A Major Deviation™ is when one battery cell differs by more than 0.07 V from the other cells in that battery.
Airdata™ presents this information in a table that lists each major deviation that occurred during the flight. A low number represents more balance between the individual cells. A high number presents less balance between the individual cells.
Because major deviations are shown in aggregate, Airdata makes it very easy to see at exactly what point during the flight a major deviation occurred and in which cell(s).
This is an example of an abnormal number of major deviations.
In this example, Cell 1 is experiencing around 15 major deviations per minute. This is considered an abnormal number of major deviations, though the deviations are all close to 0.07 V. In this case, we recommend looking at this battery's history to determine how long this has been happening as well as ensuring the proper maintenance has been conducted on this battery, such as deep cycling the battery every 20-30 cycles. If this is the first time this battery has exhibited this many major deviations and you are not receiving any notifications during flight of potential issues, it is likely safe to keep this battery in service so long as you monitor it closely.
This is an example of a high number of major deviations.
In this example, Cell 1 is experiencing over 40 major deviations per minute, the deviations are well over 0.07 V and the deviations are getting progressively worse during the flight. This battery should be taken out of service immediately and will most likely need to be disposed of, or moved to become a bench battery, to only be used for firmware updates and other maintenance and testing on the ground. There are of course other considerations to take into account, such as the battery's performance history, whether it's been properly maintained, involved in a crash, etc. Please be sure to physically inspect the battery to check for any swelling or puffing, which is a sign of the chemical breakdown of the battery, also referred to as electrolyte decomposition and ensure the battery is stored and/or disposed of safely.
NOTE: Even batteries that are functioning normally can experience a few major deviations. Less than four major deviations per flight are relatively common. 5-10 major deviations are not uncommon, though we would recommend monitoring that battery to ensure it does not degrade. A battery may be inefficient or failing when the battery exhibits:
- The bulk of its major deviations with a single cell
- Multiple major deviations per minute and/or more than ten total major deviations
- The major deviation(s) last for longer than one minute
It is important to not rely solely on deviations as a means of determining a battery's condition. Many factors should be considered when making this determination such as the physical condition of the battery, how the battery has been maintained, the historical performance of the battery and more. As part of UVT's ProLine Fleet Program, we will be here to assist you with these determinations and guide you.
WARNING: If you are prompted with any notification about a bad battery cell, a battery initialization failure, or any other battery-related error or alert while in-flight, land the aircraft immediately, conduct a thorough post-flight check and assess the situation before using that battery again.