Supercapacitor Balancing Questions

This circuit has several issues and you have made some assumptions that are leading you in the wrong direction.

The first issue is that balancing capacitors is not about switching large currents. The 80 mA current limit of the SAB series is more that what should be needed to balance capacitors. Balancing is needed because the leakage of the capacitors will not perfectly match however the leakage should be well below 50 mA overwise the capacitors are going to self discharge quickly. The use of zener diodes for voltage limiting will not work because zener diodes have two characteristics that work against you. First off they have a tolerance of at best 5% or typically 10%. The also have a high reverse leakage current and require current to flow through them reversed biased to perform there voltage drop function.The lower the zener voltage the more reverse current required and the more drift will be present in the zener diode. This means that power will be constantly wasted and you will need to set the limit voltage to around 2.1 volts maximum to account for drift and component tolerances.

The second part of the problem is that the capacitor is going to discharge down from its fully charged voltage very quickly giving you a very short discharge curve. By using a boost converter you could discharge the capacitors down two around 2.5 volts across both capacitors. However stacking all four capacitors and charging then to around 9.4 volts using a boost converter would give you a discharge curve of 9.4 to 2.5 using a buck-boost converter instead of 5 to 2.5v as in your current design.

Solder surface mounted components can be a challenge but low pin count surface mounted components are not that hard to deal with by using a few cheating methods.

Method 1. Solder then on using a regular solder iron using fine solder. Solder one corner of the chip and don't worry if you get two pins at the same time. Just work on getting the chip aligned with just solder on one corner. Once you have one or two pins soldered and the chip aligned to the pads you can solder the opposite corner. This is keep the chip in place while to finish soldering. Take your time between solder pins to let the chip cool down and just solder a pin or two at a time. Don't worry about getting more than 1 pin at a time. The trick is to use de-soldering wick to remove the extra solder that is bridging the pins. SOIC packages are not that difficult as the gap between the pins is large enough to soaking up the solder easily.

Method 2. Hot air soldering. This method requires spending around $100 for a cheap hot air soldering station but make mounting surface mounted component very easy. It also require purchasing soldering paste which is another $15. This method does not take long to learn and it provides a clean solder joint. This method works by just applying a thin coat of solder paste on the PCB pattern for the chip and then putting the chip on top of the solder paste. The exact amount of paste of not critical and the placement of the chip is not critical. You then heat the solder using the hot air slowly moving the air gun around to evenly heat the solder. Turn the air flow down to minimum and the temperature up to 40 degrees higher than the melting temperature of the paste. The package the paste comes in will tell you the melting temperature. This will  caused the flux in the paste to melt and will them lift the chip up from the board a little. As the flux evaporates the chip will align itself with the PCB pattern. Keep applying heat until the flux is gone and the solder because shiny on all of the pins. Take the heat away and let it cool, all done. If you find the alignment is not right apply the heat again to melt the solder and nudge the chip into alignment using something small like a paper clip.

If you used too much solder paste you can use solder wick to remove the excess solder. If some pins don't seem to making contact you can use the solder wick with some solder in it to fix the problem. Put the solder wick over pin and heat with a soldering iron. This will wick some of the solder from the wick onto the board making a thin but solid connection.

One of the keys in circuit design is to consider that all components are not perfect and you should never push them to the limit. The 2.5V capacitor should not seen more than 2.4V unless you want it to fail quickly. The capacitor itself may not properly withstand the voltage and the voltage you are measuring may not be are accurate as you thought it should be. Super capacitors are a special case as the only work are low voltages and thus require working near the stated voltage. Other capacitors should be derated by 20-30% and sometimes more if they are subjected to heat or surge voltages.

A second key is that heat it the enemy. Most capacitors will live around twice as long by running at 10 degrees C cooler. Many components have a much longer lifetime but keeping the temperatures down to a reasonable level. Some components by nature must be run at higher temperatures to do there job but keeping other components from being heated by the hot component will help tremendously. As simple example of applying this would be to swap R1 and D1. The diode will not heat much at the current levels it will experience but the resistor will heat considerably when the capacitors are discharged. By connect the diode in the circuit to the capacitors instead of the resistor the heat from the resistor will not as easily couple to the first capacitor. While using a 5W resistor in this case is overkill it does have the advantage of dissipating heat faster thus running cooler.

This circuit has several issues and you have made some assumptions that are leading you in the wrong direction.

The first issue is that balancing capacitors is not about switching large currents. The 80 mA current limit of the SAB series is more that what should be needed to balance capacitors. Balancing is needed because the leakage of the capacitors will not perfectly match however the leakage should be well below 50 mA overwise the capacitors are going to self discharge quickly. The use of zener diodes for voltage limiting will not work because zener diodes have two characteristics that work against you. First off they have a tolerance of at best 5% or typically 10%. The also have a high reverse leakage current and require current to flow through them reversed biased to perform there voltage drop function.The lower the zener voltage the more reverse current required and the more drift will be present in the zener diode. This means that power will be constantly wasted and you will need to set the limit voltage to around 2.1 volts maximum to account for drift and component tolerances.

The second part of the problem is that the capacitor is going to discharge down from its fully charged voltage very quickly giving you a very short discharge curve. By using a boost converter you could discharge the capacitors down two around 2.5 volts across both capacitors. However stacking all four capacitors and charging then to around 9.4 volts using a boost converter would give you a discharge curve of 9.4 to 2.5 using a buck-boost converter instead of 5 to 2.5v as in your current design.

Solder surface mounted components can be a challenge but low pin count surface mounted components are not that hard to deal with by using a few cheating methods.

Method 1. Solder then on using a regular solder iron using fine solder. Solder one corner of the chip and don't worry if you get two pins at the same time. Just work on getting the chip aligned with just solder on one corner. Once you have one or two pins soldered and the chip aligned to the pads you can solder the opposite corner. This is keep the chip in place while to finish soldering. Take your time between solder pins to let the chip cool down and just solder a pin or two at a time. Don't worry about getting more than 1 pin at a time. The trick is to use de-soldering wick to remove the extra solder that is bridging the pins. SOIC packages are not that difficult as the gap between the pins is large enough to soaking up the solder easily.

Method 2. Hot air soldering. This method requires spending around $100 for a cheap hot air soldering station but make mounting surface mounted component very easy. It also require purchasing soldering paste which is another $15. This method does not take long to learn and it provides a clean solder joint. This method works by just applying a thin coat of solder paste on the PCB pattern for the chip and then putting the chip on top of the solder paste. The exact amount of paste of not critical and the placement of the chip is not critical. You then heat the solder using the hot air slowly moving the air gun around to evenly heat the solder. Turn the air flow down to minimum and the temperature up to 40 degrees higher than the melting temperature of the paste. The package the paste comes in will tell you the melting temperature. This will  caused the flux in the paste to melt and will them lift the chip up from the board a little. As the flux evaporates the chip will align itself with the PCB pattern. Keep applying heat until the flux is gone and the solder because shiny on all of the pins. Take the heat away and let it cool, all done. If you find the alignment is not right apply the heat again to melt the solder and nudge the chip into alignment using something small like a paper clip.

If you used too much solder paste you can use solder wick to remove the excess solder. If some pins don't seem to making contact you can use the solder wick with some solder in it to fix the problem. Put the solder wick over pin and heat with a soldering iron. This will wick some of the solder from the wick onto the board making a thin but solid connection.

One of the keys in circuit design is to consider that all components are not perfect and you should never push them to the limit. The 2.5V capacitor should not seen more than 2.4V unless you want it to fail quickly. The capacitor itself may not properly withstand the voltage and the voltage you are measuring may not be are accurate as you thought it should be. Super capacitors are a special case as the only work are low voltages and thus require working near the stated voltage. Other capacitors should be derated by 20-30% and sometimes more if they are subjected to heat or surge voltages.

A second key is that heat it the enemy. Most capacitors will live around twice as long by running at 10 degrees C cooler. Many components have a much longer lifetime but keeping the temperatures down to a reasonable level. Some components by nature must be run at higher temperatures to do there job but keeping other components from being heated by the hot component will help tremendously. As simple example of applying this would be to swap R1 and D1. The diode will not heat much at the current levels it will experience but the resistor will heat considerably when the capacitors are discharged. By connect the diode in the circuit to the capacitors instead of the resistor the heat from the resistor will not as easily couple to the first capacitor. While using a 5W resistor in this case is overkill it does have the advantage of dissipating heat faster thus running cooler.