Managing your LiPo batteries are one of the largest costs and significant safety risks in operating drones commercially. The opening up of regulations and commercial availability of larger multirotors to the commercial drone operating community mean that we are now flying larger, more expensive and more energy dense batteries than ever before. Are you doing these five things to protect your battery investment and operate safely?
1- Prevent battery short circuits
Internal or external short circuits rapidly heat your battery and can cause a catastrophic battery loss and a fire.
To prevent external short circuits, you should be covering your terminals with electrical tape when not in use to reduce the risk of metal objects inadvertently connecting to the terminals. CASA has provided a helpful video for the proper way to prepare your batteries for transport in your carry-on luggage.
- Secure and protect your batteries during transport to prevent punctures and indentations
- Never charge batteries below 0°C and have them at room temperature if you want to charge them in under an hour
- Never charge batteries that have been discharged lower than 2V
2 – Plug in your cell balancing connector when charging
Charging batteries made of a number of cells connected in series requires special attention to make sure the cells are properly balanced. If the cells are not balanced during charging then some of the cells can become over charged, causing an irreversible loss of capacity and the risk of overheating and a fire.
Some smart battery packs such as those from DJI have a built in battery management system (BMS) to monitor the individual cells during charging. Other multi-cell battery packs don’t have a BMS built in and instead have a balancing connector cable that needs to be plugged in to your charger along with the main battery power cable. This is so that the BMS in the charger can manage the current to the individual cells during the charging process. If you are using multicell batteries without a built in BMS make sure you always double check both the power and balancing cables are plugged in before charging.
3 – Let your batteries cool before charging
The electrolyte in LiPo batteries begins to thermally decompose into gas as the temperature of your batteries begins to rise above 60°C. This can puff up your cells with gas and cause a cell rupture along with a flammable gas leak and potential fire
The faster you charge or discharge your battery the more heating power will be created that needs to be dissipated out of the battery to stop it getting too hot. Getting rid of this heat is inherently more difficult for larger batteries due to their lower surface area to volume ratio. If you are flying multirotors with larger cell formats it is important to give your batteries a chance to cool back to room temperature to make sure they don’t get too hot during the charging process.
4 – Heat is always the enemy – store them cool!
Heat above 30°C is your enemy and causes irreversible loss of lithium ions in the battery to the SEI layer. Aim to keep your batteries at around room temperature for their entire life. If they do get hot for any reason, the faster you return them to room temperature the better.
This heating degradation effect is worse when the battery is fully charged, so one of the worst things you can do for the longevity of your batteries is to charge them up and then keep them in your hot car all day while on a job. One of the better operational plans is to leave them in a mid-state of charge in a cool room until the day you need them, charge them the night before so they can cool down over night, and then transport them to the job site in an insulated and protective container such as an esky so they stay at room temperature until use. Let them cool in the shade after use on the job, and then pop them back in the esky for the trip home. Charge them back up to around 50% and leave them in a cool room until you need them next.
5 – Try to stay in a voltage window of 3.5-4.1V per cell
The more you push the upper and lower voltage limits for a cell, the more capacity you will get for a particular cycle, but the more you will irreversibly reduce the capacity of the cell for subsequent cycles.
Pushing the upper voltage limit causes irreversible loss of lithium ions due to oxidation of the electrolyte, while pushing the lower voltage limit causes irreversible loss of lithium ions due to lithium saturation and oxidisation of the lithium to lithium oxide in the lithium cobalt oxide electrode. On a day to day basis turn your charger down to 4.1V per cell and work towards an operating window of 3.5-4.1V to double the cycle life of your batteries.
In special jobs where the flight time on a single battery is critical and you need the extra ~two minutes flight time that a wider voltage range gives you – extend this window towards 3.0-4.2V. Note that for graphite electrode cells the voltage and power available drops off very quickly after 3.4 volts, so better to leave your low voltage indicator at 3.5V so that you have time to get your aircraft back and safely on the ground.