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Old 03-24-2014, 07:34 PM   #7
Sergio
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Join Date: Sep 2012
Location: Charlotte, NC
Posts: 9,329
Default Re: Passive Voltage Bypass for 8v battery

Parts (Picture #1):
I used an over-sized 10w resistor to dissipate the heat from the zeners and to act as the basis for the circuit.

I used 2 zeners instead of one since it provides lots of benefits: 5 watt zeners are much less expensive than the 10w and using 2 of them to add up to 10.2v allows you to compensate for the difference between the zeners.

I ordered the non-insulated 5/16 terminals because I wanted to solder those connections.

I had a lot of different sizes of heat shrinking tube already, so I used 3 different sizes.

Assembly (Picture #2):
Solder the red wire to the anode of one of the zener diodes.
Solder the black wire to the cathode of the other zener diode.
Place shrink tube over the solder connections but don't go over the zener diode.


Assembly (Picture #3):
Solder the other lead of the zener diode to each lead of the resistor as shown.


Assembly ( Picture #4):
Put shrink tube over the resistor-zener solder connection as shown.
Put some JB weld quick to bond the diodes to the power resistor (the areas circled in green), JB weld has good thermal transfer and it will help transfer the heat from the zener diode to the resistor.


Assembly Final (Picture #5):
Crimp/Solder the 5/16 ring terminals to the end of the wires.
Either put the zip-tie right after shrinking the tube around the entire part or re-heat the shrinking tube before putting the zip-tie. The warm supple heat shrink tube will make a pretty good seal with the zip-tie.

I ordered all parts from Mouser electronics, they have a price break at 10+ parts so keep that in mind when ordering. The final cost was around $2-$3/unit.

I built these units last year because the batteries that came with my cart were pretty out of balance. There was always a lot of "water" around the top of the good batteries. I finally started to check the individual voltages near the end of the charge cycle and discovered that my good batteries were above 11v while the bad ones barely got above 9v.
These devices kept the good batteries to about 10.6v while allowing the bad ones to gradually get to 10v.
One of the bad batteries eventually started to fail under heavy load so I decided to change the entire pack.

I have installed the devices on my new pack and hopefully they will keep the batteries from getting out of balance. It has only been 8 months since I changed the batteries and so far they are all 8.55v or 8.54v each.
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