I'm looking for feedback on the approach I'm taking with my first LiFePo4 battery. A small one, but the idea is that this will prepare me for the next one (~8-10kWh) that will provide electricity for the household).
The goal for the first battery is to replace a bulky 80Ah Exide Equipment Gel leisure battery used for camping purposes. Average daily consumption is ~30-40Ah. Most of this is used while the sun is shining, so I suppose that even a 30Ah battery will do the job, but I've decided to go with 100Ah just in case we get a cloudy day. For charging, I use an Epever Tracer 1206AN and 160W panels with 36V MPPV (higher voltage, less current, cheaper and lightweight cables).
I started with just a bit of knowledge - 4 LiFePo4 cells are supposed to form a battery that will suit my needs. Based on the positive feedback for the Docan store in Alibaba I ordered 4 cells. I ordered 100Ah plastic CALB cells as I was aware that they are reliable. Unfortunately, these were out of stock and I ended up with 105Ah EVE cells. While waiting for them to arrive I started picking up the other components.
The BMS was ordered from Aliexpress - Hankzor products look promising and the one I picked has the following parameters:
* Charging protection voltage: 3.65v
* Discharge protection voltage: 2.35v
* Charge release voltage: 3.55v
* Discharge release voltage: 2.55V
* Equalizing voltage 3.405v
* Equalizing current: 194ma
* The maximum continuous over-current value is 120A
* Maximum charging current: 60A
The equalization currently is likely going to be reduced after a few tests. The equalization voltage of 3.4V is what caught my eye. I will use a lower charging voltage (14V probably) to be a bit more on the safe side and to try to keep the cells below 90% SOC.
Next comes the fuse. EVE cells are 0.3mOhm and the maximum current is 12kA. 20kA to be a bit more on the safe side seems enough. Luckily my load is low and a cheap 32A 10x38mm fuse that has a breaking current capacity of 50kA will do the job. While looking for it I saw its bigger brother - 22x58mm fuses with a breaking capacity of 120kA. These will be used for the big battery. A lot cheaper compared to the T class fuses. I'm in Europe and I suppose this makes a difference to what I can easily find in the store. T class fuses are hardly available around, but the listed fuses are pretty easy to source.
Next comes the clamping force. The specs say 300kg. But the same load applies to the big EVE 280Ah cells that have a larger surface area. At 12psi the load should be 200kg and I plan to stick to that number. I'll use aluminum plates on both sides, 8 x m4 studs, and 8 springs. Between the cells and the aluminum plates I'll put an insulation layer. Most likely the same material that is used for making PCBs (without the copper part).
Then come the bus bars. The stock ones are straight and quite long. The width of the 105Ah cells is around 37mm. The current option that I'm considering is to laser cut thin aluminum bus bars (0.6-0.8mm) and stack 2 or 3 of them. These will be bent in the middle to remove the stress from the terminals.
Then comes the pack design. The picture shows the current idea. The BMS (green) and the fuse (blue) are on the same side as the springs. On the top will be placed an additional isolation block to protect the cell terminals and to provide a base for fixing the wires.
The battery connecting terminal will be an XT90 fixed to the outer enclosure. The current flowing from and to the battery will be no more than 10A and the XT90 is more than sufficient.
The outer enclosure will probably be made of plywood. Still checking the other options.
There is one concern I have. I'm reading that the charging of LiFePo4 cells should be terminated at 0.05C. I'm planning to charge the batteries with 0.1C to 0.05C. This opens a question - is limiting the charging voltage to a lower value going to limit the SOC? And if yes what would be the recommended value for the bulk charging and the float charging voltages?
The goal for the first battery is to replace a bulky 80Ah Exide Equipment Gel leisure battery used for camping purposes. Average daily consumption is ~30-40Ah. Most of this is used while the sun is shining, so I suppose that even a 30Ah battery will do the job, but I've decided to go with 100Ah just in case we get a cloudy day. For charging, I use an Epever Tracer 1206AN and 160W panels with 36V MPPV (higher voltage, less current, cheaper and lightweight cables).
I started with just a bit of knowledge - 4 LiFePo4 cells are supposed to form a battery that will suit my needs. Based on the positive feedback for the Docan store in Alibaba I ordered 4 cells. I ordered 100Ah plastic CALB cells as I was aware that they are reliable. Unfortunately, these were out of stock and I ended up with 105Ah EVE cells. While waiting for them to arrive I started picking up the other components.
The BMS was ordered from Aliexpress - Hankzor products look promising and the one I picked has the following parameters:
* Charging protection voltage: 3.65v
* Discharge protection voltage: 2.35v
* Charge release voltage: 3.55v
* Discharge release voltage: 2.55V
* Equalizing voltage 3.405v
* Equalizing current: 194ma
* The maximum continuous over-current value is 120A
* Maximum charging current: 60A
The equalization currently is likely going to be reduced after a few tests. The equalization voltage of 3.4V is what caught my eye. I will use a lower charging voltage (14V probably) to be a bit more on the safe side and to try to keep the cells below 90% SOC.
Next comes the fuse. EVE cells are 0.3mOhm and the maximum current is 12kA. 20kA to be a bit more on the safe side seems enough. Luckily my load is low and a cheap 32A 10x38mm fuse that has a breaking current capacity of 50kA will do the job. While looking for it I saw its bigger brother - 22x58mm fuses with a breaking capacity of 120kA. These will be used for the big battery. A lot cheaper compared to the T class fuses. I'm in Europe and I suppose this makes a difference to what I can easily find in the store. T class fuses are hardly available around, but the listed fuses are pretty easy to source.
Next comes the clamping force. The specs say 300kg. But the same load applies to the big EVE 280Ah cells that have a larger surface area. At 12psi the load should be 200kg and I plan to stick to that number. I'll use aluminum plates on both sides, 8 x m4 studs, and 8 springs. Between the cells and the aluminum plates I'll put an insulation layer. Most likely the same material that is used for making PCBs (without the copper part).
Then come the bus bars. The stock ones are straight and quite long. The width of the 105Ah cells is around 37mm. The current option that I'm considering is to laser cut thin aluminum bus bars (0.6-0.8mm) and stack 2 or 3 of them. These will be bent in the middle to remove the stress from the terminals.
Then comes the pack design. The picture shows the current idea. The BMS (green) and the fuse (blue) are on the same side as the springs. On the top will be placed an additional isolation block to protect the cell terminals and to provide a base for fixing the wires.
The battery connecting terminal will be an XT90 fixed to the outer enclosure. The current flowing from and to the battery will be no more than 10A and the XT90 is more than sufficient.
The outer enclosure will probably be made of plywood. Still checking the other options.
There is one concern I have. I'm reading that the charging of LiFePo4 cells should be terminated at 0.05C. I'm planning to charge the batteries with 0.1C to 0.05C. This opens a question - is limiting the charging voltage to a lower value going to limit the SOC? And if yes what would be the recommended value for the bulk charging and the float charging voltages?