I think I mentioned earlier that I have a 10A mppt that has been sitting idle. It’s wired into the DIY converter project, but I haven’t been connected to shore power in 858 days at this point so the 10A has been gathering dust.
theory / limitations
Yes, 10A is undersized for 200w of panel:
12.1v * 10A = 121w at 50% DoD
14.7v * 10A = 147w at Vabs
max realistic power of 200w of panel on mppt at solar noon: 186w. (200w * 83%)
So we could be leaving 39-65w of panel output on the table. Probably closer to the low end since >121w is probably not available in the early morning when DoD is lowest. I’ll try to watch the meter and see what they really pull in the morning.
My thinking is:
I could buy a 20A MPPT for $80, but I already own the 10A.
serial wiring reduces voltage drop on the long (20’ + 7.5’) wiring run
serial wiring will allow the controller to collect power in marginal insolation (dawn/dusk, dark overcast). The amount of power available at those times is admiittedly minimal but I’ll take it when I can get it.
the panels are poly; Vmp is 18.0v, which is decent, but running serial gives MPPT plenty of room to play.
the 10A mppt allows me control over setpoints while the 20A PWM only had presets. So after 2 hrs of Absorption the PWM effectively stopped contributing under normal circumstances. (it would have started pulling again if a massive load pulled system voltage down to traditional Vfloat)
I unhooked the DIY converter and plugged in the panel wires. I’d been running the panels in parallel to the PWM and reconfigged them in serial to feed the 10A mppt. Stored the 2-into-1 adapters.
So far I’ve observed 146.9w out of it, which is right on the money during Absorption. Not even warm to the touch. I stressed the system by running the immersion heater for a few minutes at 1415 local; solar noon was 1234. I’m surprised the panels could pull even that 2 hours past solar noon in winter. Skies were very clear, which helped.