Guru
Joined: 08/02/2015
Location: New ZealandPosts: 1204
| Posted: 12:11am 06 Jan 2026 |
I have built a number of Lifepo4 and Lead Acid multi-cell battery balancers over the past several years, both passive and active types; they all work in various degrees, some better than others. Add to the mix accurate measurement of each cells voltage and the design gets more complicated.
The passive types discharge a load resistor across any individual cell when its cell voltage goes above a high set point under charge, depending on the load resistor, quite a lot of heat can be generated, requiring a large heat sink and perhaps a fan.
They do have the advantage that the load current is known and doesn't depend on the voltage difference between other cells.
The active types using a large ferrite toroidal core with windings linked to each cell and synchronous current pulses work very well, however to get high load currents the circuit impedance must be in the mill-ohms region when cell voltage differences are low, reading the individual cell voltages is also tricky with all the voltage spikes.
Under ideal conditions where cells are matched, balancing can be achieved with a couple of amps; cells I am using are re-cycled 100 - 400 AH and various ages - manufactures, so rather large load currents are required to keep "runner" cells in line.
A passive balancer with a known load current works best here, this design is a modification of previous variants, where a mother board hosts individual cell module pcb's linked to each cell in the battery bank. A controller connects to one or two MB's allowing either 16 - 32 cell batteries. Cell boards each have their own cpu, here a very cheap Pixaxe 08M2LE and talk to the controller via opto-couplers for isolation. In the past I have used a form of PWM for cell communications, where the width of the pulse specifies the cell address and corresponding cell voltage in milli-volts, this is quite fast and should work ok on 32 cell banks.
Here are the pcb's that will be sent off to be made in the next day or so, I will draw up the schematics when its all working and any bugs ironed out.
Controller 100x100mm 4 layer:
Can drive a couple of large DC relays for main contactor control and inverter pre-charge.
Cell Board 33 x 50mm:
The 3.3v regulator is only used when monitoring 6V lead battery cells.
Mother Board 200x106mm:
Large holes in the pcb are to allow air from a top mounted fan cool the 0.35r (10 amps) 50w resistors mounted below the main pcb, using a large alloy plate as heat sink.
Cheers
Mike
Edited 2026-01-06 12:57 by Solar Mike
Footnote added 2026-02-16 09:08 by Solar Mike
Notes to self: Some of these batteries will be fully enclosed in a metal box, I have made no provision to read the cell temperature; not an absolute requirement, but would require a CPU with more pins, say 20M2, may implement in future.
Mike