Why is a laptop power supply output voltage different from its battery?

I hope you will not mind answering. This question has long been on my mind.

Normally, the adapter output voltage is higher than that of the battery. In my laptop's case, the output voltage of the adapter (or charger or power supply) is 19.2V. (That .2V itself is also a big question for me. Is that so sensitive?) But the voltage of the battery is 10.8V.

My question is, why is the adapter output voltage different from the battery's voltage?

• I explained this almost exactly this in a comment to your previous question so why are you asking it here again? The laptop battery needs to be charged by a lower voltage which depends on temperature, how full the battery is etc. Feb 20, 2019 at 21:50
• @bimpelrekkie, this question is specific, why. I didn't see your previous comments have aswer this question. Feb 20, 2019 at 21:56
• The adapter voltage is higher probably because it's easier to design the battery charging circuit that way. Solid-state switches need voltage headroom to switch properly and may be located in a circuit somewhere that requires more than the battery voltage to provide such headroom. Some switches may also need a minimum of 10V and 15V-20V is ideal for these switches. 19.2V is just because it matches a common battery chemistry and therefore mass produced and cheap for the manufacturer to buy off-the-shelf. It's the same reason 13.8V power supplies are common. Feb 20, 2019 at 22:00
• Your mobile phone is also fed with a 5V power supply, but it has to charge a 3.7V lithium battery. The laptop is no different. Feb 20, 2019 at 22:03
• I'm pretty sure this question has been asked before, too lazy to find the duplicate Feb 20, 2019 at 22:08

The voltage on your battery "10.8V" is the "nameplate" voltage, some average voltage that your battery delivers over full discharge cycle. The value of "10.8" indicates that this is a battery of 3 Li-Ion cells in series, giving their standard "nameplate" voltage of 3.6V per cell.

Charging the Li-Ion cells requires variable voltage levels, from 2.5-3 V per cell (when in deeply discharged state) to 4.2V (4.35 in some cases) per cell in "constant-voltage" stage of charging process (otherwise the cell won't be charged to full capacity). So the feeding power must have some overhead to provide the charging process (or let internal charger to do so). So, for 3-cell, it comes up to 12.6 - 12.9 V of input. The external power supply must provide this headroom, which includes minimum of "drop-out" (or regulation) voltage for switching electronics inside the external power supply and internal charger, 1-2 V per device, give or take. Eventually it comes up to 12.9+4 = ~ 16-17V.

The "19.2" nameplate is a bit of mystery, since it is not that stable in the first place. It is just an industry standard. Any AC-DC adapter in the range of 18 - 22 V will happily charge your laptop, very likely.

• When I measured it using digital multi-tester, the voltage output is 19.19V. Almost exactly as written on the name plate. Feb 21, 2019 at 0:59
• The way they wrote the output voltage 19.2V, the .2V, I believe, they intented to say so. Feb 21, 2019 at 1:01
• @AirCraftLover, the specific way of number notation wrt accuracy is a subject for notations in precise sciences as Physics. Some people believe that the 19.2 stands for "24V -20%". Nameplate voltages on laptop adapters vary, 16.5V, 18V, 19V, 19.2, 19.5 for 120W Dell adapters, or even 20V for some HP/Compaq laptops. Feb 21, 2019 at 1:11
• I don't understand your last comment. Feb 21, 2019 at 1:57

From your other question, it appears that you have a laptop power supply with power rating of 65 Watts (3.42 A × 19.2 V). As power is voltage times current, it means that if the voltage is higher, the current is lower, so thinner, cheaper, more flexible wires can be used to deliver that 65W to the laptop when charging. That's why it's not a 65 Watt, 5 Amp 13 V power supply.

Also it means that when the battery is being charged, a DC-DC converter in the charging circuit converts the 19.2 V down to match the battery voltage so that suitable amount of charging current flows into the battery. So in this case a nominal 10.3V battery could be charged at over 6 Amps with the same 65 Watts if the battery can handle that amount of charging current safely. The charging current will be limited by what is safe to the cells.

• Even in the laptop label on the bottom side of the laptop written power rating is 65W, 19V (this is 19V, not 19.2V, slightly different). So, it really required 19V. Then come question in my mind. If the required voltage is 19V, why did the battery is 10.8V? What is actually the voltage requirement? Feb 21, 2019 at 1:06
• @AirCraftLover, "What is actually the voltage requirement?" - the actual voltage requirement is written on the bottom of laptop. Plus-minus guardbands, usually +-10%. However, internally the laptop uses variety of voltages, 5V, 3.3V, 2.5V, 1.8V, 1.5V for memory, and 0.8 to 1.2V for CPU. Why doesn't it trigger the similar question in your mind? Feb 21, 2019 at 1:53
• @Ale..chenski, if it is the requirement, so why the manufacturer didn't design the same voltage with their battery? If can be +/-, then 10% minus from 19V will be 17.1V. This is lower than to series 5 cell (which will give 3.6V*5=18V). If due to 18V then the requirement 65W is not achieved, then they can increased the current. That is my question. Feb 21, 2019 at 2:02
• The laptop power requirement is determined during design. In my case, my laptop's requirement is 19.2V/3.95A. But why does battery voltage is 10.8V or 11.1V? It is becuase every batere consist of some 18650 dry cell battery which its voltage is 3.6V or 3.7V. Three batteries are arranged in series to produce 10.8V or 11.1V (3.6*3=10.8V, 3.7V*3=11.1V). As the laptop power requirement is 19.2V*3.95A (=75.84 watt), then the charger must be made according to that spec. How about the power from the battery? Need power boost to convert the 10.8V to 19.2V. And that is inside the laptop or battery. Feb 24, 2020 at 17:37

This is because the laptop is designed for a optional battery you don't have.

Laptop batteries usually come in 6 and 8 cell varieties. A 6 cell battery has two strings of 3 series cells. A 8 cell battery has two strings of 4 series cells. The higher cell count provides more power at the expense of weight. (Sometimes a 4 cell option is available consisting of 1 string of 4 series cells)

Most commonly the smaller capacity battery will sit flush with the case and the high capacity battery sticks out the back.

Ale..chenski identifies a max charge voltage of around 4.2 V/cell. So a 6 cell battery needs 12.6 V max to charge, an 8 cell battery needs 16.8 V. Then add cable losses and regulator losses to get 19.2 V.

• In the bottom of the laptop (my laptop is Toshiba), there is written power rating is 65W 19V. Mean, the laptop itself is required 19V. If they really need 19V, then why they didn't design 19V battery or closer voltage to it? Feb 21, 2019 at 1:10
• @AirCraftLover They did design a 19 V battery. You just didn't buy it, you got the smaller 12 V battery instead. The charger and adapter is made for the bigger battery if you ever get it. Feb 21, 2019 at 1:14
• I understand your explanation about cell array. But even the array is 4 series then paralleled with another 4-series, still giving 14.4V. If fully charged, then it will be normally 16.56V. This is still much below 19V. Feb 21, 2019 at 1:14
• Btw, output of the charger is 19.19V. It is the voltage at the port, after all the cables around 1m. I measured using digital multi-meter. Feb 21, 2019 at 1:17
• @AirCraftLover , "output of the charger is 19.19V" - I repeat, this is not the charger, it is "AC-DC power supply". The actual battery charger is built into your laptops. This internal charger converts input power (whatever it is, it can accommodate certain range of input voltages), and provides proper charging algorithm, with necessary variable voltage. Feb 21, 2019 at 2:43

I think the reason why the adaptor or charger would produce such big amount of volt is because, while the laptop is charging, the extra volt will help run the laptop without problem. Take for example if the battery is designed with 12volts you need at least 13 to 15 volts to charge it, and don't forget your laptop have to stay charging while you perform other operations on it. To my understanding there is a charge controller that monitors the charging. If am wrong please i need someone with a better explanation, you know we are all learning

• It doesn't need higher voltage to operate the laptop and charge the battery. It needs higher current. All it has to do is maintain the voltage while supplying the current the laptop and the battery charging circuit demand.
– JRE
Feb 24, 2020 at 12:10
• The laptop power requirement is determined during design. In my case, my laptop's requirement is 19.2V/3.95A. But why does battery voltage is 10.8V or 11.1V? It is becuase every batere consist of some 18650 dry cell battery which its voltage is 3.6V or 3.7V. Three batteries are arranged in series to produce 10.8V or 11.1V (3.6*3=10.8V, 3.7V*3=11.1V). As the laptop power requirement is 19.2V*3.95A (=75.84 watt), then the charger must be made according to that spec. How about the power from the battery? Need power boost to convert the 10.8V to 19.2V. And that is inside the laptop or battery. Feb 24, 2020 at 17:35
• @AircraftLover The power number (19.2V * 4A ~= 80 Watts) is the maximum power available. It would not surprise me if the computer operations used a fraction (30%) of the 80 watts and the remaining 70% is provided for charging. Feb 24, 2020 at 18:11
• Yes you are right all the laptop need is a constant current to charge and operate but i can't still understand the reason for that high voltage unlike our mobile phone with the battery volt of 3.7 or so that uses 5volt to charge.. Still explainable hmm🤔 Feb 26, 2020 at 0:32

It is likely that the voltages are different by design / intentionally. A higher DC voltage enables power to flow with less current (compared to the lower 10.2 Volts). This can be important when pushing DC power through appreciable distances. Battery and voltage is stepped down to 5 or 3.3. V or the CPU. Possibly other voltages for the display and other discrete devices.

There is a difference between the laptop power supply unit rated at 19v and the 10•8v Battery charger provided by laptop charging circuit which then charges the battery. What you plug to power the laptop is a power supply unit not a battery charger at all! Circuitry in the laptop determines the charging voltage for the battery designed for it.

Here is the answer to this question, after I did some research/searching.

The laptop power requirement is determined during design. In my case, my laptop's requirement is 19.2V/3.95A. But why does battery voltage is 10.8V or 11.1V? It is because every battery consist of some 18650 dry cell battery which its voltage is 3.6V or 3.7V. Three batteries are arranged in series to produce 10.8V or 11.1V (3.6*3=10.8V, 3.7V*3=11.1V). As the laptop power requirement is 19.2V*3.95A (=75.84 watt), then the charger must be made according to that spec. How about the power from the battery? Need boost converter to convert the 10.8V to 19.2V. And that is inside the laptop or battery.

What about the cell battery is not 3.6V?

Say if you can increase the voltage to be 10.4V like 9V dry cell 6LR641, and each two of them you arrange in series and then you parallel to another two series, then you will get voltage 20.8V. To meet the 19.2V you need to step the voltage down, also converter but to step down (rather then to step up).

So, when we plug that 19.2 charger to a laptop with a common battery we have (10.8V), then the voltage from the charger need to be stepped down inside the battery. Vice versa, when laptop is taking power from the battery, it need to be stepped up to 19.2V

• No. The laptop has (several) Buck converters to change the ~10 V of the battery into the 1.5 V (CPU), 3.3 V (Motherboard?) and 5 V (USB, HDD) required to run the laptop. Perhaps also a boost converter to drive the backlight, if it's a discharge lamp and not LED. Feb 24, 2020 at 19:17