Our Lithium batteries area valued system in our vagabond RV, or long-term cruising boat, and of course as part of our home solar battery bank giving us independence and utility savings.  They are typically rated for thousands of load cycles.  Those cycle ratings are based on doing a particular test in controlled temperatures and charging and discharging rates.  Actual use can be something different.  In some cases, these can be less demanding, but in others more demanding.  And over time other factors can be in play that don’t show up in short term laboratory tests.

The research over the last 10 years has brought forth an array of factors that contribute to aging of our lithium batteries.  This knowledge is very helpful as it gives us more clues on how to extend the life of this valuable asset.  We’ll run through several of the more significant ones that affect aging, and things we can do to reduce that affect.

Cell Aging Factors:

-        Lithium Plating on Anode – When discharging Lithium atoms in the LiFePO4 cathode give off an electron, and the positively charged ion is attracted to the Anode.  The Lithium ion embeds itself in the microscopically porous surface of the anode.  Over time a very small amount of the lithium residing on the Anode does not go back into electrochemical process, and stays resident on the Anode.  It is bonded in a crystalline arrangement, and is referred to as Lithium plating on the anode.  This plating is also referred to by scientists as “nanocrystals” and are the mechanism that leads to what for years was just called calendar aging.  This effect is a given, and not significant under most conditions.  We’ll touch on some parameters that increase this effect, and reduce capacity and life of the lithium battery.

-        High Temperature – Research has shown that operating our lithium batteries at higher temperatures results in increased formation of these nanocrystals, reduced battery capacity and shortened life.  The higher the temperatures the higher the deposition and degradation.  Below 25  ̊C only minimal degradation occurred.  Research showed increasing effects on loss of capacity above 25  ̊C (77  ̊F).  Ideally, we want to keep our batteries at 77-degrees or below.  As that is not always practical, providing cooling/air flow for the battery compartment can be beneficial in the long term.  It is tougher to address this in drop-in batteries that have the cells fully enclosed, so heat confined within the case; still some airflow through the locker to cool the case will be helpful.

-        Low Temperature – Extended operating at below 10  ̊C lead to degradation of battery capacity and reduced life.  Repeated charging below that reduces age; charging at below 0  ̊C will damage the battery.  It is better to keep lithium batteries inside a warmed occupancy space, rather than outdoors in very cold temperatures.  If one’s adventures take them to colder climes, it would be  good idea to place lithium batteries in warmed, thermal blankets, drawing small current off the battery.

-        Excess top charging – Research has also shown that continued charging, like trickle charging, at the top end of the battery capacity can reduce battery capacity and life.  Also, continued cycling in the top 20% of capacity also has a degradation effect.  Accordingly, one should be careful to use chargers with a CC-CV regimen, that is constant current up to near full, and then constant voltage for the upper 10% of so of charging and then turning off charging.  Avoid trickle charging, or use a charger that allows trickle to be turned off once the battery reaches full charge.

When leaving an RV or boat unattended for long periods, with solar available, it is hard to avoid this repeated top charging.  As lithium batteries have a very slow self-discharge, it would be better to disconnect the battery and avoid the excess top charging from solar day after day, while unattended.  Do the calculation on % discharge per month, and come back and charge up the batteries after several months and then disconnect again.

-        Excess bottom charging – Like excess top charging, doing a lot of cycling near the low end to the capacity also takes life out of the lithium batteries.  This doesn’t mean one has to run the batteries down and charge all the way to the top.  It is probably better to be cycling in the 20% to 80% capacity generally, and getting above or below that only occasionally.

-        Electrolyte Decomposition - Attempting to charge at very high electrolyte temperatures (65  ̊C/149  ̊F) will cause decomposition of the electrolyte and battery damage.  Most, good BMS systems will have high temperature monitoring and charging cutoff at these temperature levels.

-        Solid Electrolyte Interphase (SEI) layer – Formation of the SEI layer is a normal process in lithium battery charging and discharging processes.  IN the early life of the battery, it is a thin layer and very porous.  Over time this increases and can restrict flow of ions between the anode and cathode.  At high charge and discharge rates and high temperatures, the Sei can increase in thickness and density and further add to the degradation of battery capacity and life

-        High charge rates – Research has found that charging at above 80% of capacity (e.g., 80 amps on a 100Ah battery) can significantly degrade and damage the battery.  Most battery manufacturers indicate charging at 1C (100% of rated capacity) is possible, but they generally recommend a significantly lower normal charge rate for longer life of the battery, typically 0.25C.  Managing use of our batteries and charging schemes is important to avoid this contribution to aging.

-        Dendrites & Whiskers – As a cell gets deeper into its lifespan, some very tiny dendrites or whiskers start to form, from the lithium ions that embed and become resident in the anode.  Dendrites can penetrate the separator and impact the cathode, leading to an internal short circuit and thermal runaway.  This can be avoided by not trying to use lithium batteries that are below 40% of rated capacity.  (i.e., if fully charged, their tested capacity is below 40% of original rating

Aging Signs – What are some aging signs/parameters we can watch?

-        Reduced Capacity – Measure right after your new batteries are installed and fully charged, and do this periodically – maybe every few months, and log the results.  The steps are to fully charge the batteries, note the reading on the Ah meter, remove (turn off) solar and battery chargers, allow normal house loads to discharge the battery, and note the Ah reading when the batteries have fully discharged.  Once down to fully discharged (as recommended in battery specs (like 10.0V on our 12V Drop-In batteries, and 11.6V on our modular batteries), make note of the Ah’s discharged.  Compare this to the rated battery capacity, and determine percentage of rated capacity.  If below 40%, it is time to replace the battery.

-        High temperature of cell terminal or shell when charging – Using an infrared sensor, check the temperature of the cell terminals on prismatic cells.  Usually the negative/anode on a prismatic is a few degrees higher than the positive/cathode terminal.  If you measure temperature of the anode terminal and it is above 125  ̊F/50  ̊C, this is indicative of a high internal resistance. And the cell/battery is likely due to be replaced.

For Drop-In batteries, if you feel a hot spot or surface along the edge of the battery shell, then it is likely due to be replaced.  Check capacity of the battery as well to confirm it is due to be replaced.

-        Bulging of cell shell – If you see any bulging on the exposed prismatic cells, then that cell and the battery it is part of are due to be replaced.

-        Mini-cracks/crazing showing in plastic shell – If you notice any crazing or cracking on the exterior surface of a plastic-shelled prismatic cell, or corrosion on the aluminum shell of a prismatic cell, we recommend you replace that battery.

We DO NOT recommend replacing just one cell in a battery.  Any replacement cell that is new will have substantially different characteristics than those that have some age on them, and would create an unsafe imbalance amongst cells.  If you find one cell bad, then you should just replace the battery.

Preventive Maintenance – Besides the above regular checks on capacity, temperature and condition, and operational practices, what are some preventive measures that can be taken as routine maintenance on our lithium batteries?

-        Ensure BMS settings are consistent with Blue Heron recommendations for that battery.

-        Clean battery surface – small pieces of dirt and dust can become sites for condensing moisture and supporting trickle leakage between terminals.  Kep those battery surfaces clean – vacuum, wipe with dry cloths, DO NOT use cleaning sprays.

-        Ventilation/Heat Removal - Make sure there is good airflow through the locker containing the batteries, to keep the batteries at as low a temperature as ambient conditions will support.  Adding computer-type low amperage fans as locker inlet and outlet air flow boosters will add months to years in the life of your battery bank.[i]

-        Establish a sound plan for storage – Battery charged to about 80% before leaving; turn off all loads; have battery status checked at regular intervals (monthly); charge as need to get back to 80%; ensure temperatures are between 40 and 90 degrees F if at all possible.

When to Remove from Service -

Some manufacturers suggest removing the lithium battery from service once capacity drops to 80% of rated capacity.  That is more driven by operational needs.  For example, relying on a battery load to complete an electric bus’s assigned route with the battery bank and battery capacity selected based on that need.  For our RV, boat and solar bank needs, we can usually use these batteries to a lower level of rated capacity, if we don’t mind the more frequent charging.  Several sources suggest it is safe to continue using the battery down to 40% capacity, and we support that recommendation.

(This technical article is the property of Blue Heron Battery LLC and may not be copied or reprinted in whole or in part without the express permission of Blue Heron Battery LLC.)

[i] “A review of lithium-ion battery safety concerns: The issues, strategies, and testing standards” Journal of Energy Chemistry, August 2021. (Research paper)