The LiFePO4 body battery is deeply discharged and the charge controller no longer recognizes it or refuses to charge it? This happened to us when we bought the motorhome because the previous owner forgot to disconnect the battery over winter. The cell voltage had dropped below 1 volt (LiFePO4 cells should not be discharged below 2.0 to 2.5 volts, otherwise it is called deep discharge) and the charge controller of our Ective SSI 10 refused to start the charging process, because below 10.2 volts battery voltage it switches to an error mode. So how can you revive a LiFePO4 battery that is deeply discharged? The only way that seemed to make sense to me was to charge the individual cells using a laboratory power supply*.
Setting the laboratory power supply to rescue the deep discharged LiFePO4 battery.
In this process, the battery is initially charged with a constant current, then later with a constant voltage until the charging current drops to (towards) zero. To set the current limit of the laboratory power supply, simply short-circuit the terminals and set the current limit. This should not be too high here. I chose a very cautious 3 A, which would theoretically make the complete charging process per cell – 100 Ah in our case – take about 30 hours. However, this was more to protect my lab power supply from permanent operation at the load limit than it was necessary for the battery. So if you have confidence in your lab power supply, you can increase the charge current in the constant current phase up to 0.5 C (half capacity in A – in our case that would have been 50 A). But be sure to pay attention to safety at such high currents, especially the required conductor cross-sections. Our cross-section calculator for power lines can give you a clue.
The current limitation is set by short-circuiting the terminals of the laboratory power supply. Here 1 A current limitation is set. The voltage limit (here 3 V) can be set at any time without anything being connected.
The voltage limitation of the laboratory power supply is set to the charging end voltage. I chose 3.6 V (note: in the pictures you can see how I initially set a lower final voltage to speed up the process and get into constant voltage charging faster). The voltage limit is set without anything being connected to the power supply. This is because as soon as the deeply discharged LiFePO4 cell is connected, the voltage displayed on the power supply unit drops to the battery voltage. This is what we are doing now.
Now the voltage of the battery (and at the same time the voltage displayed on the power supply) should slowly increase. The higher your set current, the faster it will go. Once the charging end voltage is reached, which we have previously set at the voltage limiter of the power supply, the constant voltage phase in the charging process of the LiFePO4 cell begins. As the name implies, the voltage at the charger is limited and thus kept constant while the charging current slowly decreases. When the charging current finally reaches 0 A, or approaches this value, the charging process is finished and the next cell can be charged in the same way.
LiFePO4 rescue: Final top balancing
If all four cells are charged, top balancing can still be used. This serves to “synchronize” the cell voltages so that the charge level is the same and they are simultaneously full, or empty, when reconnected to a pack. To perform top balancing, the individual cells are simply connected in parallel, i.e. the positive poles are connected to each other (using a busbar or cable with a similarly large conductor cross-section – large currents can also flow here, you should not underestimate this) and the negative poles are connected to each other. So you leave the four cells then just about after.
Once top balancing is complete, we reassemble our battery. So we connect the four cells in series (plus to minus poles), so that we again reach the nominal voltage of around 12 V that we need. Last but not least, the BMS is then connected again.
LiFePO4 deep discharged: Was the rescue successful?
In this way, I was able to save our LiFePO4 surface-mounted battery. It has been doing its job without complaint for over a year on our trip, including through Portugal. Later, I took more precise measurements to confirm my impression. For this purpose, I precisely determined the internal resistance* and the capacitance* of the individual cells. Despite the deep discharge, the cells did not suffer any permanent damage. The internal resistances as well as the capacitances of the four cells are on a very similar level. This is also evident in daily operation, because the active balancer (which has been added in the meantime) is hardly used and even the passive balancer integrated into the BMS has almost nothing to do. The cell voltages are almost identical over the entire charge/discharge cycle and only move slightly apart at large loads and shortly before the end-of-charge voltage.
The whole thing took a while, of course, and if you don’t already have it, you also need to invest in – at the very least – a laboratory power supply and a multimeter*. For me, however, it was completely worth it, because in the first shock I thought I would have to replace the deeply discharged LiFePO4 battery.
Disclaimer/Disclaimer: Be sure of what you are doing! Electricity can kill you (and your loved ones) in the stupidest case and cause great damage! All work mentioned here I have described to the best of my knowledge and belief, but I assume no liability or warranty for the accuracy of the information or any resulting damage. Sometimes a crimp is not done properly or a cable is not laid according to VDE. If you’re not sure or don’t understand things clearly, ask someone who does or hire a professional.
Note: The links marked with * are so-called affiliate links to Amazon.de. If you buy a product through one of these links, the owner of this site receives a commission from Amazon.co.uk, which ensures the continuity of the site. We hope for your understanding. In addition, we hope to save you some product research time by linking. So all in all, it’s a win-win situation.
