BATTERY CALCULATOR

Pack sizing + sag on one page · weight, gearing, kV, cells and pack IR → peak amps, sag, motor load · then how many in parallel (scroll down)

Two parts, one page: the tuner below sizes the pack against your car, gearing and motor. Scroll down to HOW MUCH BATTERY? for sag and how many cells in parallel.

Starting numbers are EXAMPLE ASSUMPTIONS, not measured EVUL data. Check against a logged pass (X.BUS Data Lab or XC-Link). Size by measured IR and sag, not the label C.

MOTOR

CAR AGOOD

70%

90%

100%

GOODCAUTIONRACEBEYOND

Peak amps

mAh / pass

V/cell at peak

Deliverable

Effective C

Geared top

VOLTS PER CELL vs CURRENT · dot = your peak

PACK FUEL GAUGE

Tiers: green good (under 70%) · yellow caution (70–90%) · RED race zone 90–100%, at the limit (normal for drag racing) · striped BEYOND LIMIT = past the motor's published ratingFuel gauge: share of the pack one pass usesSag bar: white = volts per cell at peak · red zone = below your floor · amber = caution marginEffective C = deliverable amps ÷ Ah

HOW MUCH BATTERY?

Pack sag · cells in parallel until it stops sagging · % of burst rating · pack weight

Every starting number is tagged RATED (datasheet or supplier sheet) or EST (our estimate). EST values are not measured EVUL data. Put in your own cell IR and your logged ESC current when you have them. Drag, speed run, truck pull, bash or 1/5 & 1/7: set the run length and load to match what you do.

YOUR CAR & ESC

Motor power (W) EST Cells in series Motor amps once traction-limited (A) EST Launch spike fades in (ms) EST ESC motor-amp limit (A) RATED Run length at full load (s) Load shape

Real pass EST, same profile as the Ripple Pack calculator: the 550 A ESC limit is motor amps and only lasts milliseconds at launch, then falls to the traction-limited level. Pack amps = throttle (duty) × motor amps, so at launch the pack sees far less than 550 A. Throttle ramps up over the pass, and pack amps are capped at motor watts ÷ 0.85 ÷ the sagged pack volts. Sag = pack amps × (cell IR + wiring). Worst case holds the ESC limit as pack amps for the whole pass; no real pass does that.

YOUR CELL

Cell Capacity (mAh) Burst rating (C) Cell IR (mΩ, blank = EST) Weight per cell (g) Pack leads + plugs (mΩ) EST ESC leads (mΩ) EST

YOUR LIMITS

Most average sag you'll accept (% of full pack V) Most % of cell burst rating you'll run Most cells in parallel to try (1–8)

The sag limit is your call (10% is an example, not a rule). 100% of burst is the cell's RATED limit; racing runs in the red, so the bars show 90–100% in orange and anything past 100% in red. Past 100% is beyond the rating: testing only, at your own risk.

CELLS IN PARALLEL, 1P UP

Layout	Cells	Weight RATED g	Peak sag	Average sag	Sag at end	Peak A per cell	% of burst rating	Result

Voltage at the ESC through the pass, every layout. The green line is the pick. Weight is bare cells only (no wire, plugs, wrap or balance leads).

NEXT: RIPPLE PACK CALCULATOR ▶

HOW IT WORKS OUT THE ANSWER

Same model as the Ripple Pack calculator. Packs in parallel and the ESC leads share one bus. Each pack gives current in inverse proportion to its resistance at that instant.
Pack resistance = cells in series × cell IR ÷ cells in parallel + pack leads and plugs.
Cell IR blank = 0.414 V ÷ the cell's burst amps EST. Measure yours: the Battery guide shows how.
Pack voltage starts at 4.20 V per cell and follows a typical LiPo voltage-vs-charge curve as the pass uses capacity EST.
Peak A per cell = the highest pack current in the pass ÷ cells in parallel, compared with capacity × burst C RATED.
The pick is the fewest cells in parallel that meets both your average-sag limit and your % of burst limit.
Ripple is a different job. Whatever this page picks, keep a low-ESR ripple board at the ESC: see the Ripple Pack calculator.