This brief is derived from our experience in working with eBee batteries and the storage of the same. This overview of the topic is provided solely as guidance for our pilots.
As anyone that owns an eBee knows, long term storage of the battery can lead to early retirement of the battery. Batteries stored fully charged will degrade after a short period. This degradation can result in an in-flight Red Alert, and a certain level of angst from the pilot as our expensive investment attempts to make a power off return for landing. At $150 per each these batteries are too expensive to be considered disposable. Using proper management better life expectancy from batteries can be achieved.
Officially Sensefly recommends that the battery be stored at around 50% charge and at ~70 degrees (room temp). Sensefly has not, in the past, provided the best guidance on the management of the Li/Po batteries used by the eBee system. The list below is taken from the SenseFly document “How to Look After your Batteries”.
Don’t store a battery fully-charged for more than one month.
eBee batteries should be stored at about 50% charge, or about 3.85 V per cell, 11.55 V in total.
Your battery’s total voltage appears on eMotion’s Flight Monitoring tab. If you need to store a battery, and it’s level of charge is too high, ﬂy a short mission to discharge the battery.
The charger supplied with the drone is excellent in charging and maintaining the battery’s 3 cell LiPo structure. However, if you have a battery that has a weak cell, or other fault, the charger may very well charge the battery and indicate green (Full Charge) yet fail during a mission.
SenseFly’s guidance states that the best method to discharge a battery is to go fly the eBee to get the battery down to the level for storage. Well, SenseFly also requires a 100 hour TBO at a cost of over $2,000 to keep your warranty active. Based on this just ‘go and fly’ guidance, are we are supposed to increase our equipment flight time AND risk of loss?
Well, there is an easier way to do battery discharge to 50%.
The battery connection plug has 6 pin sockets and has the following layout.
Image 1 – Pin Layout
In Image 1 the individual cells are listed across the top and that the middle row of pins provide voltage reading of all 3 cells in series. It is at these middle pins that battery voltage readings should be measured. It does not matter which row is used, but measure voltage across the columns of pins.
Using the middle pins defined above, the battery voltage was recorded at various stages of discharge. The charge percentage was recorded based on the indication in eMotion. Then the eBee was powered down and the voltage of the battery was recorded using a standard DC capable volt meter. Chart 1 below summarizes these recordings.
Chart 1 – Voltage Chart
Alternate Method to Discharge Batteries to 50%
A fully charged eBee battery supplies > 12.1 vDC across the center battery sockets. To accomplish this alternate method you will need a volt meter capable of reading DC voltage, a timer and a 10W Halogen landscape light bulb (12 v) with a G4 base. The G4 socket on the bulb has two small pins designed to insert into a landscape fixture. You can get a 10W (or lower wattage) bulb from Lowe’s or Home Depot for about $5. I would not recommend anything of larger wattage as the discharge rate will be too fast. You may also use LED bulbs which are are available in 2W and greater. Note: When using LED bulbs be sure to read the actual wattage rating, not the effective or equivalent wattage. A 2W LED bulb may be listed as a 25W replacement.
Freshly charged battery will show voltage of 12.2 or greater measured across the center sockets pins (See image 1 above). Insert the G4 landscape light bulb base into the eBee battery across the center two pins, taking care not to damage the clips in the battery socket. Set your timer to 1 hour. The 10W bulb will discharge the battery to about 11.5 vDC in roughly 60-70 minutes. Check the voltage across the inner pins to verify the voltage or connect the battery to your eBee to verify. We believe it is not desirable to do a start and stop cycle on the eBee just to check the battery voltage, so use the volt meter.
NOTE: Take extra care to not create an electrical short across the center pins.
If you have lots of missions or are flying weekly, then just keep your batteries in rotation. However, if you have a slow period or end up with batteries that got charged but proved unneeded, then the above method of discharge will save risk of loss by flying the eBee and wasting your eBee’s flight time.
Additionally, you can conduct voltage checks on each cell within the battery with a volt meter (refer to Image 1 to see pin layout). CDS has had batteries charge to green on the SenseFly charger, but have a weak or bad cell. Each cell should read ~4.1 vDC or greater when fully charged. If you have a battery that is swelling, puffy or has been discharged below 20%, you should verify the strength of the individual cells.