20,000 mAh power bank · 4 × 21700
20,000 mAh, 5 V 2.4 A USB out, 2 A USB charging, full protection. 4 21700 Li-ion cells in parallel (1S4P) make one big 3.7 V cell of 20,000 mAh (74 Wh). A protection board guards it: over-charge 4.3 V, over-discharge 2.5 V, over-current and short circuit. A 2 A charger fills it to 4.2 V from USB, and a 5 V 2.4 A boost module powers the phone, cutting off at 3.0 V so the cells are never run flat.
Build it in the labGuided build, test sheet and fault finding, free in your browser.
Parts list
- 4 × 21700 Li-ion cell
- 1S 10 A protection board
- TP5000 charger module
- Bench power supply
- Power-bank boost module
- Resistor
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- USB output at 2 A, on battery
- Charge current, USB in, nothing plugged into the output
- Charger stops at full
- Over-charge protection: a faulty 5 V charger wired straight to the pack
- Over-discharge cut-off: empty pack, phone still plugged in
- Short-circuit protection: pack shorted inside the case
- Rated capacity (sum of the cells)
How it works
What it does: 20,000 mAh, 5 V 2.4 A USB out, 2 A USB charging, full protection.
Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.
Background
Power banks are rated by the cells' capacity at 3.7 V. At 5 V, after the boost converter's losses, you get about 12,580 mAh: that is why a "20,000" bank never gives a phone 20,000 mAh.