Category Archives: Sensing and Control

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.

“WINGONEER Wireless Digital DC Voltmeter Ammeter Multimeter 2.4”

Purchased this thing via Amazon.

Received device yesterday, no user manual. Searched on line for “VAC8010F-80V” and found what looks to be a .pdf manual for this product. Got thing hooked up per the schematic in that manual. Battery is 12 Vdc, 2x20Ah SLA cells, on a solar PWM charger. Batteries are fully charged (14.4 Vdc) and in good shape. Disconnected batteries from charger.

Through a 12 V – 5 V dc-dc switching converter, connected 4 RTL-SDR-equipped Raspberry Pis and a DLink gigE Ethernet switch (total of about 1.5 A load at 12 Vdc). Device displays -1.6 A current draw, a little high from what my Fluke meter says at -1.5 A. Not bad!

The wireless function is kind of awesome – it just works. The range is at least good enough for having the sensors at the battery and the display head several meters separated, with some house walls in-between. The device uses the NRF24L01 chip from Nordic, so the transmission is in the 2.4 GHz band. I have multiple 2.4 GHz Wi-Fi units in the house, I can’t see how the link degrades, but it seems to be fine. It appears that one sensor unit can communicate to many display heads, or many sensor units can communicate to a single display head. Haven’t experimented with that.

The display is bright and very readable. The three buttons, from top to bottom, are “up”, “select/change”, and “down”. The function of some of the settings is a little hard to determine from the found manual, but I’ll survive. The temperature sensor is a ~100 cm long cable with temp sensor in a ring lug. I have it in-between the two batteries, and shows 28 C, which is close enough.

So the hardware is nice, but the software sucks. Seriously not good. I have had this running now for about 40 hours; the voltmeter function works fine, the current meter appears to accurately show positive current, and the temp sensor works fine, that’s about it. The running Ah and Wh meters just don’t do anything useful. I have 40Ah of 12 V SLA battery, I programmed in via the BAT function the 40.0 Ah size, I discharged the batteries a while until it reached 38.467 Ah. Then I applied a charge to the battery through the circuit, the battery capacity stayed at 38.467 Ah. The battery voltage has continued to increase to full charge, so that’s working. The BPC value is static at 96%. I tried to reprogram the BPC to 100%, but it won’t accept the value. I’ve tried changing the battery capacity to some other value to get it to reset the counters. It will not show negative current (power flowing into the battery from the solar charger). Not sure what to do about this. Probably will return it unless I get some answers soon. I guess I’ll have to wait until Sunday night (Monday in China).

volt/amp/watt/energy meters

Got another two of the inexpensive volt/amp/watt/energy meters from mfreelaa on eBay. These are pretty nice, especially for the $12 each including shipping.

I put two of them in series to see how well each tracked the other. There should be a very small drop in voltage for the second on in series as opposed to the first. That does appear to be the case.

The unit under test is pulling approximately 1-1/4 amps at 12 volts. Assuming these ammeters use 100 A/75 mV shunts (since the meters without built in shunts do the same and it’d be easier to keep them all the same) this would imply that the second meter should read about 0.001 volts lower than the first. However, the display precision is only 0.01 A so it’s not practical to see the difference with this small load.

After two hours now, both meters are showing 50 w-h, I’ll have to wait longer to see if they diverge from one another at all. The voltage (refreshed once per second) is generally within 0.02 volts, which is 0.15% agreement. The current reading is looser, maybe 0.2%. The power reading is quite close, but one would expect that the power reading comes from the multiplication of the v and i values. The energy reading is the time cumulative sum of all the power measurements.

Next project is to measure the efficiency of my new MPPT solar battery charger. Just need to have a sunny day!


Here it is the next morning and the DUT has now pulled a total of either 261 or 262 W-h, depending on the meter read. That’s still pretty good internal consistency between meters, well under 1% difference.

So, relatively speaking, these meters are pretty consistent in their performance. Perhaps one of these days I’ll set up a calibrated load to get a better handle on the absolute accuracy. But for now, just knowing that the meters are consistent allows me to do the solar MPPT charger testing.

Update on the rooftop amp

Last time we visited the roof, the amp followed by the FM BCB notch filter was now in the die-cast enclosure, but not actually attached to anything.  Now it finally has a home, at least for now, on the tripod leg. It required a visit to Artie’s Ace Hardware in Phoenix at Tatum and Thunderbird, which until about 8 hours ago was unknown to me as a purveyor of a near infinite number of different kinds of metric fastener! Only 4 miles away, it’s a treasure to know that I can get an M4x8 mm pan head screw even late in the afternoon.

The metric hardware was required to install the steel mounting ears on the die-cast enclosure; those mounting ears accept the muffler clamps that hold the whole thing to the leg of the tripod. Later on this winter I’ll bend up some 0.032 Al sheet to act as a sun shield and remount the box on the north leg with the shield to keep it cooler during the summer. I still need to do something permanent about the power for the amp, it’s currently the solar power setup I made a couple weeks ago.

Left is input, right is output. Runs on any voltage up to about 32 vdc and down to about 7 vdc. The internal dc-dc converter keeps the amp supplied with an even 5.0 volts.

With this amp in place, my stack’o-scanners is just bangin’ along. I’ve got great reception, and no FM BCB interference. And, there’s space in the enclosure for a future Arduino or Raspberry Pi, as well as the necessary network connection.

Shocking new amplifier

Is finally built. I bought three of these a couple years ago from kuyaya520 on eBay and they’ve languished since then, heat-shrink protected and tie-wrapped in place, like for the truck’s half-deaf GRE PSR-600 scanner.

Finally put one in my new die-cast case that I got 10 of last month, and used a 82 ohm resistor to set the operating voltage to around 10 vdc when running off 13.8 vdc. According to the eBay page about the amp, its best gain and noise figure is around 9-10 vdc.

Checked out the gain, and it’s pert darn near what the vendor says it is. Don’t have a simple way to do noise figure. Need to get myself an ENR noise diode so I can do y-factor.

This amp will likely go into the truck to replace the tie-wrapped kludge… I’m starting to get reasonably good at assembling these things.

Next step on the rooftop LNA setup

This past weekend I finally started building the ultimate case to house the LNA and FM BCB filter for the rooftop multireceiver project.

Last night I did the final bit of wiring, installed a DC-DC buck converter to take +12 vdc and knock it down to 5.0 vdc.

Here’s some pics of the project.

The two separate die-cast enclosures are the LNA (lower) and the FM BCB notch filter (upper). There’s a LM2596 DC-DC converter in the lower left, and a weathertight Ethernet connection in the center-right wall. All external RF connectors are N. All internal connectors are TNC. All coax is RG316 double-shielded. I used an Ethernet connection to get power to the box and to allow the future addition of either an Arduino or an Raspberry Pi for telemetry purposes.

The overall enclosure with installed components. Got this particular enclosure for about $22 delivered – someone in Santa Maria CA didn’t want it. Was missing the base plate upon which I mounted all the components – fortunately, I had a piece of Al in the garage that was a near-perfect fit! Would have cost nearly $70 new.

The left N connector is the antenna input, the right N is the assembly output to the shack. The Ethernet connector is for future expansion and dc power.

Installed on the roof temporarily (will be mounted on the tripod to right ultimately) with power supplied by batteries in the Tupperware container.

The batteries (Eneloop AA x 10) are charged with a newly modified solar panel install. Again, over the weekend I cut some Al extrusions to replace the old way I’d attached a single 5 w solar panel, now it supports two 5 w panels for a total of 10 w. This almost guarantees sufficient charge for long winter nights.

In addition, the box mounted on the tripod already had a PoE Ethernet connection from the main rooftop unit. I use that 12.6 volt PoE  through a single 1N4001 rectifier to source power to the LNA box when the solar panels can’t provide enough charge to keep the batteries up. I want to add some telemetry (through a future Arduino or similar on the roof) to measure voltages, temperatures, etc.


10 MHz bandpass filter from China

Saw this 8 to 11 MHz band pass filter (item number: 201406314462) on eBay a few weeks ago and thought it might come in handy to help my SDR radios hear WWV and WWVB while not being overwhelmed by other signals.

Here’s what the 1 to 100 MHz band looks like using the PCR-1000 connected through the new antenna multicoupler, without the new filter. Remember, this antenna setup already has some significant attenuation below about 100 MHz since it’s using a VHF/UHF discone and the 10 to 2000 MHz LNA.

Here’s a plot using the ICOM PCR-1000 using the new BPF. Indeed, it has good performance, especially for the US$11.48 incl shipping.

LNA unhappy with 10 uH Coilcraft 0805 chip inductor

Today I finally synced up with my friend Doug and collected from him quantity five Coilcraft 0805 10 uH chip inductors. Tiny things. Had little caffeine today, so by the time I’d returned home I was pretty steady.

Brought the LNA down from roof, opened lid, clamped assembly down to bench so it wouldn’t move, put a drop of 60/40 on one inductor pad, and with my TU-10b tweezer I picked up the part and set it in place, then tapped the one end with the soldering iron. It was harder than I thought; the part weighs nothing and has no surface friction with the tiny bead of molten solder, so it instantly moved on me.

After some re-approaching of the problem, I got the part securely attached. Checked continuity, everything looked good! Lid back on. Connected it to network analyzer, and everything did NOT look good. 20+ dB gain above about 200 MHz. The whole 10-200 MHz output level was badly attenuated, and there were strange artifacts in the low end of the spectrum. Removed the inductor and everything returned to normal. Posted a note to the designer over on and am awaiting a response. 10 uH at 100 MHz is 6k  ohms impedance, so it should be fine. The LNA was drawing its typical current (~ 160 mA).

Took another identical  inductor and pressed it down on the pads with a plastic tuning stick, and did exactly the same thing as soon as it made contact. There’s something about that output circuit that doesn’t like the chip inductor.