54 replies to this topic

[buckland]

Hey everybody... Just off the phone with BB. I outlined all I had in the camper with solar and controller and DC to DC charger and he recommended ..... relax.... select lithium preset and go camping. I asked about my install of the second battery in parallel and all he said to do was disconnect both ...put each a on tender to bring both up to same voltage.... then re-install in parallel. He said no need to tell the controller it is now 200 Amps. (I do not have a separate Battery monitor... just the Victron 100-20 w/bluetooth). The only reason for the second BB battery is we have two eBikes to charge through the Renogy 1000W inverter. Otherwise we would only need one.

[ri-f]

I’m an old aircraft electrician, and I didn’t know either...  Prof Google says:

 

The tail current is decaying current that represents open voltage and time dependent conductance elicited by the first step now closing when stepped to a hyperpolarizing voltage and vice versa if the step eliciting the tail is opening the channel.

Nice try: ), but this definition refers to tail currents in medical-neuroscience applications. Rolls Batteries has an explanation closer to what we are talking about. Rolls is referring to their AGM batteries here, not Lithium, nonetheless, it's a pretty good simplistic explanation for Tail Current:

 

END AMPS OR RETURN AMPS

 

As batteries near full capacity, charge current decreases. End Amps, Return Amps or Tail Current refers to the lowest output of current (Amps) flowing to the batteries as they have reached full capacity. Some chargers will determine this set point has been reached by monitoring current output to the battery bank. If the charge current drops, reaching the End Amps/Return Amps/Tail Current set point before the programmed Absorption time has completed, this will trigger the charger to shut off or switch to the Float voltage phase which holds the battery bank at 100% SOC. The charger will complete the full programmed Absorption time if this set point is too low or programmed at 0%.

 

The recommended End Amps/Return Amps/Tail Current set point for Rolls Flooded models is 2% of the 20 Hr AH rating (C/20) of the battery bank. Typically, when current drops to the 2% set point for 1 hour the battery bank has reached 100% SOC.

 

Rolls Flooded models with Advanced NAM may have a slightly higher End Amps/Return Amps/Tail Current set point as the cells are less resistant to charge and current output will remain slightly higher at 100% SOC. (2% recommended, 2-5% range) An adjustment to this set point may be required when these models replace other Flooded batteries. Test specific gravity at Float Charge to confirm 100% SOC.

 

WARNING: The End Amps/Return Amps/Tail Current setting, combined with sulfated batter(ies), may confuse the charger as added resistance will reduce the flow of current. This may falsely trigger the charge to end the Absorption charge prior to reaching 100% SOC. Test specific gravity regularly to confirm the battery bank has reached 100% SOC and adjust this set point and/or Absorption time as necessary.

 

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The tail current percentage range can be anywhere between 0.5 (narrow end) to 10% (wide end). 4% being the typical default for LiFePo4 batteries. And 2% to 4% being common set points values. 

 

Rich

 

 

[Wallowa]

So setting the Tail Current point protects the battery from overcharging [?] by shutting off the charger or going into float?

Edited by Wallowa, 07 May 2021 - 08:23 PM.

[ri-f]

So setting the Tail Current point protects the battery from overcharging [?] by shutting off the charger or going into float?

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Something like that. It won't shut off the charger, your BMS will shut it down if it exceeds > 14.7v on a BB. The tail will just reduce the higher absorption voltage so it doesn't keep putting in, for example: 14.4v endlessly and unnecessarily when a full charge has already been met. When the criteria for dropping to a float voltage are met (i.e., minimal current, for a set length of time) it will drop the voltage from absorption, towards the float set point, for example: ~ 13.2v if your on an AC charger, or at least less than absorption voltage, if you're on a fluctuating charger like your solar panels.Anyway,  adjusting the tail current percentage correctly, somewhere between 0.5 to 10% (typically 2-4% on the average), helps to ensure that your SOC reading is synced up correctly and giving you an accurate meter reading, and that the charging routine isn't ending prematurely or running too long. Any battery electrical engineers out there to explain this more precisely?

 

Rich

Edited by ri-f, 07 May 2021 - 09:40 PM.

[rando]

With a battery charger, the tail current is what tells the charger the battery is fully charged and to switch from absorb to float.     This is not very important for LiFePO4 as unlike a lead acid battery, they don't really need much of any time in absorb.   A lead acid will switch from bulk  charge to absorb charge at about 80% SOC, and then take 3 - 6 hours in absorb to get to to 100% SOC, which is very important for lead acid.   In contrast, an LiFePO4 will be at 95 - 99% SOC when you switch from bulk to absorb, so it will only take 15 - 30 minutes to get to 100% SOC, and it doesn't care if you don't make it all the way to 100%

 

For LiFePO4 you can set the tail current fairly low, like 1-2% of the battery capacity and then set the absorption time limit to 30 - 45 minutes.   That way if you have loads on the battery when you charge, or varying current available from solar you won't fool the tail current detection and you will drop to float in 30-45 min no matter what. 

 

Tail current is a little different on a battery monitor like the BMV-712.  In that case it is used to detect when the battery is fully charged to resync the SOC to 100%.   If the voltage is above the charge detect voltage AND the current into the battery is below the tail current, the BMV will assume the battery is full and reset the SOC to 100%.