I mentioned before that I’m trying to squeak by on limited propane to avoid making a special trip.
excessive caution during early morning Absorption
I have been percolating coffee over propane because:
- I had lots of it before
- it’s been in the 30s in the morning and I wanted the interior heating
- coffee time is morning, and I didn’t want to impose loads while the bank was still actively charging
My thinking has evolved a bit on the last point. Previously I’d acted as if running loads off the panels was eqivalent to “impose no loads that will affect state of charge”, but I was internally mistaking that for “impose no loads during Absorption that will affect the stead increase in state of charge”.
I failed to recognize that there are loads that may stall charging but not negatively affect state of charge. Consider this scenario:
- bank at Vabs, approx 80% SoC based on amps returned
- load drops bank voltage back to Vfloat (or really anything above 12.7v)
At this point charging is stalled and will continue when the system is allowed to to return to Vabs. Overnight-style sulfation is not an issue.
So I decided to do some experimenting.
My little lab hotplate runs at a measured ~290w, which is something like 990btu/hr. Not a lot of heat. But it’s free heat….
The burner is 4” across and the percolator has a 4.75” base so the fit is very good. Almost no radiated heat was noticeable during operation, as opposed to the propane rig that heats the van interior
It took the plate ~17 minutes to bring the pot to percolation so it pays to start it ASAP. But after achieving boiling the 990btu is too strong to maintain gentle percolating. I placed the small grate from my woodgas stove between the hotplate and percolator and this regulated the perc temperature nicely.
Two unexpected observations:
- during the perc power demand suddenly dropped to zero. I thought something had failed then I remembered the hotplate had an internal 750F thermostat – with the percolator suspended on the grate the hotplate surface was bouncing off the 750F limit.
- I cut power after 4 mins of perc and returned to email. A few minutes later I realized it was still percing. The little hotplate is quite heavy for its size and has a lot of thermal inertia. The next morning I removed the perc from the burner at shut-off to avoid overextraction.
max 1-day solar harvest
The day of the first test it was quite chilly outside with clear skies. That’s one of the reasons I decided to electro-perk. I ended up running a bunch of loads that day:
- the coffee (21 minutes @ 290w)
- heated bathing water on the hotplate, another 20mins @ 290w.
- a small $10 250w “desktop/personal” heater I’d been wanting to try out (probably 3 hours @ 225-250w)
- batch of crockpot beans (5 hours @ 138W)
- heavy laptop use
I wasn’t on a mission to see how much power we could make but I figure the system was 85% maxxed all day. Result: 4.28kWh for that sunup/downdown period, the highest I’ve seen. I average something like 1.75kWh so that was a pretty big jump.
I think I’ll keep the electro-percolating experiment going as long as bank voltage is already into the 14s at breakfast time and skies look clear. The days are growing shorter so it can’t last forever.
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