Charging LiPo batteries safely

LiPo packs hold a lot of energy in a soft foil pouch. Treated properly they are reliable and last for years. Treated carelessly they can vent burning gas, and you cannot put that fire out with water. None of what follows is difficult — it is just non-negotiable.

The rules that matter

• Never charge unattended. Almost every LiPo fire happens to a pack left charging in an empty room.
• Charge in a fireproof container. A LiPo bag or an ammo tin on a hard, non-flammable surface. Not on the carpet, not on the sofa, not in the car.
• Never charge a damaged pack. Puffed, split, dented, or pierced means the pack is finished. Retire it.
• Let a hot pack cool first. Straight off the track a pack can be well above room temperature. Give it twenty minutes.
• Always balance charge. More on this below.

Setting the charge rate

Charge current is expressed in C, a multiple of the pack's capacity.

Charge current (A) = capacity (Ah) × C rate

A 5000 mAh pack is 5 Ah, so 1C is 5 A. 1C is the safe default and is kind to the pack. Many modern packs are rated for faster charging — 2C, sometimes more — and the label will say so. If the label does not say, use 1C.

Charging faster than the label allows generates heat inside the cells, which shortens the pack's life long before it does anything dramatic.

Set the cell count yourself

Tell the charger how many cells the pack has rather than letting it guess. A 2S pack is 7.4 V nominal, 3S is 11.1 V, and so on — 3.7 V per cell. If a charger reads a deeply discharged 3S pack as 2S and charges it accordingly, it will massively overcharge every cell. Almost every charger has a setting for this; use it.

A full charge is 4.20 V per cell and no more.

Balance charging

A pack's cells drift apart in voltage as it ages. Balance charging uses the small white JST-XH connector to bring every cell up to the same voltage instead of only watching the pack total.

Always plug in the balance lead. A pack that reads a healthy 11.1 V might be three cells at 3.9, 3.9 and 3.3 V — the last one being quietly ruined. Balance charging catches that; charging through the main lead alone hides it.

Most chargers show individual cell voltages while charging. Glance at them. More than about 0.05 V of spread on a fully charged pack means the pack is on its way out.

After charging

If you are running the pack that day, leave it charged. If it is going to sit for more than a couple of days, put it into storage charge instead — see the guide on looking after LiPos between race days.

If a pack does vent

Get away from it and let it burn out somewhere it cannot set anything else alight. Do not use water. A LiPo fire produces its own oxygen, so smothering it does not work either — the goal is containment, not extinguishing. This is exactly why the charging container matters.