How to build an external lithium battery pack for a laptop with 19V DC input?
If you are asking this question here it means that you MAY manage to hurt yourself trying to do it. Not a certainty but certainly a possibility.
Voltage &quot;somewhat over&quot; 19V MAY cause damage depending on the laptop design. As they made it to work AT 19V they have no need to make it survive much higher voltages. There will be an upper safe limit. 21V will PROBABLY NOT destroy your laptop, but may.
Voltages more than a little below 19V will probably not work. How much lower depends on brand and model but some cut off at voltages only slightly below 19V as they expect a correct charger and if your &quot;charger&quot; can not provide 19V it is clearly not a correct one.
Best methods to use are:

4 or less cells with a boost converter
80% - 90% efficient
Regulator failure could but usually won't destroy laptop.

6 or more cells with a buck converter.
90% - 95% efficient.
Regulator failure MAY destroy laptop.

6 or more cells with a linear regulator.
Not preferred but may be bearable.
75% to 98% efficient across falling battery range.
Regulator failure MAY destroy laptop.


Boost converter solution allows car battery, auto cigarette lighter and &quot;12V&quot; solar panel to be used if desired.
You can buy COTS* converters that take nominal 12V and supply laptop level power.
Quality varies widely.
Most of these would accept input from 3 or 4 LiIon or from 12V SLA.
IF making your own LiIon supply you MUST used internally protected cells PLUS add external protection and will need a charger.


COTS = Cheap, off the shelf.


baaggage_lump said:

It led me to something called a 3.5-28V to 1.25-26V DC-DC Converter Boost Buck Step Up Step Down Voltage Module on eBay . It says the output voltage is adjustable and independent of the input voltage, so I'm guessing that should do the trick.


Looks good.
It uses the NatSemi LM2577 boost converter adjustable version datasheet here worth $6.63/1 AMD NatSemi LM2596 buck converter IC worth about the same datasheet here . That board is extremely good value for money.
The boost section should suit your need. Typical cct and probably very close to what they are using:

Boost to &gt; cells at max charge = 4.2 x 5 = 21V, then buck from there.