locked Re: Batteries starting to get weak - MMM - What to do -

Mark McGovern


That's part of the issue.  If you read each manufacturers instructions for how to store their LiFePO4 batteries you will get very different answers.  There is some level of commonality between some of them, but the specific recommendations are quite different from manufacturer to manufacturer.  See below for the details. 
Given that these batteries are all made from cells of the same chemistry (LiFePO4) and, in some cases, possibly the same exact cell manufacturer, that just doesn't make sense to me.  There should be one "best way" to store these batteries.  My conclusions from this lack of consensus seven or eight years into selling these batteries commercially are:

1.  The battery manufacturers don't really know for sure what the best storage method is


2.  The storage method does not affect the longevity/capacity of the batteries significantly enough to warrant investing the time and resources to figure it out

I am not stating that storage method does not affect the longevity/capacity of LiFePO4 batteries.  I actually believe that it does based on the reading and research that I have done.  But what I am saying is that if storage method was significantly affecting the manufacturers warranty costs and/or reputation, I believe that they would have figured that out by now and they would all have very similar recommendations for how to best store these batteries.  

Contrast this complete lack of consensus on storage method to the manufacturers recommended/built-in Low Voltage Cutoffs and High Voltage Cutoffs.  It is accepted fact that over-charging (voltage too high) and over-discharging (voltage too low) is what kills LiFePO4 batteries prematurely.  Every LiFePO4 battery manufacturer that I have researched has set a High Voltage Cutoff at 3.75-3.90 volts per cell and Low Voltage Cutoff at 2.5-2.8 volts per cell. While they are not exactly the same voltages, they are all based on a defined voltage level and they all are within the consensus "acceptable" voltage of 2.5 to 4.0 volts per cell for LiFePO4.

As I stated earlier in this thread, I've only seen one person actually experiment on what happens to LiFePO4 batteries when stored at a high SOC% (link referenced earlier in this thread).  The two experiments he conducted showed a permanent capacity loss of 4-12%.  This was after leaving LiFePO4 cells that were charged to 100% SOC and then left alone for about 12-13 months with no charging or discharging during that time.  To me, 4-12% is not a huge loss of capacity given how long the batteries were completely neglected.  After all, most of us would never leave our boats unattended for 12-13 months at a time (at least on purpose).  And most notably, that capacity loss is well below any threshold that I have seen where a manufacturer would have to replace the battery under warranty.

If anyone is interested in the details on the various LiFePO4 manufacturers storage recommendations, below are some links and "copied and pasted" excerpts from some of the better known LiFePO4 battery manufacturers for your reference.  I've bolded and italicized some of the more pertinent information:  

Battle Born:  https://battlebornbatteries.com/faq/

The storage temperature range is -10°F to 140°F (-23°C to 60°C). We recommend bringing the Battle Born Batteries to a 100% charge and then disconnecting them completely for storage. After six months in storage your batteries will remain 75 – 80% charged. 

Victron:  https://www.victronenergy.com/upload/documents/Manual-Lithium-iron-phosphate-batteries-Smart-EN-NL-FR-DE-ES-IT.pdf

Recommended storage/float voltage: 13,5V resp. 27V per battery. Batteries must be regularly (at least once every month) charged to 14V (max. 14,4V) in order to fully balance the cells. Two or four batteries in series should be charged regularly to 28V resp. 56V.

MasterVolt:  https://images.mastervolt.nl/files/10000015396_14_manualMLIUltra27505500_EN.pdf

The battery should be stored in a dry and well-ventilated environment. The rate of self-discharge is less than 5% per month. High or low ambient temperature affects the self-discharge rate of the batteries and natural aging. If the battery will not be used for a period exceeding 3 months, we advise the following:

If external AC power is available switch off all loads and switch on the charger. Apply a float voltage as specified in the following table.
Model - Float voltage setting
12V - 13.5V 
24V - 27.0V

If no external AC power is available: - Charge the battery to > 80% of its capacity before storage. - Set the safety relay knob to “LOCK OFF”, see page 12. - Make sure MasterBus powering is not set to "Always on" (see Configuration tab in MasterAdjust). In this setup the batteries can be kept at least 6 months without maintenance. However, it is highly recommended to charge the battery to > 80% of its capacity every 100 days.

RELiON:  https://ceb8596f236225acd007-8e95328c173a04ed694af83ee4e24c15.ssl.cf5.rackcdn.com/docs/product/RelionInstallationManual-8.5x5.5-081720.pdf


5.1. Storage Temperature LiFePO4 can be stored between 23 to 95°F (-5 to 35°C). For storage longer than 3 months, the recommended temperature range is from 32 to 77°F (25 to 40°C).

5.2. Storage Conditions It is recommended to store LiFePO4 batteries at 50% state of charge (SOC). If batteries are stored for long periods of time, cycle the batteries at least every 6 months.

Lithionics:  https://lithionicsbattery.com/wp-content/uploads/2019/02/Lithionics-Battery-Storage-Procedure.pdf

Storing your battery at the correct specifications is important as it keeps the battery in the healthiest state possible for the fastest deployment when needed. Consult the table below for proper storage conditions.

Typical storage scenario < 3 months:
1. Fully charge the battery.
2. Turn the battery OFF by the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.

Typical storage scenario > 3 months:
1. Reduce the battery SOC to 3.3V/cell which is 50% ±10% SOC. Note: See chart below for cell voltage calculation.
2. Turn the battery OFF via the On/Off/Storage switch.
3. Keep the battery in an environment according to the specifications shown above.
4. Every 6 months charge the battery to 100% SOC, then discharge the battery to LVC, then charge it back to 50% ±10% SOC. 

Mark McGovern
SM #440 Cara
Deale, MD USA

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