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I am wanting to make a 3s battery pack for a 12v Drill should I use just 3 cells in series or should I put 6 in parallel

Well, 18650 is rated 3.6 V, end of discharge around 3.0 V. Charge stop is at 4.2 V. Discharge current can run at 10 A, but temperature must be controlled!! Having said that. The design depends on the drill power and the load you will apply. Li-ion batteries do not like heat. When under load, they can start overheating and you may end up with a runaway train (chain reaction of explosions and nice colourfull fire).

Putting this aside. Now talk voltage. If the drill is fine working at 15 V - get 4 batteries in series. If it is OK working at less than 10 V - get 3 in series. Making them in parallel (2x3 or 2x4) - that will be dictated by the load and work time you need. If the drill is 10 W - 1x3 is enough. If the drill is 80 W - 2x4 would be much better. 2x4 will be loaded at about 3-4 A and will not produce this much heat as 1x4 at 7 A.

Thank you PLP wonderful answer, I am new to this and not sure about its possibilities the drill is substantially built, On the name plate there is nothing to tell me what the wattage or (ah) is but on the Ni-cad battery case circuit board is 1.3ah 60min. . I am not likely to use the drill over long periods or excessive use and I have a dewalt Li-ion drill but hate to throw a perfectly good drill. Which would be the safest way 1x3 2x3 or 2x4. Hope you can answer Thanks again,

Can you take a picture of the plate or provide a model number?

OK, that's enough info. So if the torque is 24 Nm at 100 rpm, power load would not be more than 250 W. I assumed peak torque will be kept till 100 rpm - not sure what kind of motor it is. To get +1000 rpm I bet it uses gear.

250 W at 12 V will be roughly 20 A peak discharge. I would use 2x3. Many 12 V Li-ion packs are 3x18650 and if you have 1.2 Ah you will have 1 set or 2.0 will be 2 sets...

How will you solve charging of them? Any temperature monitor? Safety?

Just be aware - while they are great batteries because they pack a lot of energy (great power density), it is their danger as well - uncontrolled discharge and overheating creates great fire potential (plus the chemistry inside).

So, in short - 2x3 will be fine. Just check specs of the 18650's you have - their manufacturer/capacity and max discharge rates. If for any reason they are small capacity and low discharge, you can go 3x3 to keep it safe.

Thanks again PLP the batteries I was going to use GIF IGR 18650 3,7V 12000mAh and it says on the battery "The voltage of full charge and cut-off-discharge is 4.2V and 2.75V. I only have 4 of these and I assume you do not mix different manufacturers I can order a couple more but I would like to go 3x3 if you say that would be safer but they would not fit in the battery case so I would make a modification to the case or should I select 2x3.

https://www.candlepowerforums.com/vb/showthread.php?432972-Test-review-of-GIF-ICR18650-12000mAh-(Yellow)

Thank you PLP Now I get further confused so they are bad batteries so I will kick them into touch, going off what you have found from my intention can you recommend the batteries I should purchase and should I go 3x3 or 2x3 the latter would be better for me as they will fit into the case but if you say 3x3 I will go along with that and by the way I am going to charge them using the Imax B6 (Genuine) charger. Hope you can recommend , Roy

I am not sure how they will be put together. I assume some case with easy access/replacement for charging. Having said that - I'd use them since you have them. If they turn out to be poor quality (you will see the drill does not have power/torque or the capacity is not there) you can replace them with a different model.

Thank you PLP for all your gladly received info, just a point I have 6 batteries from a laptop and all reading 3.61V But I don't have there Amh capacity and do not know how to obtain that info. There is some numbers on the battery LGDA531865 and J118D283AD, I am thinking of using them to make 2x3 would I be ok doing this. Thanks again.

As I said before - try them out. There are some tools that could estimate capacity, but the best approach is a controlled discharge of fully charged battery. Just like the reviewer did - different load until certain voltage. Calculate capacity.

If you have a multimeter you can measure max load on the motor to see what kind of setup would be appropriate. Keep in mind that you want to stay at 80% of max allowable load on the battery.

To find out more of it do a simple google search using model number, like LGDA531865, and you will find a few specs. https://www.google.com/search?client=firefox-b-1-d&q=LGDA531865

Thanks for all your help PLP I will try and digest .

Thank you for your reply PLP. the Plate on the drill contains only this:- Erbauer 12V Model :- ERB 1200- 12V== -0.8-10mm 0-400/0-1150rpm I would think that the Model No refers to it being 12V the middle No's = size of chuck and then the variable rpm. It is not an Hammer Drill just a 2 speed and screw torque settings. Cheers.