The setting for the tail current in the Victron Connect App for the BMV-712 is expressed in percentage of the capacity. This doesn’t make any sense as the capacity is measured in Ah (actually in Wh even, but we all know that it needs to be multiplied by the voltage) and the tail current is measured in A.
How do I interpret that percentage setting? Do I implicitly divide the value by h(ours)?
I own a battery with 100Ah capacity. I assume that 4% tail current is supposed to mean 4A. Is my assumption correct?
PS: The UI for the Smart MPPT and the Phoenix Smart directly show that value in A(mpere) which is physically correct. IMHO it is weird that comparable settings are named differently and use different units within the same app for different products.
Yes the 4% tail current setting means 4A for a 100Ah battery. I agree the units don’t strictly match up. People seem to be a bit lax about this when it comes to batteries. There is also the common misuse of C (where 1C is the amp hour capacity of a battery) and max. charging current will be specified as a faction of C e.g. 0.5C
I know this question is solved but what other factor would you suggest to use to determine the proper tail current for a battery bank? If the question is asked “What should my tail current be set at?”, how do you accurately answer this question? I don’t know how to determine the proper tail current settings for a battery bank other than using a pre determined percentage of the battery’s capacity based on the batteries chemistry and charge/discharge characteristics. The rule of thumb for lead acid, because of its higher resistance, has always been around 4% of capacity. This is probably why the BMV comes with that as a preset. As an installer I can do the math for any battery bank of any size that I may encounter. If anything the MPPT setting should be changed to a percentage. The tail current setting is in the “expert mode” settings so I guess this implies you should know what the heck you are doing if you have gotten that far.
Furthermore, the tail current setting for a BMV or Smartshunt serves a totally different purpose than that of a MPPT. One is a battery monitor and the setting is used to help determine the point the battery is fully charged and to calculate the SOC. The other is a charger and the setting is used to actually charge the battery properly, telling the charger when to switch to absorption charging.
What I would expect? Proper physical units to avoid guessing. Either the the unit should read as “A” with an absolute value or - if it should be a factor of the capacity - the unit should be “%/h” or at least the manual should explain what units are implicitly assumed.
Yes, I know that the setting of the tail current in a charger and the monitor serves different purposes: the charger uses this as a threshold to switch from floating to storage charging, the monitor as a threshold to consider the battery fully charged. Still in both cases the tail current denotes the same physical effect and its unit is A.