Battery Choice

Checked 2026-09-26. The question: how big a pack does this airframe really take?

The bay

These values are measured from the STLs, which are exported in assembly position. The pack sits on the Battery Tray in the Fuselage Front, under the front hatch, ahead of the 14mm front spar.

The tray is our own part, parts/Battery Tray (velcro).stl from tools/make_battery_tray.py (Ryan, 2026-09-28), in place of the kit’s Battery Tray. It has the kit’s outline, its 2mm and its top face at the same height, so the bay is unchanged, and no text. Two 20mm hook-and-loop straps hold the pack, at Y 137 and 213. Each lies in a 22 x 1.4mm groove across the top face, under the pack, and comes up between the pack’s side and the fuselage wall at the tray’s edges. A 1.5mm strap stands 0.1mm above the plate and a 2mm strap 0.6mm, inside the 1mm that the P50B pack has under the hatch-mounted sense board. The strap does not pass under the plate: a plate lifted by a strap lifts the pack into that board (th-main, 2026-09-28). The tray is glued flat to the floor like the kit’s. It weighs 20.5g sliced in PETG, 0.6g less than the kit tray, and prints on its flat bottom, grooves up, with no supports. The Charge Sense Fence stands on this plate, not on the floor through the kit tray’s letters.

Largest box that fits	Volume
148 x 76 x 55 mm (length x width x height)	about 620 cm³

The tray limits the length and the width. The hatch limits the height. The front spar (which crosses 18mm behind the tray) limits the back end. Leave a few millimetres for the strap and the wiring.

Candidates

Pack	Size	Mass	Energy	Continuous	Fits	Price
Tattu 6S 4500 25C LiPo (optional tuning pack since 2026-10-01)	fits by design	0.67 kg	100 Wh	112 A	yes	$108
6S2P Molicel P50B 21700, 10Ah (Nexus, NV; in the cart since 2026-10-01)	139 x 48 x 72	0.90 kg	216 Wh	120 A (60 A per cell x 2P; Nexus, read 2026-10-01; no pulse figure given)	yes	$252; the $165 Snack Packz pack on eBay gives no dimensions
Titan 6S1P Amprius SA08, 11.1Ah	165 x 53 x 50	0.71 kg	225 Wh	22 A (2C, airflow dependent)	no, 17mm too long	$600
Titan 6S4P Amprius SA08, 44.2Ah	about 165 x 100 x 100	about 2.75 kg	902 Wh	88 A	no	$2,400

The RED V3 P50B 6S2P from Upgrade Energy is 156mm long, so it does not fit either. Before you buy a pack, check its length against 148mm.

What each does to the aircraft

The model uses momentum theory for hover (four 8-inch discs, figure of merit 0.6, drivetrain 0.8). Cruise is L/D 9 at 20 m/s with 50% propulsive efficiency. Each flight has 2 minutes of hover. LiPo is used to 80% of capacity and Li-ion to 85%.

The airframe without the pack is an itemised estimate of 1.63 kg. Trust the ratios more than the minutes. The parked lift props and arms add drag that the model leaves out.

Pack	All-up weight	Hover power	Hover current	Cruise power	Endurance
6S 4500 LiPo	2.30 kg	412 W	20 A	100 W	42 min
6S2P P50B 10Ah	2.53 kg	476 W	23 A	110 W	93 min
6S1P SA08 11.1Ah	2.34 kg	423 W	20 A	102 W	106 min
6S4P SA08 44.2Ah	4.38 kg	1,084 W	52 A	191 W	232 min on paper

Why the 44Ah pack is out

• It does not fit the bay by a wide margin.
• It almost doubles the weight of the aircraft. Hover power goes up with weight to the 1.5 power. Thus hover needs 2.6 times the power of the baseline. The lift motors lose the thrust margin they need to recover from a gust near the ground.
• Stall speed rises with the square root of weight, about 38% here. Thus AIRSPEED_MIN and Q_ASSIST_SPEED would move up. The printed wings, spar mounts and landing feet are sized for about 2 kg.
• It sits ahead of the CG, so it would make the aircraft badly nose-heavy.

Why the wing makes big packs cheaper than they look

The pusher does not blow air over the wing. It only pushes the aircraft forward, and the wing makes all the lift from that airspeed. In cruise the pusher only has to overcome drag, about weight divided by L/D. For this aircraft that is about 250 to 300 g of thrust. In hover the four lift motors carry the full weight.

That is why an extra kilogram of pack costs a lot in the two minutes of hover and very little in the hour of cruise. And that is why the right pack is the biggest one that still hovers with margin and fits the bay.

Tattu 6S LiPo packs (genstattu.com, checked 2026-09-26)

These are the 6S packs of 3,500mAh and up from Tattu’s 330 to 350mm FPV category. They are checked against the 148 x 76 x 55mm bay and the same endurance model. The smaller 1.6 to 2.2Ah packs in that category are 5-inch racing packs, so they are left out. 8S packs do not match the 6S powertrain.

Pack	Plug	Mass	Energy	Wh/kg	Size (mm)	Fits	Modelled	Price
G-Tech 3500 150C	XT90	580 g	78 Wh	134	139 x 43 x 46	yes	~33 min	$96.99
4500 25C (cart)	XT60	650 g	100 Wh	154	138 x 42.5 x 52	yes	~42 min	$108.19
R-Line 4500 95C	AS150	685 g	100 Wh	146	150 x 49 x 44	no, 2mm long	~41 min	$144.19
R-Line 5100 95C	AS150	868 g	113 Wh	130	175 x 50 x 55	no	~43 min	$144.19
6000 35C	XT60	885 g	133 Wh	151	148 x 45 x 59 (±5 length)	only if it measures under 148	~52 min	$133.99
G-Tech 6000 150C	AS150	945 g	133 Wh	141	161 x 45 x 61	no	~50 min	$159.99
6S2P P50B Li-ion, for comparison	XT90 (ordered option)	900 g	216 Wh	240	139 x 48 x 72	yes	~93 min	$252

Higher C ratings buy current that this aircraft never draws (about 23 A in hover), at the cost of weight. Thus, among LiPos, the low-C packs give the most energy per gram. The 25C pack in the cart is the best LiPo that fits for certain. The 6000 35C adds about 10 minutes if a sample measures within 148mm. Tattu quotes its length as ±5mm.

Recommendation

Replace the Tattu LiPo with a 6S2P Molicel P50B 21700 pack that measures 148mm long or less. The manual lists 6S2P 21700 as the largest supported pack. It gives about two times the flight time for 230 g. Keep one LiPo if you want more hover power for tuning flights.

Li-ion changes some ArduPilot values from the LiPo numbers in the manual:

Parameter	LiPo (manual)	Li-ion 6S
Q_M_BAT_VOLT_MAX	25.2	25.2
Q_M_BAT_VOLT_MIN	19.8	18.0 (3.0 V per cell)
BATT_LOW_VOLT	about 21.0	about 19.2
BATT_CRT_VOLT	about 20.4	about 18.6

Check the CG again with the heavier pack. 230 g more at the tray moves the CG about 9 mm forward. Slide the pack aft on the tray until the balance matches the manual.

216 Wh is over the 160 Wh airline limit. Thus the pack cannot fly as luggage, and it ships as hazmat.

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Source: docs/battery.md in the repo. Edit it there; this page is generated.