MPPT Solar Battery Charging in the Arizona Desert

I’ve got a 125 W solar panel feeding a 12 Vdc 85 Ah deep cycle battery at a comms site in the southwest Arizona desert. I’m using a Rich Solar MPPT-20 solar charge controller (SCC), which so far appears to be doing a great job. The load on the battery is a Raspberry Pi with two NESDR Smart radio dongles, running software called RWMon, and decoding ATCS indication messages being sent from railroad control points along the I-8 corridor, but that functionality isn’t the subject of this article. It’s the battery management that’s the thing!

Figure 1 – 24-Hour Battery Voltage

It’s a tough environment out there in the desert, and determining an appropriate energy storage technology is part of the challenge. I settled on a lead-acid battery, mainly because it tolerates higher working temperatures, and since there are plenty of charge management devices that understand lead-acid chemistry really well.

In Figure 1, battery voltage is shown over a 24-hour period. This particular 24-hour period was dead clear during the daylight hours, so the curve is very clean.

Figure 2 – Manufacturer’s Chart on Battery Charge Management

Figure 2 is the chart right out of the Rich Solar MPPT-20 charge controller manual. From about 0700 (the sun begins to directly illuminate the panel) to around 1130 the SCC is in the fast charge mode. From 1130 to about 1330 the SCC is in the sustained charge mode; from 1130 to 1730 the SCC is in float charge mode. It’s nice to see that the real-world situation replicates the manual pretty accurately.

There are other nice things to discuss about this SCC, including very low RFI generation, that future notes will touch on.


I realized that I also have for the same setup the voltage characteristics for the PWM SCC that I had out there last month before switching to the MPPT unit.

In Figure 3, the units are wrong, but the curve is the important bit.

Figure 3 – PWM SCC Voltage Curve

Here, it’s a much cruder controller, but the same kind of curve is evident. The drop off from boost to float is a very slow ramp, that’s probably not as good as what the MPPT does.

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