Originally Posted by Davidhoy
For those of us unfamiliar with "tail current", would you mind explaining?
A major part of charging
lead acid batteries
correctly is the transition from the absorption voltage (typically around 14.7v) to the float voltage (typically around 13.8v). Leave the batteries at the absorption voltage for too long and the batteries will be “overcharged”, shortening their life. On the other hand, changing to float voltage too early will undercharge the batteries, once again shortening their life. It is important to realise that slight overcharging is much better than slight uncharging, but the ideal is for the solar controller to pick the correct transition point.
The ideal way is to measure the voltage and current entering the battery. At absorption voltage the current entering the battery will gradually reduce and when this reaches around 1% of battery capacity (so 4A for a 400 AHr battery) this is an appropriate point to drop down to float. The 1% number is debatable and depends on the battery type, but the ideal range is narrow (0.5% to 2%) as the current drops off rapidly as the battery becomes charged. This 4A number is the tail current.
Unfortunately, this requires communication with a shunt close to the battery, which is not available in many systems. In the absence of this, ideal system controllers try to pick the point of transition without knowing how much current is entering the battery. This is normally done by holding the absorption voltage for a set (but adjustable) time, usually around two hours. This sounds hit and miss and in many ways it is, but with some care selecting the absorption time it can still work
The Victron controllers have incorporated a second method by using a tail current setting into the algorithm, but the solar controller on its own cannot measure the current entering the battery, only the current leaving the solar controller.
The Victron controllers were previously fixed with a tail current value of around 1A (this depended on the size of the controller). So if the solar controller was putting out 1A or less and the battery voltage was at the absorption value, the solar controller would assume the battery was charged and drop down to float. The problem with this system is that it does not work with multiple charge sources and most boats have multiple charge sources. Particularly in the case of multiple solar controllers, the tail current caused problems with one or more of the solar controllers dropping to float too early because the controllers tail current criterion had been met.
For example, with two solar controllers set for the same absorption voltage, when the battery needs say 5A to maintain absorption voltage, ideally each controller would supply 2.5A, but this never happens. In practice one controller might supply 4A and one supplies 1A. This is due to slight manufacturing and wiring
differences. It is of no consequence to the battery, as it receiving the correct voltage and current, but the solar controller supplying only 1A concludes the battery must be charged, because the voltage is high and the current is low. The but solar controller does not realise the voltage is high because another charge source (the other solar controller) is supplying 4A of current. Thus the change to float is incorrect.
Very few solar controllers incorporate a tail current criteria into their algorithm unless they can communicate with a battery shunt for this reason, but it was until recently a fixed and unchangeable part of the Victron controllers.
Without a battery shunt communicating with the controller the “tail amp” setting is not the real tail amp setting so unfortunately you cannot set the value to the number quoted by the battery manufacturer.
If you have a boat with multiple solar controllers this is very likely to be causing one solar controller to drop to float incorrectly. Even with one controller and a smart alternator
controller, there is a reasonable chance this setting was causing problems. If you leave the boat for long periods with no loads, or have a motor
home or home solar system, the tail current setting is likely to be helpful, but for most boats that meet the above criteria it is much better turned off. Now you can.