Making a rugged solar charging system that will survive years of outdoor use exposed to extreme temperatures is not trivial. This board is intended to be a base for such a system.

Features

• Based around LiFePO4 battery technology for long life with thousands of charge cycles and improved temperature behavior.
• Input voltage range of 4.5V to 28V for compatibility with a wide range of solar panels.
• Output voltage of 2.7V-3.6V can be used to directly power 3.3V electronics with load currents up to 3A.
• Solar panel MPP (Maximum Power Point) is settable with a single resistor.
• Maximum charge current of up to 3A, multiple default values available and customizable (up) by adding a single resistor.
• Low voltage load disconnect at 2.7V battery voltage to prevent over discharge and maximize battery life.
• Over temperature (58°C to 64°C) and under temperature (-3.2°C to 0.2°C) protection to prevent charging outside of recommended battery temperature range.
• Heater control that turns on battery heating from the solar panel's power during under temperature conditions.
• Large through hole solder pads to facilitate easy assembly even when using large gauge wire.

Calculating MPP resistor

To extract the maximum amount of power from your solar panel, you need to customize the maximum power point setting to match that of your panel. Use the following formula to calculate the value of the MPPT resistor:

MPPT = 120500 / (Vmpp - 4.295)

Some examples values from my own testing with different panels:

• 6V panel, Vmpp = ~5.2 V -> MPPT = 120K
• 9V panel, Vmpp = ~8V -> MPPT = 33K
• 18V panel, Vmpp = ~16.2V -> MPPT = 10K

Customizing maximum charge current

There are many ways that charge current is limited by different parts of the system, so often it's not necessary to worry about limiting the maximum charge current. For instance, even in the brightest sun, the output power of the solar panel may be limited to the extend that the charge current never reaches the limit of the battery. You can check this by taking the wattage of the panel, and using it to calculate the theoretically maximum charge current (Icharge = Ppanel / 3V).

If you find you do need to limit the current, you can calculate the maximum current as follows:

Ichargemax = 0.12 / Rsense

Rsense for different default charge current versions I have for sale:

• 3.07A: 0.039 ohm
• 2.14A: 0.056 ohm
• 0.5A: 0.24 ohm

The maximum charge current can be increased by adding a resistor IMAX in parallel with the sense resistor already present on the PCB. The new sense resistance can be calculated by:

Rsense = Rdefaultsense * IMAX / (Rdefaultsense + IMAX)

You can then use this new Rsense value to calculate the resulting charge current limit.

Thermistor

A 10K NTC thermistor needs to be connected for the system to operate correctly. The system has been designed for thermistors with a B value of 3950, but reasonable performance can be had for B values down to 3435. There is a footprint for surface mount (R2) as well as through hole (THRM) thermistors. Only one of them should be used at a time.

Many battery packs come with a 10K thermistor built-in, and connecting this thermistor to the charger is the ideal situation. If no thermistor is present in the battery pack, we sell a thermistor with 8 cm leads that can be taped to the battery pack. A surface mount thermistor on R2 may work if there is little temperature difference between the PCB and the battery. If you don't want thermal protection of any kind, a 10K resistor can be installed instead of a thermistor.

Below are calculated threshold levels for different B values of the thermistor:

• [B value 3950] Under temp limit: 0.2°C (falling), 0.7°C (rising) - Over temp limit: 52°C (falling), 58°C (rising)
• [B value 3435] Under temp limit: -3.2°C (falling), -2.6°C (rising) - Over temp limit: 57°C (falling), 64°C (rising)

Battery heater

In under temperature conditions, the solar panel's voltage is applied to an optional heater connected to the HEATER pads. This can be used to bring the battery up to temperature before charging is started, ensuring that no low temperature lithium plating occurs and thus maximizing battery life.

In its simplest form, the heater can be a power resistor thermally connected to the battery pack. Since no MPP is maintained, the user needs to take care to choose the resistor value so it will not collapse the solar panel voltage under reasonable light conditions where enough energy is present for heating, while at the same time ensuring it doesn't pull more than 1.1A under ideal light conditions.

Limitations

• Does NOT come with a battery, this one is BYOB (Bring Your Own Battery).
• ONLY use LiFePO4 batteries, no other Li-ion or LiPo!
• Only use a single cell 3.2V LiFePO4 cell, or place cells in parallel to create a larger capacity LiFePO4 battery that still has 3.2V output voltage. Cells in series are NOT supported.
• Battery over-discharge protection has nominal shutoff threshold of 2.7V, turning the load back on at 2.835V nominal.
• The load output has a 3A thermal fuse, the heater output a 1.1A thermal fuse.
• If you don't connect an MPP resistor, the MPP voltage will be 4.295V. Unless you have a 5V solar panel, you will not extract energy from the panel efficiently.
• If you don't connect a thermistor, the system will be in under temperature protection.
• Don't trust your solar panel spec, it is likely highly overrated. Do your own testing to find maximum power output and maximum power point voltage. In reality you can expect about 70% of rated power.

Prototype units

These solar charging units are offered at a low cost to get them out in the field and start to collect usage information. The design has had some testing done, but not enough to make solid claims about performance in extreme conditions. The specifications listed are design specifications but actual performance may not meet these numbers. My hope is that by offering these with very little markup, people will use them in their projects and together we can gain field experience.

Disclaimers

These devices deal with possibly high voltages and fairly high currents, in addition to high energy and power density batteries, used in possibly extreme outdoor environments. As such, using them correctly requires experience with electronics and an understanding of the rigors of protecting electronics in outdoor environments. I cannot be held responsible if something dies because it got wet or you put something on fire or you blow something up because you don't know what you're doing.

That said, since these are prototypes, something might be wrong with them, or they may not last as long as expected, so if you have a problem you can contact me, and we can discuss what happened. If you can provide me with valuable failure data, and you haven't done anything stupid, I could likely be persuaded to replace a bad unit. :)