Multiple Battery Cell Balancer

[Solar Mike]

I need a cell balancer for a three separate 48 volt banks of 8 series 6v lead carbon batteries; these are 300 AH sealed gel batteries and are proving difficult to get 100% balanced; once in a balanced condition, they will probably remain in that state and require little intervention.
Another requirement is to measure each individual cells voltage and raise some sort of alarm if any drop or go above set limits.

A simple balancing solution would have a voltage reference and voltage comparator attached to each cell, turning on a resistive load of several amps via a mosfet switch whenever the cells voltage went above a set point (7v in this case). Once the battery bank is near fully charged to 56 volts the charge current is only 1-2 amps, so it doesn't take much of a load on any runner cells to prevent them over charging.
Perhaps a 08M2 would do the job here, measure the cell voltage, turn on a mosfet, communicate with a Master Manager via opto-couplers, means using lots of cpu's but they are not that expensive compared to the $700 cost of each 6v battery.

Here is my initial schematic, uses all smd devices to keep the pcb size down, each one fits onto a 50x62mm pcb designed to fit vertically across the resistor bank mounted on a heatsink. They are linked together via 10 pin ribbon IDC back to the master (yet to be designed), however each can be setup to run stand alone initially.



Note each cpu board sits at various battery cell potentials, so have to be initially programmed in a test jig prior, I imagine once a Master is connected it could alter voltage set points from a central point.

Will post the pcb soon...

Cheers
Mike

 

[papaof2]

Looks like a good start.

Would the master be polling the PICAXEs or would each PICAXE set a pin high/low to indicate it has new data? Or would the PICAXE run data collection in a loop and only notify the master if the most recent voltage reading is out of limits? How would the master notify a PICAXE that the voltage limits it's looking at need to be adjusted?

Polling ensures that the master knows all the PICAXEs are alive. Lots of things can affect PICAXE serial data in and out so you may want one of the serial interfacing gurus to give a blessing on your plans for that - there is a 2-wire power + serial data network which includes addressing other PICAXEs as well as sending and receiving data among them ;-)

I suspect the knowledgeable folks here might want a day or two to do the wiring and component value verifications and start the "Would it be better if..." discussions - and they'll certainly want to see the code.

 

[Solar Mike]

Each battery cell module communicates via opto coupled common Input and an outputs all in two parallel streams; makes sense for a master to poll each one as required to get any data; so each has to have a unique cell address to prevent multiples attempting communications at the same time.

The protocol as yet undecided would have a number of master commands, ie broadcast to all units a min-max voltage, a single address to get data from an individual cell monitor.

The communications may not be serial, as it is hard wired, could go with a simple pulse width scheme where the width of a pulse determines a slave address, width of 2nd pulse determines function required and a reply is a pulse width that is say a voltage reading in ADC units like 10us per unit. The system doesn't have to be fast, just reliable.

 

[Solar Mike]

Here is the cell module pcb, 50 x 62mm, the boards will mount edge on vertically direct to the pcb hosting the 50w aluminum shell resistors bolted to a heatsink, the ribbon cable runs across their tops. I am using 2 ohm load resistors - 3.5A for the 6v battery cells balancing at 7v.

This scheme has a central balance point, rather than having individual balance boards wired at each battery. Having long 2.5mm^2 balance wires running to each battery cell means voltage drops should an adjacent module be balancing at the time of a voltage measurement; part of the coms protocol could be a broadcast command, turn off any balance loads for 100mSec to allow an accurate measurement.... perhaps.

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[Solar Mike]

Here is the load module pcb for eight cells, its quite large, 240x76mm; sits above the 50W load resistors bolted to a thick alloy plate acting as a heat sink and will form one side of a box possibly. Automotive 10 amp fuse holders for standard fuses solder direct to the board, a disconnect plug\socket goes to the battery.
I may place an intermediate pcb between this board and the actual battery cell leads, it will have heavier plug\sockets for the wires; as I'm also experimenting with an 8 cell inductive balancer that I designed a while ago as it requires bigger cables to keep the lead resistance down, see link here for more details Inductive Balancer.



Master controller is next, will wait a while on that, too many other projects require finishing first.

 

[Solar Mike]

Decided to make a pcb for initial connection of the battery balance wires, fuse in series with each wire, will make it easier and safer during experimentation with various balancer designs. The 9.5mm screw pcb sockets can take up to 4mm wire - required for the inductive balancer.

81x87mm pcb fits inside a 115x90 plastic box with a clear lid (AliExpress), I can bolt this at the end of each battery bank of 8 cells.



Once I have a working tested system, will post the gerber files here.

Cheers
Mike