strange battery behavior trying to run 12v frig on solar - Fiberglass RV

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Old 05-20-2019, 12:17 PM   #1
Name: dave
Trailer: scamp
New Mexico
Posts: 76
strange battery behavior trying to run 12v frig on solar

quick and dirty:
have solar panel that turns out 220W
2 6v 225Ah deep cell batteries

was in direct sun so i tried turning the 12v frig on. immediately batteries started going down. only think i could figure was that the batteries still needed a lot of juice to finish charging so i turned off frig and let batteries go thru most of absorb cycle until they were only requiring about 30W to keep them happy. i.e. even though 200+watts were hitting the panel, my system was only using about 30w and that was just going into the batteries to maintain roughly 14.8v absorb.

so, i figured there was plenty of extra power to run the frig which takes a max of about 120-130w when cooling (9-10A at 13V). i flipped on the frig and the wattage being used by the system went up to 220 and the batteries voltage started going down again. i just don't understand why, when the batteries are only using about 30w and i put the 120w frig on line and i have 220w available and going into the system, where the rest is going and why the batteries are falling.

any ideas?

btw, i know the frig takes about 120w (DC) because i've measured it directly and i also have an inline volt/amp/power meter that lets me see what the total draw is on the batteries - the frig was all that was running at the time.

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Old 05-20-2019, 01:07 PM   #2
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Name: John
Trailer: 1978 Trillium 4500, 1979 Boler 1700
Posts: 1,337
Consider this:

A Renogy 100 watt monocrystalline panel optimally produces 5.29 amps at 18.9 volts. If you are NOT using a solar charge controller or if you are using a PWM solar charge controller, I think you will still only get at most 5.29 amps at lower voltages (13 volts, 12 volts, etc). So at 13 volts you would get about 68 watts. So with 2 100 watt panels, you would get about 136 watts.

EDIT: OK, so since you are using a 220 watt panel, you need to look at the specifications for that panel. If you are using NO solar charge controller or are using a PWM solar charge controller, your resulting wattage will be calculated as the solar panel amperage multiplied by your actual system voltage (e.g., 13 volts). To take full advantage of the output wattage of the solar panel, you would have to use an MPPT solar charge controller matched to your solar panel specifications.

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Old 05-20-2019, 01:30 PM   #3
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Name: John
Trailer: Escape 21, behind an '02 F250 7.3 diesel tug
Mid Left Coast
Posts: 1,912
a 220 W solar panel only puts out 220 watts under /optimal/ conditions, direct perpendicular sunlight high in the sky with perfectly clear and dry air, and a fairly deeply discharged battery.

check the output current and voltage (NOT the solar panel voltage) and multiply them to see the actual watt output... (volts*amps = watts in DC land).
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Old 05-20-2019, 01:34 PM   #4
Name: dave
Trailer: scamp
New Mexico
Posts: 76
thanks John, i should have mentioned i am using a sunsaver 15 mppt charge controller. so i was taking in a full 220w at the time. however, you did get me thinking along the lines of how much actual power is available in the system even with 220w coming in. 220W/14.8V = almost 15A. assuming the frig was taking 9-10 of those i would think there should still have been roughly 5A available to hold the batteries on full absorb charge which shoule have been plenty since they were only requiring about 2A before frig was turned on. what am i missing here? i just can't figure out why that much power couldn't keep the batteries at 14.8 while running the frig. i'm wondering if i have something wired weird such that the power comes out of the batteries first to run the frig and even though power is diverted to them, it's not enough to hold the full charge? i can't quite wrap my head around that either but maybe it's a side effect of the way batteries work?

i should have mentioned that the frig is wired directly into the trailer wiring (the norm) not onto the load side of the charge controller.
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