New LiFePO4 Cells For Use With Solar Power. Test And Charging.

I picked up 28 unused, but older LiFePO4 cells, and started setting up a test system for them.

I also picked up some BMSs (Battery Management System), 2 for 12V 50/100A, and one for 24V 50/100A systems. I may need to get one or two more.

Solar Power Charging: 

The charging system for those batteries are configured like this:

I am using my solar system in my lab, described in the previous post, i.e. 2x 100W solar panels, charge controller and 3x 30Ah / 12V batteries. This may be expanded a bit with more batteries in parallel. providing a nominal capacity of about 180Ah. Because the batteries are GEL type, I count on a total usable capacity of about 90-100Ah.

The single cell charger consists of a DC/DC buck (step down) converter capable of delivering a maximum peak current of 20A, with CV (Constant Voltage) and CC (Constant Current) settings available on the PCB. To be sure of avoiding overheating I have set those like this:

CV: 3.60V (the upper limit of charge voltage for LiFePO4 cells)

CC: 10A (Just to keep it cool)

The circuit does have a fuse in the battery lead, a 15A one. A bit of redundancy is good. The system should not make a show of spontaneous combustion ;) .

This system is now running, and the first cell is being charged. Things seem to run smoothly right now - no overheating.

More Efficient Energy Use:

The intention is to get all 28 cells tested for capacity, then match them for the best possible batteries.

For this I could just use a resistive load and waste all the energy. Here is the idea to avoid some of this waste:

When I have 4 cells fully charged I will get them balanced for voltage. This happens with 4 low-value resistors, one in series with each battery (plus connection). When connected all in parallel the 4 battery-resistor sets will slowly balance the voltage of the cells. This is the simplest way to do this, and may not keep the batteries in balance long term, but that can come later, when all cells have been fully tested.

The first 4 cells can then be connected with a BMS to form a second 12V power supply, provisionally. With this and a second charging circuit, and considering the losses in the system, this system should be capable of charging 3 cells from (near) 0 to full charge. The 4th cell can be charged from the solar system, providing a new 4-cell battery for test charging the next cells. I could also be connected as a provisional (second) solar power system.

This is a long process, but it will test the cells for reaching the full charge voltage.

Checking Capacity Of Single Cells:

The intention is expanding the 12V system for the radio station, and also create an expanded battery capacity in the lab upstairs.

I expect the 12V battery at the station to be about 300Ah (12 cells), and the lab system to be 200Ah (8 cells).

The first system should likely be the one for the lab. So a full test will have to be made of enough matched cells (8 pcs) to make the 12V - 200Ah battery with all cells the closest possible with respect to capacity.

For this a discharge system has to be made. This could be a simple resistive load with a digital meter. The meters I have seen, however, need 6.5V, so they cannot be used for testing a single cell. Hmmm!

Can I program an Arduino (or the like) and make a simple circuit for testing capacity? Of course...

The simplest solution will be making a simple constant current load (or a simple resistive load) and measure the time before the output voltage reaches the lower limit of 3V, then read out the time and switch off the load. Not too difficult with an Arduino, even with my limited programming skills.

I do not need to measure the absolute capacity, just to match the cells, so the resistive load should be sufficient. The absolute capacity can then be measured when the cells are connected as a 12V battery.

It will take a while, as I am not constantly home to monitor the process.

Also, making this with simple means, things do take time, although they battery/cell testing is not too time consuming, as other things can be done while the tests are running.

I suspect some building of supports for solar panels, and some simple receiver circuits and/or microwave stuff, is in my future, along with the battery testing - and doing some QSOs with the radios.

No time to be bored...

Update: After some hours of charging the first cell still charges with about 8A, and the voltage rises very slowly.