100 Ah 12 V LiFePO4 battery (4S17P, 32700 cells) with charger
12.8 V 102 Ah (1306 Wh) battery from 68 × 32700 cells, 4S BMS with balancing, 20 A CC/CV charger from mains. 68 LiFePO4 32700 cells (3.2 V 6 Ah each). 17 in parallel make a 102 Ah group, and four groups in series (4S17P) make a 12.8 V 102 Ah battery (1306 Wh). Inside each group the cells are joined by copper busbar strip: nickel strip is rated about 15 A and a 20 A charge or a 300 A load would cook it. A 4S 300 A balancing BMS on the negative side watches each group: over-charge 3.65 V, over-discharge 2.5 V, over-current and short circuit. The charger is built from parts: a 24 V switching supply feeding a CC/CV buck module set to 14.4 V (3.6 V per cell) and 20 A (0.2 C). A 125 A MIDI fuse on the positive terminal protects the main cable; the BMS’s B+ lead only senses the top group.
Build it in the labGuided build, test sheet and fault finding, free in your browser.
Parts list
- 68 × LiFePO4 cell
- 4S LiFePO4 BMS 100 A
- Fuse
- Utility grid
- Switching power supply
- CC/CV buck module
- Resistor
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Battery voltage at 60 %
- Charge current (constant-current stage, battery at 50 %)
- Charger holds 14.4 V when full (constant-voltage stage)
- Over-charge: charger mis-set to 16 V
- Over-discharge: empty battery under load
- Over-current: a 450 A load
- Mains off: charger stays off, nothing drains back
- Rated capacity (cells in one parallel group)
How it works
What it does: 12.8 V 102 Ah (1306 Wh) battery from 68 × 32700 cells, 4S BMS with balancing, 20 A CC/CV charger from mains.
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
Packs of cylindrical LiFePO4 cells (32700, 26650, 18650 LFP) are how many e-trike, golf-cart and small solar batteries are built: cheap cells, many in parallel, a busbar along each group. Match the cells by voltage before welding: one weak cell in a parallel group drags the whole group down.