Solar Li-ion Charger and Power Supply for Arduino

1. Description

The most Arduino-based projects are powered by external power supplies or battery-packs. The use of such power supplies might be sometimes impractical (i.e. remote microcontroller system placed in a less accessible area). In such cases, a solar power supply backed-up by a rechargeable battery could be more suitable. The purpose of this tutorial is to show how to build such a power supply.

Difficulty level: beginner.

2. Parts

– 1 x Solar Panel Module (6V, 1W);
– 1 x Li-ion Battery Charger Module (TP4056 5V Mini USB 1A);
– 1 x Boost Step-up Power Supply Module (DC 0.9V-5V to DC 5V);
– 1 x Li-ion Battery (18650, 3.7V, 2,250mAh);
– wires.

3. Li-ion Battery Charging Cycle

Lithium based batteries are one of the most popular types of rechargeable  batteries due to the high energy density, tiny memory effect and low self-discharge. The downside is that they cannot be charged using a regular power supply; their charging cycle involves two stages:
constant-current stage: the battery is charged using a current-limited  power supply. The recommended charging rate is 0.5C – 0.7C (C is the  battery power capacity). The end of this stage is reached when the maximum voltage is reached; at this point the battery charge is about 70-80%;

Ex. The battery used in this tutorial has a nominal voltage of 3.7V, maximum voltage of 4.2V and a power capacity of 2250mAh (C = 2250mAh). The constant-current stage should be done at a charging rate between  0.5C = 1125mAh and  0.7C = 1575mAh; in our case the charging current will be limited at 1000mA that corresponds to a charging rate of 0.44C.

constant-voltage/saturation stage: When the battery reaches its maximum voltage, the charger acts as a voltage limited power supply; the battery voltage remains at maximum while the charging current drops gradually. When the charge current is below 10% of the nominal capacity, the battery is considered fully charged.

Li-ion battery charging cycle (charge rate: 1C). The constant-current and constant-voltage charging profile depend on the charge current, battery maximum voltage and power capacity.

– Li-ion batteries do not need to be fully charged as it’s the case with other types of rechargeable batteries. Actually it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold or eliminating the saturation stage prolongs battery life but  reduces the runtime;
– trying to charge Li-ion batteries differently than according to their charging cycle specifications might cause overheating, explosions and serious injuries.

4. Li-ion Battery Charger Schematics

(click to enlarge)

5. Assembly

(click to enlarge)


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