The battery of my old drill is dead. It is not charging anymore. It contains 10 batteries NiCd 1.2V, 2A serially connected. I found the correct replacement but those are expensive. I am planning to replace the batteries using 4 18650 Li-Ion batteries of 3.7V 2.2A each (serially connected as well). I understand the voltage will be not the same but it is into the drill input voltage range.
My concern is more about the impedance of the 10 serial batteries VS the 4 li-ion batteries. The resistance of the 10 batteries for sure is bigger than the others. So the electric current delivered to the drill can be more that what it can handle.
What do you think, could my idea work or better buy the expensive but secure batteries?
Best Answer
The 10s NiCd battery pack will return approximately 10V - 12V from "flat" to fully charged. It will still provide some energy below 10V but only a small percentage of total.
The 3.6V or 3.7V voltage of a LiIon cell is the nominal or mean voltage during operation - NOT the maximum voltage
A loaded LiIon cell has a voltage range of about 3V to 4V (4.2V absolute max unloaded) so a 4S battery will return 12V - 16V from flat to fully charged. This may be more that your drill will reasonably tolerate (and may not). A 3S battery will return about 9V-12V.
The maximum allowed loaded current of a LiIon cell depends on cell type. Some are optimised for maximum energy and usually have a low maximum allowed discharge current. Others are optimised for maximum power output with (usually) lower energy capacity.
In this application, using "power" rated high discharge rate cells is required. A power cell, depending on brand and model, may allow a 5C - 10C discharge rate, and perhaps higher again in very short bursts. For a cell with about 3 Ah capacity and 10C maximum allowed discharge rate this corresponds to 15 - 30A. For a 3S pack this is a nominal power of
3.7V x 3 cells x 30A =~ 300 watts.
This is liable to be acceptable in many drills.
What your drill requires can be determined by operating it from a variable voltage power supply and measuring loaded current.
In the absence of a suitable power supply a well charged good condition 12V lead acid car battery would be a reasonable source.
If current is well above the LiIon cells rated maximum current under load their use is "risky". If maximum current is under say 30A (which is likely) then suitable rated liIon cells are acceptable.
Charging MUST be carried out with an appropriate LiIon charger (not just a BMS) plus the use of an appropriate BMS is "a very very good idea".
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Molicell (and some others) make 21700 size cells (eg Molicell INR-21700-P42A cells) rated at 45A continuous. If you can fit 3 of those in the drill it should meet any need.
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