Picture of Cheap Black & Decker 18Volt battery
So if you're like me with several aging batteries for you cordless drill and to cheap to pay the full price for a replacement
Then check this out.
 I have several Black & Decker 18 volt batteries, part #244760-00, that were at least 5 years old and only 1 good one.  I decided to try and find a cheaper alternative to the 35-45 dollar new battery price. I just happened to be at Harbor Freight one day and noticed there line of 18 volt cordless power tools.  The battery they use is Drill Master part # 67029, and is only 10 dollars.  I decided to just buy one, since it was cheap and took it home to do a comparison with a B & D battery.  They batteries are very similar in size and the way it is wired. 

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Step 3: Making changes

Even though both packs use the same connector, they are not arranged the same way so some adjustments will have to be made.  On the Harbor Freight battery wires there is a right angle bend on the bottom of the connection wire that needs to be removed, if not the positive and negative wires will touch during final assembly.

Step 6: Finishing up

Before testing I like to Check Polarity of any battery work, as you can see it was 18.52 volts. The old battery was 11.5, bad.  You may also find that when charging the battery it may be a tight fit because of the additional wiring in the new battery pack. 
newestbie made it!9 months ago

I've done same. Original B&D batteries so expensive, Harbor freight batteries best solution. I replaced 3 of my batteries same price that I would pay one original.

Standard HPB18 Black & Decker 18V batteries only 1300Ah, FSX models are 1500Ah.

Harbor Freight has couple different models. DrilMaster 18v (item 68413) batteries are 1300Ah and only $12. You can also find Chicago Electric 18v batteries (item 68860), these are 1500Ah and sold for $15.

They both will fit in your B&D battery case. Only 1500AH one has three wires others has two wires. Third wires connected to thermistor, used by fast chargers to measure battery temperature during charge. Regular Black&Decker chargers don't have this feature So you don't need to connect thermistor wire.