Service Bulletin 002 - VTOL LiPo Battery Lifespan

Date: 14 May 2020

Affected Aircraft: Lynx VTOL 068+

Background

We are attempting to quantify the estimated lifespan of the VTOL lithium polymer battery. This only applies to the VTOL (smaller) battery. The main batteries are unaffected.

Increased voltage sag under load (takeoff) has been noticed by a Lynx VTOL user. As the battery voltage sags, the aircraft cannot generate enough thrust to complete a takeoff. In this specific case, the aircraft automatically aborted the takeoff due to triggering the takeoff abort timeout and landed.

We had also noticed a similar issue with some of the original VTOL batteries in Autumn 2019. Those particular batteries were used for testing Lynx VTOL prototypes. The battery supplier was contacted, but ultimately proved not very helpful and ascribed the issue to either a bad batch, abuse from flight testing, or poor storage habits.

They provided the following storage guidelines:

When not using your LiPo battery pack, store it at 60-70% of the pack’s rated
capacity at room temperature.  Make sure to cycle your packs at least once per
month since leaving them on the shelf for a prolonged period of time can cause
the packs to get severely out of balance, puff/swell or even go dead.
High-discharge batteries require such specific care

It was not the response we wanted, but chose to proceed with same cell type given that it had only happened to our early batch. Unfortunately, due to the second occurrence, we are now asking the supplier to perform additional load testing in an attempt to help ascertain the issue.

We do not believe, given the battery’s advertised ratings, that Lynx VTOL is abusing the battery on takeoff or landing. Furthermore, flight testing was performed that far exceeded the demands of normal operations and showed that both temperature and capacity requirements were acceptable.

All batteries, especially lithium polymer, have relatively short lifespans. However, we now think that these batteries simply have a shorter lifespan than what we anticipated.

Recommended Action

• Store battery at 60-70% capacity using the charger’s storage function
• Store battery at room temperature
• Cycle battery at least once per month if not flying regularly. Do so by charging and then using the storage function.
• After use, wait at least 30 minutes before charging
• Avoid car charging near a running engine due to excessive heat
• Avoid charging the battery in temperatures below 10C or above 50C
• Rotate what batteries are used for flying (label them if needed, example A, B, etc.)
• Replace after 40 flights or one year, whichever comes first

It is unclear how much degradation is due to flying, storage, or frequency of use. At this time, the safe thing to do is limit the batteries to 40 flights, or one year, whichever comes first. We are not satisfied with a battery that is limited to 40 flights. Alternative batteries are now being evaluated. Unfortunately the selection is narrow due to the space requirement for the VTOL battery. We want the replacement battery to be backwards-compatible and fit within your existing aircraft.

In addition, a VTOL battery failsafe will be implemented in a coming Swift GCS update that will help mitigate unacceptable low voltages during takeoff and cause the aircraft to land in place.