LiPo, NiMH, LiHV and Li-ion: Reading a Battery Label From Nothing

Every electric RC runs on a pack, and the label on that pack is written in a code nobody explains before they sell it to you: *3S 5000mAh 50C LiPo, 11.1V, XT90*. Six pieces of information, and each one changes what the car does and whether the electronics survive.

This explains all of it from nothing — what each number means, how to work out whether a pack suits your car, and how to judge whether the number printed on the label is actually true.

The chemistries, and which you actually want

There are four you will meet. Most people only need to understand the first two.

NiMH — nickel-metal hydride

The old standard, still sold, still genuinely useful.

What it is: Usually six or seven cells in a hard plastic case, delivering 7.2V or 8.4V nominal. Heavy for the energy it holds.

Why you might want it: It is *forgiving* in a way lithium is not. It does not need balance charging. Leaving it half-charged for a month does not hurt it. Overdischarging it is recoverable rather than fatal. There is no meaningful fire risk from normal use.

Why most people move on: Weight, and voltage sag. A NiMH pack's voltage falls steadily as it drains, so the car noticeably slows through a run. A LiPo holds its voltage up and feels strong until it is nearly done.

Honest position: For a young child's first model, or for a toy-grade replacement, NiMH is a defensible and often better choice. For anything else, LiPo.

LiPo — lithium polymer

What almost everything runs on now.

What it is: Soft-pouch lithium cells, 3.7V nominal each, 4.2V fully charged. Far more energy per gram than NiMH, and it holds voltage under load.

The trade: It demands respect. Balance charging, correct storage voltage, never over-discharged, never charged unattended. Get those wrong and you have a fire risk rather than a dead battery. Our LiPo safety guide covers that properly.

LiHV — high-voltage LiPo

A LiPo variant that charges to 4.35V per cell instead of 4.2V.

That extra 0.15V per cell is a small but real power increase — on a 4S pack that is 0.6V more at the top. It matters most in FPV racing where every bit of punch counts.

The catch that catches people: your charger must have an explicit LiHV mode. Charging a standard LiPo to 4.35V damages it and is a genuine hazard. Charging a LiHV pack on the standard LiPo setting is safe, you simply do not get the extra voltage. Check which one you actually bought — they look identical.

Li-ion — lithium-ion (18650 / 21700 cells)

Cylindrical cells, common in long-range FPV and increasingly in crawlers.

What it is good at: energy density over *time*. Li-ion holds more total energy per gram than LiPo but cannot deliver it as fast. That makes it excellent where you want long runtime at modest current — a crawler creeping for two hours, a long-range drone — and poor where you want punch.

Nominal voltage is 3.6V or 3.7V per cell depending on the cell, and they are usually built as packs of 18650 or 21700 cells.

Cell count: the "S" number

S = cells in series. Series adds voltage.

Voltage sets motor RPM. That is the whole reason cell count matters. Going 2S → 3S is a 50% voltage increase, so roughly 50% more RPM, which means much more current and much more heat.

P = cells in parallel. Parallel adds capacity, not voltage. A "3S2P" pack has six cells: three in series, doubled up. You see this mostly on Li-ion packs.

⚠️ The rule that saves ESCs: never fit a higher cell count than your ESC and motor are rated for, and when you do go up in cells, gear down to compensate. Adding cells without changing gearing is the single most common way people destroy an ESC on the first run after an upgrade.

Capacity: the mAh number

mAh = milliamp-hours. It is how much charge the pack holds — the size of the fuel tank.

The arithmetic is simple. A 5000mAh pack is 5.0Ah. Drawing 5A continuously would empty it in one hour. Drawing 50A empties it in one-tenth of an hour — six minutes.

Worked, for a real car: a 1/10 buggy averaging around 20A on a 5000mAh (5Ah) pack gives 5 ÷ 20 = 0.25 hours, so roughly 15 minutes of running. Real figures come out lower because bursts pull far more than the average and you should stop before the pack is empty.

Bigger is not automatically better. More mAh means more weight, and weight changes how the car handles and how hard it works. Racing classes often use smaller packs deliberately. Fit the biggest pack that fits the tray and the class rules, not the biggest one sold.

C-rating: the number most likely to be a lie

C-rating is the manufacturer's claim about how much current the pack can deliver continuously, expressed as a multiple of capacity.

Continuous current = capacity in Ah × C

A 5000mAh (5Ah) pack rated 50C claims 5 × 50 = 250A continuous.

Many packs also quote a burst rating — a higher figure for a few seconds only. Burst is a marketing number more often than a specification; design around the continuous figure.

Why the printed number often is not true

C-ratings in this hobby are not independently standardised. No body tests them, no definition is enforced, and two packs printed with the same number routinely behave very differently.

The concrete version, from the drone world where this gets measured carefully: a quality 75C pack from a reputable brand will commonly outperform a no-name 100C pack, because what actually limits delivery is internal resistance, not the sticker. (Maker Pro, TH3SEUS)

Internal resistance — the number that is actually true

Internal resistance (IR) is the pack's own electrical resistance, measured in milliohms (mΩ). It is the honest version of the C-rating, and most decent chargers will measure it per cell.

Here is why it matters. Whenever current flows, some voltage is lost across that internal resistance instead of reaching your motor. The more current you draw, the more you lose. That loss is voltage sag — the pack's voltage dipping under load and recovering when you let off.

The everyday version of the same physics: an old phone showing "100% charged" that dies the instant you use the flash. The charge is there; the pack can no longer deliver it fast enough, because IR has risen.

What to do with it:

1. Measure IR when the pack is new and write it down. The absolute number varies by pack size and cell count, so what matters is *your* pack's baseline.
2. Re-measure every couple of months. IR rises as a pack ages — that is what ageing physically is.
3. When IR has climbed noticeably above baseline, the pack is on its way out. It will sag harder, run hotter, and trip your ESC's low-voltage cutoff early even with capacity left.
4. Compare cells within the pack. One cell reading much higher than its siblings means that cell is failing, and the pack should be retired.

⚠️ What we could not confirm: there is no cross-manufacturer "good" or "bad" IR figure we can publish, because it depends on capacity, cell count, chemistry, temperature and the charger's own measurement method — different chargers give different readings for the same pack. Use *change over time on your own pack, measured on your own charger* as the signal, not somebody else's absolute number.

Voltage sag, and how to read it

Sag is what you feel, IR is why it happens.

• A healthy pack sags a little under hard throttle and recovers immediately when you ease off.
• A tired pack sags deeply, recovers slowly, and gets hot doing it.
• A pack sagging enough to hit your ESC's cutoff while it still holds charge is finished, regardless of what the capacity says.

Sag also gets worse when the pack is cold. A pack run straight out of a cold car will sag hard and can be over-discharged before you notice. Let packs reach room temperature before running them.

Connectors — the part nobody mentions

The plug matters, because a connector too small for the current is a resistive hot spot, and resistance means heat.

Buy your rig, your packs and your charger with one connector in mind and fit adapters only where you must. Every extra adapter is another junction that can heat up. A connector that has gone brown or feels loose is telling you it has been running hot — replace it before it becomes an intermittent fault you chase for weeks.

Putting it together: choosing a pack for your car

Work through it in this order.

1. Cell count — whatever your ESC and motor are rated for. Not more. If you go up, gear down.
2. Physical size — measure the battery tray, including the strap and the wire exit. This eliminates most options immediately and people always leave it until last.
3. Capacity — the biggest that fits the tray and the class rules, then sanity-check the weight.
4. Connector — matching whatever your ESC already wears.
5. C-rating — ignore the headline and buy a brand with a reputation. The sticker is not a measurement.
6. Verify by temperature. After a full run, the pack should be warm, not hot. Hot means it is being asked for more than it comfortably delivers — either the rating was optimistic or the gearing is too aggressive.

The short version

• NiMH for a child's first model or maximum forgiveness; LiPo for everything else.
• LiHV charges to 4.35V and needs a charger mode to match — check which you bought.
• Li-ion for long runtime at low current, not for punch.
• S = voltage = RPM. More cells, less pinion.
• mAh = runtime. Bigger also means heavier.
• C-rating is a claim. IR is a measurement. Track your own pack's IR over time.
• Warm after a run is fine. Hot is a message.

Sources

• Maker Pro — Why High C-Rating Batteries Still Experience Voltage Sag
• Brushless Whoop — LiPo Battery Specifications
• TH3SEUS — LiPo C-Rating Explained