FLOAT CHARGING:
Float Charging – A continual charge voltage applied to the battery that is
in excess of it’s natural resting 100% SoC voltage.
LFP batteries are not lead acid batteries and they were not designed nor intended to be “
float charged“, in the typical
lead acid sense (definition above). There is scant data on float charging LFP cells. At CMI we have two years of standby testing (see below) on LFP and unfortunately the data collected is all over the map. While we have a few premium branded LFP cylindrical 18650 cells that suffered zero capacity changes, after being held at 3.400V (13.6V for a 12V nominal bank) 24/7, for six months continuously, we also have capacity losses holding the same voltage on counterfeit cells and no-name LFP cylindrical cells exceeding 16%. We also have an 11% Capacity loss on some CALB SE prismatic cells from leaving them at 100% SOC and letting them sit for 12 months. In other-words the data is confusing at best and our float life testing here is still on-going.
Lack of a Definition for “Floating” LFP?
Unfortunately there is no real definition for a voltage setting that is
not holding an LFP battery at a voltage above the natural resting 100% SoC point, as we do with lead acid. There are really actually two types of voltages necessary for LFP;
Storage Voltage – A voltage setting, usually programmed using the
float voltage setting on a lead-acid based charger, that results in the battery discharging to approximately 50% SoC (or a mid-range SoC) then being held there. A storage voltage would ideally be used anytime the batteries will not be used for any more than a a few weeks or so.
Standby Voltage – A voltage setting, usually programmed using the
float voltage settings on a lead acid charger. This voltage should be one that results in the battery discharging to just below the full charge point of the battery or 90% SoC to 98% SoC, an being held there.
As can be seen “
float” can be complicated with LFP. LFP batteries ideally need two differing types of voltages
storage &
standby but not “
float“, in the lead acid sense, where we purposely hold the battery above the 100% SoC point.
Look at any of your tablets, cell phones iDevices etc. and they all
terminate charge when the battery is full. They cut back in when battery terminal voltage has fallen to a preset level, but they do not hold a high voltage on a full battery.
Floating LFP is a certainly a complex subject with scant data. Bottom line is to avoid
floating LFP banks
if you can, but a standby or storage voltage setting can be used
.. For a typical standby voltage you would be best to be below 13.6V or 3.400VPC. Some have argued that a continual standby voltage of 3.35VPC or lower (13.4V for a 12V nominal bank) is not badly damaging over the long haul, but it may be, so a
storage voltage should also be considered as a longer term option.
Please understand that any voltage
below the 100% SoC point, of the LFP battery, would not be considered “
floating” it. If using a standby voltage at say 3.35V per cell, the current into the battery will end up at 0A and be slightly below the 100% SoC point.
Unfortunately, we don’t have enough data, across all cells, to confirm any capacity losses due to using a
standby voltage that is held at or near 100% SoC. There is very little research and literature on holding LiFePo4 at or near 100% SoC. If a standby voltage is high enough it keeps you in the upper SoC range for long periods of time, and these batteries, according to every LFP cell maker we know of, prefers to see them sit at a mid-range SoC when not being used. This is where a storage voltage comes in. These cells were originally designed to be actively cycled.
Can you hold a
standby voltage at 3.400VPC or 3.35VPC or lower? Absolutely, but we don’t really know the long term affects other than to say it is it
may shorten the life of some cells and may cause little to no harm to others. The premium cylindrical cells we tested at 3.400VPC (using a very expensive very linear power supply), lost no quantifiable capacity but some of the cheap cells lost as much as 16% in the same time frame using the identical charge source. Do you or will you know the quality of the cells inside your own battery and how they actually handle a “
standby voltage“?
Premium Cell #1 – 1100 mAh Rated – Base Line Capacity = 1.140 mAh – 6 Months at 3.400VPC = 1.130 mAh
Premium Cell #2 – 1500 mAh Rated – Base Line Capacity = 1.391 mAh – 6 Months at 3.400VPC = 1.387 mAh
Premium Cell #3 – 1500 mAh Rated – Base Line Capacity = 1.404 mAh – 6 Months at 3.400VPC = 1.403 mAh
No-Name Cell #4 – 1200 mAh Rated – Base Line Capacity = 1.101 mAh – 6 Months at 3.400VPC = 0.921 mAh
Counterfeit Cell #5 – 1500 mAh Rated – Base Line Capacity = 1.298 mAh – 6 Months at 3.400VPC = 1.192 mAh
If you will note above that only one of these cells delivered it’s rated capacity, cell #1. Three of them, the premium branded cells, lost virtually no capacity after six months at 3.400V and the two other cells, no-name brand and a counterfeit of one of the branded cells, lost quite a bit of usable capacity when floated at 3.400V or 13.6V for a 12V bank. The loses of capacity are likely due to the vendor using bargain basement, low quality internals.
Of the piles of white papers I have on LFP batteries not a single one of them has dealt with fractional “C” use of LiFePO4 and being held at 3.400VPC. The only float paper I have was using Mn doped LiFePO4 cells and in 24 months cells floated / maintained at 100% SOC and at 25ºC lost 30% of their capacity, without any cycling. What we do know is that storing these batteries at 100% SoC
resting voltage (not even charging) can lead to capacity loss and is advised against.
The question of f
loating or standby voltages with LFP, and its impact on cycle life, is still very much unclear.
Bottom line on this subject?
If you choose to use a
standby voltage be sure you are below 3.40VPC or 13.6V for a 12V nominal pack. Any voltage above this point will result in lead acid type
float charging and holding the battery above the 100% SoC threshold.
FULL ARTICLE
