Budget Bottom Balancing

Using a LightObject 5740 Volt Meter to build a $100 LiFePO4 bottom balancer

You will need a wall wart power supply, a power resistor, a high amperage continuous duty relay, a LightObject 5740 volt meter, a pair of 30A alligator clips, a pair of smaller alligator clips, some 8 or 10 gauge wire, and some small gauge wire.

Major components:
Wall wart power supply 16vdc 5w $10
http://www.digikey.com/product-detail/en/DA4-165US/454-1400-ND
.1 Ohm 200W resistor $28
http://www.digikey.com/product-detail/en/2-1630019-7/A102386-ND/
Continuous Duty Solenoid 80AMP 12V $16.50
http://www.amazon.com/Continuous-Duty-Solenoid-80AMP-12V/dp/B0050I94XG
Programmable 4 Digit Red LED AC/DC Volt Meter with dual control. $38
http://www.lightobject.com/Programmable-4-Digit-Red-LED-ACDC-Volt-Meter-with-dual-control-Good-for-HHO-System-P408.aspx

I built mine for considerably less than $100, I had everything except for the 5740 meter. I used a wall wart power supply left over from a dead router, several 0.1 Ohm 20W power resistors that I ganged together, and a contactor that will eventually go on my motorcycle pack.

It isn't very pretty, but it works. The resistors that I had, when connected 5P2S across a LiFePO4 cell, draw about 40A. The power resistor listed above from Digikey draws about 20A.

Below I have a JLD404 hooked up with a shunt to show the 40.5A draw at 3.24v with my resistors. I'm going to have to get some heavier wire to connect my resistors... 40A was making them get warm. A fan blowing across the resistors and wires helped a lot. I probably should add a fuse or two in there somewhere.

Here you can see where I rebuilt the balancer with the power resistor, better wires, alligator clips, and a fuse on the power lead. The JLD404 shows that it is drawing 22.7A at 3.27v. This power draw is closer to the recommended 0.3C discharge for the cell. 

Wiring the balancer:

1. Connect your dc power supply to terminals 1 and 2.

2. Jumper terminal 1 and 15 to connect the positive DC power to the common terminal (15) of J1.

3. Connect one side of the contactor to the normal closed terminal (14) of J1.

4. Connect the other side of the contactor to negative DC power (terminal 2).

5. Connect the negative voltage sense lead to the COM terminal (8).

6. Connect the positive voltage sense lead to the 10V terminal (6).

7. Connect one side of the contactor power terminal to the power resistor with heavy wire.

8. Connect the other side of the contactor to a heavy alligator clip with heavy wire.

9. Connect the other side of the power resistor to a heavy alligator clip with heavy wire and a 12V/30A fuse.

Programming the balancer:

1. Press Manual/Enter and enter 0001 to get the relay menu.

2. Set AH1, the high voltage trigger, to your lower voltage (2.60v).

3. Set AL1, the low voltage trigger, to your higher voltage (2.75v).

4. Select End and press Manual/Enter to exit the relay menu.

5. Press Manual/Enter and enter 0036 to get to the configuration menu.

6. Set PuL to 0.000 for no offset
7. Set PuH to 1.000 for displaying 0.000 to 9.999 vdc.

8. Set dot to 3 for 3 decimal points.

9. Set FILt to 0 for no noise filtering.

10. Select End and press Manual/Enter to exit the configuration menu.

When the voltage is higher than 2.75v the contractor will energize, drawing power from the cell. When the voltage gets down to 2.60v, the contractor will open, removing the load. If the voltage rises above 2.75v, it will close again. After several cycles, the voltage should settle just below the 2.75v.