Lithium Batteries In a Non-Heated Environment.

When LifePO4 batteries are located in an unheated outdoor equipment shed in climates like Colorado their winter temperature can fall below freezing quite often. LifePO4 batteries will be damaged if they are charged when they are colder than freezing. A couple of uninviting options exist. First, the shed can be insulated and heated, which could be a lot of work and expensive. Second, the batteries could just not be charged until the temperature warms up. Even on a sunny day that typically means around noon and leaves only time for a partial charge on short winter days. A third option appears to be the least painful and that is to provide some external source of heat directly to the batteries through the use of heaters. There doesn’t seem to be any product marketed specifically as a LifePO4 battery heater. Researching alternatives, one possibility is the silicone heaters used to warm the bed of 3D printers. It is flat, comes in various shapes, voltages, power ratings, and it is inexpensive. A sampling was ordered and tried out. Finally selected is a 20 watt 12 volt heater shown below.

These pads fit nicely between alternating cells so each cell is adjacent to one heater. Leads are brought out and connected in parallel with wire nuts. Each heater draws 1.5 amps and in the lineup below 4 heaters draw 6 amps.

Getting this far was the easy part. Figuring out how to power the heaters is the next challenge. It was quickly learned that using the batteries themselves was a net negative. Heaters use too much power. The batteries don’t get fully charged before the sun goes down. An external set of batteries was tried but that just shifted the problem. After a few days the external batteries don’t have enough charge to run the heaters. Another failure was the use of timers to only turn on the heaters right before the sun came up. A new idea was needed. Time for …


While the batteries are too cold to charge and the heaters are running, the solar cells are sitting idle wasting generated power. Duh. Why not use the solar power to run the heaters? This idea was tried and has been working successfully for several cold winter months. Power was tapped where the solar panels go into the solar controllers. The tap is the small red and black wires in the picture below.

Raw voltage is typically 20 volts and that would burn out the heaters. A 10 amp buck converter was inserted in the line to keep the voltage at 12 volts, one buck converter for each of the battery banks. A metal box limits the rfi emitted from the digital buck converters.

A W1711 thermostat rounds out the installation. This little guy is set for 5 degrees Celsius which allows some margin to make sure the batteries are kept above freezing when the sun is up. When the sun isn’t up there is no concern because there is no solar power available to damage the batteries. What happens when there is solar power but the batteries haven’t warmed up above freezing? The Morningstar controllers were specifically chosen because of their feature called “low temperature fold back”. Even when there is solar generation, if the batteries are below freezing the Morningstar controller will refuse to charge the battery.


Insolation Monitor Project

Insolation is a big word meaning how much sunshine is there? That’s an interesting bit of information when one is trying to keep batteries charged with solar panels. It’s just a cross check to see if the charge amperage is consistent with the amount of sunshine each day.

The project consists of a photo cell and an Arduino-based device called a ESP32. The hardware looks like this. Very minimalist. The breadboard is just to hold the ESP32 in place. A USB cable brings in 5 volt power.

The ESP32 connects to the Internet over wifi and uploads data every 10 seconds using the protocol MQTT, “the standard for IoT messaging” . Data consist of the resistance of the photo cell. A server processes the data and provides a web page GUI. The server is called a broker and in this case the broker is provided free for personal use by Adafruit. The ESP32 is also a product of Adafruit. The ESP32 cost $20 at Microcenter.

Below is a screenshot of the GUI page, putting it all together.

Ideas for the next version: Mount the ESP32 inside a solar powered yard light and eliminate the USB cable. Disconnect the light and power the ESP32 instead.

For a closer look the link is here:

This solar powered led yard light was chosen at random and it was chosen for it’s reasonable price. When it arrived it looked like this:

Opening it up revealed a pleasant surprise which had not been mentioned in the sales description. It has an actual 18650 lifepo4 battery. Perfect. This battery should power a ESP32 for many hours. The ESP32 draws 100ma at 5 volts which is one half watt. The 18650 is rated at 4.4 watt-hours (4.4 watts for an hour). That would be 4.4/.5 or 8.8 hours. In reality that time would be extended by the ESP32 going into sleep mode when it’s not sending data. It would never need to send data constantly for 8.8 hours.

Unfortunately the controller board that comes with the unit will have to be discarded because it doesn’t have the features needed for the ESP32.

Will the ESP32 fit inside the waterproof cabinet? Looks like it will.

In fact, a LORA board will fit very nicely, too, and that can come in useful for the next project, building a LORA network.

Reading up on how to power a ESP32 from a solar yard light has revealed some challenges but also solutions. First, the cell voltage is 3.7 as can be seen in one the pictures above. The ESP32 needs either 5 volts or 3.3 volts, neither of which is close to 3.7 volts. What is needed is either a boost converter to get up to 5 volts or a buck converter to get down to 3.3 volts. The battery voltage of 3.7 is nominal. The voltage can vary from 4.7 to 3.2. When it’s 3.7 or above the buck converter works fine but when the voltage drops below 3.7 the buck converter shuts down. That rules out the 3.3 volt option. Looking at the 5 volt option, there is a possible solution. Connect a standard charge controller between the solar panel and the battery such as the TP4056 Charging Module 5V Micro USB 1A 18650 Lithium Battery Charging Board with Protection (5 pieces for $5.95 on Amazon) which looks like this. It’s output will vary with the voltage of the battery.

Boost converters exist ($7.29 for 5 pieces on Amazon) that will provide a constant output of 5 volts with an input as low as 1 volt or as high as 5 volts and look like this.

The concept is the charging module will regulate the solar input to keep the battery properly charged. As the battery charges and discharges the output voltage will vary. The boost converter will take that varying voltage as input and it will output a constant 5 volts.

Moving on to the next step, those parts will be ordered today. Total additional cost $2.86 per unit.

Baby Solar Panel

A baby solar panel was added for the purpose of charging the battery dedicated to LifePo4 heating.

The panel is only 10 watts and will probably not do the job but it was available and worth a try. Heating the LifePo4 batteries is done to allow more hours of charging and discharging during the winter. LifePo4 can only be charged when the temperature is above freezing. Discharging is ok but the battery is more efficient if it is warmer than 0 degrees Celsius. They are already protected from charging below freezing by the solar controller. Heaters just add some hours of operation in the Colorado winter that otherwise would not be available. A separate third battery is being used for the heaters. Experience last winter showed without heaters the batteries do not get warm enough to take a charge until little or no daylight is left.

Update: A 100 watt panel has replaced the baby and has been added to the end of the rack. A thermostat is on each heater but keeping the heat on 24 hours a day is too much drain on the heater battery. The 20 watt heaters draw 25 amps at 13 volts or 325 watts. There are 16 heaters deployed between the cells. Each heater is placed between two cells. Currently the heaters are on a relay that is controlled remotely to turn on the heaters when they are needed and to save battery capacity.


From Lead Acid to Lithium

The original golf cart and deep cycle marine lead acid batteries didn’t seem to have the oomph they once had.  They are running down too quickly and are getting harder to recharge even though they are only 2 years old.  Batteries Plus tested a couple of the golf cart batteries and said they had about a year of life left.  They tested by drawing 125 Amps for a minute on each battery.  Those results were an inspiration to look around for new batteries.  Upgrading to AGM batteries would cost $1600 (from a dealer quote in Denver).  Then a YouTube video was discovered about LifePO4 lithium iron phosphate chemistry.  See Will Prowse the DIY Solar master at

Four LifePO4 100AH cells were purchased on eBay along with a BMS (Battery Management System) in an attempt to move up the learning curve and find out if LifePO4 would be a good upgrade.  In ensuing months it was learned that there is a lot to learn.  Lithium chemistry is far superior to lead acid for a solar application but there is a learning curve.  For example, LifePO4 batteries can not be charged if the temperature is below freezing.  That’s a significant hurdle for an outdoor storage shed on the Colorado plains.  After a month of use and several paradigm changes the Lithium batteries are proving to be a great upgrade.  One advantage is the space saved with Lithium.  Compare the Lithium sitting on the shelf (green) to all the lead acid batteries on the floor it replaces.IMG_0929

The solar controller was replaced with a model that was designed for Lithium charging, which is a Morningstar ProStar.  In addition a second order of 4 batteries was added for a total of 200AH rated capacity.  Heater pads (12 volt, 25 watt) were place between the cells with a thermostat set to 5 degrees Celsius.  Two autumn frosts have occurred and the heaters have kept the batteries above freezing.  As of this writing the ambient temperature is 1C and the battery temp is 6C.  Will they hold up over the cold winter ahead?

Followup after a snow storm and 108 hours below freezing at the end of October:  Yes, the heaters kept the batteries warm.  However, the Lithium did not have enough power budget to supply the heaters, the radio equipment, and the computers along with snow on the solar panels.  A low voltage disconnect occurred causing no damage but the station was off the air.  The heaters and the computers were moved back over to the lead acid batteries to get back on the air during the storm.  To avoid spending any more money this hybrid arrangement will be used for a while.  Long term solution is more Lithium batteries.

Solar Panel Tilt Adjustment


Solar Panels

Time for a seasonal adjustment of the tilt on the solar panels.   Today they were re-tilted to 17 degrees instead of the 40 they had been set to which is shown in this picture.   They will be changed for winter/summer.  This new tilt was obtained from one of the many online calculators.  Next step is to see what time battery “float” condition is reached each day.  It had previously been 1 pm.  With the new tilt float still comes after noon so more panels are probably needed.  On Summer Solstice, June 21, 2018 at 1:00p Mountain Daylight Time, the sun peaked at 17 degrees as measured with an angle detector.  The 17 degrees calculation seems to be ideal for Denver summer’s insolation.





  1. exposure to the sun’s rays.
    • the amount of solar radiation reaching a given area.

Update: Additional solar panels were added so the batteries can reach float more dependably especially in the winter.  These also can be tilted to match the prescribed angle for the season (shown at their old summer setting).  The new panels have their own controller which is connected in parallel with the original controller at the battery terminals.  A parallel connection works because each controller monitors the battery voltage and applies charge as needed. They do not fight each other or require any synchronization.


One thing leads to another.  Upon attempting to connect the new panels it was discovered that there are no more vacant power jacks on the battery bank RigRunner panel.  John, KC0RF,  a telecommunications power engineer, observed we needed some serious reworking of the battery wiring to stay safe.   John insisted on having separate buses created for the charging and for the distribution, and fusing the charge connections to the battery.  Here is a white-board sketch that shows how the recommendations were implemented.


Ground wires are left out of the drawing but each battery string also has a (negative) ground wire that is home run to the buses along with the positive leads.   “Essentials” in the sketch above are the router, Ethernet switch, and internet modem.  They are not switched by the RigRunner 4005i like everything else.  “Controller” is the solar controller for the two panel arrays.  The small rectangles represent 30 Amp inline fuses.  The “Charge Bus” is along the top and the lower bus (not labelled) is the “Distribution Bus”.  All wires are 10 AWG.

It looks like a mess but John said it was pretty, knowing the installation was now safe and up to telecommunications standards except for cable dressing. Pictured are 8 golf cart batteries which are 6 volts each, wired according to the diagram above.


Now there is a location to connect the new solar controller and yes, the batteries reach float earlier in the day.  But that’s all changed.  See “From Lead Acid to Lithium”

September update:  The panels were tilted down to 26 degrees for the winter.  Unfortunately there wasn’t enough room between the bottom of the new panels and the ground to allow for snow.  A cedar post was installed and the panels were moved up.  Plenty of room for snow now, just in time because it was already snowing today.

IMG_0016 (1)

Update:  The tilt on these panels looks too steep.  The sun doesn’t peak at that low of angle above the horizon on December 21 in Colorado.  Consulting solar web sites we determined the ideal angle for this latitude is 26 degrees for winter.  The tilt was readjusted and float is being reached much earlier in the day ever since.  Where did those old angles come from, anyway?

Update:  The 26 degree angle and the second panel has worked perfectly over the winter.  On December 21 (worst case because it’s the solstice) the batteries reached float by 11am.

Another Update (summer 2019):  The two solar controllers connected in parallel have been replaced by one controller with large capacity.  Although two controllers did not fight each other they did give each other false readings.  If one controller was charging the battery the other controller saw that higher voltage and decided the battery was fully charged.  Vis-versa.  Even though they didn’t fight each other they did confuse each other.  Only one panel could be charging at one time.   This was effectively only a one panel system.  A new controller that can handle all the panels all the time will keep the batteries charged better it is hoped.

Spring 2020 update: Cells tied together as one big battery and both solar controllers paralled as before to make one system rather than two separate ones.  Eight CALB brand lithium 100AH cells were added over the winter and now all 16 cells are one big battery.

The controllers are behaving nicely.  In the morning when the battery is lowest and needs the most charge the controllers both deliver their current at the same time.  As the battery nears the top the controllers alternate.  One stays in the bulk state and the other goes to absorption.  After a few minutes they switch.  This seems to be keeping the battery charged as intended.

Solar tilt:  As an experiment the tilt of the panels has been left at 26 degrees for the summer.  The thinking is the panels will be able to produce enough power even at these angles and it will save a lot of work every spring and fall.  At the end of April it is working and the summer should only get better as the sun angle gets higher.


Solar Panel Addition

Today we added a fourth solar panel for the purpose of improving early day battery charging.

IMG_0205The other three panels are aimed due south re-tilted at 26 degrees for maximum December 22 output at noon.  This new panel is aimed due east for the first sun of the morning tilted to obtain peak output at 8 am.  We often want to operate just after dawn when the batteries are at their lowest.  This panel should fill in the gap until the main panels are getting sun.

Additional Batteries

Three deep cycle marine batteries were just not providing enough reserve capacity.  They charged quickly and on sunny days were fully charged by 9am even in the winter.  After dark, they drained quickly, too. Time to upgrade. Research showed a good battery for the money is a golf cart battery.  They are rated at 215 AH, 6 volts each, and sell for $89.99 at Batteries Plus.  We purchased two pairs and wired them in series/parallel to provide a rating of 430 AH capacity at 12 volts.  We didn’t run out of juice nearly as early at night but the first snow storm proved we needed still more.  Running without recharging for 3 days was impossible even with 430 AH.  Next purchase was a Morningstar Sunsaver Duo solar controller to replace the Sunsaver SS-20L.  The new controller allows two banks of batteries, meaning we can combine the golf cart batteries and the original deep cycle marine batteries.  We’re waiting for the next snow storm to see if this was the correct decision. Here’s a current peek inside the shed.  The batteries are on the floor on the right hand side.


The Sunsaver Duo controller comes with a bonus.  It has a data port that provides information about the battery’s state of charge.  An adapter has been added that provides Web access and we can now see how much battery capacity remains any time.

After a few weeks of operation the new battery arrangement seems to be a success.  The radio bank and the peripheral bank both reach float before noon on a sunny day and neither bank has dropped below 12.0 volts during stormy spells.