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I have:

  • Dead notebook with live LiIon battery (HP HSTNN-CB50):
    14.8V, 8 x 2500 mAh 18650 LiIon cells.
    Rated at 73 Watt-hour.
    I haven't managed to open the battery pack yet, but it's clear from the voltage and from the specification plate that the cell arrangement is "2p4s"
    (2 x strings of 4 cells in series in each string.)
  • Live electric screwdriver (China/noname) with dead battery cells:
    12 NiCd Sub-C cells, 14.4V 1200 mAh

It's possible to fit 4 x 18650 cells into the screwdriver.

I would like to have:

  • Live screwdriver
  • with Live LiIon battery from notebook in a shape of screwdriver battery.

I am on a budget for this project and the idea is to replace the dead NiCd cells in the screwdriver bat.pack with the LiIon cells and circuitry from the notebook battery. Does anybody have experience with this?

More precisely:

  1. Is it possible to easily recycle the charging circuits from notebook?
  2. What about the screwdriver discharge?
    I do not have an idea in general if the Li-Ion battery from notebook will be able to handle the peaks from the screwriver?
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  • \$\begingroup\$ So you already checked that the form factor (probably 18650 for the laptop battery cells) is not a problem? About question 2: If there is a model code on the cells you can find a datasheet that tells you the max. current/"C rating". \$\endgroup\$ – AndreKR Sep 29 '11 at 13:16
  • \$\begingroup\$ Yes, it is 18650 assuming the external size of the battery pack. At least four 18650 cells will fit into the srewdriver pack originally holding 12 sub-C. I'll try to open the pack to get the codes. \$\endgroup\$ – Matej Sep 29 '11 at 14:11
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    \$\begingroup\$ +1 from me (yesterday). The people who marked this down may wish to mark it up again, as it hopefully now meets whatever it was deemed to lack. \$\endgroup\$ – Russell McMahon Sep 30 '11 at 6:33
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* NOTE SAFETY CAUTIONS AT END *

Caveat Emptor - No responsibility taken for advice given.

Playing with LiIons is an almost safe activity for careful competent adults.
Do it wrong and they die.
You may too, but probably not - but your equipment may.


The configuration should be 2p4s (or === 4s2p) as cells are nominal 3.7V each x 4 = 14.8V.
3.7V x 2.5 Ah c 8 cells = 74 Wh which matches your figure.

It is common for functionally dead LiIon battery packs to contain usefully alive LiIon cells.

It is a VERY GOOD IDEA [tm] to use cells which have inbuilt protection circuits. Many (probably most) do have, and if it's an HP battery pack as the part number suggests then they very probably will have.

If they are cylindrical 18650 cells rated at 2500 mAh then they will have at least 1C discharge capability (2.5A) and probably 2C (5A). YMMV!. With 2 strings in parallel that amounts to 5A or 10A.

Loaded typical voltage for a 4 cell pack will be in the 12V - 14V range and V after a full charge will be 4 x 4.2V = 16.8V.

As 12V = about as low as you should go x 5A (all cells) that's 60 W. That should be enough for your screwdriver - this can be measured easily enough.

You may be able to use the notebook charging circuitry if notebook death is unrelated to this. This may extract from the laptop functionally well BUT it's possible that the processor goy=t it's fingers in the pie due to gas gauging or whatever so you *may * find the circuitry is too embedded to be extractable. If you don't mind having a charger that looks suspiciously like a dead laptop then you can probably use the whole unit.

The battery will probably have monitoring points for each cell = at least 5 contacts (-ve, +1, +2, +3, +4 and may also have a thermistor for temperature monitoring - probably v+ and maybe ground referenced = 6 contacts. You could keep the battery pack whole and put it back in the laptop to charge (so you'd need a cord or other way of connecting to the s.driver. ) or you can run N leads into the laptop from the s.driver or ... .

You can buy N cell LiIon charger IC's relatively cheaply and you may find it easier to build a new charger from scratch.

If you can manage the lxury of accessing each cell separately (floating mini chargers) a LiIon cell is VERY easy to charge well when it is working OK.

  • Charge at 1C (2.5A) or less until Vcell = 4.1V.

then

  • Charge at 4.1V until I charge = C/x

where x depends on what you want to achieve. X=2 gives long cell life and notably lower capacity. C/4 or C/10 may be used. C/20 is for the energy density freaks.

  • If you stop at the top of the constant current mode (when Vcharge = 4.1V then you will get ~= 70% capacity and far better cell life. This is also the optimum level to leave cells at when in storage or no use for a while.

DANGER WILL ROBINSON

I said 4.1V above. Official figure is 4.2V for full cell BUT if you do that yu MUST temperature compensate. Magic smoke, VERY thick and hot, may or may not happen not much above 4.2V.

You are probably aware that 'vent with flame" is a LiIon cells favorite parlor trick. They can self immolate just because they can on a bad day - but many many many last for years with no problems.
DO NOT SHORT.
DO NOT CHARGE PAST 4.2v
DO NOT CHARGE AT HIGH CURRENT (> 1C) WITHOUT DATASHEET SUPPORT. Do not bend fold spindle mutilate staple - but especially in this context do not spindle - poking them with sharp objects may get exciting action.

People have had good results doing what you are trying to do.
Do it properly and you should get a good result.

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  • \$\begingroup\$ I am flying a model with LiPo battery so I am aware of these Li* gotchas. Using a cord and the laptop as a charger is probably a clever idea for the first milestone. The question is - will the battery / protection circuit cut itself nondestructively after it is discharged? \$\endgroup\$ – Matej Sep 29 '11 at 14:28
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    \$\begingroup\$ @Matej - The battery protection circuit MUST NOT be used as the low voltage cutout. your system should turn off without relying on the battery circuitry. \$\endgroup\$ – Russell McMahon Sep 29 '11 at 16:51
  • \$\begingroup\$ Thanks for info, I'll try to install some LED that will signal that some cell is about to be overdischarged. There are simple control components available that can be connected to a service connector of a RC model LiPo battery and it should work in this case as well. \$\endgroup\$ – Matej Sep 30 '11 at 15:55
  • \$\begingroup\$ @Matej - overdischarge can be fatally damaging to LiIon health. Some form of auto shutoff is a very very very good idea. \$\endgroup\$ – Russell McMahon Sep 30 '11 at 23:57
  • \$\begingroup\$ Yes, I am aware of this. Now I have decided to start with "corded" POC, keep the original LiIon pack in a pocket and control the voltage with a multimeter. If I will be satisfied with the performance, I'll decide how to embed the cells to the srewdriver's pack and think about a better overdischarge protection. \$\endgroup\$ – Matej Oct 2 '11 at 8:42

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