Note 15Power Rev 1 Watch the video · 2:26

Battery balance

Why does a LiPo pack need balancing?

Because a plain charger watches only the total. On a 3S pack with one cell faded to 90%, a plain charge stops at 12.60 V with that cell at 4.25 V, past its 4.20 V limit. A balance charge watches every cell and stops each one at 4.20 V.

The figure charges the same pack twice at 2.2 A. First plain, through its main plug alone. Then with its balance lead plugged in, so the charger can bleed 300 mA off the cell that leads.

A 3S pack, charging

Plain charge, then balance charge

Cell 3, at the top, holds 1980 mAh. The other two hold 2200 mAh. All three start at 3.80 V. On the balance pass, the dashed line is cell 3 on the plain pass.

The numbers

Charge	Took	Cells 1 and 2	Cell 3, weak	Pack	Lost as heat
Plain	29 min	4.17 V	4.25 V	12.60 V	none
Balance, 300 mA bleed	29 min	4.20 V	4.20 V	12.60 V	0.41 Wh

A 3S pack and its balance lead

Two plugs, two jobs

A 3S pack is three cells in series. Two thick leads carry the charge and the load through the main plug, an XT60 here. Across them, a charger sees only the total.

Four thin wires run to the balance plug, a 4-pin JST-XH. They reach both ends of the pack and both joints between cells, so a charger can read every cell.

The pack

A 2200 mAh 3S pack, 106 × 34 × 26 mm. Under the wrap are three flat pouch cells, stacked as in the opening figure.

The numbers

Plug	Wires	Carries
Main, XT60	2 thick	The charge and the load current, across the whole pack
Balance, 4-pin JST-XH	4 thin	Each junction between cells, for reading and balancing each cell

Each balance pin sits one cell above the last. With every cell full, they read 0, 4.2, 8.4 and 12.6 V.

A charger works out each cell from the difference between two neighboring pins.

The pack as a circuit

The thick leads carry the pack’s current to the XT60. The thin wires to the balance plug only read and balance.

The numbers

Balance pin	Joins	At full
1	The pack’s −	0.0 V
2	Cell 1 + and cell 2 −	4.2 V
3	Cell 2 + and cell 3 −	8.4 V
4	The pack’s +	12.6 V

S and P

Cells in series, cells side by side

S counts cells in series. Each adds its voltage, and the same current flows through all of them.

P counts cells side by side in each group. They share the current and add their capacity. Joined, they always sit at the same voltage, so they balance as one.

The balance plug has a pin for each end and each joint, so a 3S pack takes 4 pins, whatever its P. Hobby makers’ guides draw 3S2P as two 3S strings side by side. Large packs join each group first, as drawn here.

Three wirings

2200 mAh cells. Each open circle is a balance pin.

The numbers

Wiring	Cells	Full	Capacity	Balance pins
2S1P	2	8.4 V	2200 mAh	3
3S1P	3	12.6 V	2200 mAh	4
3S2P	6	12.6 V	4400 mAh	4

A plain charge

What the charger sees

A plain charge leaves the balance lead out and works on the total. It pushes 2.2 A until the pack reads 12.60 V, then holds 12.60 V while the current tapers off. It stops at 220 mA, after 29 min.

V_pack=V₁+V₂+V₃

4.173 V+4.173 V+4.254 V=12.600 V

The total comes out right. The cells don’t. Cell 3 holds 10% less, so the same current fills it faster. It passes 4.20 V after 26 min and ends at 4.25 V, 82 mV above the others.

One LiPo maker gives 4.25 V as the absolute maximum, never to be passed. A cell only 5% short still ends at 4.23 V.

Cells from one batch still differ in capacity, resistance and self-discharge, and TI notes that the gaps grow with use. The weak cell is the one that ends high, and each overcharge weakens it further.

A plain charge

Blue is cells 1 and 2, which charge as one. Orange is cell 3.

The numbers

Minute	Pack, V	Cells 1 and 2, V	Cell 3, V
0	11.426	3.809	3.809
5	11.560	3.852	3.857
10	11.748	3.910	3.927
15	11.969	3.982	4.006
20	12.206	4.057	4.092
25	12.462	4.138	4.187
28.7, end	12.600	4.173	4.254

What 4.25 V costs

Cycle life against charge voltage

Nobody has published cycle life at 4.25 V. The nearest data is a 2020 study by Juarez-Robles and others. They cycled 5 Ah LiCoO₂ pouch cells at 1C to seven charge voltages, until each held only 4 Ah.

Charged higher, worn out sooner

Their Table I. Charged higher, a cell holds more at first and lasts fewer cycles. Orange is 4.3 V, the first row above this pack’s 4.25 V.

The numbers

Charged to	Capacity at first	Cycles to 4 Ah
4.2 V	5.05 Ah	290
4.3 V	5.46 Ah	209
4.4 V	5.83 Ah	198
4.5 V	6.48 Ah	134
4.6 V	6.97 Ah	28
4.7 V	8.77 Ah	29
4.8 V	9.05 Ah	12

302

Overcharge

A little more capacity, a lot fewer cycles

Field marks. A cell that reads high at the end of every charge and low at the end of every flight. Past 4.5 V, a pouch that has puffed up.

Look-alike. The rule that each 0.1 V halves cycle life. Battery University gives it with no source. Measured, 4.3 V instead of 4.2 V cost 28%: 209 cycles against 290.

Range. 4.3 V gave 8% more capacity at first. Past 4.4 V, lithium metal built up on the graphite. Past 4.5 V the cells swelled, and the one charged to 4.8 V almost doubled in volume. A fresh cell pushed slowly failed at 5.1 V.

Habitat. Any series pack charged on its total alone: hobby packs on a plain charge, and packs with no cell monitor.

303

In flight, it empties first

10 A from the plain-charged pack

The weak cell runs out first, too. The same 10 A comes out of every cell, and it has the least to give.

A flight controller running Betaflight reads only the pack. Its docs’ example settings warn at 3.4 V a cell and alarm at 3.3 V. On a 3S pack, that’s 10.2 and 9.9 V.

Here cell 3 is empty after 12 min, while the pack still reads 10.0 V. The warning came 5 s before. The alarm never sounded.

The same pack in flight

Voltages under the 10 A load. Cell 3 is empty where its line ends, at 3.02 V.

The numbers

Minute	Pack, V	Cells 1 and 2, V	Cell 3, V
0	12.478	4.132	4.214
2	11.917	3.962	3.994
4	11.504	3.830	3.845
6	11.228	3.743	3.743
8	11.081	3.695	3.691
10	10.925	3.647	3.630
12.2, empty	10.018	3.501	3.016

Balance charging

Passive: burn the extra

A balance charge plugs in both leads. It charges through the main plug and reads every cell through the balance plug.

When one cell leads, the charger switches a resistor across it. Part of the charging current flows around that cell as heat, and the others catch up.

I_bleed=V_cellR

=4.20 V14 Ω=300 mAand 1.26 W of heat

A bleed across each cell

Cell 3 leads, so its switch is on. The lit loop is its bleed current.

The numbers

Part	Value
Bleed resistor, each cell	14 Ω
Bleed current at 4.20 V	300 mA
Heat while on	1.26 W
Switch	A MOSFET, on while its cell leads

304

Passive balancing

Simple and cheap, and the extra becomes heat

Field marks. A resistor and a small MOSFET across each cell, in a charger or on a BMS board. Hobby chargers bleed 0.3 to 1 A a cell.

Look-alike. Active balancing, on the same balance lead. It moves charge between cells instead of burning it.

Range. A 300 mA bleed balances this pack in 29 min, about as long as the plain charge, and turns 0.41 Wh into heat. TI’s BQ76920 bleeds up to 50 mA, and never two neighbors at once. On this pack it takes 3 hours.

Habitat. Hobby chargers, and most commercial packs. Even the Tesla Model S balances with resistors: 39.5 Ω across each group of 74 cells, about 94 mA.

305

All three balancers end with every cell at 4.20 V. They differ in time and heat.

A bleed fixes the overcharge. It leaves the run time alone: cell 3 still holds 10% less, and the pack still stops when cell 3 does.

Three balancers, one pack

Each charge at 2.2 A from 3.80 V, as in Fig. 15d, until every cell is within 5 mV of 4.20 V.

The numbers

Balancer	Current	Full and balanced in	Lost as heat
Charger’s bleed	300 mA	29 min	0.412 Wh
BQ76920 bleed	50 mA, 70% of the time	183 min	0.435 Wh
Inductor, active	825 mA out of the high cell	29 min	0.035 Wh

Balance near the top

Where a gap shows

A balancer goes by voltage, and a cell’s voltage barely moves through the middle of its charge. A gap between cells hides there. A LiFe hides it almost to the end, as Note 18 shows.

The same 5% gap in charge reads as 20 mV at 3.70 V and 70 mV at 4.15 V. Near full, the gap is easy to see.

A cell’s voltage at rest

Each pair of dots is 5.1% of a full cell apart, the gap the plain charge leaves between cell 3 and the others. Past 100%, a cell is over 4.20 V.

The numbers

Cell at	Charge	5.1% more charge	Voltage gap
3.70 V	20.9%	3.720 V	20 mV
3.80 V	52.0%	3.822 V	22 mV
3.90 V	69.0%	3.945 V	45 mV
4.00 V	80.6%	4.048 V	48 mV
4.10 V	91.2%	4.152 V	52 mV
4.15 V	96.1%	4.220 V	70 mV

Active balancing

Move the charge instead

An active balancer moves charge out of a full cell and into an empty one. With an inductor, it takes three steps, 50,000 times a second.

An inductor between two cells

10 µH switched at 50 kHz, between cells at 4.10 and 4.00 V. The high cell is on top. The lit loop is where the current flows. One settled cycle of it, repeated.

The numbers

Step	From	To	Conducting	Inductor current
1, fill L	0 µs	9 µs	The switch, from the high cell	0 to 3.63 A
2, empty L	9 µs	17.3 µs	The diode, into the low cell	3.63 A to 0
3, wait	17.3 µs	20 µs	Nothing	0

Less arrives than leaves. The rest heats the switch, the diode and the inductor’s copper:

η=V_lowI_inV_highI_out

=4.00 V × 0.741 A4.10 V × 0.825 A=88%

A capacitor can carry charge too. It connects across the higher cell, then across the lower one, and takes a little charge down each time:

I=C·ΔV·f

=100 µF × 100 mV × 10 kHz=100 mAagainst 97 mA simulated

The current follows the gap: 19 mA at 20 mV, 4.8 mA at 5 mV. It slows as the cells close in.

A capacitor between two cells

100 µF switched at 10 kHz. The dots are the simulated currents, the dashed line C·ΔV·f.

The numbers

Gap	Simulated	C × gap × f
5 mV	4.8 mA	5.0 mA
20 mV	19 mA	20 mA
100 mV	97 mA	100 mA

306

Active balancing

Charge moved, not burned, for more parts

Field marks. An inductor or a transformer beside the cells, or capacitors switched between them, with a switch for each.

Look-alike. Passive balancing, on the same balance lead, with only a resistor and a switch a cell.

Range. The inductor balancer moves 0.825 A at 87.7% and loses 35 mWh balancing this pack. ADI’s LTC3300 uses a transformer, at up to 92% and up to 10 A. A switched capacitor reaches only its neighbors, and slows as the gap closes.

Habitat. Electric vehicles and grid storage, and packs where run time is worth the parts.

307

Fast enough, an active balancer can work during a flight. It feeds cell 3 from the others while the pack gives 10 A.

The balanced pack then gives 2058 mAh instead of 1936 mAh before any cell reads 3.3 V, 122 mAh more. A bleed can’t do that, since it only takes charge away.

Balancing in flight

The pack balance-charged first, then run at 10 A. The active balancer is the inductor one above.

The numbers

In flight	Delivered	Ends with
No balancing	1936 mAh	cell 3 at 3.29 V
Active, during the flight	2058 mAh	cell 3 at 3.30 V

Which balancer

A key

Key to balancing a pack

Which balancer fits?

1aThe pack charges on a hobby chargerBalance mode, lead plugged in
1bIt has its own BMS boardGo to 2
2aOnly overcharge mattersPassive, like the BQ76920
2bRun time matters tooGo to 3
3aCharge has to cross the whole stackInductor or transformer, like the LTC3300
3bNeighbors only, and slow is fineSwitched capacitor

This pack’s answer is 1a. The Tesla Model S balances with resistors, as in 2a.

Fade a cell yourself

24 charges, every one drawn

Pick how far cell 3 has faded and how the pack charges. The further it has faded, the further a plain charge takes it past 4.20 V.

A bleed and the active balancer both end with every cell at 4.20 V. The difference is the heat.

Cell 3 holds

90%, 1980 mAh

Charge

Each cell, charging

Cell 3 ends at 4.25 V, 54 mV over

Cell 3 ends at: 4.25 V
Cells 1 and 2 end at: 4.17 V
The charge took: 29 min
Lost as heat: none

2.2 A from 3.80 V, as in Fig. 15d. Bleed is 300 mA. Active is the inductor balancer. Under 86%, a plain charge takes cell 3 past the end of the cell model.

Every case, as a table

Weak cell	Charge	Weak cell ends at	Others end at	Took	Lost as heat
100%, 2200 mAh	Plain	4.20 V	4.20 V	30 min	none
100%, 2200 mAh	Bleed 300 mA	4.20 V	4.20 V	30 min	none
100%, 2200 mAh	Active	4.20 V	4.20 V	30 min	none
98%, 2156 mAh	Plain	4.21 V	4.19 V	29 min	none
98%, 2156 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	78 mWh
98%, 2156 mAh	Active	4.20 V	4.20 V	29 min	7 mWh
96%, 2112 mAh	Plain	4.22 V	4.19 V	29 min	none
96%, 2112 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	160 mWh
96%, 2112 mAh	Active	4.20 V	4.20 V	29 min	13 mWh
94%, 2068 mAh	Plain	4.23 V	4.18 V	29 min	none
94%, 2068 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	244 mWh
94%, 2068 mAh	Active	4.20 V	4.20 V	29 min	20 mWh
92%, 2024 mAh	Plain	4.24 V	4.18 V	29 min	none
92%, 2024 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	327 mWh
92%, 2024 mAh	Active	4.20 V	4.20 V	29 min	28 mWh
90%, 1980 mAh	Plain	4.25 V	4.17 V	29 min	none
90%, 1980 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	412 mWh
90%, 1980 mAh	Active	4.20 V	4.20 V	29 min	35 mWh
88%, 1936 mAh	Plain	4.26 V	4.17 V	28 min	none
88%, 1936 mAh	Bleed 300 mA	4.20 V	4.20 V	29 min	497 mWh
88%, 1936 mAh	Active	4.20 V	4.20 V	28 min	42 mWh
86%, 1892 mAh	Plain	4.28 V	4.16 V	28 min	none
86%, 1892 mAh	Bleed 300 mA	4.20 V	4.20 V	33 min	587 mWh
86%, 1892 mAh	Active	4.20 V	4.20 V	28 min	50 mWh

Conditions

Pack: Three LiPo cells in series, two of 2200 mAh and cell 3 faded to 1980 mAh, all from 3.80 V. Sized like the Gens Ace 2200 mAh 3S 25C, GEA22003S25X6, with a plain label.
Cells: An equivalent circuit for each: the open-circuit voltage of a LiCoO₂/graphite pouch cell from PyBaMM’s Ai2020 parameters, then 4 mΩ in series and 2 mΩ in parallel with 10 kF. The resistances and the capacitance are assumed. No LiPo maker publishes them.
Charger: 2.2 A until the pack reads 12.60 V, then 12.60 V until the current falls to 220 mA. A balance charge also waits until every cell is within 5 mV.
Bleed: 14 Ω, 300 mA at 4.20 V, the SkyRC B6 Evo’s most. The BQ76920 case uses 84 Ω, 50 mA, on 70% of the time. Each bleeds any cell more than 1 mV above the lowest, once the spread passes 3 mV.
Active: 0.825 A out of the highest cell, 87.7% of the energy into the lowest, from the inductor balancer’s settled cycle
Balancers: ngspice, cells as sources behind 4 mΩ. Inductor: 10 µH with 20 mΩ, 50 kHz, on 45% of each cycle, an ideal 10 mΩ switch and a diode. Capacitor: 100 µF with 5 mΩ, 10 kHz, 10 mΩ switches.
Flight: 10 A, until a cell is empty or, for Fig. 15l, until a cell reads 3.3 V under load
Renders: Blender. Cells, tabs and plugs from the pack’s published size and photos.

Simulated only. Nothing on this page has been measured on a board yet.

Limits

Past 4.20 V the cell model extends its fitted curves, so voltages there are indicative.
The cells’ resistance is a guess. A higher one would make every cell read higher while charging.
One temperature throughout, with no heating and no aging during a run. Self-discharge is left out.
Fig. 15e is another cell, 5 Ah, tested in 0.1 V steps. No one has tested 4.25 V.
The active balancer’s current is held at its value between cells at 4.10 and 4.00 V.

Files

sim/pack.pyThe cell model, charger and balancers
sim/scenarios.pyThe charges and flights
sim/balancers.pyThe inductor and capacitor balancers in ngspice
sim/site.pyThe explorer’s sweep
render/scene.pyThe renders
refs/research.mdEvery datasheet and paper figure and its source

Sources

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Rev 1 · 5 Nov 2026

Rev 15 Nov 2026First published

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