Finally getting around to doing the math for a sand battery...
My panels operating voltage and current are 38.6v and 8.44A. That means for one panel the ideal heating element would be 4.6 ohms. For 2 panels in series, 9.1 ohms.
The little immersion heater coil I got says it's 1000W at 110VAC. Which would mean 9.1A, and therefore be about 12.1 ohms. But the ohmmeter says it's 17.6 ohms.
That might be a bit high for the panels. hard to say what power point that will produce. Will just have to try.
If I were to power from my 25.6V batteries, and it's really a 17.6ohm element, that would only be 1.4A, so only 37W. So that's out.
The only other option with this element would be to keep it AC, and have the FM80 aux output programmed to open a relay to power it once the batteries reach float. On most days that'll be by 1p at the latest, and that gives me 4 hrs at 1kW of heat going into the sand for 4kWh of heat storage or about 13650BTU. That would probably be meaningful.
Hippy Steve
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2026-02-04 04:44:04 UTC
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2026-02-04 04:44:04 UTCEvent JSON
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"content": "Finally getting around to doing the math for a sand battery...\n\nMy panels operating voltage and current are 38.6v and 8.44A. That means for one panel the ideal heating element would be 4.6 ohms. For 2 panels in series, 9.1 ohms. \n\nThe little immersion heater coil I got says it's 1000W at 110VAC. Which would mean 9.1A, and therefore be about 12.1 ohms. But the ohmmeter says it's 17.6 ohms.\n\nThat might be a bit high for the panels. hard to say what power point that will produce. Will just have to try.\n\nIf I were to power from my 25.6V batteries, and it's really a 17.6ohm element, that would only be 1.4A, so only 37W. So that's out.\n\nThe only other option with this element would be to keep it AC, and have the FM80 aux output programmed to open a relay to power it once the batteries reach float. On most days that'll be by 1p at the latest, and that gives me 4 hrs at 1kW of heat going into the sand for 4kWh of heat storage or about 13650BTU. That would probably be meaningful.",
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