Fantastic. I got 3 batteries to do
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Yes, it is possible, however, your power tools will perform less well, it will be weaker.
often not much weaker. car batteries have lower resistance than nicd cells so the voltage remains close to 12 V when the tool works
Agreed, under significant load a 12V (actually closer to 13V fully charged) car battery will retain as high or higher voltage than the typical NiCd pack, but why are we talking about car batteries still, I don't recall the original question stating car battery.
12 V batteries might be car batteries
but youre right they are not mentioned explicitly
You'd be better off hooking two 12 volt batteries in series and running on 24 volts. DC motors are much less susceptible to overvoltages than undervoltages. As long as there are no electronics involved.
A typical brushed motor used in a DC power tool is not less susceptible to overvoltage. That causes more current, more overheating. That is, if the addt'l power is put to use, if it is not then there was a throttle that effectively reduced power anyway.
Hint - trigger on variable speed drill constantly reduces voltage.
if you do it work for short time each time and let the motor cool down
for continuous operation use 12 V
I have run 18 volt, 14 volt, and even nine volt battery tools for years on twelve volt car batteries. They work fine, although as mentioned below they are somewhat weaker.
This works best with drills and other tools that do not draw a lot of current and are used intermittently. Because at lower voltage they are running at lower rpms, they tend to run at a higher current and you run the risk of burning out high amperage tools like saws if you push them too hard or for too long.
When higher than rated current runs for too long on electric motors it can heat up enough to melt the coil wire insulation. This is what burns out most motors.
This is incorrect. When you run a 18V, 14V, tool at 12V, it will consume less, not more current. The motor windings are a fixed resistance per RPM if spinning at a given load, the only risk of high current causing a problem is if the lower voltage causes them to stall in which case the stalled motor puts all that current through the same armature contacts and windings, disproportionately overheating that section of the motor instead of the whole thing evenly.
In theory, you are correct. But in practice there is a little more going on here than meets the eye. If you lower the voltage on a motor that is idling, the current will decrease. Under a load it is a different story.
Try this simple experiment. Connect a battery to a small dc motor and measure the voltage and current while it is idling. Then grab the shaft with your fingers and put a load on it. As you increase the load the voltage will drop and the current will go up. This is because the reactance (effective resistance of an inductor) of a dc motor decreases with lower rpm.
In addition, as rpms are lowered by using a lower than rated voltage, the fan that cools the motor can not remove the heat as effectively and this leads to additional heating of the coils.
Anyone who uses power tools such as table saws knows that you have to use a big enough gauge extension cord to the saw or the voltage will drop and the motor will tend to overheat and even eventually burn out. I have burnt out several motors and have discovered you need not stall a motor to burn it out.
And yet, when you put that load on the motor with the lower voltage, the current can't go up as much because it is a lower voltage.
It does without saying we cannot let a motor sit stalled, that it will then overheat, but otherwise at a low effective tool RPM you still have a reasonably high motor RPM as most motors are not direct drive in situations where high torque is needed, instead the percentage of slowdown is not exceptional to the point where it should be starved for airflow to cool it, as a properly designed tool will be built with these margins.
If this weren't the case, many cordless tools simply wouldn't work or last for long. Take a drill for example, under a load like driving a lag bolt the RPM drops very low, there is indeed more current then but there is voltage droop because of the battery impedance among other chemical reaction periods, reducing the current compared to higher voltage.
The reason you were burning out motors is because you subjected them to too high a load using a battery with (for practical purposes relative to a NiCd/NiMH pack) infinite current, that this high current but lower voltage battery retained a HIGHER, not lower voltage during use under any significant load.
You can test this easily enough, instead of the large 12V battery, run the same power tool off N-2 sub-C cells.
We were talking about DC tool motors yes? Not AC table saw motors.
If they're nicad batteries (the 18v ones) you can revive them using this Instructable: https://www.instructables.com/id/Revive-Nicad-Batteries-by-Zapping-with-a-Welder/