- Created the "Shunt" circuit.
- As I "wind up" the voltage into the circuit, current is conducted ... but the voltage seen by the battery does not remain the same (which is what I expected).
- When I measure with the ammeter, the current seen by the battery remains the same ... so even when the system is "balancing" ... there is still current sucking into the cell which is at full voltage.
- How does this balance cells ?
- Thinking ... and carefully doing calculations (see below)
- The trick is to do calculations on a number of cells at the same time (not just one cell).
- As the "Shunt" circuit conducts, it actually reduces the resistance of the "Cell" ... thus decreasing the voltage drop across the cell (in comparison with other cells).
- The current in the circuit actually goes up.
- So the supply voltage to other cells will go up.
- The voltage reference and power transistor, should "balance" the circuit at the selected voltage.
- According to my calculations, the current through the balancing transistor should always be negligible, because the circuit effectively "balances" on the edge of the voltage selected.
- What I need to do now ... is test the circuit in parallel, with dummy loads (and check that it does, indeed, fix the maximum voltage at the cell terminals).
Testing
- Created test board ... of simulated cells etc.
- Turning up the voltage ... means shunt sends power away, keeping voltage constant (almost !).
- The TL431 does have some drift (as per its specifications).
- Changing the 120 ohm resistor in the voltage divider circuit before the TIP137, does make a slight difference (a slight improvement in steadiness of voltage).
- Reducing the resistor to 0.5ohm from 1.0 ohm makes a large difference in stability at higher amperage (up to 0.5Amps).
- Over 2.5amps, the circuit becomes completely unstable (TIP137 swamped ?) and voltage runs away.
- Therefore, it will be critical to have careful "back up" safety strategies.
