The first choice to make when it comes to fitting battery-packs to eVTOLs is between lithium-ion and lithium-polymer. The former carry more weight, but deliver power over a longer period and are therefore the cells of choice for electric cars, aeroplanes and upcoming air taxis. The latter pack more punch and are preferred at the lower end of the scale when budgets dictate shorter flight times at lower weights altogether. They also come as flat-packs, instead of bumper packs of cylindrical cells the size of AAs.
The second choice is whether to 'bus' the batteries together, and supply the motors by a sort of spider-web array of wires from the centre-section, or to have packs address motors directly. Smaller drones are conventionally wired from a single battery in this way, but many personal air vehicles fit the batteries directly adjacent to motors, like for instance LIFT's 'Hexa' eVTOL.
An advantage of a single energy source is that it might feature one large battery-pack or a pair thereof wired in series to attain a higher voltage, instead of say eight smaller.
Thirdly there is a choice between higher and lower voltages, with the number of cells in each pack determining the voltage in multiples of 3.7V. T-motor recommend running our motor of choice on six-cell packs (6S) like the one above, which supply around 22 of the rated 24V. Voltage determines the RPM of the motor and is thus tailored for the fixed pitch propeller to optimise thrust at expected operational values.
On balance I prefer to go down the distributed energy route, for even though a single battery failure would affect one of the eight motors, this will not be an issue because there are eight more independent motors and power sources available for recovery.
Finally we can calculate the flight endurance from the battery rating, by examining the T-motor data below. This says that the motors will be drawing around 22A at a typical setting, and a 22000mAh battery-pack like those I have used in the past would run the motor for one hour therefore. The smaller pack (2200 mAh) seen here would therefore be expected to run for a tenth of that time, or six minutes. LiPo batteries however are not run flat, however, so that the ones in your phone and laptop for example are wired to a battery-management circuit (BMC) that prevents this from happening.
Accordingly eight of the above packs (else the slightly larger at the same price) would suit the prototype, at a cost of £520. This brings the budget nearer to £3000 all in and shows why few people build large drones for fun, let alone people-carrying types. My plan this time however is to let you power up my prototype, the way Liberace used to tell people that he hoped they liked his jewellery... as they had paid for it.
(Note that the figures below are the results of tests at 25V instead of 22, although we can address this by upping the number of cells in the battery-pack with a 7S type, so that we've nearer 26V available to us.)
