HOW MUCH BATTERY
One battery guide for everybody: drag, speed run, truck pull, boat, bash, and 1/5 & 1/7.
SAME ORDER
Gearing first, then battery, then ripple. Gear it so it doesn't grenade. Then size the pack so it holds. Then size the caps at the ESC. If you haven't geared it yet, go do that first: Gearing calculator.
THE BASICS
S IS VOLTAGE
S is how many cells are stacked in series. Each LiPo cell is about 3.7 V resting and 4.2 V fresh off the charger. 4S is about 14.8 V resting / 16.8 V full. 6S is about 22.2 / 25.2 V. 8S is about 29.6 / 33.6 V. 12S is about 44.4 / 50.4 V. Match S to what the ESC and motor are rated for. That is the first battery decision, and you don't get to fudge it.
P HOLDS IT UP
P is how many cells sit side by side. Sag is amps × the resistance between the cells and the ESC, and most of that resistance is inside the cells. Cells in parallel cut it for the whole run: 2P halves it, 4P quarters it. Small high-burst cells in parallel usually beat one big low-C cell at the same weight.
IT GULPS CURRENT
What runs out first on a hard hit is usually not mAh. It's voltage: how far the pack sags when the car hooks and pulls. Size the pack by sag, not the label.
LABEL C IS A STICKER
Brands don't test it the same way. Size by measured IR and sag, then work out the real, effective C: deliverable amps ÷ Ah.
RUN IT IN THE RED, NOT PAST IT
Sag is one test. The other: does every cell stay inside its burst rating at the peak pack amps? Under 90% of the supplier's burst rating is green. 90–100% is in the red: working hard, still inside the sheet. Past 100% the cell is being asked for more than it was sold to do. It will sag more and age faster.
WEIGHT IS THE MULTIPLIER
Same speed with double the weight takes about double the energy and current. Every ounce you add, the battery pays for. That's why a heavy 1/5 or a truck pull sled changes the answer.
WIRE IS THE THIN SLICE
Short pack leads, good plugs and short ESC leads are worth doing. Once your cells get good, a bigger share of your sag is happening outside the battery. Wire won't fix a pack that's too small.
WHAT YOU'RE DOING
| Style | What the pack sees | Battery question |
|---|---|---|
| Drag | A launch spike, then a few seconds and it's over. | Sag on the launch. Capacity used is tiny; current is the job. |
| Speed run | Part throttle while it builds, then wide open the whole straight. | Can it hold voltage for the full straight, not just the hit. |
| Truck pull | Under load the whole distance, and it gets heavier as the sled moves up. | Sag at the end of the pull, when the load is highest. |
| Boat | Minutes on the pipe, held wide open. | Sag and heat over minutes, plus enough capacity for the run. |
| Basher | The whole session in punches. 6S on an X-Maxx, flips, field blasts. | Repeated punch current. A weak pack sags on every punch like a drag launch. |
| 1/5 & 1/7 | 12S ESCs and 60 mm motors moving a heavy car, long sessions. | 12S is about 44.4 V resting / 50.4 V full. Rated gear, enough parallel, and heat. |
Same rules for all of them. What changes is how long the pack has to hold up and how often it gets hit.
HOW LONG THE PACK WORKS
Teaching sketch of relative load through a typical run — illustrative, not measured data from any car. 100 is as hard as that run gets, not a real amp number.
A drag pack takes one big hit. A speed-run or boat pack has to hold voltage for the whole run. A truck pull pack is at its worst at the end. A basher pack gets hit over and over. A 1/5 or 1/7 pack has a lot of truck to move the whole time.
MEASURE YOUR CELL IR
Charger IR meter: balance-charge, let it rest, run the charger's IR function on the balance lead, and write down each cell or parallel group at the same temperature every time. For a parallel group, multiply by P to get one cell. Good for tracking and matching; compare like with like.
Load step (ΔV ÷ ΔI): measure resting cell voltage V1, apply a known load ΔI, read V2 a second or two in. IR = (V1 − V2) ÷ ΔI, measured at the cell tabs, not the pack leads.
Put your number into Cell IR on the calculator. Blank uses an estimate. Recheck every few days; a group that climbs well above the rest has a tired cell.
RUN THE NUMBERS
One battery calculator, two parts on the same page. The top sizes the pack against your car, gearing and motor: peak amps, sag and motor load. Scroll down to How Much Battery? for how many cells in parallel so the pack doesn't sag at your ESC limit, % of burst rating and pack weight.
All numbers in the calculators are tagged RATED or EST. Confirm against a logged run.