LiPo safety

Classroom resources

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Students will be able to

• I can inspect a LiPo pack and know when to retire it.
• I can follow the charging routine.
• I can say the full, storage and empty voltages for a pack.

Timing

Predict 3 min · Sort 10 min · Practise 5 min · Routine 5 min · Sign-off 7 min

Before class

Follow your school's risk assessment and the manufacturer's instructions; they override this page. Figures: full 4.2 V a cell, nominal 3.7 V, storage about 3.8 V, do not discharge below about 3.0–3.3 V a cell (set the low-voltage cutoff or alarm). Charge at 1 C or less unless the pack says otherwise.

Have ready: a balance charger, a LiPo bag, a non-flammable surface, a bucket of sand, and the school's battery disposal route. A swollen pack goes to disposal, never the general bin.

This lesson is a prerequisite: no student uses a LiPo until the sign-off step is done with you. AA or NiMH packs meet every brief in this course.

Running the lesson

1. Predict: let students argue for 'it still works' before revealing why not.
2. Sort: ask for the reason behind each unsafe habit.
3. Sign-off: talk to each student or group; this is the record that you did.

The hard part (on purpose)

Students treat LiPos like any battery because they look harmless. Show the school's fire-safe bag and disposal arrangements in person.

When students get stuck

Answers to the checks

1. a puffy pack: A LiPo pack is slightly puffy, but it still powers the robot. What should you do?
   Stop. Swelling means gas from damaged cells; it can fail and catch fire, especially when charged. It goes to safe disposal, never the bin.
2. Sort them: Safe or unsafe?
   Charging in a LiPo bag on a tiled bench, with someone there → Safe; Charging overnight in a backpack → Unsafe; Using a balance charger set to LiPo and the right cell count → Safe; Charging a LiPo on a NiMH charger → Unsafe; Running the robot until it stops completely → Unsafe; Storing packs at storage charge (about 3.8 V a cell) in a fire-safe box → Safe; Putting a dented pack back in the robot → Unsafe; Unplugging a pack that is getting hot and moving everyone away → Safe
   Inspect, charge supervised on the right charger in a safe place, never run flat, store at storage charge, retire damaged packs.
3. Balance charging: Why does a LiPo need a balance charger?
   It charges each cell to the same voltage, so no cell is overcharged
   Cells drift apart; charging the pack as a whole could push one cell over 4.2 V.
4. Full: A 2S pack is full at 4.2 V a cell. What does the whole pack read when full?
   8.4
   2 × 4.2 = 8.4 V.
5. After a run: A 2S pack reads 6.2 V after a run. What should you do?
   Stop using it now: that is 3.1 V a cell, near empty
   6.2 ÷ 2 = 3.1 V a cell: recharge it (on a balance charger) before storing.
6. the charging routine: Put the charging routine in order.
   see the program below
   Inspect, contain, connect correctly, set the charger, supervise, finish.

   Check the pack: no swelling, dents, torn wrap or damaged wires.
   Put the pack in a fire-safe LiPo bag on a non-flammable surface.
   Plug both the main lead and the balance lead into the balance charger.
   Set the charger to LiPo, the right cell count and a current no more than the pack's capacity (1 C).
   Stay with it while it charges.
   Unplug it when the charger says it is full, and let it cool before use.
   

Stretch and support

Stretch: Look up the C rating on a LiPo label. What does 25 C mean for a 1000 mAh pack's maximum current?

Support: A printed card of the routine and the three voltages, kept by the charging station.

Exit ticket

Give three rules for charging a LiPo.

Any three of: balance charger set to LiPo and the right cell count; in a fire-safe bag on a non-flammable surface; never left unattended; inspect first and never charge a damaged or swollen pack; charge at 1 C or less.

What each step is for

• a puffy pack: Committing to what to do with a slightly swollen pack surfaces the 'it still works' instinct before the rules are taught.
• safe or not?: Sorting real habits into safe and unsafe builds the rules from cases, each with its reason.
• the numbers: The voltages that mark full, storage and empty, for a 2S pack.
• the charging routine: Putting the routine in order makes it something students can follow on the day, not just know.
• Sign-off: LiPo use is a teacher decision, not a self-paced tick: this step records the conversation.

Where it leads

Back to Choosing a battery, or on to a Make project: a two-motor drive base (pairs with Mechanisms for gearing and the robotics simulator).