I'm trying to figure out how to build an external battery pack for a laptop, plugging into its 19V DC input. In a previous question I learned that I probably want to go for 5 lithium cells in order to closely match the 19V input expected by my laptop. But even if such a battery pack nominally provides 18.5V, it can peak at 21V and go all the way down to 14V and below. Is it correct to assume this could potentially be problematic?

How can I build a battery pack for this scenario?

  • \$\begingroup\$ . . Did you ever find a solution for this ? Did you try anything out ? I'm basically trying to do the same thing so I'm curious. \$\endgroup\$
    – user8897
    Commented Mar 27, 2012 at 22:00

2 Answers 2


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 "somewhat over" 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 "charger" 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 "12V" 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.

enter image description here

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:

enter image description here

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

  • \$\begingroup\$ Great answer. Made it much more clear what the requirements are and how to get there. 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. \$\endgroup\$ Commented Feb 29, 2012 at 13:56
  • \$\begingroup\$ The part about LEDs does not make any sense to me. Maybe it's a mix-up of some sort? In any case, you say the "boost section" should suit my needs. I'm thinking of using 5 cells, in which case the boost and the buck sections should suit my need. The nominal voltage from the batteries would be 18.5V, and I'm guessing having it as close as possible to the output voltage will improve efficiency? Also, how does one calibrate a module like this? I guess I need some kind of instrument to measure, and rotate what I assume to be a potentiometer on the picture? \$\endgroup\$ Commented Mar 1, 2012 at 10:59
  • \$\begingroup\$ Sorry - LED answer belonged to another question !!! :-). 5 cells = 21 volt max and about 15V min. You can boost to > 21 - say 22V and buck from there. Efficiency should be quite reasonable. Even a linear regulator from 22V to 19V is 19/22 = 86% efficient so buck should be better than that. \$\endgroup\$
    – Russell McMahon
    Commented Mar 1, 2012 at 12:13
  • \$\begingroup\$ Just FYI: COTS = Commercial Off The Shelf. en.wikipedia.org/wiki/Commercial_off-the-shelf \$\endgroup\$
    – user3624
    Commented Mar 1, 2012 at 16:45
  • \$\begingroup\$ @DavidKessner - Cheap off ... Google =~ 470,000 hits. | Commercial ... Google = about 8.4 million hits SO yes, commercial is far more common BUT Cheap off ... is also well enough used. In this case Cheap is preferred to commercial ;-) :-). \$\endgroup\$
    – Russell McMahon
    Commented Mar 2, 2012 at 0:32

Car battery, cheap inverter, stock power suppy. Anything else and you are reinventing the wheel for no good purpose.

  • \$\begingroup\$ -1, there's a lot being wasted in this setup. \$\endgroup\$ Commented Jun 26, 2012 at 16:22

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