LiPo Internal Resistance: How to Measure and Read IR

What internal resistance tells you

Every cell has some resistance inside it. The more current you pull, the more that resistance costs you: voltage drops (sag) and energy turns into heat instead of thrust. A low-IR pack feels punchy and stays cool; a high-IR pack sags on a punch-out, runs warm and hits the low-voltage alarm early.

IR rises as a pack ages. Cycles, heat, hard high-current use, over-discharge and storing packs full all push it up. That makes IR the best number you can take at home for tracking the health of a pack over its life, as long as you measure it consistently.

How to measure LiPo internal resistance

Most hobby balance chargers have an IR function, and a dedicated IR meter works too. Neither is lab-grade: they're good for comparing a pack against itself on the same device, and not much else, so don't worry when a friend's charger reads your pack 2 mΩ higher. The method matters more than the tool. Do it the same way every time:

1. Use the same charger (or IR meter) and the same leads every time. Different chargers read the same pack differently.
2. Let the pack rest at room temperature. Not straight off the charger, not warm from a flight, not cold from the car.
3. Pick one charge state and stick to it — at storage voltage or fully charged. IR reads higher on a low pack.
4. Connect the main lead and the balance lead, then run the charger's IR function.
5. Note the reading for each cell, not just the pack total.
6. Repeat two or three times and use the typical value. The first reading after plugging in can be an outlier.
7. Write down the date, per-cell values and total, next to the cycle count.

Reading your results

Compare a pack with its own baseline (the reading from its first few cycles) and each cell with its neighbours. The table is a set of rules of thumb, not pass/fail limits: capacity, C rating, cell brand and your charger all shift the numbers.

Indicative IR values (rules of thumb only)

People always ask for a 'good' number. Here are rough ranges for new, high-discharge LiPo packs measured per cell on a hobby charger. They vary a lot by capacity, C rating, brand and charger, so use them only to spot a reading that is wildly off, never to judge a pack on its own.

• Large packs (about 4,000 mAh and up, e.g. 6S heli and plane packs): often low single-digit mΩ per cell.
• Mid-size packs (about 1,300–2,200 mAh, e.g. 4S and 6S FPV): often single digits to the low teens.
• Small packs (about 300–850 mAh, e.g. whoop and micro packs): often tens of mΩ per cell.
• Bigger capacity and higher C rating usually mean lower IR. Two packs of the same model can still differ, which is why each pack's own history matters most.

Temperature and charge state change the reading

Cold cells read noticeably higher IR, and warm cells read lower. A pack tested in a cold garage in January and a warm room in July can look like two different packs. The same goes for charge state: a nearly empty pack reads higher than the same pack at storage or full.

This is also why some pilots warm packs before flying in winter — lower IR means less sag. It's a real effect, but it's temporary; it doesn't repair an aging pack.

How often to test, and keeping the history

Take a baseline once a new pack has done a few cycles, then test every ten cycles or so, or once a month in the flying season. A single reading tells you little; a line of readings over a year tells you exactly where the pack is heading.

RC Flight Log records IR tests per cell and draws each pack's history as a chart next to its cycle count, so the cell that is slowly pulling away is obvious at a glance instead of buried in a notebook.

When rising IR means retirement

IR is a strong signal, but not the only one. Retire a pack when IR has roughly doubled from its baseline and flights feel it, when one cell is consistently well above the others, or when a sudden jump is confirmed on a re-test. Puffing or physical damage overrides everything: that pack is done regardless of its IR. See the guide on when to retire a LiPo for the full list of signs and how to dispose of a pack safely.