Measuring Your Own Power System: IR, Sag and Amp Draw

Our battery label guide explains what the numbers on a pack *claim*. This one is about finding out what is actually true — measuring your own power system instead of trusting a sticker, and reading what the measurements mean.

Three tools do all of it, and together they cost less than one decent pack.

Why the printed numbers are not enough

C-ratings are not independently standardized in this hobby. No body tests them, no definition is enforced, and two packs printed with the same figure routinely behave very differently. A well-made 75C pack commonly out-delivers a no-name 100C pack, because what actually limits delivery is internal resistance, not the label (Maker Pro).

Capacity claims have the same problem. A pack labeled 5000mAh may deliver noticeably less, and it will deliver less again as it ages.

None of which matters much, because you can measure the things that count.

The three measurements that tell you everything

1. Internal resistance — the health number

What it is: the pack's own electrical resistance, in milliohms (mΩ). Most decent balance chargers measure it per cell during or after a charge.

Why it matters: every amp you draw loses some voltage across that resistance instead of reaching the motor. Higher IR means more loss, more sag, more heat, less usable power. IR rising over time is what battery ageing physically is.

How to use it — and this is the whole method:

1. Measure it when the pack is new. Write it down, per cell, with the date. This is your baseline.
2. Measure the same way every time — same charger, similar pack temperature, same point in the charge cycle. Different chargers give different readings for the same pack, so consistency matters more than the absolute figure.
3. Re-check every month or two. Log it.
4. Watch two things: the trend upward over time, and whether one cell is drifting away from its siblings.

Reading it:

• All cells close together, IR near baseline → healthy
• All cells risen together → the pack is ageing normally. Expect more sag and shorter runs.
• One cell noticeably higher than the rest → that cell is failing. Retire the pack.
• A sudden jump after one hard session or a crash → damage. Inspect the pack.

⚠️ We cannot publish a "good" IR number. It depends on capacity, cell count, chemistry, temperature and the charger's own measurement method. Your own pack's change over time, on your own charger, is the signal. Anyone quoting a universal figure is guessing.

2. Voltage under load — the sag test

Voltage at rest tells you very little. Voltage *while the car is working* tells you almost everything.

With telemetry (the easy way): if your radio system supports it, a pack voltage sensor gives you a live reading and an audible alarm at a threshold you set. This is the single most useful telemetry sensor there is — set an alarm rather than watching a screen, because if you are watching the screen you are not watching the car.

With a wattmeter (the direct way): an inline wattmeter between pack and ESC reads volts, amps, watts and cumulative mAh in real time.

The test:

1. Wattmeter inline, car on a stand with wheels clear.
2. Note resting voltage.
3. Apply a hard, sustained throttle burst.
4. Watch the voltage dip and how fast it comes back.

What you are looking for is the shape, not one number:

• Small dip, snaps straight back → healthy pack, comfortably within its capability
• Deep dip, slow recovery → the pack is being asked for more than it likes, or it is tired
• Deep dip plus heat → definitely too much. Either the pack is over-rated or the gearing is over-ambitious
• Sag that has got worse over months on the same car → the pack, not the setup

Cold packs sag much harder. Always compare like with like; a pack straight from a cold car is not a fair test.

3. Amp draw — what your car actually pulls

This is the measurement that turns pack shopping from guesswork into arithmetic, and almost nobody does it.

With a wattmeter inline, run the car normally for a full pack and read the peak amps and the cumulative mAh consumed.

Now you know two genuinely useful things:

a) Whether your pack is adequately rated. Compare peak draw against the pack's claimed continuous current (capacity in Ah × C). If your car peaks at 90A and your pack claims 250A continuous, you have plenty of headroom — and if the pack still sags badly at 90A, the claim was optimistic.

b) Whether you are using the capacity you paid for. If a full run consumes 2,800mAh from a 5,000mAh pack, you are carrying weight you never use. A smaller, lighter pack would be quicker and cheaper. If it consumes 4,600mAh, you are running the pack close to empty and shortening its life.

Worked example. A 1/10 buggy pulls 20A average and 85A peak, consuming 3,400mAh over a 12-minute run. That says: a 4,000mAh pack is enough with sensible margin; the peak is well inside any modern pack's rating, so sag under that load would point at pack quality rather than the car; and a 5,000mAh pack is carrying about 25% more weight than this setup uses.

The three tools

Together they cost less than one good pack, and they answer questions people otherwise argue about for years.

A simple log that pays for itself

A note on your phone is enough. Per pack:

• Purchase date and the pack's ID (number your packs — a marker on the shrink wrap)
• Baseline IR per cell, and the date
• IR re-checks with dates
• mAh consumed on a typical run
• Anything unusual — a hot run, a crash, a cell drifting

Within a season this tells you which packs are ageing fastest, whether one charger or one car is harder on packs than another, and when a pack is genuinely finished rather than "feels a bit down." It also stops you replacing a healthy pack because a run felt slow on a cold day.

Common findings, and what they mean

"My new pack sags worse than my old one." Check IR on both. A high-C claim on a cheap pack often measures worse than a modest claim from a good brand.

"The car cuts out early but the pack still has capacity." Sag is dropping the pack below the ESC's low-voltage cutoff under load, even though the resting voltage is fine. That is rising IR — the pack is on the way out. Confirm by measuring rather than raising the cutoff to hide it.

"One pack always comes back hotter." Measure its IR against its siblings. Higher IR converts more of the energy into heat inside the pack.

"Runtime dropped suddenly, not gradually." Sudden change means damage or a failing cell, not ageing. Check per-cell voltages at the end of a charge and per-cell IR.

"It's fine in summer and gutless in winter." Cold raises IR substantially. Let packs warm to room temperature before running, and expect genuinely different behavior in the cold — this one is physics, not a fault.

What we could not confirm

We are publishing no target figures for IR, sag or amp draw. There is no cross-manufacturer standard for any of them; IR readings differ between chargers for the same pack; and amp draw is entirely specific to vehicle, gearing, tires, surface and driving style.

Everything here is a method for measuring your own equipment and reading the trend. The numbers that matter are yours, measured the same way each time. Where your pack or ESC manufacturer publishes limits for their own product, those beat anything general.

Sources

• Maker Pro — Why High C-Rating Batteries Still Experience Voltage Sag
• Brushless Whoop — LiPo Battery Specifications