I have several old cordless power tools and they're all in good working condition. The trouble is the batteries all need to be replaced and the batteries are obscenely expensive. I have a really hard time paying for batteries that cost almost as much as the tool and I didn't want to discard perfectly good tools.
One other issue I had with my old batteries as that every time I went to use them the batteries were dead as the NiMH batteries would self discharge rather quickly, especially in cold weather.
Most modern cordless tools use Lithium batteries and I happen to use LiPo battery packs all the time for other projects so I figured I'd convert my old cordless tools to Lithium power using inexpensive LiPo battery packs.
This is a really simple conversion and the cost is a fraction of what power tool manufacturer replacement batteries cost.
Be sure to check out the FAQ section at the end as Lithium batteries do have to be treated in a particular manner!
Step 1: Tools and Materials
There are only a few tools needed:
Wire stripping tool- I really like this inexpensive wire stripper
Soldering iron- You could use pretty much any inexpensive soldering iron as this is a very easy job. Mine is an analog Pace ST30 which I purchased used for $100 (I had to look for a long time- new is about $260) The digital version is the ST50 model.
Dremel tool with cutoff wheel (a narrow bladed saw will also work and cuts cleaner)
Battery charger- I really like the Hitec X1 Multi-Charger as it's a smart charger can handle most any battery chemistry but any charger capable of charging multi cell Lithium batteries will do
For materials you'll need the following:
Heat shrink tubing
XT60 connectors (these come with heatshrink)
Battery monitor- I really like the inexpensive Hobby King low voltage battery alarm
3s (11.1V) or 4s (14.8V) LiPo R/C battery pack- Hobby King sells lots of different LiPo packs so pick the one that best matches your tool's voltage and available battery space
Step 2: Open the Battery Case
Cut apart the old battery case.
A few cordless tools have battery packs that are bolted together but no such luck with my Makita tools- the battery packs are seam welded shut so they had to be cut open.
To cut open the battery pack I carefully cut around the top edge of the casing with a Dremel tool cut off wheel. Once you get the top of the casing off remove the battery cells (properly recycle them) and keep the empty battery casing.
Measure the inside of the casing to determine how large a LiPo battery pack you can fit inside that closely matches the power tool voltage rating. For my Makita 14.4V drill I used a Turnigy 1300mAh 4S (14.8V) LiPo pack that measures 74mmx 34mm x 33mm and it fit the empty battery casing perfectly. You don't want the battery to use all of the available space as you'll need a bit of room for the wiring and low voltage alarm,
Step 3: Solder the Battery Connector
Now it's time to do some wiring!
Take your power tool casing apart. For my drill I only had to remove a few screws to lift off one side of the drill housing. Once this was done I could insert the top of the old battery casing in order to correctly determine the battery polarity for the wiring.
Take a length of 14ga wire, strip the ends of the wire and solder to the battery contacts inside the power tool. Slide heat shrink tubing over the opposite ends of the wire and solder the wire ends to a XT60 battery connector (most 3S and 4S LiPo battery packs ship with XT60 connectors) and then slide the heat shrink tubing down to protect the wire ends at the connector.
Step 4: Reassembly
Now reassemble your power tool.
Once the tool is reassembled connect the LiPo battery pack. Now plug in the low voltage alarm- there are notations on the alarm as to how to connect it to the battery pack balancing connector. The alarm will beep rather loudly at first and then either three of four green LEDs will light up, showing that each cell in the battery pack is above the cutoff voltage.
As the battery is drained, the LEDs will turn red and an alarm will sound, indicating it is time to charge the battery. Most LiPo cells have a cutoff voltage between 2.8 and 3.0V and the cells can be damaged if they are drained below that level. The alarm is there to notify you when the cells have reached this level. The low voltage alarm will work with 2S to 4S batteries.
Once everything is working properly you can clip the bottom of the battery casing back into the power tool and you're good to go!
Step 5: Finished!
Now experience the joy of Lithium power!
I have several drills that I've converted using this technique and they all work better than they did when they had NiMH batteries.
I really like the old robust Makita stick style drills- they can be found dirt cheap (or even free) and they hold a decent size battery as well. The beauty of these old style drills is you don't even need to take apart the old battery case as the battery just slides right in the grip. The mini Makita stick drill is one my son found for $3 at a recycling center. When he found it it didn't have a battery but I knew we could fix it up as soon as we got home so he bought it and he's been using it ever since for his own projects.
Won't the battery alarm drain the battery over time?
Yes, it most certainly will. I've developed the habit of keeping the batteries unplugged until I'm ready to use my tools. It takes me maybe ten seconds to plug them back in and since the LiPo packs have such a low self discharge rate my tools are always ready to go.
Don't LiPo packs require special chargers/procedures?
They absolutely do. Here's a good guide to understanding LiPo packs and how to care for them- A Guide to Understanding LiPo Batteries. There is more info about LiPo batteries and safety here. If you're not comfortable handling LiPo batteries and going to the extra trouble of maintaining them properly then this instructable probably isn't for you. If they are treated properly they are perfectly safe for every day use, but it's not something I would recommend if you're using tools on a construction site as they're not really practical for that application. I've been using these tools for quite some time now with absolutely no negative side effects.
If you are really worried about an overload condition (motor shorting/stalling, etc.) then you can install an automotive blade type fuse on the positive power lead from the battery connector to the tool (not on the battery pack.) These fuses will increase the load resistance so you will have to do some testing to obtain the right value but it should be below the total current capacity of the battery.
What about LiFePO4 batteries?
Yep- you can use those too. While I used a 2s Li-Ion battery in my son's drill and it works just fine ( I already had this battery pack from a broken RC helicopter that was given to me at home ) LiFePO4 batteries are a better choice. The advantage of these batteries is that they tend to be a bit more durable than LiPo packs but they can be a bit larger so they may not fit inside many power tools. If you can find a pack that fits then great- just be sure to use a pack that either has a built in protection circuit or be sure to use a low voltage cut off alarm. Again, you will need a special charger for these batteries.
Isn't the battery capacity less than the original manufacturer's?
In the case of my 14.4V Makita drill- yes. But I don't really care as I found I'm rarely trying to drive 200+ decking screws from a single battery charge. Since the batteries are so inexpensive I can also afford to keep several packs on hand.
What do I do if I have a tool that has a plug in battery but I don't have the original battery?
In this case you'll have to get creative. Two options are 3d print a suitable battery case or vacuum form one from plastic sheet.