I've obtained a laptop that has no charger. Short of buying another charger online, can a phone charger be adapted to charge the laptop's battery, provided it's disconnected from the laptop (i.e., the charger hooks straight to the battery, rather than charging through the laptop)?

Both are of/for Li-ion batteries. I've seen hand-crank chargers for electronic devices, so I'm wondering if this type of cross-charging could work for a laptop battery.


closed as off topic by Leon Heller, Olin Lathrop, Dave Tweed, Brian Carlton, zebonaut Feb 20 '13 at 19:28

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    \$\begingroup\$ Your question is not a good fit here (see FAQ). It would be best to post or have this one migrated to Superuser. \$\endgroup\$ – shimofuri Feb 19 '13 at 4:52
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    \$\begingroup\$ Short answer is no. There is no way the phone charger will be able to source enough current at a high enough voltage to charge the laptop. \$\endgroup\$ – Matt Young Feb 19 '13 at 4:57
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    \$\begingroup\$ Hand-cranking-charge a laptop battery may cause RSI (Repetitive Stress Injury). Taking breaks is recommended. :-) \$\endgroup\$ – Chetan Bhargava Feb 19 '13 at 5:05
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    \$\begingroup\$ @shimofuri I think the question does fit here, a lot better than it would SU. OPs asking if (and how) a battery pack can be charged by a given charger. While the answer is no, not practical or recommended, it is a engineering question. \$\endgroup\$ – Passerby Feb 19 '13 at 7:06
  • \$\begingroup\$ You should add pictures of the type plate of your battery, laptop, original charger and proposed charger. How do you propose to physically connect the charger to the battery? \$\endgroup\$ – jippie Feb 19 '13 at 7:54

Phone chargers cannot be used to charge a laptop battery for following reasons:

  1. Phone chargers can't supply enough current to charge a big battery.
  2. Phone charges can't be connected directly to battery. A charging circuit is needed to charge the battery. Charge manager & circuit is built into the laptop but not in the battery. Battery only has the protection and telemetry circuits.
  3. Charging laptop battery using a non-standard charger may cause fire or explosion.

To be safe, buy a used charger for your laptop model from local swapmeets, craigslist or ebay.

  • \$\begingroup\$ Does it mean that I just can't charge a battery with a low charging current applied for long time? \$\endgroup\$ – sharptooth Feb 19 '13 at 6:33
  • \$\begingroup\$ @sharptooth maybe, as specs are unknown or not provided in the post! \$\endgroup\$ – Chetan Bhargava Feb 19 '13 at 6:45
  • \$\begingroup\$ Agreed with sharptooth. Even a low current can charge a big battery, given enough time. Trickle Charge. It is just not efficient or practical. Also, some phone chargers have li-ion charging circuits. Mainly chargers that let you charge a second battery without needing the phone. Not to say I would recommend it, or that it will work well, let alone work in a usable sense. \$\endgroup\$ – Passerby Feb 19 '13 at 7:04
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    \$\begingroup\$ @jippie, the first point is solely about current. Provided the right voltage, you could charge a li-ion with barely more current than it's self discharge rate, in the single to double digit milliamps. Not practical, but doable. \$\endgroup\$ – Passerby Feb 19 '13 at 8:00
  • \$\begingroup\$ What about the possibility of modding another laptop charger to do the job (same voltage outputted, but a different brand laptop charger)? \$\endgroup\$ – Coldblackice Feb 22 '13 at 2:21

As @Matt said above, the short answer is no.
Phone batteries tend to be single-cell with charging voltage of 4.1V or 4.2V. Cell phone charge will not provide a higher voltage.
Laptop batteries tend to be multi-cell, so they need a charging voltage, which is a multiple of 4.1V or 4.2V.

Then there are the safety precautions.

Finally, if one had to charge a laptop battery to save the world, one could try to charge it with a cell phone charger one cell at a time. But such exercise is for the enlightened and brave.

  • \$\begingroup\$ What about modifying another laptop charger to do it? Specifically, a charger that outputs the same voltage as the original? \$\endgroup\$ – Coldblackice Feb 22 '13 at 2:18

Short: No, you can't :-).


  • Cellphones usually use a single Lithium Ion cell (battery) with a nominal voltage of 3.6V, max voltage during charging of about 4.2V and minimum (safe) voltage of about 3V.

  • Some few phones MAY use 2 cells in series - just double th above voltages. Top end digital cameras often use 2 LiIon cells in series and a very few uses 3.

    • Most laptops use 3 or 4 LiIon cells in series and may use a number of series strings of cells in parallel. For charging voltage purposes parallel combinations of series strings may be considered to be a single series string of larger capacity.

    • As a rough approximation a charger will input a voltage of about 5V per cell to give it a little "headroom" compared to the battery packs 4.2V max per cell.
      So a 1 cell battery will input ~~ 5V to the charger, a 2 cell = 10V, a 3 cell 15V and a 4 cell 20V.

  • Because switching regulators are essentially always used, if the highest voltage needed is used it can be downconverted to the correct level. 4 cells in series is the usual maximum so about 20V input will cover all likely cell arrangements. Industry standard is to use nominally 19V. For 4 cells that's 19/4 = 4.75V/cell available or a "headroom" of 4.75-4.2 = 0.55V "spare" per cell. If you use a 3 cell battery the switching regulator can downconvert to the required lower voltage.

    • Example: I have an HP netbook that has 4 cells in the basic battery and 6 in the extended battery. The 4 cells are arranged 4 x 1 = called 4s1p, and the 6 cells are arranged in 2 paralle strings of (3 cells in series) = 3s2p.
  • In SOME cases you CAN input a lower voltage such as 12V DC and the laptop will still charge. In this case the power supply must be a "boost converter" to give the ~15 or ~20 Volts required. However, many laptops demand the full 19V and if you drop below about 18V they shut up shop until you give them what they want.

  • It is POSSIBLE but very very unlikely that you will find a laptop that will boost from a 1 cell charging voltage of ~= 5V to ~=19V internally - BUT they may exist. It's technically possible but makes little sense to do so and I've never seen one that I'm aware of.

    • The truly desperate could series connect the output of 4 cellphone chargers. This may work after a fashion - but they may fight each other.
  • \$\begingroup\$ What about modifying another laptop charger to do it? Specifically, a charger that outputs the same voltage as the original? \$\endgroup\$ – Coldblackice Feb 22 '13 at 2:18

A telephone charger usually outputs about 5V, where the average laptop charger outputs 19V. Same goes for the battery packs in them: a phone battery will be rated at 4'ish volts where a laptop battery will usually be rated around approximately 18V. There will simply be no current flowing into the battery pack. Probably even worse, if the battery pack is connected directly to the phone charger without a proper protection/charging circuit, the battery pack will try to 'charge' the charger.

Therefore even trickle charging is not going to happen unless you use the McGyver method mentioned by Nick, which is really not advised if you want your battery pack to survive more than a handful of cycles.


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