Why it is important to properly store an aircraft battery and how to accomplish this?
Proper storage of an aircraft battery is important because it directly impacts the battery's performance capability as well as its total service life. To understand this, let me describe what happens to the battery during storage. When a battery is in storage, it gradually loses charge even though there is no load on the battery. This process is known as self-discharge and is illustrated in the chart shown on the screen. The capacity retention ratio, or state of charge, decays with time and the rate of decay is strongly affected by the storage temperature. The higher the temperature, the faster the battery loses its charge. As a battery self-discharges, the plates become sulfated and the internal resistance of the battery increases. If the sulfate level in the plates gets too high, the battery will take a lot longer to charge and will not last as long.
To actually see sulfation, take a look at the high magnification photos of sulfated plates and un-sulfated plates shown on the screen. The photo on the left shows the sulfated plate with large sulfate crystals. This is what causes the high resistance in the battery. To prevent the sulfate levels from getting too high, the battery needs to be boost charged to get it back to 100% state of charge. The photo on the right shows the plate after charging, which erases the sulfate crystals. Fortunately, a simple check of the battery's open-circuit voltage with a digital multimeter can be used to determine the condition of the battery.
The table shown on the screen summarizes the instructions from the CMM. A fully charged battery will have an OCV of about 13 volts for a 12-volt battery and about 26 volts for a 24-volt battery. While in storage, the OCV should not be allowed to drop below 12.5 volts for a 12-volt battery or below 25.0 volts for a 24-volt battery. The OCV should be checked every two to four months depending on the storage temperature. If the OCV is getting close to these values, the batteries should be boost charged with a constant potential charger.
Note that if the OCV is allowed to drop below the minimum, a capacity test will be necessary before it can be installed in an aircraft. This is to make sure that the sulfated plates can be restored to a good condition, so obviously, it is best to not let the OCV drop below the minimum. The worst thing that you can do is to put the battery on a shelf indefinitely and ignore the battery's state of charge. Eventually, the battery can become so sulfated that it will not recover and will have to be scrapped.
Another thing that is covered in the CMM is the storage temperature. Ideally, the storage temperature should be below 68 degrees F or 20 degrees C. Referring back to the capacity retention chart, you can see why cooler temperatures are preferred. At 20 degrees C, it takes about 15 months to reach 50% state of charge. At 30 degrees C, it only takes nine months to reach 50% state of charge. And at 10 degrees C, it takes well over 18 months to reach 50% state of charge. So, the cooler it is during storage, the longer you can go before a boost charge is necessary. However, if you cannot avoid storing the battery at warmer temperatures, it just means you will have to boost charge the battery more often.
One of the popular myths I hear is not to store it on a concrete floor because this will cause the battery to discharge very quickly. Is this correct?
No, that is not correct. That myth is a carryover from early automotive batteries that sometimes had acid residue on the case, which could contact the concrete. This would cause a rapid self-discharge of the battery. Today's lead-acid batteries do not have this issue. The only factor that affects the rate at which a battery self-discharges is the ambient temperature. Cooler is always better.
I think the easiest way to cover this topic is by referring to the flow chart shown on the screen. First, we want to do a visual inspection to check the overall physical condition of the battery to make sure there are no obvious signs of damage. If you see any damage, don't install the battery because it may not be airworthy and it's not worth the risk. Next, measure the open-circuit voltage of the battery with a digital multimeter. As long as the voltage is equal to or greater than 25.5 volts, the battery can be installed. If the voltage is equal to or greater than 25.0 volts but less than 25.5 volts, then it will need a constant potential boost charge before installing it in the aircraft. If the voltage is less than 25.0 volts, then you will have to charge the battery at constant potential and follow that up with a capacity test.
As long as the battery passes the capacity test, it is acceptable for aircraft installation. Note, that for a 12-volt battery, all of the voltage requirements are cut in half.
The flowchart is included in Appendix B of the CMM. I also want to point out that the CMM is a governing document for servicing Concorde aircraft batteries, and this video does not take the place of the CMM. If you have any questions regarding the CMM, please contact Concorde's customer service department. Thank you, everyone. Be safe.