1] VOLTAGE: 3.6V or 3.7V - 18650 Li Ion Batteries
All single cell lithium ion batteries are going to be 3.6-3.7v. There are applications where multiple cells will be tied together in series. This will result in voltages that are multiples of 3.6-3.7v. So as long as you match the number of cells and approximate mAH you should be fine.
2] Possible Voltage Shortage?
The voltages and battery life responses for all batteries are going to have slight difference. For the most part this won't matter. Most projects that use batteries are not terribly voltage dependent. They will either boost or regulate their voltage to get the voltage they want out, or they will be able to run at a wide range.
As a note, "Shortage" in this context usually means you are creating a short across your battery. Might want to be careful with that terminology.
3] Fundamental Reason for this Voltage Range
I am not an expert on this, but I know it deals with the chemistry of the battery itself.
4] Parallel Cell Charging - One BIG Li-Ion Battery Pack
This can be done. There are some issues that can come up when doing it. This might be worthy of a question by itself. If you do ask, might want to ask if the same can be done for packs in series.
5] Charging... How?
Same as previous answer.
* 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.
Best Answer
I have 3 18650 in my galaxy s4 mini with cyanogen 14 android version 6.0.1, use mine as a normal phone as wlell as an ap at home as its faster internet at up to 65 meg, works a treat and charges fine but had to add a charging pcb that also doubles as a USB power sorce, was from a pound shop portable mobile charger.had to add a switch between battery's and phone to turn it off fully to reset the phones battery meter as does not recognise when its fully charge. Lasts a couple of days with normal use. All in all very pleased with results so far. About 4 months.