scratch building a battery pack.

[Brother2]

I have an electric drill with an old battery pack. I would like to rebuild the battery pack that is in there right now, but I want some advice before I do.

It is a 7.2v craftsman pack. The pack inside is a 6-cell 1300mah NiCd. It is wired with one ground and two positives going to a three prong plug.

I think the individual cells are sub C. Is there a good resource for learning how to rebuild a pack.

With 7.2 volts. Surely I can use a 2S lipo pack of greater capacity than 1300mah with no self discharging problems.

I have a Hobbico Accucylce Elite to charge and maintain whatever packs I end up building.

Thanks for whatever advice you can offer.

[Malc C]

I don't kmow the drill or the pack, but if it has three wires could it be that one is a tap at a lower voltage, if not then I can't see the point of having two wires to supply the 7.2v unless the current the drill draws is that high.

The thing with lipo's is the peak and sustainable current draw. If the current the drill draws under stall load is high then the packs may not be able to cope.

[jeffs555]

The extra connection often goes thru a thermal sensor for charging.

Stall current could be over 100 amps. This page shows stall current for a Harbor Freight 9.6v drill as 80 amps for the motor only. To be safe, you should probably measure the stall current or the stalled resistance of your drill. A 2Ah 20c lipo would only supply 40 amps.

http://www.enigmaindustries.com/Motors/Harbor_Freight_45935.htm

[Brother2]

I'll probably just use the sub c cells since this will be my first battery pack build. I have no previous experience with lipos.

There is a little circuit card in the pack that has the led that lets you know you are charging.

[jeffs555]

I have an old Ryobi 7.2v drill that I had thought about converting to lipos. Your post got me to pull it out and measure it. At 8 volts, the stall current is around 60 amps, and no-load current is around 4 amps. It looks like a 2000mah 20C continuous 30C burst lipo should handle it. The drill works fine, but I use it so rarely that I have to charge the nicads before every use. The lipos with their low self discharge would make it much more convenient.

[jeffs555]

Brother2, I am glad you started this thread, because it got me to look at my 7.2v Ryobi drill again, and it is now working with lipos.

I didn't have a 20C lipo pack with the right form factor to fit in the old Ryobi battery case. I did have a 1900mah Thunder Power pack that fits, but it is only rated for 19amps continuous and 30amps burst. As a test, I added enough wire to limit the current to 30 amps at stall and tried it out. It works better than I expected. It has plenty of torque to overrun the maximum setting for the screwdriver torque clutch. The battery just barely gets warm. When drilling, the torque is obviously less than with the nicads, but I am sure that is just because of the extra wire I added to limit the current. I may look for a 2000mah 20C 2S pack with the right size(70x50mm) as that would let me get the same torque as the nicads, but for now I am happy.

If anyone knows of any 2000mah 20C lipos with a length of 70mm or less please let me know. I have seen some cheap 1000-1400mah in this form factor from UnitedHobbies, and I may get some and run 2S2P with them.

[Brother2]

What is the C rating of NiCd and NiMh cells?

Well, Jeff, that's great. Now your drill costs as much as the battery, but it probably suits your uses better. Care to post a picture?

[JustinONE]

123's are good for 80A discharge, wallmart sells the 123's under the black and decker vxp or is it vpx line. two cell packs cost $17 and are 1100mAh at 3.6vlts per cell, they can also be quick charged at about 4-5 amps.

the big 2100mAh 123's are $19 for each cell, can be purchased individually

123's also hold a charge for about 10 years. ;)

[jeffs555]

Thanks for the info Justin. I had seen the VPX stuff at walmart, but didn't look at it closely and didn't realize they were A123 cells. The raw 18650 cells are 65mm long and 18mm diameter so two cells would probably fit where the 6 subC nicads were in a 7.2v drill. The nominal voltage at 3.3v per cell would be slightly less than the nicads, but would probably work fine. Only problem is I would need a new charger. Some sites say the VPX is being discontinued, so I will watch for clearance sales. http://toolmonger.com/2008/02/15/dealmonger-black-and-decker-vpx-on-clearance/


I will post a picture of my lipo setup when I get a chance. Best thing about it was that it was free as I already had the TP pack and the drill, and it charges with my existing lipo charger.

[Brother2]

If you used the vpx cells, you could use the black and decker charger.

What is a good place for buying individual cells?

[JustinONE]

http://www.battlepack.com/LiFEPO4.asp these guys are good to deal with for high capacity 2500mAh individual cells, but it's much cheaper buying 1100mAh VXP packs from wallmart for $17 that's about $9 for a cell

[Brother2]

Well, I finally did it. I bought 6 new 4500 mah subC cells off the internet. They were nearly 6 bucks each.

My soldering skills suck apparently. The biggest problem with making the pack is keeping everything together and then not busting a solder joint or pulling a wire off while trying to put it back in the plastic casing.

This project took me a couple hours, but I could definitely do it in about 20 minutes now.

BTW, I figured out why the battery pack has three prongs. One is a ground, One is a 7.2 volt positive, and the third is a 4.8 volt positive connection. There is a bung between the 4th and 5th cell. This makes a physical speed controller so the first half of the trigger pull gives you a lower voltage slow speed drill, and the full squeeze gives you maximum voltage and higher speed.