Top tips for happy Phantom, Inspire and Mavic batteries

DJI batteries require cycling (i.e. discharge fully and then recharge fully) every 10-20 cycles.
The reason this is necessary for DJI batteries is because of the difficulties in determining actual remaining capacity (deep dive #3). As a battery is used, its chemistry and its behaviour changes and the calculations regarding % capacity remaining that were valid when the battery was new begin getting inaccurate.

To overcome this, DJI asks you to discharge the battery all the way down and then charge fully. In the process the battery counts how many mah are put back into the battery and recalibrates its calculations.

This is quite an interesting topic as pilots are being put between a rock and a hard place by asking them to discharge their LiPos as low as possible.
So, on the one hand discharging will affect your battery's life and performance (see tip #4) and on the other hand not discharging will get your capacity indications wrong and won't let you enjoy the battery to its fullest (or even, risk a crash landing). Rock and hard place.

How to discharge

There are a couple of ways to discharge the battery for calibration purposes:

• Flying the drone until it reaches the required capacity. This really is not sustaintable for any pilot who flies frequently and has multiple batteries. It takes a really long time and wears your precious electronics, motors and ESCs unecessarily.
• For zero wear, zero effort and optimal cycling discharge use a dedicated discharger such as the Phantom Angel (which, full disclosure, I make :) ).

maH – Each LiPo battery is rated in maH. This tells us how long a given level of milliamps can be put out by the battery. You can see this as how much juice the battery can hold. Other things being equal, the more maH, the longer your flights. For example, P2 batteries are 5400maH (5.4aH), meaning they can put out 5.4 amps for one hour or 10.8 amps for half an hour and so on.

Cells – A single LiPo cell has a nominal voltage of 3.7v. When higher voltage is required, multiple cells are connected to form a single battery. For example, Phantom 2 batteries are 3 cell (shown as 3S) and Phantom 3 batteries are 4 cell (4S).

Watt hours – Watt x hours gives you the total energy a battery can store and is the product of volts x amps, or, power times the amount of time it can provide this power for. So a 5A (5000mA) 3S battery is 55.5Wh. This means it will run a 100W light bulb for just over half an hour.

C rating – The battery’s C rating tells you how quickly the battery can be discharged. A 10C battery could be discharged at a rate 10 times its capacity. So a 5500mah 10C battery could be discharged at 5.5 x 10 = 55A, so in 6 minutes. Most LiPos give two C ratings, continuous and peak or burst, which refers to short peaks in current.

A multi-cell LiPo battery will appear as a single battery of, say, 11.1v but in reality is three batteries (or cells) of 3.7v. When charging or discharging such a LiPo, tips #1 (never overcharge) and #4 (never over discharge) apply to each cell independently.

Over time cells can lose “synchronization”, so one cell may be at 3.4v and another at 3.3v. In order to bring them back into sync and make sure that each cell is never over or under charged, balance chargers are used which measure each cell in its own right.

DJI intelligent batteries handle this themselves.

It may sound surprising, but telling how much current is left in a LiPo at any point in time is surprisingly difficult to estimate accurately. That’s why you’ll see laptops and mobile phones going from as much as 10% to empty in a matter of minutes.

The simplest method, and the one used by practically all devices, is to measure the voltage output at any point in time. There are many reasons this is not accurate:

a) Load affects voltage. A LiPo at rest and a LiPo under load will give two completely different readings

b) Age, wear and amount of usage affect voltage

Temperature also makes a difference. Note that flying in cold weather will reduce your flight time (if you ever owned a GoPro you’ll notice how it keeps itself warm – that’s why!)

Each LiPo brand and model has its own voltage to capacity curve – and none of them are linear

The other method is to discharge as much as possible and then measure how many amps can be put back into the cell. That’s exactly why DJI requires a full discharge and re-charge of its intelligent batteries every 20 cycles or so – so that it can re-calibrate its calculations.

• Don’t charge LiPos that are hot (as they tend to be right after a flight) or colder than 10C / 50F
• Take extra care when travelling with LiPos as airlines do not allow LiPos in the hold - you have to carry them with you on the plane and often there are restrictions on the number of LiPos you can take. Refer to your airline's policy before travelling
• LiPos don’t have a memory effect (like older batteries used to), so there’s no need to fully discharge and charge them
• LiPos maintain their charge exceptionally well. They lose around 1% per month of storage
• When using multiple batteries in series or in parallel make sure they are batteries of the same type, manufacturer, age and state. They should be as close to identical as possible
• When the time comes to dispose of a LiPo it is advisable to first discharge them as much as possible (as long as they’re not damaged!) and then soak them in salt water for two weeks (half a cup of salt per gallon of water). This will neutralise them and make them safe for disposal