--- title: "LiPo safety" subtitle: "Power and motors · Batteries · Lesson 5" --- # LiPo safety **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Drones, racing robots and phones all use lithium batteries because nothing else is as light for the power. The same energy, released all at once, is a fire. What keeps it in? :::teacher-note **In one sentence:** A teacher-led safety lesson. Students decide what to do with a swollen pack, sort charging and storage habits into safe and unsafe, work out full and empty voltages for a 2S pack, put the charging routine in order, and are signed off by the teacher before any LiPo use. **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **A LiPo pack is slightly puffy, but it still powers the robot. What should you do?** ☐ Squeeze it flat and tape it ☐ Charge it slowly to fix it ☐ Stop using it, tell your teacher, and have it disposed of safely ☐ Keep using it until it stops working My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Stop. Swelling means gas from damaged cells; it can fail and catch fire, especially when charged. It goes to safe disposal, never the bin. ::: ## Safe or not? :::keep-together ### Sort them **Safe or unsafe?** Groups: Safe, Unsafe - Charging in a LiPo bag on a tiled bench, with someone there: {{field-15}} - Charging overnight in a backpack: {{field-15}} - Using a balance charger set to LiPo and the right cell count: {{field-15}} - Charging a LiPo on a NiMH charger: {{field-15}} - Running the robot until it stops completely: {{field-15}} - Storing packs at storage charge (about 3.8 V a cell) in a fire-safe box: {{field-15}} - Putting a dented pack back in the robot: {{field-15}} - Unplugging a pack that is getting hot and moving everyone away: {{field-15}} ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### Balance charging **Why does a LiPo need a balance charger?** ☐ It makes the pack hold more ☐ It's only needed for big packs ☐ It charges faster ☐ It charges each cell to the same voltage, so no cell is overcharged ::: :::teacher-note **Answer:** 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. ::: ## The numbers :::keep-together ### Full **A 2S pack is full at 4.2 V a cell. What does the whole pack read when full?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8.4 2 × 4.2 = 8.4 V. ::: :::keep-together ### 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 ☐ Keep going: it still has charge ☐ Charge it on a NiMH charger ☐ Store it like that ::: :::teacher-note **Answer:** 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. ::: ## Make: the charging routine Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Make: Sign-off Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Give three rules for charging a LiPo.** {{lines-3}} ::: :::teacher-note **Answer:** 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. :::