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Author Topic: Don't throw away cordless drill battery yet until you try this Part II  (Read 2102 times)
vonoretn
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« on: September 30, 2010, 11:59:03 AM »

I'm starting this topic over since the previous one is over 120 days old.  I have been rebuilding drill batteries for a couple of years.  I am not a professional power pack rebuilder, but here are my observations about zapping the power packs with current.  I am not recommending anything that I discuss, as doing it wrong could be hazardous if you don't have sufficient electrical knowledge.

1. The procedure of zapping power packs with high amperage works sometimes.  It is true that you burn off internal oxides, conductive whiskers inside the batteries that short them out internally faster than you can charge them. 
2.  It often does not work when you zap the whole power pack, because almost all of these power packs are made up of a series of 1.2 volt sub C NiCad cells.  Sometimes some of these cells are just shot.  That is they will act like a short, instead of a battery, and they cannot be saved by zapping them with current.   

3. Occasionally but more rarely a battery will act like a capacitor, or an off switch and stop the series current from charging the other batteries. 
4. My procedure is to open up the power packs, which you can usually do by removing 4 or more screws. 
5. Test each cell, by applying a voltage slightly higher than the rated cell voltage, then ramp up the current to about 3 amps for a second, and repeat the cycle several times until the cell either holds it's rated voltage or it stays under approximately 0.2 volts.  This only takes about 20 seconds for this first test.  You need a power supply to do this.
6. Replace all the cells that won't take a charge. 
7. Charge the power pack over-night at a low amperage, approximately 80 ma or lower.  Let sit a day, no charging.  Then if any cells go back to zero voltage, replace those also. 
8. When replacing cells, carefully grind the connecting nickel strips off with a drummel or small die grinder, so as not to damage the top of the cell by pulling off the strips, which can tear the top of the cell as you break the spot welds. 
9. Solder the strips back to connect the cells, cut strip to cut strip, to keep as much damaging heat away from the cells. 
10. It takes this second step to identify cells that still have internal leakage and just won't hold a charge. 
11. Now you have a power pack that has been renovated, to the point that it will act like new for several months or even years. 
12. As a regular procedure, leave your drill on overnight, to run completely down once a month. 

An even better technique is to shop hardware stores for cheap power packs intended for discontinued drills and hence are cheap.  A good guideline is one dollar or less per cell.  For example an 18 volt power pack (15 cells) for $14 would be a good deal, I just got one last week.  Then replace all cells in your old power-pack with the new ones. 

To have your power pack rebuilt by a professional would cost you about $40.  This is a fair price, and a tribute to the advantage of mass produced new drills that come with new power packs for not much more than that.
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Don4107
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« Reply #1 on: September 30, 2010, 01:38:19 PM »

I would agree with everything except #12. 

Rechargeable pack cells always have slightly different capacities.  As the pack runs down, the cell or cells with the least capacity reach zero volts first.  Continued current draw reverse charges the dead cell. That is very damaging to that cell.

This is why you want to stop using the pack as soon as you notice it loosing power.  Any further use damages the cells.  You have to resist trying to drive that last screw.  Grin

The only safe way to completely discharge a pack requires monitoring each cell as the pack discharges and putting a jumper across each cell as it reaches zero volts.  That or loading each cell individually.  Neither of which works for an assembled pack.

So, stop using the pack as soon as it starts to lose power.  Don't leave batteries sit that are low or dead, it breeds shorts.

Good luck
Don 4107
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Don 4107 Eastern Washington
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Jeremy
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« Reply #2 on: September 30, 2010, 01:53:30 PM »

Back in my modelling days it was accepted procedure to 'equalise' nicad packs by flattening them periodically with a small load, such as a bulb. The more sophisticated battery chargers used to flatten the batteries before charging them up again. I've no idea whether this is good science or not, but I suspect the 'small load' bit may be significant in avoiding any cell damage.

And it's worth stressing that this practice was for nicad (Nickel Cadnium) batteries only (definitely not recommended for lead acid bus batteries!). I've no idea what is good or bad practice for modern NiMh or Li-ion battery packs


Jeremy
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Hobie
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« Reply #3 on: September 30, 2010, 05:56:46 PM »

The magic number is 1 volt.  Do not discharge below 1V for NiCads or you will damage the cell.

 
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vonoretn
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« Reply #4 on: October 01, 2010, 10:16:32 PM »

Thanks for your responses.  I am never to old to learn from other's experiences.  Don, I will quit doing number 12, as I have never observed much of an improvement by doing it.  The same first 11 steps also applies to Dust Busters. 
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RoyJ
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« Reply #5 on: October 01, 2010, 10:21:46 PM »

The magic number is 1 volt.  Do not discharge below 1V for NiCads or you will damage the cell.

 

I believe you meant ni-mh? Ni-Cd love to be discharged to near 0 volts; it's been a common practice in the R/C industry to store Ni-Cd packs "shorted out".

Ni-mh however, will sustain damage if repeatedly discharged below 0.9V, especially in a series pack format.

If you have Li-ion, even bigger caution: NEVER discharge them completely. Even though almost all li-ion packs are circuitry protected, it's still a good practice to never dis/over charge them. Li-ion packs love to be kept between 30% - 80% state of charge, which not surprisingly, is how most EV li-ion packs are designed to run at.
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