7.4 Charging, Discharging, Balancing and Battery Limitations

Charging Procedure

Charging a lithium polymer pack correctly is a defined procedure, not an approximation.

1. Inspect before connecting. Any swelling, puncture, torn heat-shrink, damaged lead or discoloured connector means the pack does not go on charge — it goes to disposal.
2. Let the pack cool. Charging a pack still hot from flight accelerates degradation. Wait until it is at ambient temperature.
3. Connect both leads. Main lead to the charger's output, balance lead to the balance port. Never charge a multi-cell pack through the main lead alone.
4. Set the correct chemistry and cell count. A charger set to the wrong cell count will either refuse to start or attempt to charge to the wrong voltage — the second is dangerous.
5. Set the charge rate. The default is 1C.
6. Charge in a fire-safe location — a LiPo bag, metal ammunition box or ceramic container, on a non-flammable surface, away from anything that will burn.
7. Never charge unattended. Most LiPo fires start on charge.
8. Stop at 4.20 V per cell. A correctly configured charger does this automatically; a charger that overshoots is faulty and should be retired.

The 1C rule

A 1C charge rate means charging at a current numerically equal to the capacity expressed in amp-hours:

• A 2,200 mAh pack at 1C charges at 2.2 A.
• A 5,000 mAh pack at 1C charges at 5.0 A.
• A 16,000 mAh pack at 1C charges at 16 A.

At 1C a fully discharged pack takes roughly an hour to reach full charge. Higher rates — 2C, 3C, or the very high rates some smart chargers offer — are permitted only where the pack label or manufacturer explicitly allows them, and every increase shortens cycle life and raises heat.

Discharging and Cell Balancing

Discharging matters in two contexts. In flight, the aircraft discharges the pack, and the discipline is to respect the low-voltage warning and land — never fly a pack below about 3.0–3.3 V per cell, because below that the chemistry is permanently damaged. On the ground, a storage discharge brings a fully charged pack down to about 3.85 V per cell when it will not be used for more than a few days; most quality chargers have a dedicated storage mode that will either charge up or discharge down to that level.

Cell balancing is the process of equalising the individual cell voltages within a multi-cell pack. Cells never age identically: small differences in internal resistance mean one cell reaches 4.2 V before the others. Without balancing, the charger — seeing only total pack voltage — keeps pushing current, over-charging the leading cell while the laggards remain low. Over-charging a lithium cell is precisely the condition that produces gassing, swelling and thermal runaway.

A balance charger monitors each cell through the balance lead and either bleeds off the leading cells or slows the charge until the pack is even. The pass criterion after charging is all cells within about 0.05 V of each other. A pack that will not balance, or that drifts out of balance within a flight or two, has an internal fault and should be retired.

State of charge from voltage and capacity

Two ways to judge how much energy is left:

• By voltage — quick, approximate, and non-linear. LiPo voltage sits on a plateau for most of the discharge and then falls away sharply, so the last 20 per cent of capacity disappears fast. Voltage under load also reads lower than voltage at rest.
• By capacity consumed (mAh) — far more accurate. If a charger returns 4,000 mAh to a 5,000 mAh pack, the flight used 80 per cent. Logging returned capacity flight by flight is the best way to learn what your aircraft actually consumes.

C-Rating and Battery Limitations

Schedule 4 topic 6 asks explicitly about the continuous C-rating versus the maximum burst C-rating.

• Continuous C-rating — the current the pack can supply indefinitely without overheating or damage. Multiply by capacity in amp-hours to get amps.
• Maximum burst C-rating — a substantially higher figure the pack can supply for a few seconds only, typically 10 seconds or less.

Worked example. A 5,000 mAh pack rated 20C continuous / 30C burst:

• Continuous: 20 × 5 Ah = 100 A
• Burst: 30 × 5 Ah = 150 A, for a few seconds only

If the aircraft's hover draw is 40 A and its full-throttle draw is 95 A, the pack is adequate — 95 A sits under the 100 A continuous figure. If full-throttle draw were 130 A, the pack would only support it as a burst, and any sustained climb would drive it past its continuous rating into sag and heat.

Voltage sag is the practical symptom of a pack being asked for more than it can comfortably give. Under load the pack's voltage drops; a healthy pack sags a few tenths of a volt per cell and recovers promptly when the load eases. A pack that sags sharply, recovers slowly, or triggers a low-voltage warning within seconds of throttle-up is either under-specified for the aircraft or nearing end of life.

The size-versus-endurance trade

Schedule 4 topic 6(b) asks about the trade-off between battery size and flight endurance. It is not a straight line. A bigger pack stores more energy — but it also weighs more, and the aircraft must lift that weight, which raises the hover current draw. Past a certain point, the extra energy is entirely consumed lifting the extra mass, and endurance stops improving. Every airframe has an optimum pack size, which is why manufacturers specify one rather than saying "as big as fits".

Estimating endurance

A reasonable planning estimate:

$\text{Usable capacity} = \text{Pack capacity} \times 0.8$

$\text{Endurance (min)} = \frac{\text{Usable capacity (mAh)}}{\text{Average draw (mAh/min)}}$

The 0.8 factor reserves 20 per cent — you land with reserve, never on empty.

Worked example. A 10,000 mAh pack with an average hover draw of 900 mAh per minute gives (10,000 × 0.8) ÷ 900 ≈ 8.9 minutes to the reserve point. Add wind, payload or a climb and the real figure drops. Measure your own aircraft's consumption from returned-capacity figures rather than trusting a brochure number.

Battery Checkers and Serviceability

A battery checker plugs into the balance lead and displays per-cell voltage, total voltage and often the difference between the highest and lowest cell. Use one:

• Before every flight — confirm all cells are near 4.2 V and within 0.05 V of each other.
• After every flight — confirm no cell went below about 3.5 V, and note any pack that consistently comes back lower on one cell.
• Before storage — confirm the pack is at storage voltage.

Retire a pack when it swells, when a cell will not balance, when internal resistance has risen markedly, when capacity has fallen well below its rating, or when it has been over-discharged or physically damaged. Retirement means safe discharge and disposal at a battery recycling point — never in general waste, where a damaged lithium cell can start a fire in a collection truck.