I am considering to replace the 18650 battery cells in a 3S1P 11.1V cordless drill battery pack rated at 1300mAh, with three 2200mAh cells from a salvaged laptop battery pack.

One of the four cells in the laptop battery pack was bad (0V), but each of the other three had a nice 4.1 volt charge.

Should I consider all the cells from the laptop battery pack bad?

Is it likely that the charger for the 1300mAh pack can't handle charging a 2200mAh pack?


closed as off-topic by Leon Heller, Matt Young, tcrosley, Dave Tweed May 11 '14 at 11:11

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  • 1
    \$\begingroup\$ Tip. If you go through with this retrofit, be sure to keep a fire extinguisher handy. At all time. \$\endgroup\$ – Nick Alexeev May 10 '14 at 17:23
  • \$\begingroup\$ Regarding putting this topic on hold; This question is about updating a working battery pack for a cordless drill with better battery cells, taken from a bad laptop battery pack. \$\endgroup\$ – frodeborli May 12 '14 at 13:36

Changing from #S1P with larger capacity cells is usually "safe enough" [tm].
Using recovered cells also probably safe enough [tm][again] BUT see notes below.

As Ignacio said, the cells which show around 4V are probably OK - but they will have lower capacity than new.

Note - LiIon / LiPo charging:

  • Normal LiIon / LiPo charge mode is to charge in CC (Constant Current) mode at Imax of typically say C* (C = current in mA numerically equally to battery capacity in mAh) until Vmax is reached then in CV (Constant Voltage) mode until I drops to X% of Imax.

    • X% may be 50% to 5% with smaller percentages resulting in higher max stored capacity - but low X% is hard on battery life and does not gain much capacity.
  • Stopping charging at 25% of Imax or even 50% is probably a reasonable choice.
    0% and 5% is for 'road warriors' who do not pay for their replacement batteries.

  • [*Imax in terms of C is usually set by battery manufacturer. Normal is Imax = C.
    Some few have Imax = C/2 and a few have Imax = 2C.
    Special cells with Imax >>C exist but are not usually found in consumer equipment.

When charging xS1P stack with a 2 wire connection, if the cells are of differing capacity (due to cycles of use) they will reach the CC/CV changeover knee at different times in the charging cycle.
eg in a 3S1P pack one cell may be at 4.2V (should be in CV mode) and two at say 4.0V. The charger sees 12.2V total, is waiting for a 3 x 4.2 = 12.6V pack voltage before changing to CV mode, SO keeps charging at CC when one cell is already at Vmax. This will potentially drive the cell above 4.2V until the three cells combine to provide 12.6V total. As the cell that is being overdriven is the one that probably has least capacity (as it reached the Vmax limit first when charged the same as the other cells) this arrangement is most liable to damage the already most damaged cell. :-(. As the cells are from a low cost or free source this is liable top be acceptable from a lifetime achievable perspective but increases the "halt and catch fire" / vent with flames possibility. As you are charging at lower C rating (1300 mA charger with nominally 2200 mAh cells = 1300/2200 = 0.59C = C/1.7, you are probably "safe enough" [tm].

Changing from XX1P to XX2P often works OK but introduces a greater chance of cell imbalance if you charge the pack as a whole (which you would probably do with a simple charger). The low I charger will charge the high capacity OK but slower - but better than N times slower as charging at < Imax increases charge% at which Vmax is reached.
However if the charger is set to terminate at say 10% of 1300 mA = 130 mA this will now be 130/2200 ~+ 6% of 2200 mA and ~-3% of 4400 mA. SO using a small current charger with high capacity battery packs will give a slow charge BUT a higher charge % than originally.

If you care (!!!) then checking individual cells occasionally to see if some are reaching 4.2V first MAY help lifetimes if you are willing to boost them with an isolated charger.
The "ideal" arrangement is N floating chargers for N cells with taps to all cells and a multi way plug. This is functionally what top equipment actually does.
If one cared this could be done with relative ease. For a 6 cell pack 6 floating 2A wall wart LiIon chargers connected one per cell would do an excellent job.


Only the single cell needs to be considered bad; Li-ion cells that are bad won't charge safely/properly, much less handle an actual charge. Be sure to dispose of the cell according to local safety and environmental regulations.

Provided the battery chemistry is the same (there are several different chemistries called "Li-ion") and that the cells have very similar C ratings (or even if the lower capacity battery has a proportionally higher C rating) there should be no problem using the charger for the lower capacity battery with a higher capacity battery. The charge time will simply be extended accordingly.

  • \$\begingroup\$ Okay, great. Just of curiosity, can I push my luck a little and modify the pack from a 3S1p to a 3S2P? Based on your answer, I assume 2P will sort of ensure that the new 4400mAh pack can handle the C from the existing charger? \$\endgroup\$ – frodeborli May 10 '14 at 17:49
  • \$\begingroup\$ Did the old pack have a C rating 3 times as high? \$\endgroup\$ – Ignacio Vazquez-Abrams May 11 '14 at 0:42
  • \$\begingroup\$ Sorry, I changed the accepted answer. I upvoted your answer instead. The other answer is really quite extensive. \$\endgroup\$ – frodeborli May 11 '14 at 18:40

IF the original battery pack for the drill was li-ion, then it will have a protection board and also it will likely include balancing on that board in the charger, so the above problem mentioned by Russell will not be an issue if you re-attach the protection board.

By the way,the problems mentioned by Russell are very real and can cause very big fires.

If the old battery pack was of a different battery chemistry, then you for certain run into problems as the voltage of the charger will not be correct for the new battery pack you make.

You can purchase relatively cheap an Imax B6 balance charger that will balance your new battery pack as you charge it. You will need to solder on balancer wires, but easy to do, and the charger has all the connectors to charge any battery pack.

  • \$\begingroup\$ I've been looking at the Imax B6 for another project. In this particular case, it's important to be able to continue to use the old charger - that is; this will be a very simple drop-in upgrade. \$\endgroup\$ – frodeborli May 12 '14 at 13:38

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