Today there are LiFePO4 batteries on the market that require the installation of an external BMS and batteries that already have a BMS inside them.
The latter is called “Drop In” or even “Plug and Play” precisely because the marketing of the manufacturing companies would like them to pass as perfect replacements for the old lead-acid batteries: you buy them, put them in place of the old ones, and everything will work as Before.
Is everything they say true? Of course not, and we will soon understand why.
Drop-In or External BMS?
So the first thing to decide, before making the purchase, is which type of battery we should buy. Integrated or separate BMS? Generally speaking, batteries with integrated BMS are more convenient because they simplify installation.
However, installing an external BMS makes the system more versatile and scalable, and it will be easier and less expensive to carry out updates and improvements in the future. Not to mention that, in the unfortunate event that the BMS or the cells of a battery should break, we could intervene by replacing only the defective piece. In contrast, in the case of Drop In batteries, we would be forced to throw everything away: battery and BMS.
The other advantage in favor of the separate BMS is that, in this case, we are free to choose the BMS we like best or with the technical characteristics that best satisfy us, and we will not be forced to take “blindly” what the manufacturer decides to put inside the batteries, whose actual quality is often ignored. Especially in cheaper Drop In batteries of more or less unknown Chinese brands, it is often the BMS that is sacrificed to have a competitive price. The final consumer doesn’t see it and often doesn’t even know it exists. The savings in producing LiFePO4 batteries are almost always realized in the electronic components (BMS) and the assembly.
In this regard, if you want to waste some time, there are videos on Youtube where cheap lithium batteries are purchased on various sites such as AliExpress, eBay, etc., and are opened to check what is inside. Surprises are interesting. Often the opening of the external container will reveal cheap BMS, not to mention the assembly of the various cells, made with very thin aluminum busbars instead of copper, and cables with a section unsuitable for the currents involved.
Even the current limit I mentioned earlier is typical of the inadequacy of the electronics contained inside the Drop-In batteries. Specifically, it is almost always caused by the contactors (the relays that disconnect the battery in case of problems), which cannot handle the power very high. To partially get around this problem, if you plan to buy batteries of this type, consider that it will be better to take, for example, 2 100 Ah 12v lithium battery and connect them in parallel rather than taking just one 200 Ah one. In the first case, the maximum current value will be given by the sum of the currents accepted by the single batteries in parallel (example: 50 + 50 = 100 Ampere).
In any case, installing lithium batteries that require an external BMS is always preferable, even if it requires a greater design effort.
In this case, a BMS can control a whole bank of batteries in series/parallel. Not only will you be able to choose the BMS you want, but it will almost certainly be able to actively communicate with the other components of the system (battery chargers, inverters, battery switches, and so on) to switch them on and off, thus ensuring a very long life to your precious batteries.
How to install Lithium Iron Phosphate batteries
Checks on the electrical system
Lithium batteries are light years ahead of lead ones but are more delicate in some respects. They do not tolerate voltage being maintained beyond certain values for long periods.
Charging them is much simpler than lead-acid batteries, as there is no need for three-stage algorithms (bulk, absorption, and float), but it is enough to apply a constant voltage to their poles, usually 14.2 Volts. As soon as the battery voltage reaches this value, it will be 100% charged, and the charging process must be stopped as soon as possible. (lazye.com) What does this mean? Lithium batteries do not require any float phase (maintenance). If your battery maintainer wants a float voltage set, you can set it to 13.5 Volts. Still, it would be better to avoid keeping them at these voltage levels for long periods (days or weeks) because, as mentioned in the previous post, they don’t like staying at 100% charge for a long time. Using lithium batteries in this way also means significantly reducing their useful life.
If you have to use them for weeks or months without cycling them, i.e., always connected to a charger, it would be highly advisable to keep them at a voltage of 13.3 Volts, corresponding to a SOC (State Of Charge, i.e., a charge level) of 90%. Approximately.
Attention does not neglect this aspect because it has been demonstrated that for LiFePO4 batteries, prolonged overcharging or long maintenance at 100% of SOC, rather than a constant very deep cycling, up to 10% of SOC, is more harmful. Lithium batteries are made to be charged and discharged continuously.
What does this mean for us, in practical terms? It means that our charger devices, of whatever type they are, i.e., not only the 220 Volt charger but also the alternator, the solar panel regulator, must be checked and set correctly (if they allow it).
Checking the 220 Volt charger or solar/wind charge controllers.
Most recent battery chargers now have a charging program suitable for lithium batteries, which in any case, should always be checked against the values I have written above. Sometimes, charger manufacturers make the predefined programs for charging LiFePO4 spectacularly wrong. This a sign that there are still many people (even among professionals) who understand very little about lithium Iron Phosphate batteries. In the best battery chargers, manually setting all the charging parameters is possible. If your charger is one of these, I recommend setting it up like this:
Bulk voltage (usually these terms are used): 14.2 Volts
Absorption time: should be set to the lowest possible value. Ideally, it should be eliminated, so a zero value would be fine. If your charger forces you to set a time, choose the lowest possible and set a bulk/absorption voltage of 14.1 Volts instead of 14.2.
Float Voltage: 13.5 Volts or slightly lower. Normally the value of 13.5 Volt corresponds to a 100% charge when the lithium battery is at rest. So if you set this value, the batteries will always remain at 100% as long as the charger is connected. If you leave the batteries connected to the charger for long periods, lower the value to 13.3 or 13.4 Volts, corresponding to a 90-95% SOC. Remember that a lithium battery is always better to keep it a little discharged, rather than overloading it or keeping it at 100% for long periods. In prolonged storage, the batteries should be left at a temperature between 10 and 25 degrees, disconnected, at 40% – 60% charge. This is the best way to store them.
Checking the engine alternator
Adjusting an alternator is a more complex matter. Standard alternators cannot adjust output voltages and application times. Only a few alternators equipped with external charge regulators can do this, but 99% of the alternators installed on our vehicles have a simple voltage regulator built into them, which is quite stupid and only outputs a voltage of about 14. 5 Volts (may vary depending on the alternator model, from 14.2 to 14.7 Volts). So boost/absorption voltage is usually too high for the Lithium, and, above all, no Float phase.
If this isn’t a big problem for lead-acid batteries, it is for lithium-ion batteries, for the reasons I’ve already discussed in detail.
Furthermore, charging lithium batteries(12V battery, 24V battery, 48V battery) using the alternator could also cause problems for the alternator itself since these absorb very high currents during the charging phase. This is one of their main advantages, as for this very reason they recharge in a very short time, but could lead the alternator to work for long periods at maximum power. The alternators that equip our engines as standard are not designed for this overwork and, therefore, can overheat until they burn out.
Do you want to know another problem you could have when charging your lithium batteries with the alternator? Well, it is that the BMS disconnects the battery for some particular reason. For example, it is 100% charged or because the voltage is too high. Well, know that the alternator does not like being suddenly and brutally disconnected from the load while delivering power. Most of the time, this action translates into a very high voltage peak (in jargon, it is called spike), which can even reach hundreds, thousands of Volts of very short duration, in the order of milliseconds, but which will be sufficient to burn the alternator diodes and – perhaps worse – to destroy the electronic devices you have on board!
These problems are not insurmountable, but they must be studied, and resolved. There are so many variables that avoid stunning yourself with too much close information. I prefer you to digest what has been said so far.