LiPo packs are light and give big currents, but they store a lot of energy in a soft pouch. Damaged, over-charged or run-flat cells can swell and catch fire. The rules: inspect every time, charge on a balance charger in a fire-safe bag while someone watches, never run below the cutoff, store at storage charge, and retire any pack that swells. Required before any LiPo use; your teacher signs it off.
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?
LiPo (lithium polymer) packs are rated by cells: a 2S pack has two cells, 7.4 V nominal (4.2 V a cell full, about 3.0–3.3 V a cell empty). They are the right choice when weight and current matter, and the wrong one when nobody is following the rules below.
Your school's risk assessment decides whether LiPos are used at all. AA and NiMH packs meet every brief in this course.
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Investigate: Work out why it happens, one step at a time.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
A puffy pack
InvestigateWork out why it happens, one step at a time.
Safe or not?
Safe or unsafe?
Charging in a LiPo bag on a tiled bench, with someone there
Charging overnight in a backpackUnwatched, on something that burns, with nowhere for heat to go.
Using a balance charger set to LiPo and the right cell count
Charging a LiPo on a NiMH chargerWrong voltages: it can overcharge the cells.
Running the robot until it stops completelyBelow about 3 V a cell, LiPos are damaged: use a low-voltage alarm or cutoff.
Storing packs at storage charge (about 3.8 V a cell) in a fire-safe box
Putting a dented pack back in the robotPhysical damage can short the cells inside.
Unplugging a pack that is getting hot and moving everyone away
PractiseA few on your own, to check you have it.
The numbers
A 2S pack is full at 4.2 V a cell. What does the whole pack read when full?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
The charging routine
Put the charging routine in order.
Pieces (click to add)
Your program
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Sign-off
For teachers
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. About 30 minutes.
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.
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
Predict: let students argue for 'it still works' before revealing why not.
Sort: ask for the reason behind each unsafe habit.
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
A student wants to charge at home. Ask: Who will be watching it, and on what surface? Fix: School rules: charging only at school, supervised.
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.
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).
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.
Now you can
Inspect a LiPo pack and know when to retire it.
Follow the charging routine.
Say the full, storage and empty voltages for a pack.