Instructables
Welcome fellow slacker. If I had this to do over I would not do it. It cuts well yes, but the battery only last a matter of minutes...

Someone gave me a cordless circular saw with a dead battery and no charger. So I could pay $150 for a new battery and charger, or I could take the battery from my Mikita drill that would work fine and run it off that. You'll need:

wire strippers
soldering iron
screw/drivers of several types
cutters (dikes)
$9 auto-ranging probe I love you Harbor Freight
wire
solder
duct tape
beer or perhaps the kindest kind
 
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Step 1: Will it work?

Well the battery for the Dewald is 18 volt and 2200 milla amps (1/1000 of an amp) or 2.2 amps. So volts times amps is 36 (or so) watts. The Mikita is 14.4 volts and 2 amps. 28 watts, ya it will probably be a little under powered maybe not spin as fast? I don't care, but I should have. The battery dies after just a few minutes.

I am not happy with this at all. The battery only last a few minutes. I am going to put a cord on it. A 12v 1.5 amp ac adapter barely spins it. The battery charger senses it not hooked up to a battery and does nothing. Maybe I could I could increase amps by putting several smaller transformers in parallel?

Dewalt is no help at all "This saw is designed to run using DEWALT 18v batteries, we do not recommend using any other power source for this saw. We do not have any other motor specifications for this unit." Liar...oops did I say that out loud?

Step 2: Determine polarity

Picture of Determine polarity
Get out your meter and see which is positive and which is negative on the battery. Write it on them with a marker.

Step 3: Take the donor battery apart

My Dewalt donor battery has star screws in it. Remove the contact piece/plug inard out of the battery, and remove any wires and/or attaching foil that held the old battery together.

Step 4: Connect the plug

I used about 6" of the cord off an old AC adapter. Strip and solder it to the donor battery contact plug.
PKM5 years ago
I think there's a problem with your maths. The numbers on the battery are the amp-hour rating, ie how long it will provide one amp for. Amp-hours with voltage are a measure of capacity, not power. The actual current drawn will be determined by the saw. You are right that the 14.4V battery is underpowering the 18V saw, how serious that is depends on how picky about voltage it is (you said it runs fine so I guess it isn't a problem). I see this confusion between amps and amp-hours everywhere- I blame the media for never bothering to differentiate properly between energy and power.
gabes413 (author)  PKM5 years ago
so enlighten me, how do I measure current, isn't that watts? Isn't that true power? help me understand?
PKM gabes4135 years ago
The current flowing is determined by the circuit. I nearly blew up my dad's multimeter as a child trying to measure the "current of a battery"- he pointed out to me that a power source doesn't have "a current" like it has "a voltage". The magic formulas you need at this point are watts = volts * amps and amp-hours = amps * hours.

Say your 14.4V drill uses 500W. The current in that case is
500W / 14.4V = 34.7 amps. '

If you then drill into something harder then the drill might start using 600W instead of 500, and then the current draw from the battery would be
600W / 14.4V = 41 amps, and the battery would last less time.

If your battery holds 2 amp-hours and you are drawing 41 amps, then the time it will last is
2 amp-hours / 41 amps = 0.049 hours or about 3 minutes.

If you plugged an 18V battery into your 14.4V drill, assuming it didn't blow up, then when it used 600W it would only draw
600W / 18V = 30 amps, so a 2 AH battery would last

2AH / 30A = 0.15 hours or 4 minutes.

All of these calculations are making a few assumptions about the way your devices behave, and the numbers are probably all wrong (a drill that lasts for 3 minutes? I'd get my money back) but I have done them to try and show how watts, volts, amps and amp-hours relate to each other. A major part of electrical physics is knowing how these quantities relate to each other and how to determine unknowns from knowns.

I end up saying this a lot but a decent physics textbook should clear all this up a lot better than I can. Maybe I'll make an Ible about basic electrical theory.
PKM PKM5 years ago
Is a watt a watt hour? It shoudl be.

Can you measure distance in miles per hour? Speed in yards? Height in cubic inches? Temperature in seconds? No- they aren't just the wrong unit, they are the wrong quantity.

Watts are for measuring power- they can be converted to calories per second or BTU per hour or horsepower or soldering irons, but they intrinsically measure energy per unit time. Watt hours are a unit of energy, and can be converted to calories or BTU or electricity meter units or AA batteries or cheeseburgers, but they are a measure of energy.

To convert horsepower into cheeseburgers you have to multiply by a time quantity (seconds, days, time it takes to brush your teeth) to determine how many cheeseburgers your horse has to eat to work for one teeth-brushing.

OK, I'm all out of analogies but I hope that clears up the difference between energy and power. You measure a drill in horsepower and a battery in cheeseburgers.
gabes413 (author)  PKM5 years ago
So I understand better, but it still seems silly. 1 watt continuous for 1 hour is a watt hour. 1 watt as measured represents what is being draw or flowing at that moment in time (power). 1 watt per second is one jewel joil maybe, whatever per second, so 3600 in a watt hour. Energy is what has been used in relation to time.
PKM gabes4135 years ago
That's pretty much it, except one joule (sounds like "jewel", hence joule thief, ha ha) is one watt for one second, not watt per second. Everything else you wrote is accurate.

"but it still seems silly.
Communicating in a non-ambiguous way in a scientific context is silly? The Mars lander crashed because there was a confusion between inches and centimetres, and those are both measures of distance. Mixing up watts and watt-hours isn't just confusing, it's meaningless.

It may seem confusing because the SI convention maintains the names of a lot of the units (hence my use of horses and cheeseburgers rather than watts and watt-hours) but the two are fundamentally different things. Personally I find turning BTUs into the number of cubic yards of salt water you can heat by one fahrenheit more confusing than one watt for one second being one watt-second, but to each their own.

Sorry if it seems like I'm picking on you, I'm really just venting my frustration at people using the wrong quantity to make nonsensical statements. It bugs me every time I hear pressure stated in "pounds"- I know they mean pounds per square inch but that's not what they say, and this bizarre habit of measuring temperature in cubic kilograms or whatever they insist on doing can only lead to confusion.

The statement "this wind turbine produces enough energy in a year to power 2,000 homes" should set your teeth on edge- power 2,000 homes for how long? If you mean a year then the time quantity is irrelevant, if you don't mean a year the statement is meaningless.
Current is the flow rate of charge per unit time, measured in amperes. Power is energy per unit time, measured in watts. Google Is Your Friend
8bit gabes4135 years ago
The current would be entirely dependant on the voltage of the power source and the resistive effects of the circuit. If you wanted to measure the current directly, you could put an ammeter (voltometer set on amps) in series with the battery. The wattage is power. For DC circuits, the wattage is calculated by voltage multiplied by amps. ( the voltage may slump a little when connected to batteries due to the current draw.) In a circuit with any form of AC, the calculations become more complex.