LiFePo4 direct charging from solarpanels

How to recharge LiFePo4 batteries

directly with solar power

Be mobile with pure sunpower:

Electric mobility could be much more eficient if we do not have to reload the LiFePo4 batteries from the grid. Of course it is possible to do it with a solar system, too. The usual way to reacharge these new lithium-bike-batteries with a solar system, is like this: First charge a big 12V buffer battery, then transforme it up with an inverter from 12V to 230V, only to be transformed then back down to 36V or 48V by the lithium-battery-charger.
That is very awkward and complicate, and in each step are many unnecessary losses. Why this? During charging a lead-acid battery has an efficiency of max.80%, a good sinewave inverter creates a maximum of 90%, rectangle inverters only about 70%, most recently comes the lithium-charger with about 75% efficiency, which usually has to be cooled by a fan. So we end up losing summarizes the half of the solar energy through all of these procedures. But there is now another completely different way to do it!

And this is how to do it:

It's unbelieveable simple, and basically really easy! We need three solar panels in series (60V) for a 36V battery, and 4 panels (80V) for a 48V battery. Please make sure that all solar panels have about the same power and not only one can be shaded during the charging. The max. current of the panels should be appropriate for the lithium-battery-pack. For small batteries up to 7Ah better use only 50W panels with 2.5 A. Larger batteries with e.g. 20Ah can tolerate more, e.g.: 90W panels, which are charging with 4.5A.

But even this small portable and foldable solar charging system can be very handy.

With four small 20W solarpanels in series you can charge up a 48V/10Ah-Battery with 16 LiFePo4-Cells at only one good sunny day.

With 4 change-over switches and an extra regulator you can change the system into a 12V charger for LiFePo4-Batteries with 4 cells.

In a controller box on the back of the modules you find the 12V to 48V switches, two sets of regulators with their heavy-duty resistors. And a set of 4 balancers for 12V-LiFePo4-Batteries.

Without a controller you only would kill your batteries:

We have to have a regulator that is able to do two things. It has to disconnect the solar panels safely from the lithium-battery when it reaches the maximum charging voltage. But that's not enough. In order for the inbuilt balancers to do their job, the charging current should be limited to 500mA about 1V before reaching the maximum tension.
The circuit here has been developed especially for a very simple replica. The components you can get for very little money, even most of them can be recycled from old electronic scrap. The circuit is designed for 48V batteries, but is pretty easy to modify for 36V batteries by the use of a 39V Zener diode instead.

More about the circuit:

The left part of the circuit we need to make two times. One for Relay 1, which controlls the max. tension of 58.4 V. For further safety precaution, we stop the charging at 57.5V. Thats 0.9 V before the maximum.

A high voltage Schottky diode prevents an accidental reverse current at night, just in case you have forgotten to disconnect the battery from the charger.

The second circuit including relay 2 to opens up the charge current at 56.5 V, so that only 0.5 A can flow through the big 40 Ohm power resistor. So the balancer can do their work and balance all the cells in the battery well, without being overwhelmed by a big charging current, which would overload individual cells.

TIP: For old or heavily used battery-packs it is recommended to make this current brake even a little earlier (e.g. at 55.5 V), which takes a little longer then to be fully recharged. With even a suspicion of bad balanced cells (rather sudden loss of capacity to early shutdown while driving, or too short charging times after a empty battery), it is very helpful to put all the cell connections in the battery-pack outside with thin wires and a multi plug. In order to check the individual cell voltages during the charging. And if the BMS inside is optionally overwhelmed or broken, the cells can be charged or discharged manually.

The 0.22 uF capacitors prevent sparking at the relay contacts and increase the life span. The LEDs indicate the switching status and here you can see if the battery is shortly before finished its charging or completely finished.

The 150K ohm resistor generates a hysteresis of about 3V, so once cut off, the charger does not switch back. On the 5K ohm potentiometer the maximum voltage can be adjusted with a range of 53V to 60V. The 220uF and the 22μF prevent a flickering of the circuit. Please only use high voltage capacitors (63V) and transistors (80V e.g.: BC546 and BD139). The 12V relay, here with 290 ohms resistance can also be operated on 48V with a 7W strong 1K ohm resistor. The diode to the relay coil prevents hazardous high voltage surges for the circuit.

Here are these two circuits with a Schottky diode and a big 40 ohm power resistor together with 16 self-made-balancers (see my blog: www.Lithium-Balancer.blogspot.com) housed in a Aluminum box for cooling. A selfmade volt and ampere meter gives information about the sunlight and the charging status.

The circuit operates with ±0.05V super accurate. Its very sensitive to dust and moisture. Temperature fluctuations can alter the switching voltages slightly, but with 1 volt safety to the maximum rated voltage its not dangerous. I wish you much fun and happy driving with the pure energy of the sun!

For further questions please ask me at: solarmichel@hotmail.com