NOTE: We are now selling the new revision 4!

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, customizable (up) by adding a single resistor.
• Low voltage load disconnect at 2.7V battery voltage to prevent over discharge and maximize battery life (at which point current is reduced to ~1uA).
• 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.
• In revision 4, heater control now pulses the heater current to prevent the solar voltage from collapsing, configured by the same MPP resistor as charging!
• 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

With revision 4 I only have units with a default charge current of 2.14A for sale, which uses a 0.056 ohm Rsense.

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 = 0.056 * IMAX / (0.056 + IMAX)

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

If you need to reduce the maximum charge current because your solar panel is too powerful for the battery capacity, you will need to replace the Rsense resistor, which is marked R16, with a value larger than the default 0.056 ohm.

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 one side of the thermistor is connected to ground internally in the pack, connect the external side to the THRM pad closest to R2. 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. If nothing is installed, this looks like an under temperature condition to the device and your battery will not charge.

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. In revision 4, a poor man's MPP was added to the heater control that will pulse width modulate heater power to prevent the solar voltage from falling below 70-80% of the MPP voltage of the panel. This is configured with the same MPP resistor as charging. The HEAT LED will pulse at the same rate as the power applied to the heater so if the heater resistance is very low, the HEAT LED may not turn on visibly as the pulses will be very short.

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 pack 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 uses a load switch since revision 4, which provides better protection. The overload current limit is 3.1A ~ 3.8A due to component variations, 3.45A typical. Note that the load switch consumes 80uA when the load is turned on. This is usually minimal compared to the current drawn by the load but more than in previous revisions which used a MOSFET, so this could be a downgrade in certain applications.
• 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.

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.