This started off looking for LiFePO4 cells on BatteryHookup.com. There was kit for building a 24Ah Headway 38120 battery pack, and so we started searching YouTube to find more information on such a Headway build. Later it led to Jehu Garcia’s PCB Board Bus Kit and PCB BMS Board Kit using the same cells.
Since BatteryHookup.com was selling cells for $5.00 per cell (at the time) we decided to try this out.
This build proved to be a lot more difficult than anticipated. Other than waiting for China shipments for the 2 Daly 100A BMS‘s and Hi rise M8 180A terminals, it turns out that soldering the small connections for the BMS leads meant very fine motor skills and very close through-holes that can very easily be bridged or covered over with errant solder. This might seem like a convenient build, but it is not for the inexperienced solderer.
This battery allows for 200 amp draw, though the optimum 64Ah would not sustain that for very long. It has a 250A ANL fuse.
General Specs:
Created: April 1, 2023.
Cells: Used Battery Hookup Headway 38120 8Ah 3.2V LiFePO4 cells, 4 cells in series, 8 in parallel.
BMS: 2x parallel Daly 100A 4S LiFePO4 12V common port BMS.
PCB Bus Board and PCB BMS Board designed and sold by jag35.
Configuration: 4S8P.
Nominal Voltage: 12.8V.
Overall weight: 26.2 lbs (11.88 kg).
Current owner: AD6DM Dennis
The completed battery only produced 37Ah after initial test (57.8% of rated/desired capacity). After swapping out a parallel group of older used cells for less old used cells, 44.43Ah was achieved (69.4% of rated capacity).
This led to the addition of a 5A Active Balancer, a newer device that has made it into the DIY battery building scene. It uses capacitors and switched parallel and series relays to redistribute and equalize the voltage differences between series groups for a better overall total performance.
Upon installation and testing of the Active Balancer, the full 64Ah was achieved.
This goes to show that battery building with passive BMS balancing is best for new and closely-matched cells. Passive BMS balance current only amounts to a few tens of milliamps of balancing current, and this is typically resistive balancing, meaning the higher voltage cells have energy burned off (power is dissipated in the resistors) while other cells continue to charge and catch up in voltage. This amount of balance current is nothing compared to large charging currents, and only helps series cells that are minimally mismatched.
A significantly weaker series cell (or parallel group in a 4S pack) will be the limiting factor for the whole battery pack. When dealing with “second life” used cells where the degradation level of each of the series members is difficult to determine or remedy, an active balancer is almost a necessity in addition to a BMS.
All in all, this 824Wh 64Ah battery cost about $375 to build, including the addition of an active balancer, but not including the “good cells” swapped in from a different 12V 24Ah BatteryHookup Headway kit. Also, as of this writing, BatteryHookup raised the price per cell to $5.50. Assume you will get more than the minimum 32 to arrive at a “good set” of 32 cells.
Cell Specs:
Used Headway 38120 8Ah cells with M6 screws included.
Source: BatteryHookup.com
Shell Material: Aluminium
Rated Capacity: 8Ah @ 1C
Nominal Voltage: 3.2V
Dimensions: 136mm(H) x 38.5mm(D)
Weight: 335g ±5g
Max. continuous discharging current: 120A (15C)
Max. pulse discharging current: 240A (30C)
Max recommended charge current: 80A (10C)
Standard charging current: 24A (3C)
End of discharge voltage: 2.5 ±0.05 V
Charge voltage: 3.65 ±0.05 V
Cycle life (80%DoD): ≥2000 cycles 80%DOD
Internal resistance: ≤2.5mOhm(AC，1000HZ)
Charging temp: 0°C~45°C
Discharge temp: -20~60°C
BMS Specs:
Daly 4S LiFePO4 12V 100A Common Port BMS
Source: Aliexpress.com
Discharge current: 100A
Discharge overcurrent protection: 300A±50
Internal resistance: <=20mΩ
Charge voltage: 14.6V
Charge current: 50A
Overcharge detection voltage: 3.75V±0.05
Overcharge protection delay: 1 sec
Overcharge release voltage: 3.65V±0.05
Balance opening voltage: 3.5V
Balance release voltage: 3.5V
Cell balance current: 30±5mA
Over-discharge detection voltage: 2.7V±0.1
Over-discharge detection delay: 1 sec
Over-discharge release voltage: 2.7±0.1
Discharge overcurrent protection delay: 1 sec
Discharge overcurrent protection release condition: Remove load
Short circuit protection condition: External load short circuit
Short circuit detection delay: 250μs
Short circuit proection release condition: Remove load
Self-consumption working current: 500μA
Sleep current (battery over-discharge): 0μA
Operating temperature: -20 to 70°C
Storage temperature: -40 to 80°C
Pure aluminum heatsink
High current copper plate with 3mm pure copper interface
Waterproof, dust-proof, shockproof closed injection glue design
Dimensions: 6″ x 2.5″ x 0.8″
Active Balancer Specs:
3/4S “Flying Capacitors balancing v1.7” Active Balancer
Battery type: NMC/LiFePO4
Working voltage: 2.7V – 4.2V (for 3S, leave B4+ lead empty)
2oz copper with balance current from 0 – 5.5A.
30Ah to 300Ah battery per active balancer.
Working consumption: 12mA.
Sleep state trigger voltage: 2.7V.
Sleep state consumption: <0.1mA.
Dimensions: 66mm x 46mm x 16mm.
Balance cable length: 1.31ft.
Acrylic top/bottom case included.
