at 5:42
  • \$\begingroup\$ @DKNguyen - thanks for cleaning up the formatting of my question. I should also mention that during the daylight charging hours (12 hours), it can be assumed that the battery SoC never dips below 50% (that is, when the sun comes up, the solar panel will be producing 50 watts minimum, but 100 watts on average for the 12 hour daytime hours). Perhaps even 200 watts maximum to balance out the slow start of only 50 watts. \$\endgroup\$ – David Feb 20 at 5:56
  • \$\begingroup\$ In winter you'll get more like 4 hours of good daylight, so have to factor in the 20 hour battery power. \$\endgroup\$ – Brian Drummond Feb 20 at 11:05
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Is having 100 watts of charge power and a 50 watt load simultaneously similar to no load and a 50 watt charger?

Yes. The battery will get the same 50W charge power in both cases.

However, this scenario...

the battery will be charged full within 12 hours of daylight using a solar panel (assume 100 watts average over the 12 hours). So in case 2, the battery will drain down to about 50% SoC overnight and then slowly go back to 100% SoC during the 12 daylight hours.

...is unrealistic. In practice the solar system probably won't be able to produce 100W for 12 hours, so the battery will have to (at least partially) power the load for more than 12 hours.

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  • \$\begingroup\$ Assume a 150+ watt solar panel that will produce (on average), 100 watts over the 12 hour period from say 7am to 7pm. I updated my original question to reflect this change. However, I do see your point. At 7am (when the sun is just coming up), there wont be 50+ watts of solar available, thus the battery may drop below 50% SoC for a few hours, good point. I suspect in that case, the battery will be more stressed and maybe have fewer charge/discharge cycles because of the deeper DoD. \$\endgroup\$ – David Feb 20 at 5:57
  • \$\begingroup\$ I suppose if someone had a well angled mirror, they could reflect the sunlight at 7am onto the solar panel, and then as the sun came up higher, the panel would then get the more direct rays. That would be something "slick" to try. Perhaps in that scenario, the 150 to 200 W max panel WOULD produce 50+ watts when the sun came up at about 7am. \$\endgroup\$ – David Feb 20 at 6:04
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    \$\begingroup\$ Exactly. It doesn't matter what power the solar panel can produce in the middle of the day, if it can't produce at least 50W from 7am to 7pm then the battery will get overdischarged. A grossly oversized panel (or your mirror idea) might do it in summer, but the correct solution is more battery capacity. \$\endgroup\$ – Bruce Abbott Feb 20 at 6:06
  • \$\begingroup\$ Yes I agree, or if the load was less like 40 watts, to give the sun a chance to come up before the battery drops to 50% SoC overnight. Good analysis thanks. This example is hypothetical anyway, since 3 cloudy days in a row is possible or it might be raining at 7am then my battery will be stressed like a turkey the day before thanksgiving. \$\endgroup\$ – David Feb 20 at 6:07

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