Instructables
WARNING: PERFORMING THIS MOD WILL PROBABLY COMPLETELY VOID ANY WARRANTY YOU MAY HAVE.

You may notice the charger included with Harbor Freight rechargeable drills (at least the Drill Master 66965) is not a "float" charger, meaning you CAN NOT leave the battery on the charger for too long otherwise you will destroy the battery.  

This Instructable fixes this issue.

This limits the charging current to approximately 100 milliamps which is less than 10 % of the 1200 milliamp rating of the battery thus OK for a constant “FLOAT” charger. Note - Allow no less than 14 hours for full recharge of a fully discharged battery but you may leave ON after that time.
 
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Step 2: Make your resistor

Take 3 Radio Shack 10 ohm/ 1 watt resistors (Radio Shack # 271-151, BUY 2 PACKS) clip them short, and solder them in series.
(yes, I know my soldering STINKS.) Cut a piece of shrink tubing to cover the resistor  and heat to shrink around it.

Step 3: Insert resistor into charger base and close

Clip red wire in the charger.  Strip the covering to expose appx 1/4" of the wire on each.
My pictures aren't great but I forgot to add shrink tubing to one end to cover the solder connection.
Shove it in and replace the cover.

This limits the charging current to approximately 100 milliamps which is less than 10 % of the 1200 milliamp rating of the battery thus OK for a constant “FLOAT” charger. Note - Allow no less than 14 hours for full recharge of a fully discharged battery but you may leave ON after that time.
wario2357 months ago

I just got my hands on a HF drill (68851) and it came with a really crappy charger (68859). I'm really careful about reading reviews on HF stuff before purchase (thus that specific drill). Reading the reviews on the charger though, say very similar things to the complaints you remedy in your instructable here.

1st night charging the battery, I only charged it for an hour, and had to unplugg it for 20 min to let it cool; it was already getting that electronic burning smell.

Would I need to plug my information into an equation for a different amount of resistance to drop the current to "Float charge" specifications? Specs: 7.2-24VDC, 1.3A.

xrobevansx (author)  wario2359 days ago

Not sure what the output Amperage is on that model. You need to know what it's delivering to the battery. Then add resistors accordingly.
http://www.sengpielaudio.com/calculator-ohmslaw.htm

ladyfb9 days ago

I cannot locate any type of connection in which to insert the adapter plug into the base (battery section). The box pictures a connection on the certer of the back but no opening exists anywhere in which the plug would fit.

xrobevansx (author)  ladyfb9 days ago
The battery won't have an adapter hole. The charger will.
What model do you have?
jongscx1 year ago
You could very easily use a switch that would short the ends of the resistor network and be able to chose fast or trickle charging...
gsnoorky jongscx9 months ago
Finally, I picked up my resistors this evening.

Putting in a switch may not prove worth it: HFT now has these for sale--separately boxed--discounted to move for about $5. Thus, one may leave one as is, while modifying the other--marking one or both chargers accordingly. (Remember, this is "Drillmaster"--not "Chicago Electric.")

Given that I screw up, somehow, my original charger still should work. I believe that my chargers are both 68420 models--thus, I'll relate later whether the mod works well or not....
xrobevansx (author)  jongscx1 year ago
The only potential problem is that "fast charge" is a few hours and if you forget to take it off it blows up the battery. It's a bad design by HF (go figure.)
bigun9131 year ago
Will this mod work on the HF 68420 charger? It to is an 18v and I have killed 3 or four of them by leaving them on overnight. It appears its the same voltage and I wonder if this mod will work.
xrobevansx (author)  bigun9131 year ago
I JUST saw this question and apologize for the insanely late response. I believe it should work, yes.
gsnoorky2 years ago
You're Leonardo Da Vinci with that iron compared to me--instead, I'm analogous to a little kid finger-painting!

Thanks, also, kill-a-watt: Many of us need to be reminded of the quality way--the right way to do such things--that saves future aggravation--and, time.

Thanks for this: It would have cost the PRC a fraction of a cent to do this. If HFT had insisted upon this from the PRC co. involved, they would have avoided great resentments and carping about this product. It seems that HFT would desire less resentment and more public satisfaction--spending a few cents and maybe even adding a $ to the purchase (for their trouble) would smooth things over with the public. It seems to me, at least, it's in their best interests.... Anyway, I'm certain that this solution may be easily incorporated into other such products which people buy. (I know!: It is what it is!)

PRC cos. copy and steal ideas from each other endlessly: Something which costs $20+ at a certain retailer costs $5+ elsewhere. (I can submit easy examples, if desired: I think that I'll join!)

I'm moving, and we have to sell our property, so I won't have as much time as I desire to contribute at this point. I can tell that the contributors here are great, though!: Thanks again! I wish that I had known about this site years ago!

Phil B3 years ago
I think what you call poor soldering will work just fine for you. There is nothing that will disturb the joints or apply pressure to them, and the shrink tubing will strengthen the connections. I did something like what you did here. You can see it at this link.  Although it has been several years, the same battery is still in use and still working just fine.
xrobevansx (author)  Phil B3 years ago
Awesome Phil! Thanks for your encouraging words.

Your soldering looks pretty good to me. There is, however, a lot of solder. Even though the picture is blurry, I can see that enough heat has been applied so there isn't a "cold joint" here.

A passive component like a resistor is the perfect thing to practice on though. As a practical manner you don't have to worry about too much heat destroying the part.  

Next time:  1) twist the leads together to make a mechanically strong joint. 2) "tin" the tip of your soldering iron, then wipe it on a sponge to clean it. 3) add one "drop" of solder directly to the freshly cleaned tip 4) touch the tip of the iron to the future solder joint and hold it there until you see that drop of solder wick into the joint 5) add a small amount of additional solder, but allow the joint (not the iron) to melt the solder and allow it to be wicked into the joint 6) remove solder and iron, allowing the joint to cool to solid without a bump or jarring,
xrobevansx (author)  kill-a-watt3 years ago
Thanks for the tips! I admit my soldering is BAD (even if your kind words say otherwise) but the charger is indeed working very well! Thanks again. I will practice using your directions!
I just looked at all of your Instructables. I like your ideas and hope you will continue to post more.
xrobevansx (author)  Phil B3 years ago
Thanks again Phil. I try to make instructables as I need to. Problem...fix...make instructable.