WHY NOT SAG CAPS?

Buy more battery. Here's why, in pictures. Math at the bottom.

THE FIX

8S4P · TWO 8,000 mAh PACKS IN PARALLEL

Average sag 3.1 V instead of 5.7 V for one pack, for +832 g of cells, with each cell at 86% of its burst rating. 8S, 550 A ESC limit, 3 s pass. EST

1 · MORE CELLS WIN ON WEIGHT AND SAG

Weight added and average sag for each option — Every option starts from one 8,000 mAh pack (4 Ah high-burst cells, 8S2P). Cell and cap masses RATED, LIC bank mass EST, sag EST.

The second pack cuts the sag more than any bank and weighs less than the LIC bank. The buyable EDLC bank adds more weight than the second pack and saves less than a fifth as much sag.

2 · CAPS FADE BEFORE THE FINISH

Share of the current from the cap bank falls from launch to 3 seconds — Red = amps from the cells, orange = amps from the cap bank, dashed = what the ESC asks for. EST

A cap bank only gives current while its voltage is above the pack's. As it gives, its voltage falls, so it hands the load back. EDLC bank: 20% of the current at launch, 2% at 3 s. LIC bank: 47% → 13%. Extra cells carry their share the whole way. EST

3 · IT HAS TO FIT THE CAR

LIC bank footprint next to one 8,000 mAh pack, to scale — Top view, to scale. LIC bank ≈ 280 × 130 × 64 mm, ≈ 2.15 kg EST. Extra pack: 16 bare cells stacked, 149 × 72 × 38 mm EST, 832 g RATED.

THE SCORECARD

Option (on one 8,000 mAh pack)	Can you buy it?	Fits a 1/7 Outlaw car?	Still helping at 3 s?	Cost per volt saved
+ second 8,000 mAh pack (8S4P)	YES	YES · +832 g	YES · the whole pass	ordinary cells
+ EDLC supercap bank (14 × 400 F 2.7 V)	YES	heavy · +1008 g RATED	2% of the current EST	costly per volt saved
+ LIC bank (10 × 1100 F 3.8 V)	QUOTE ONLY	NO · ≈280 × 130 × 64 mm, ≈2.15 kg EST	13% of the current EST	quote only
+ 220 F 3.8 V hybrid caps, one per cell	YES	light · +285 g for 10s3p	under 6% at any point EST	RETIRED

RETIRED: 220 F hybrid caps, one per cell. The datasheet limits each cap to 8 A peak and 100 mΩ max ESR RATED. Even three strings take under 6% of a 550 A launch EST. They don't fix sag; we no longer recommend them for it.

KEEP THE RIPPLE BOARD

Ripple caps at the ESC are a different job. The ESC switches thousands of times a second and the battery can't follow that; a low-ESR board right at the ESC leads can. That's where capacitors earn their weight.

RIPPLE PACK CALCULATOR ▶BUILD GUIDE (PDF) ▶

HOW MUCH BATTERY? CALCULATOR ▶THIS PAGE AS A 2-PAGE PDF ▶SIMULATOR ▶

APPENDIX · THE MATH

Current splits by resistance. Pack and bank sit on the same bus. A bank only pushes current while its own voltage is above the bus: I_bank = (V_bank − V_bus) ÷ R_bank.
Series strings divide the farads and add the resistance. 14 × 400 F in series = 28.6 F at 45 mΩ with wiring EST, against 11 mΩ for one 8,000 mAh 8S2P pack EST. 10 × 1100 F in series = 110 F at 12.5 mΩ EST.
The fade. The bank's share falls with a time constant τ = C × (R_pack + R_bank): about 1.6 s for the EDLC bank and 2.6 s for the LIC bank on one 8,000 mAh pack EST. After 1 τ its share is down to 37%; a 3 s pass is longer than both.
Cells in parallel. Pack resistance = cells in series × cell IR ÷ cells in parallel + leads. Doubling P halves the cell part, for the whole pass.
Assumptions EST: 8S at 4.20 V/cell, 550 A ESC limit (XC E6 MAX, RATED), 5000 W, 12 lb, grip 1.2, 3 s pass, cell IR = 0.414 V ÷ burst amps, bank precharged with 0.46 mΩ wiring and fuse. Cap values from maker datasheets RATED. Same model as the How Much Battery? calculator and the Simulator.