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2:26 · With sound

Battery balance

Why does a LiPo pack need balancing?

Read Note 15

Transcript

This pack reads full: 12.6 V. But one cell is at 4.25 V.

The charger only watched the total. Balancing watches every cell.

3S1P means 3 cells in series, and 1 in parallel.

Series adds volts. 3 full cells at 4.2 V make 12.6 V.

Parallel adds capacity. 3S2P is still 12.6 V, with twice the mAh.

Cells in parallel share one voltage, so you only balance the series groups.

That's why the balance plug has one more pin than cells. Each pin sits one cell higher.

No two cells are identical. Even from one batch, they differ in capacity, resistance and self-discharge.

In series, every cell gets the same current. So the smallest cell fills first, and empties first.

I simulated a 3S pack where one cell has faded to 90%.

All three start at the same storage voltage, 3.8 V.

Charged without balancing, the pack reaches 12.6 V, and the weak cell reaches 4.25 V.

That costs cycles. In one test, charging LiPo cells to 4.3 V cut their cycle life by 28%.

In flight, the weak cell empties first. It's flat while the pack still reads 10 V.

Betaflight only reads the whole pack. At 3.3 V a cell, this cell is already empty.

And it feeds itself. The overcharged cell wears faster, and falls further behind.

Passive balancing puts a resistor and a switch across each cell.

The fullest cell bleeds into its resistor, while the others catch up.

Hobby chargers bleed 0.3 to 1 A per cell. Here, that burned 0.4 Wh.

TI's BQ76920 chip bleeds only 50 mA. The same charge took 3 hours.

Balancers work near the top, where the curve is steep. The same gap reads 20 mV mid-charge, and 70 near full.

Active balancing moves the extra charge instead of burning it.

An inductor pulls current from the high cell, then pushes it into the low one.

In my sim, 88% arrives. ADI's LTC3300 reaches 92%.

A switched capacitor slows down as the cells get close. 100 mV apart, it moves 97 mA. 5 apart, under 5.

Active can even balance in flight. Here, it added 120 mAh.

So which? Passive is cheap and simple, and on a small pack the heat is tiny.

Active saves that energy and helps in flight, but it needs more parts.

Even a Tesla Model S balances with resistors. About 0.1 A per group.

Use balance mode, with the balance lead plugged in. Otherwise, all the charger sees is 12.6 V.