Tuesday, October 23, 2012

Smallest & Cheapest LiFePO4 Cell Bottom Balancer

Here is a shrunk version of my original automatic bottom balancing device

Automotive relay, 3 resistive wire from an old air dryer, and 2 small switches

A red flashing LED to show when we are  in drain mode 

Some welding

Resistors and switches close up

and the final product
There is an input for a 12V power supply, I used a solar charged 12V battery for that 

Now some testing with remaining LiFePO4 cells 

Power switch ON

View from the top

Added a little extra cover on the red wire to avoid damage from the resistor heating up

From the left side

From the right side

I later on added a little piece of aluminum to dissipate the heat a little bit more
The resistors are going up to 75°C and more or less 4A are going through them :
Measured Resistance: 0.8Ohms 
Current: I = U / R = 3.2 / 0.8 = 4 Amps

It works fine and I drained one cell yesterday with it, in 2 hours I think

The total cost is close to 60 USD:

Programmable dual relay voltmeter                                             47.50
Automotive relay 12V / 30A                                                        3
Red flashing LED                                                                         5
Switches (x2)                                                                               2
Resistive wires                                                                              free
Wires and connections                                                                  3
Battery Clamps                                                                             free
Japanese fishcake board                                                               free 
Total                                                                                          60.5 USD


  1. That is a nice little setup for discharging single cells but really only useful for final trimming once most of the energy has been removed. I use a 9" DC motor to drain full cells fast then use my PowerLab 8 to trim them to the final voltage. But I do like the design for those that don't have a PowerLab 8 to trim the cells. Kudos on that one. Clean and small.

    1. Thank you for your comment.
      Yes, I usually connect the cells in series, then bleed them with the DIY ESS Grid Tie Inverters (like you do with your Motor), while watching their voltage; When they are close to 3V, I stop, and then use my Bottom Balancer on each cell, and draining 4A, the job is done pretty rapidly after that :-)

  2. What are the two voltage thresholds you use to enable and disable the relay?

  3. This comment has been removed by a blog administrator.

  4. This comment has been removed by the author.


Please Share this Post if you Liked it !

Please Share this Post if you Liked it ! Thanks !
Related Posts Plugin for WordPress, Blogger...