50 Ah 12 V LiFePO4 battery (4S9P, 32700 cells) with charger
12.8 V 54 Ah (691 Wh) battery from 36 × 32700 cells, 4S BMS with balancing, 10 A CC/CV charger from mains. 36 LiFePO4 32700 cells (3.2 V 6 Ah each). 9 in parallel make a 54 Ah group, and four groups in series (4S9P) make a 12.8 V 54 Ah battery (691 Wh). Inside each group the cells are joined by copper busbar strip: nickel strip is rated about 15 A and a 10 A charge or a 120 A load would cook it. A 4S 120 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 10 A (0.2 C). A 60 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
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 180 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 54 Ah (691 Wh) battery from 36 × 32700 cells, 4S BMS with balancing, 10 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.