A LiPo pack is the most capable and the most unforgiving component in your model. Treated properly it will give you hundreds of cycles. Treated carelessly it will swell, fail, or in the worst case burn. None of that is scaremongering — it is chemistry, and the rules that follow from it are short and worth knowing exactly.

What the numbers on the pack mean

A typical label reads something like 5000mAh 3S 11.1V 50C. Each figure tells you something you need.

• mAh — capacity. How much energy the pack holds. 5000mAh delivers 5 amps for one hour, or 50 amps for six minutes. More capacity means longer runs and more weight.
• S — cells in series. Each LiPo cell is nominally 3.7V, so 3S is 11.1V. This is the number that must match what your ESC and motor are rated for.
• C — discharge rate. Multiply by capacity for maximum continuous current: 50C on a 5000mAh pack is 250 amps in theory. Treat manufacturer C ratings as optimistic, and leave headroom.

Why your new pack arrived half empty

Lithium packs ship at storage charge — around 3.8V per cell, roughly half capacity. This is not a fault and it is not a shortcut. Transport regulations require it, and a pack stored at full charge degrades measurably faster. Charge it before the first run; that is the whole story.

Charging

• Use a balance charger. It brings every cell to the same voltage. A pack whose cells drift apart is a pack heading for failure.
• Charge at 1C unless you have a reason not to. A 5000mAh pack at 1C is 5 amps. Faster charging is possible on packs rated for it and costs you cycle life.
• Never leave a pack charging unattended. This is the rule that matters most. Nearly every LiPo fire happened while nobody was watching.
• Charge on a non-flammable surface. A concrete floor, a metal tray, or a LiPo bag. Not the carpet, not the sofa, not on top of the model.
• Let a hot pack cool first. Straight off a run it may be 50°C or more. Charging it then stresses the cells.

Discharging and the 80% rule

Do not run a LiPo flat. Below about 3.5V per cell under load, damage begins; below 3.0V resting, the pack is usually finished. Set your ESC's low-voltage cutoff and believe it when it starts pulsing.

The practical habit: use about 80% of capacity and land or stop. On a 5000mAh pack, that is 4000mAh going back in on the next charge. If your charger regularly puts back more than 90%, you are running the packs too far down and they will not last a season.

Storage

• Store at 3.8V per cell. Most chargers have a storage mode that will take a pack up or down to it.
• Never store fully charged. A pack left at 4.2V per cell for weeks loses capacity permanently.
• Never store flat. Self-discharge can take it below the point a charger will accept it.
• Cool and dry. Room temperature is fine; a hot car is not.
• If it will sit for more than a week, put it into storage charge. It takes two minutes and doubles the life of the pack.

When a pack goes wrong

Swelling is the main warning sign. A puffed pack has vented gas internally and it will not recover. Stop using it immediately.

• Do not charge a swollen pack. Not once, not gently, not to "get it home".
• Do not post it. A damaged lithium pack cannot legally travel as an ordinary parcel and must not be sent back to us or anyone else.
• Isolate it. Somewhere non-flammable, away from anything that matters, until you can dispose of it.
• Recycle it locally. Most councils and many hobby shops take lithium packs.

A pack damaged in a crash deserves the same caution even if it looks fine. Puncture damage can take hours to show. Leave it somewhere safe and watch it before charging again.

Matching the pack to the model

Check three things before buying: cell count against what the ESC accepts, connector type against what the model uses, and physical size against the battery tray. A pack that is right electrically and 5mm too long is still the wrong pack.

Running more cells than the ESC is rated for is the fastest way to destroy both the ESC and the motor, and it is not covered by any warranty. If a model is sold as 2S–3S, 4S is not an upgrade — it is a repair bill.