Why doesn't passive cell balancing re-distribute electron charge?

Question

I've done a lot of searching for an answer that's just eluding me. To put it simply, when series batteries are balanced using a passive balancing scheme (resistor and FET), why don't the electrons get re-distributed to the adjacent cell.

For example, assume Battery A sits on top of Battery B. It's then determined that Battery A needs to be discharged (via a parallel resistor and FET) because it's voltage is too high. When the FET parallel to Battery A is turned on for discharging, why don't the discharged electrons get re-distributed to Battery B causing its voltage to rise?

All I keep reading is that the electrons(current) get dissipated as heat in the resistor. However, it's not as if the electrons get destroyed in the resistor? So why don't the electrons end up re-charging battery B causing its voltage to rise?

What am I missing here?

Answer 1

When current is mentioned it is the convention current as I have not want to include mention any detail as to whether it is positive and/or negative charges which are moving within the batteries.

If you have battery $A$ (charged) in series with battery $B$ (discharged) with the positive terminal of battery $A$ connected to the negative terminal of battery $B$ then the current which flows within battery $B$ is from its negative terminal to its positive terminal.
This will not "charge" battery $B$ because in order to reverse the electrochemical reaction within battery $B$, ie recharge battery $B$, the current within battery $B$ must flow from its positive terminal to its negative terminal.

If the batteries are connected in parallel, positive to positive and negative to negative, then battery $A$ can recharge battery $B$ and this is what happens when a battery pack is used to recharge a mobile phone battery.

Answer 2

OK. I think your example has Battery B on top of Battery A. My example had Battery A on top of Battery B. But that's ok we'll stick with your example. So with Battery B on top its negative terminal is connected to the positive terminal of Battery A. Also, we'll say that conventional current is from the positive terminal to the negative terminal.

Now, let's assume Charge Shuttling(active balancing) is used where a capacitor is placed across the terminals of Battery B and allowed to fully charge. Once the capacitor is fully charged it is then switched (using a network of switching FETs) to be placed across the terminals of Battery A. In this case the charge on the capacitor re-charges Battery A.

So what I'm trying to understand is why does current discharged from a capacitor (active balancing) into Battery A re-charge it, but current sourced from the positive terminal of Battery B to the positive terminal of Battery A (resistor passive balancing) NOT re-charge Battery A???

In both cases current is being forced into the positive terminal of Battery A, yet only one method (active balancing) can re-charge Battery A. This is what I'm trying to understand.

Another way to think about it is in regards to a battery charger. When using a charger the conventional current is forced into the positive terminal of the battery. In passive balancing the same thing is happening. The only difference is the source is another battery rather than an actual charger. So what's the difference?

Answer 3

We call these things "circuits" because electric current flows in circles. For your passive circuit, the discharge current flows from the positive terminal of the battery you're discharging through the resistor and FET to the negative terminal, completing the circuit. The other battery is not part of this circle, so it receives no charge.

In the active scheme you have two possible circuits. When the capacitor is connected to battery A, it forms a closed circuit with the battery. When it's connected to battery B, that's the closed circuit. In either connection, current flows until the voltage on the capacitor and the battery are equalized: the direction of the flow depends on which has a higher voltage at the time the circuit is connected. Alternating connections transfers charge from the higher voltage battery to the lower voltage battery. The capacitor alternately forms a circuit with each battery, unlike the passive circuit which only forms a circuit with the battery it's connected to.