Students compare a 9 V PP3 with 4 × AA for a robot, work out run time from capacity and draw with an 80% margin, find why NiMH cells fail a voltage range, and choose a battery against voltage, run time and weight, which leads to a LiPo.
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Students will be able to
I can work out a battery's run time from its capacity and the current.
I can choose a battery against a brief's criteria.
I can explain the difference between voltage and capacity.
Timing
Predict and Try 5 min · Worked example 10 min · Vary 8 min · Practise and Make 17 min
Before class
Figures are typical, not exact: alkaline AA capacity falls a lot at high current, and PP3s are poor at motor currents (often well under 500 mAh). Check the packs you buy. The board's run time is capacity × usable % ÷ draw.
LiPos appear here as the battery that fits; the next lesson is required before anyone uses one.
Running the lesson
Predict: ask what the 9 in 9 V tells you, and what it doesn't.
Make: ask students to say which criterion rules out each battery.
The hard part (on purpose)
Students rank batteries by one number. Keep all three criteria on the board for every candidate.
When students get stuck
What you will see
A question to ask
The fix
A student divides current by capacity.
Which is bigger, and which way gives hours?
Capacity ÷ current.
Answers to the checks
the 9 V battery: A robot draws 350 mA. Which runs it for longer: a 9 V PP3 (500 mAh) or 4 × AA (6 V, 2000 mAh)? 4 × AA: about 343 minutes against about 86. Voltage doesn't decide how long; capacity and current do.
Try: compare them: The PP3 is fitted. Note its run time, then try 4 × AA.
86 minutes for the PP3, 343 for the AA pack: four times the capacity, four times the time.
Hours and minutes: A 1000 mAh LiPo powers a robot drawing 900 mA. Using only 80% of it, how many minutes does it last? (nearest whole minute) 53 1000 × 0.8 = 800 mAh; 800 ÷ 900 = 0.89 h ≈ 53 minutes.
Find the mistake: Sam's robot is no faster with the new pack. Click the line where Sam's reasoning first goes wrong. line 3 Line 3: mAh is how much charge it holds, so the 2200 pack lasts longer. Speed depends on voltage (both packs are 7.4 V here), not capacity.
900 mA, one hour: The robot now draws 900 mA and must run for an hour. Switch on 'use only 80%'. Does 4 × AA still pass?
Yes: 1600 mAh ÷ 900 mA ≈ 107 minutes.
Rechargeables: Try 4 × AA NiMH rechargeables. Why do they fail?
Voltage: NiMH cells are 1.2 V, so four make 4.8 V, under the driver's 6 V. Six cells (7.2 V) fix that, but weigh 165 g.
Hours: A 2200 mAh pack at 1100 mA, using all of it. How many hours? 2 2200 ÷ 1100 = 2 hours.
Capacity needed: You need 90 minutes at 600 mA, using 80% of the battery. What capacity, at least? (mAh) 1125 600 mA × 1.5 h = 900 mAh needed; 900 ÷ 0.8 = 1125 mAh.
a light robot: A small robot: 900 mA, at least 45 minutes using 80%, 6–9 V, at most 80 g. Find the battery that meets every criterion.
Only the 1000 mAh LiPo: light, enough voltage, and just enough run time. That is why small robots and drones use LiPos, and why they need the safety rules in the next lesson.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: a hungrier robot › 900 mA, one hour
Open task: any program that does what the step asks.
Vary: a hungrier robot › Rechargeables
Open task: any program that does what the step asks.
a light robot
Open task: any program that does what the step asks.
Stretch and support
Stretch: Two batteries in series: what happens to the voltage, and to the capacity? In parallel?
Support: A table with a row per battery and a column per criterion.
Exit ticket
A 1500 mAh pack and a robot drawing 500 mA. Using 80%, how long does it run?
the 9 V battery: Committing to which battery lasts longer tests the new move: run time comes from capacity and current, not voltage.
Try: compare them: Switching between the two batteries on the board settles the prediction.
Worked example: run time: Working run time from capacity and draw, then planning on 80%, models the move. Sam's 'more mAh is more powerful' is planted after the method.
Vary: a hungrier robot: A bigger current draw, a longer run and the 80% rule change which batteries pass, one criterion at a time.
run times: Run time both ways.
a light robot: A brief where weight rules out the AA pack and run time rules out the PP3 leads to the LiPo, and to why it needs the next lesson.