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I started researching ways to balance batteries and settled on capacitive balancers. I found a diagram on them and most of them use NE555 to generate a square wave with a frequency of ~7 kHz and a duty of ~50%.

Why 7kHz? How does frequency affect efficiency? I assume that the frequency is calculated based on the charging time of the capacitor. Accordingly, the pulse interval must be long enough for the capacitor to charge to the battery voltage... are my assumptions correct?

Balancer scheme for understanding: balancer

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  • \$\begingroup\$ In my circuit I use 3300uF capacitors. With MOSFET resistance + internal capacitor resistance, I got 1.3 kHz at ~50% duty. At 2200uF it turned out to be 2.5kHz... How can this balancer work if at 7kHz the capacitor clearly does not charge to the battery voltage \$\endgroup\$
    – minto
    Commented Jul 10 at 16:15
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    \$\begingroup\$ Perhaps you would explain to us the operating principle of the balancer. \$\endgroup\$
    – Neil_UK
    Commented Jul 10 at 16:20
  • \$\begingroup\$ @Neil_UK No problem. The balancer has only 2 operating steps. In step 1, the capacitors under the first mosfets (C25, C19....) are charged, the bottom row of mosfets is blocked. In step 2, these capacitors are connected in parallel (the first row of mosfets is blocked and the second row is opened) and their voltage is equalized; at the beginning of the first step, if the capacitor voltage is higher than that of the battery, the stored energy goes into the battery. \$\endgroup\$
    – minto
    Commented Jul 10 at 16:24

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You only have very small voltage differences compared to the nominal cell voltage. The capacitors are only charged/discharged by some percent per cycle.

The charge transfer current is high at the start of a switching cycle but quickly drops down to insignificant values.

So a higher switching frequency can transport more charge at small voltage differences and limits the component stress at larger differences.

Typical electrolytic capacitors would just explode if you would fully charge or discharge them in kHz range.

These kind of circuits use only a very tiny amount of the energy strorage capabilities of the capacitors. But they have the advantage, that no supervising control instance is required.

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