# why do laptops have 10 or 20 volt batteries if most components only use logic level voltages

I've noticed that often laptop chargers and batteries are higher voltages than logic levels, 1 to 5 v. I am building a laptop and was originally using 3.7 v lipo battery. I am curious if I should put them in series to get a higher voltage and get an appropriate charging chip or just stay at 3.7 v.

What is the advantage to higher voltage batteries if it's regulated for logic level computing?

• Why do we have 380kV power transmission lines when most household items only use 230/120V? – PlasmaHH Apr 23 '17 at 21:35
• @PlasmaHH A rhetorical question is unlikely to be helpful to someone asking a basic question like this. – Techydude Apr 23 '17 at 21:54
• I don't know but will share my thoughts. There are basically two easy choices. All cells in series, or all in parallel. Anything else is more trouble than it is worth from a pack design standpoint. If all cells are in parallel, you need to boost to get 5V (needed for USB), and use a switcher or linear regulator to get 3.3. I think it just ends up being a lot easier to go all in series and use buck regulators for everything. Typical supply is like 45 W. At 3.6V 45W is 12.5A, and that might require 14 AWG cable. So I agree with others that cable diameter is also a consideration. – mkeith Apr 25 '17 at 5:00
• you have hard drives that may need 12V, and/or 5V, usb needs 5V, the processor needs 3.3 and possibly 1.8 and maybe 1.0 or others, other devices need the above or a mixture of them so you need to at least cover 5V or 12V plus the losses plus the battery drain. sure you could and do all of this with a DC to DC converter, but most designs like that convert from higher to lower. – old_timer May 12 '17 at 17:47
• the other part of this is traditionally these battery packs are made up of common sized rechargeable batteries, not necessarily C or D or AA or whatever but commonly made/used cells you gang up 6 or 8 or 9 of them and you get the voltage you get. rather than having to make custom battery packs from custom batteries. – old_timer May 12 '17 at 17:48

Because while total power stays the same, voltage droop across a cable due to high currents can be avoided with higher voltages and lower current. Copper is expensive, and it's simpler to deal with 2.5 Amps at 20V than it is 10 Amps at 5V.

• I really doubt the cost matters that much compared to the laptop. That copper is weight, though! – Loren Pechtel Apr 24 '17 at 4:23
• @LorenPechtel every penny counts when production is half a million units. – Passerby Apr 24 '17 at 4:42

Ohm's Power Law:

$$P = V*I = I^2 * R$$

Power lost in copper, and needing to be dissipated as heat, goes up with the square of the current.

Consumers don't like thick cables or hot laps.

So if you can deliver Power to the laptop, and use it within the laptop, with the least amount of current, but still within ELV (Extra-Low Voltage) levels (< 50 V) for safety/compliance reasons, then your cables are cheaper, your internal power management components are cheaper, your heat dissipation complexity & overhead is less because you lose less power as heat.

Buck switch-mode voltage regulators (taking the N * 4.2V/cell of several Lithium cells in series, & regulating it down to logic levels) can be made efficient enough (>90%) within certain constraints, which is a much better compromise than feeding, say, 5V @ 10A from a power brick a few meters away from the laptop.

• You might want to review your power relation, there. P = R* I^2 = V^2 /R. And not I^2 /R. – Sredni Vashtar Apr 25 '17 at 1:25
• You're welcome. To be pedantic, though, now you should explain the OP why, since power varies with both the squares of the voltage and the current, you choose to reduce current and not voltage. Perhaps adding the circuit with the cable resistance in series with the load would help the OP in grasping that. – Sredni Vashtar Apr 25 '17 at 7:30

You can't run the logic directly off the battery, nor can you charge the battery directly from a fixed-voltage power supply. The actual computing part of a modern laptop needs a range of stable voltages from less than a volt to about 5 volts. Traditionally they also needed a high AC voltage to drive the screen back light (though that may have changed with the move from CCFL to LED)

The voltage of a lithium ion cell is usually listed as something like 3.6V but it actually varies from about 2.8V at the end of the discharge cycle to about 4.2V at the end of the charge cycle.

It is easier/cheaper/more efficient to make converters that only step voltage in one direction than to make converters that can step voltage both down and up and resistive losses generally go down with higher voltage.

So back to the question of how many cells to put in series.

One cell in series means that you need and up/down converter for 3.3V and an up-converter for 5V. It's common in things like smartphones/smartphone-like tablets that use relatively little power and have less need for 3.3V/5V.

Two cells in series mean you can create all the common voltage rails with down-converters but you don't have much headroom for the converter supplying 5V. Since 5V often supplies peaky loads like motors having headroom there is useful.

So it makes sense to use 3 cells in series. Most laptops seem to go for three or four with the choice of which being decided mostly by physical and/or capacity constraints (some laptops can actually support both).

The input voltage then needs to be comfortably above the fully charged voltage so the charging circuit only has to down-convert. Fully charged voltage for a 4-cell is about 16.8V, so an input voltage of around 19V-20V makes sense.

It's important to note that battery voltage is not constant. Common Li-ion cells vary from around 4.2-4.3V at full charge down to below 3.0V. Exact cutoff voltage varies depending on the protection circuitry, you want to avoid dropping to the point where recharging is impossible, but although that's 2.7V or lower, there's very little usable energy between a 2.7V cutoff and a 3.0V cutoff. Still, part of the useful range of the battery is below the 3.6V (or thereabouts) minimum input needed by a low dropout regulator.

So, in order to get a regulated 3.3V rail from a single series cell arrangement, the regulator has to support input voltages both above and below the output, e.g. a buck-boost regulator. Wherease if you put at least 2 cells in series, your minimum voltage is over 5.5V and you can use a buck regulator for 3.3V and even 5.0V output.

Buck regulators are simpler, smaller, and cheaper than their buck-boost brothers.

In batteries in real electronics, 2 series ("7.4 V") arrangements are popular. 3 series and 4 series are even better in some regards (losses in the charging cable) but the incremental advantages aren't nearly as great, and start trading off against compatibility with e.g. 12V chargers such as USB QuickCharge and USB C PD.

Let's do a power budget for a laptop...

CPU and GPU - Lots of watts (10-50W) at around a volt.

DDR, chipset, WiFi, etc - A few voltage rails like 2.5V and 3.3V, I'd say 10-20W.

HDD - SATA spec requires 5V and 12V, although most 2.5" HDDs do not actually use the 12V supply, if it is not there your laptop will only work with some HDDs, so you have a marketing problem...

DVD drive - same problem, but worse, as spinning a large CD or DVD at high speeds requires about 10W...

Fans - 5V or 12V

Stuff connected to USB - People see USB ports, plug in USB devices drawing the max current as per spec, and expect it to work.

Onboard audio and loudspeaker amplifiers - 5 or 12V

Display backlight - This one is quite the power hog, for CCFL it will use an inverter, for LED backlight it will use a LED driver obviously. Input voltage should be taken from battery without further conversion, as this is the most efficient option.

OK... So you have to design a battery-powered solution for this.

If your "laptop" is a actually a smartphone or a tablet like an iPad, no HDD or DVD drives, just a few flash chips on the motherboard instead... a very efficient ARM cpu... no fans, no 12V... things are simpler, and one 3.6V Lithium cell is likely to be the simplest solution. The only voltage which needs a boost DCDC will be 5V for USB host, but the current is rather limited.

For a standard x86 laptop though, you want to use a battery voltage above your 12V rail, so all your DCDC converters are buck, which are more efficient than boost. So you're gonna have to use 4 or more Li cells in series.

If you don't have DVD drive and can spec the HDD then you don't really need a 12V rail, but you still need 5V for USB, so you could use 2-3 cells.

Your largest DCDC converter will be the CPU VRM which will output lots of amps at around 1V. You really want this one to be efficient, and buck converters tend to degrade when the Vin/Vout ratio becomes too high, also low voltage MOSFETs have higher performance... this means you don't want your battery voltage to be too high.

Everyone seems to use either 4 or 5 cells, ie 14 to 20V, roughly. It's the sweet spot for efficiency.

Then you shop for batteries, decide whether you want custom made pouch cells to make your McBook Air fashionably slim, or fat cheap 18650s...

Laptops use 19Vin to DCDC battery charger and also DCDC LV mobo outputs with inputs from battery or 19V charger if active.

Cell voltage depends on watts needed to run a larger screen backlight and thus less conduction loss with lower current at higher cell string voltage.

## edit

Understand this.

There are many reasons, besides ; cost, efficiency and flexibility.

• The DC-DC from 19V typ to logic levels , 5,3.3, 1.2 etc is independent of the battery charge and battery. You don't need either to operate laptop.

• The battery charger will regulate current charge based on how much is leftover after the MOBO is satisfied. This DC-DC regulator for complex V and I battery profiles is independent of the one above for the MOBO

There used to be a plethora of "Universal" laptop chargers with selectors roughly for 12 to 24V with many settings for different OEM's needs.

Now in North America, these are almost extinct and replaced with 19V "universal chargers " which satisfy all recent and new laptops that need 65W or more. Apple is still unique with their connector and others use smart ID communication to enable only the battery charger portion of all the DC-DC converters and ought to still run the MOBO.