May 11, 2021

My buddy Mark's setup

He has the following mounted on a trailer he built: All of this was purchased in the summer of 2014. He uses this to power his RV part time. It is not adequate to power his RV in summer months when he wants to run his A/C.

His Solar Power Trailer plugs into the same plug that the Power Plug from the Meter plugs into. He just switches the Power off at the meter, unplugs the Power Cord at the RV, then plugs the Plug from the Inverter into the same plug on the RV.

Some of my calculations

These batteries are 100 amp-hour, there are 4 of these 12 volt batteries, so we have 4800 watt hours available.

His panels can generate 375 watts, so it will take 4800/375 = 12.8 hours to fully charge the batteries (starting from full discharge). From half discharge, it will take 6.3 hours to fully charge the batteries. He says if he runs the batteries down to 48 percent (about half) at the end of the day, does not use them at night, then by about 10 AM the next day they will be at full charge (the RV disconnected). With sunrise around 6 AM this is 4 hours to get to full charge, which is better than I would have predicted, but I may not understand his system properly.

His inverter tells him that running his air conditioner draws 1600 Watts. This is 13.3 amps of 120 volts and 133 amps of 12 volts. With 400 amp-hours of battery, he can run this for 3 hours on battery. (I have heard that you should try to only use half of your battery capacity, i.e. 200 amp-hours in this case, if you want to prolong battery life. 375 watt panels won't do much to help the batteries -- it is just not practical to run a 1600 watt A/C using 375 watts of panels. Something like 2000 watts of panels would be required to run the A/C and also charge batteries for the coming night.

With the system in operation on a bright sunny day in mid-May, the display on the charge controller show 40.3 volts from the panels, and 13.7 volts and 20.9 amps going to charge the batteries. This is 286 watts.

That Tracer 3215RN

I am shopping for my own charge controller, so I find this very interesting to examine. This is a 30 Amp MPPT charge controller made by EP Solar. It is discontinued as of 2021, but used to sell for about $180. Being a 30 amp unit, with a 12 volt battery setup, it can push 30 * 12 = 360 watts into his batteries. With a 24 volt battery setup you could push 720 watts. It is reasonably sized for Mark's panels, and Mark could even add panels if he switched to a 24 volt battery setup.

Mark says that his panels are the weak part of his system and he really needs four 250 watt panels (a total of 1000 watts). If he did upgrade his panels, he would need to switch to a 24 volt battery setup, and even then he would be pushing about 42 amps, so he would need to upgrade his charge controller also.

Batteries

You can buy a 100 Amp-hour deep cycle AGM lead acid battery for $180 these days. Compare that to a 100 Amp-hour Li-Fe-PO4 battery for $700. The difference is longevity. The lead acid battery will endure 500 to 1000 charge cycles. The Li-Fe-PO4 will go for 8000 cycles perhaps. In other words, you get what you pay for. Dollars per day of use is the same either way (more or less), but you will be replacing those lead-acid batteries every few years. However, if you want to get in the solar game and don't have a pile of cash, the lead acid deep cycle batteries are just fine.
Have any comments? Questions? Drop me a line!

Tom's notes / tom@mmto.org