Self made, low cost LiFePo4 battery

My self made LiFePO4 battery

We need a battery with just enough capacity to run our fridge through the night, give us the light for the night and charge several electronic devices of two teenage kids during our vacations. Basically that is it. We don't live in the camper and we also don't go on winter vacations. So no big battery is needed. However, I wanted LiFePO4 batteries since I have never had much luck with lead batteries. The trouble with LiFePO4 is, that you should protect the cells with a Battery Management Systems (BMS) unit from high and low voltage situations. Before I built my own battery, I watched a few Youtube videos. Like so many times, the people making thes videos use good but expensive components, likely because they get sponsored by someone. Since I'm not getting sponsored for anything, I was searching for affordable components. I have seen Battery Management Systems (BMS) for self build batteries that cost upward of €120. To me it does not make sense to spend that much for a battery with 40Ah.
Unfortunately most BMS units in the lower price range only have a common port for charging and discharging. That has a major disadvantage. Because the BMS will shut of charging and discharging at the same time, in case it runs into an under or over voltage situation. Let's say you wanted to fill up your battery to the max because the next day the frogs predict clouds and rain. If one cell reaches the BMS protection voltage, the BMS will disconnect the negative (-) line of your battery from your electric system. But not only the charging of the battery gets stopped at that moment, you also can not use anymore power out of the battery, to discharge it. You can now hope that the cell will drop a few millivolts so the system starts up again in a few minutes. But then you better be quick to burn some of the excess capacity in the battery. Worst case you will have to manually bypass the BMS and use up some power. The situation is worse when the system runs into a low voltage disconnect. Let say you park your camper for an extended time. You don't worry about the battery, since it's being charged every day by the solar system. But unfortunately you also forgot to turn the fridge off. Your battery may now get into a situation during a bad weather period that it drops  voltage below the BMS protection point. The BMS will disconnect the battery from any consumers to protect the cells. But it will likely not be able to re-connect to allow charging. That means your battery will sit at minimum voltage until you manually re-charge the system by bypassing the BMS. Depending on how long the batteries sit a low voltage, you may already cause some damage to the cells.
Therefore I would only suggest a BMS with separate ports for charging and discharging. Unfortunately many of the BMS that offer separate ports, only allow very low charge currents through the system. But wit a little trick you can also use these relatively low cost BMS and allow charging with more than a few amps.
Check the video above to see how I solve that little problem. One can actually also use a relay for discharging the battery. In this case a really low cost BMS can be used to control the relais. The disadvantage is some small amperage draw by the relays. But my system is being re-charged every day by a little sunlight, even in the winter.