Sunday, January 15, 2017

RV - Solar and DC Upgrade (Part 3) updated

More playing around with the RV low voltage stuff. I set the DIP switches on the charge controller for the AGM battery that I'm currently testing things with. Right now, the charge voltage is set to 14.4 volts but I should probably set it to 14.35 per the Morningstar documentation for sealed AGM batteries. The flooded 6V golf cart batteries will be set to 14.8 volts as recommended by Handy Bob and mentioned in one of Ray's videos. Equalization is set to manual and is triggered with the black push button switch right below the serial port. I applied power by setting the output circuit breaker and the charge controller went through its self test routine but lacking a solar panel, I can't test. I did find out at the Coffeehouse this morning that DavidR has an unused Kyocera 12 volt, 130 watt panel that I may try and borrow to test the charge controller.

With the strain reliefs on the bottom of the charge controller, I was able to terminate the output cables and the positive input cable on the charge controller. Ox-Guard was applied to the copper before they were put into the screw terminals. The wires look overkill because they are. 2 awg is much larger than is needed but I had it lying around the garage. I still need to connect the chassis ground, temperature sensor and the battery sense wires.

I also set up some of the many of the parameters of the Trimetric 2025-RV such as the fully charged voltage (14.4 VDC) and the amp-hour capacity of the battery bank (I'm guessing around 70 amp-hours). I then turned on the inverter and charged up some Makita tool batteries which cranked up the inverter to draw about 8 amps for a short time. This minimal use drew down the voltage of the AGM starting battery (not deep cycle) pretty quickly. It actually wasn't as dead as the battery charger claimed (65%) as it charged back up pretty quickly. Here, now that the battery has gone through a discharge-charge cycle, the most useful display (%Full) is showing that the battery is fully charged.

The test AGM battery is about 70 amp-hours (guess based on some specs for a deep-cycle battery) and by multiply that by the nominal battery voltage (12 VDC) and you get 840 watt-hours as a theoretical maximum capacity. About half of that as a realistically useable or 420 watt-hours. Some initial playing around with the Kill-A-Watt meter yielded 60 watt-hour for two of cups of coffee from a Keurig machine and 320 watt-hours to cook three cups of brown rice. Plus the inverter itself consumes about 10 watt-hours per hour. So three cups of rice, and two cups of coffee over a three hour period would use up the useable capacity of the group 24 AGM battery. The four flooded golf cart batteries would have 430 amp-hours times 12 VDC or 5160 watt hours max or 2580 watt-hours useable. Or approximately 86 cups of coffee. That sounds like it might be enough...

Just as another test, I left the inverter on all day (Friday) with no load. The Trimetric monitor reported that the inverter was drawing 0.4 amps or about half of what it said in the specs. After 24 hours, it would've drawn about 9.6 amp-hours or 115.2 watt-hours which is about 14% of the battery capacity (115.2 watt-hours/840 watt-hours x 100%) . The Trimetric monitor reported that 85% of the battery was available. I like this device. The accuracy of this percentage is based on the amp-hour capacity that was entered in the configuration. The voltage at after a day was only 12.3 VDC which would indicate a state of charge of around 50 65% based on tables on the Internet. Does this mean that this battery only has 30 amp-hours instead of 70? This battery has been run almost dead a couple of times so maybe this is an example of how the battery capacity gets decreased when running them down too far. Also note that this is a starting not a deep cycle battery.

And the last photo was just from walking through the backyard. The snow is about mid-thigh so it's a challenge to walk around. At least it's a warm outside today, +11°F (-12°C) instead of the forecasted -40°F/C! Can't complain about that…

Updated info - The 20 hour amp-hour capacity of starting batteries isn't normally given in the specs since the focus is on CCA or cold cranking amps. But I did find at least one manufacturer of starting batteries that did list it. And for a group 24 AGM battery, it is listed at 55 amp-hours so I reset the Trimetric to 50 amp-hours. Pretty low capacity for the size and weight but I guess that's the difference between deep cycle and starting.

Sunday Afternoon - I switched to another charger a Solar Pro-Logix PL2208 smart charger that does have temperature compensation. The other smart charger would charge the battery up to 14.3 volts then back off immediately to just float. After many hours, the charger would claim 100% but the voltage would only be 13 volts. The Solar Pro-Logix charged all the way up to 15.8 volts before backing off to 14.8 volts. After a while, the charger dropped to float mode at 14.1 volts which is about what the Morningstar temperature compensation table suggested. 13.4 volts (float) + 0.75 volts at 32°F. Maybe I'll restart the discharge test tonight.


  1. So to accurately measure current draw, without the use of a shunt, would you measure with a clamp-on amp meter the draw on the main positive battery cable and the positive cable going to the inverter, add the two resulting figures?

    I am really starting to lean towards using a 1KW generator (when outside published "quiet hours") to just run electronics and charge the battery for nighttime usage (lights, AP).

    High draw appliances like microwave, water kettles, AC would cause the use of the Onan 4K generator.

    Now looking into the securing of said 1K generator (very portable) so I can leave it running and charging the house battery while I am out riding during the day.

    1. Yes, measure all of the 12V draws on either the positive or negative cables and add the two numbers. With the battery bank I'm looking at, I should be able to run anything except the air conditioner on battery power.