Help with Low Power Mountain Top Solar Radio Repeater
For the past two years I have been experimenting with making a small GMRS radio repeater. The radio, a Retevis RT97, does not use much power. It uses about 2 amps at 12 volts when transmitting and 75 milliamps in standby. It is in standby most of the time. The charge controller is a simple PWM. It charges it charges at 14.6/14.4/ and floats at 14.0 volts. It has a low voltage load cut out circuit that will turn off the load at 11.2 volts. It will reconnect the load when it sees voltages over 12.6. More information on the controller is at the bottom.
My first attempt was using a 9 amp hour battery and a 30 watt panel with a 10 amp PWM controller. This worked well enough during the late spring, summer early fall when we have an enormous amount of sunlight. During the winter it would start to shut down and take nearly a week or so to come back online for a few days before going down again. Around February of 2021 I went up to check on it. I found the solar controller off and the battery was showing 2.5 volts. Brought the battery home and when warm it moved up to around 5 volts. I was able to "recover" this battery but I haven't done any load tests on it to see how it fared. I assumed the battery and panel were too small to make it through cloudy days, the voltage dropped, cut off, and then the battery froze. Once the battery froze the controller shut down and never came back online.
For my next attempt I figured I would increase the solar generation capacity, increase battery storage capacity and put the battery underground to protect it from the cold snaps. I added a 50 watt panel, changed to a 35 SLA amp hour battery and put it just over a foot under ground. That setup was installed in May of 2021. It worked without going down all spring, summer, fall and into the winter. I subjected the radio to some "strenuous" use during December and it saw several days where it had about 4 hours of transmitting time.
On December 20th I was using it for several minutes and it shut down mid transmission. This lead to me believe that it hit the low voltage cut off. Being in Alaska, and the day before winter solstice, I wasn't all that surprised. At this time of year the panels only get 3 hours of sunlight directly hitting them. December was also a very cloudy month. It was about 20 degrees F that week. The following week we had nearly 7 days of clear skies and full sun. The air temperature dropped to around -15 degree F at night that week. Near the end of the week the system had not come back online and I grew suspicious. I figured maybe the battery froze or maybe the panels can't make enough power to get the voltage up above 12.6. The outdoor air temperatures have been in the upper teens to 30s for the past few days. The ground temperatures are showing around 20 degrees F based on research I did online but I suspect them to be closer to 0 or 10 degrees as that is more in line with a average daily temperature.
I took a 14 amp hour SLA battery, ran it under a 2 amp load at 66 degrees F until the cut off circuit of a spare charge controller turned off the load at 11.2 volts. I placed the battery outside in the negative temperatures for 24 hours. When I checked the battery it had a voltage of 11.8 and could still run the light bulb for a short time before hitting the low voltage cut off again. It didn't appear to freeze in the negative 10 to 15 degree evening.
I then took that cold battery, the charge controller, and a spare 30 watt panel out to sunlight around noon. I hooked up the charge controller and it was still indicated a low battery and kept the load circuit shut off. Once hooked up to solar, the 30 watt panel took only a minute or two to get the voltage above 12.6 volts. This turned the load circuit back on. Several minutes later the voltage of the battery was up at 14.4 volts while connected to the panel. I measured to amps between the controller and the battery. In full sun the 30 watt panel and PWM was creating one amp of power to the battery. Clearly, in full sun, the panel can create enough power to reach standard charging voltages. So that doesn't appear to be the issue....and neither does a frozen battery based on my next tests. The attached images are from this test.
I did a second test. One where I tried to freeze a SLA battery. I have an older 3.3 amp hour battery. I attached the charge controller to it and discharged it with a .5 amp load for several hours before the controller shut off the load. The voltage stabilized at 12 without the load after about 30 minutes. I connected the load right to the battery and drew it down to sub 9 volts. The battery stabilized to 11.3 volts after a few hours. When I connected the charge controller, the small, ~20 milliamp load, brought the voltage down to 10.5 volts. I chucked this whole contraption in the freezer for 12 hours. Checked it again and the controller is still on, still indicating low voltage and the load circuit is off. The voltage still reads 10.5 volts. It weakly illumining my .5 amp load (a light bulb) and the voltage dropped to around 4 volts before recovering back to 10.5 upon removal. I assume this means the battery, even in such a low state isn't frozen?
The final test I did was taking the charge controller (I have a few of these lol), and seeing how it reacts to low voltages. I used a buck converter to raise and lower the input voltages. I found that it can run with voltages as low as 2.5 to 3 volts. Below those it will shut down. Once the voltage creeps above 2.5 to 3 it turns back on. As the voltages increase it will turn the load back on as normal. This seems to indicate that if a battery froze and stopped producing power, once the battery thawed the controller would come back to life. With the controller coming back to life I would assume it would start adding power from the panels. The only thing I didn’t test to far it to see how the controller reacts when solar is connected under such extreme low voltage situations.
I almost am starting to feel like maybe the solar controller has failed somehow and the battery is “okay”. The battery is surly in a very low state of charge but I don't think it is frozen and incapable of taking a charge at the moment. I tried to confirm this by visiting the site but due to weather and terrain it is inaccessible.
The current charge controller is a Hunie 10 amp PWM I got on Amazon.
I decided to try an EPEVER MPPT controller for next year…mostly b/c I can program a higher cut off voltage to protect the batteries better when I fail to get them charged. I chose the 10 amp Traver2606BP.
Again this is a remote mountain top setup. The only way to get to it is backpacking so size and weight constraints are a major contributing factor. My questions are, where are the flaws in my system, am I using flawed logic, does anyone have any similar situations and what suggestions do you have to overcome these issues.