I am looking to get a better understanding of the use of tail current and absorption time in setting up chargers and shunts, specifically when charging LFP. My understanding is that the tail current will specify when absorption charging stops based on the battery current reaching at or below the specified value. For example a 2.0% tail current for a 200AH battery bank will stop absorption charging when the batteries will only receive 4 amps. However, I also understand that using a fixed absorption time will also stop absorption at this fixed value. For example, I have seen a fixed absorption time of around 20 min per 100 AH of battery recommended, resulting in 40 min fixed time for a 200AH battery bank.
My questions are these:
1.Am I understanding the basics of what I stated above correctly?
2.If both options will stop absorption charging why use both values? Why not just use one or the other?
3.Is one option better than the other in any situation?
4.How does the “charged voltage” value on a shunt or BMV factor into this?
The time limit on the Absorption cycle takes care of two scenarios where the Tail current does not reach the minimum value: first where there is an external dc load that the charger does not measure, the second where there is a battery fault (eg shorted cell in a lead acid pack), and again the tail current does not drop. Stop bulk after excessive time feature also relates to a battery fault.
Both options should be used, settings depend on application.
4)Charged Voltage parameter is also coupled with a time parameter. The Charged voltage should be set slightly below the Absorption voltage of the charger, and the charged time set to less than the time before the charger switches to float - usually 3 -5 minutes should suffice. Once both the time and voltage conditions are met i.e. Vbatt> Vcharged for T min, then the SOC will be reset to 80 or 100% depending on system.
Whilst all of the above is designed for Lead batteries, lithium battery charging is fairly similar - though the Abs voltage and current should be under the control of the BMS rather than the charger. This allows proper balancing to take place.
The charged voltage needs to be exceeded in the shunt while the current is below the tail current for the shunt to synchronise to 100%. The charged voltage should be set 0.1-0.2V below your absorption voltage so for the synch to occur you need to be in absorption.
The answer to tail current or fixed absorption is more complex. There are two reasons for getting to absorption, balancing the cells and completely charging the battery.
Typically, cell balancing only takes place in absorption so you need long enough in absorption for balancing to take place. The rule of 20mins per 100Ah is quoted by Fogstar batteries, possibly others, Victron default is 1hr. This is because the BMS has a limited balancing current, so the more Ah in your cells on a BMS the longer the absorption needed for balancing.
Charging your battery is essentially complete when the tail current reaches 4% and this can occur after 10 minutes or less of absorption. There really is no need to go longer than this as the extra Ah charged is minimal, and holding the batteries at absorption for extended periods negatively impacts on cell life. However, too short an absorption can result in cell imbalance which can cause operating problems and reduced life of extremely high or low cells.
As you can see, the 2 reasons have opposing requirements and have different end criteria. With simple batteries you have to choose the lesser evil, which is where the 20 mins/Ah or 1hr absorption time because these periods are still long enough for most balancing but not too long for damage accumulation of the charging is infrequent. The trouble is, many systems get a daily charge so your batteries end up getting a long absorption every day, which is not required to keep them balanced. Victron recommended something like 4-8 hrs a month so 8-15 mins a day. What you can do is set end of absorption based on tail current, monitor your cell balance and if they drift then set a fixed absorption for a few charges to get the balance back, then go to tail current again.
To complete the picture, it is worth discussing fully featured BMSs that take control of the chargers such as the Victron Lynx NG. These monitor tail current, absorption time AND cell voltages. They may have maximum absorption times set as a safety limit. The BMS continues absorption until the current is below the tail current setting AND the cells are balanced. If the cells are balanced then absorption stops very quickly, perhaps only 5 minutes.
Depending on the BMS, that will take a while to get them straight. You could consider to charge the lowest cell by hand for a while to get there sooner. Did you do a good top balancing by commissioning the system by the way?
Charging your battery is essentially complete when the tail current reaches 4% and this can occur after 10 minutes or less of absorption. There really is no need to go longer than this as the extra Ah charged is minimal, and holding the batteries at absorption for extended periods negatively impacts on cell life. However, too short an absorption can result in cell imbalance which can cause operating problems and reduced life of extremely high or low cells.
Exactly. And because of that I’m curious what the reason might be that this tail current cannot be set in the Multiplus II series. (like it can in the Bluesmart chargers). It’s an excellent expert setting!
I have commissioned the Victron system 5 weeks ago, balanced one battery 2 weeks ago.. (the battery with the 2 cells outliners) after 2 days of balancing all the cells were within 3 mV.
Right now I’m balancing the battery again…the 2 cells have again much higher voltage then the other 14 cells (at close to absorbtion voltage).
I have a second battery, it’s now charged to 100% with a small power supply, no cell problem here.
Both batteries are the same with each 16kwh.
When both battery’s are 100% and balanced I connect them together on the DC busbar.
i do not know the answer definitively but I think the Multiplus do not have an accurate DC current measurement device inside. There are posts suggesting that they back calculate DC current from the ac and efficiency. Look at how the battery monitor synchronises, you tell it the battery % at end of bulk, no tail current setting.
