Turning an alkaline pack into a 2500 mAH NiMH pack

[jgedde]

Intro:
The following are instructions to modify a White's alkaline battery pack into a 2500 mAH Nickel Metal Hydride (NiMH) battery pack that can be charged in the V3's charger. Go slow, be careful, and bear in mind I hold no responsibility if you make a mistake... I haven't yet received the batteries, so this post takes you as far as modifying the pack and wiring the thermistor. Additional steps and photos will be added when the batteries arrive that show how the thermistor is placed into the pack. For now, I've reached the maximum amount of photos I can attach....

To keep this thread easy to read, please refrain from replying directly to this thread until the final steps are added. This will keep the modification steps contiguous.

For now, just start a new thread if you have questions or comments, or just PM me.

Overview:
To safely charge NiMH batteries and prevent damage to the cells it is necessary to monitor the pack temperature. When the batteries reach full charge, no more energy can go into the cells and they start to heat up. This effect is employed by the White's charger to terminate the charge and enter into trickle charge mode. A third contact and a thermistor needs to be added to the alkaline pack to allow the temperature sensing to terminate the charge.

Unlike Lithium-Ion batteries, nickel metal hydride batteries are not prone to violence when they are overcharged. They simply overheat and possibly vent (they are designed to vent more or less safely - but will damage the pack). Even if they don't vent, charging them at too high a temperature will likely damage the cells internally and prevent them from meeting their voltage/mAH rating. In other words, use the right thermistor and make sure it's mounted in contact with a cell in the pack and everything will be fine.

These steps below illustrate how to modify the alkaline battery pack included with the V3 and turn it into a high capacity NiMH pack that should last longer than the factory NiMH pack which I believe use 1600 mAH cells. The modified pack's nominal output voltage is 9.6V (just like the White's NiMH pack). With a full charge you may see voltages approaching 11.5V. This is normal.

I carefully reverse engineered the pack and charger and selected the proper thermistor using sound electronics engineering practices. I did, however, rush through the modification process and the quality of my work shows this (poor soldering, gluing, drilling, etc) Don't make a hack job out of it like I did.

Materials needed:
1 White's alkaline battery pack
8 Energizer 2500 mAH AA size Nickel Metal Hydride batteries (an eight pack is available for about $20-$22 on-line. Energizer PN NH15BP-8 )
1 Thermistor (Digikey PN 615-1016-ND 80 cents each)
1 New clad dime or a small piece of gold or nickel plated (oxidation resistant) sheet metal.
6-8" of 22 gauge stranded wire
Solder
Insulating tape
3/16" diameter shrink tubing
Epoxy adhesive (suggested: JB Weld)
Foil tape (optional)

Tools needed:
Flat blade screwdriver
Soldering iron
Heat gun or other heat source to shrink tubing.
Drill
9/32 drill bit
tin snips

Procedure (Part one)
Step 1: Open the battery pack as if you were replacing the cells. Then, using a flat bladed screwdriver between the cover and the terminal side of the pack, gently pry up while sliding the cover completely off.
Step 2: To allow the charger to know when to stop charging it is necessary to add a third contact to the pack. Mark the exact center between the two existing contacts and drill a 9/32" hole slightly below the vertical centerline of the pack. Drill very slowly. The drill bit may grab when it penetrates and destroy the pack if you go too fast. Drill it exactly between the + and - contacts, not off center like I did. See photo.
[attachment 130781 holeinpack.jpg]
Step 3: Using tin snips, cut a new clad dime as shown in the photo below.
Step 4: Solder a piece of 22 gauge stranded wire to the dime. See photo.
[attachment 130782 dimewithwire2.jpg]
Step 5: Using epoxy adhesive, glue the dime into the pack so it shows through the hole drilled in step 2. Be generous with the epoxy. It doesn't need to be a mess like my pack. See photo below. Allow to fully cure.
[attachment 130783 dimeglued.jpg]
Step 6: Cut the wire so that it will reach the middle of the 4 cells closest to the terminals. See next photo.
Step 7: Solder the wire from the dime to one side of the thermistor. See next photo.
Step 8: Solder another wire from the opposite end of the thermistor. See photo below.
[attachment 130784 thermistoronendofwires.jpg]
Step 9: Solder the second wire to the (-) pack terminal inside the pack. See photo.
[attachment 130785 packwiring.jpg]
Step 10: Use a piece of shrink tubing to cover the thermistor ands its wire connections. Using a suitable heat source, shrink the tubing. While the tubing is still hot, fold over the end of the tubing away from the thermistor and pinch it closed. Be careful not to burn your fingers - better yet, use a set of needle nose pliers. See photo.
[attachment 130786 thermistorwithshrinktubingcloseup.jpg]

More to follow when I get the batteries....

John

 

[jgedde]

Step 11: Install the batteries into the pack paying close attention to polarity.
Step 12: Dry fit the thermistor as shown in the photo below. The thermistor should be facing towards the cells and end up more or less in the center of the cell body..
[attachment 130890 thermistorplacement.jpg]
Note: It is recommended but not strictly necessary to use adhesive backed foil tape for the next steps - better thermal bonding will be achieved with foil tape.
Step 13: Fold a 1.5" length piece of foil tape lengthwise.
Step 14: Place the fold down between the two cells which with the thermistor will be in contact. Press the tape down tightly.
Step 15: Place the thermistor down into the fold between the two cells.
Step 16: Stick another piece of foil tape over the thermistor and the first layer of foil tape. When complete, the pack should look like the photo below.
[attachment 130891 thermistortaped.jpg]
Note: The wire to the negative terminal was moved as the placement of the wire as shown in the photos in the first post caused interference with the cover. Solder your wire as shown in the photo above.
Note: In order to fit into the charger, it is necessary to narrow the tab on the cover. The photo below illustrates the difference between the White's NiMH pack and the alkaline pack.
[attachment 130892 tabs.jpg]
Step 17: Using a file, narrow the tab on the alkaline pack so it is the same width as that on the NiMH pack. When complete, it should look like the photo below.
[attachment 130893 modofiedtab.jpg]
Step 18: Install the cover.
Step 19: Insert the pack into the charger. The red light should turn on. If the red light doesn't turn on, check cell polarities, thermistor wiring, contact hole placement, and whether you narrowed the tab enough.
Step 20: This step is important: don't skip it. Monitor the charging of the pack the first time to verify that the red light goes out after a time and the pack doesn't get ridiculously hot. If it does, take it apart and look for errors. At room temperature, cell temperature will reach about 45-50 degrees C (113 to 122 degrees F) before the charger will cease charging. If you have any doubts about how this feels in a finger test, charge the White's NiMH pack and feel the pack when the red light first goes out. The modified pack should get no warmer than that.

That's it! I now open the thread to questions and comments... Good luck and happy detecting!

Cheers,
John

 

[jgedde]

I ordered 5 thermistors from Digikey. One was used for my pack, one I'm setting aside for teckpro, so I have 3 available left over. They ended up being about $2 each with shipping and tax. PM me if you want one. $2 and a SASE will get you one.

John

 

[jgedde]

I found that the heat from the charger caused the charge to terminate earlier than was intended leaving the battery at about 10.5V after charging. A small design change was in order. Basically I moved the thermistor to the side of the pack away from the terminals as in the photo below. The thermistor is underneath the foil tape on the left. The foil on the right just holds down the wires.

This fixed the issue so that the pack reaches about 11.2V at end of charge.

[attachment 132064 IMG_0384.jpg]

John

 

[robertk]

I performed this modification last night, and it works great! Charges right up in the standard charger, shuts off right at 11V. And now when the time comes, the cells can be easily replaced.

THANK YOU for this writeup!!

 

[captainron]

Thanks for passing this along, John.

Captain Ron

 

[jgedde]

Glad you like the mod! You'll be pleasantly surpized how long those 2500 mAH cells run. They keep going, and going, and going... Well, you know the rest.....

I performed this modification last night, and it works great! Charges right up in the standard charger, shuts off right at 11V. And now when the time comes, the cells can be easily replaced.

THANK YOU for this writeup!!

 

[teckpro]

I made one , I soldered My cells together, mostly because the were older used cells. Works fine in charging craddle.

Thanks John.

 

[megadivers]

If anyone needs a thermistor, i have several extras that I can also do for the $2.00 sase. I ended up with more than I needed.
Just PM me.
Thx

 

[slumpshooter]

Do you have any more Thermistors? Will buy one or two if you still have them. I got the part number, but what is the value? Good post on how to... Look forward to hearing from you... Thanks!
And I did

 

[jbow]

Alan,

I need one of those too if you still have them. I keep meaning to order one but why not get one from you if you still have them. BTW, Roy found a fresh site in Kennesaw. If you want the info let me know. The bush hogged and removed the trees Thursday and Friday.

PM me if you want.

Julien

 

[mgtmadness]

You do not need the thremistors if you use a better battery charger.

Also, What I have read someplace is that the Diode thing that whites has in there battery pack is to help prevent a reverse voltage spike into white's charger.
http://www.kpsec.freeuk.com/components/diode.htm

We used to use Diodes on our Rc car/truck motors to prevent revere motor voltage spikes and to help with the current flow.

This is an excellant idea you have though.
What is the current temperature that it shuts off at?
You can buy rc car chargers that have a temp sensor with it that you can monitor the temp of the battery pack and have it shut of at any temp you feel safe to setting it at. You also can set it so it shuts off at a certain Voltage.
I think most whites machines are maxed at 13.5 voltage and have a voltage regulator inside the machine to prevent more than 13.5 volts going into the machine.
You can also get a cheap lazer rc car thermometer to check battery temp at finished charge.

 

[mgtmadness]

The V3 charger is nice though, just drop it in. If I get a V3 i think ill hook that battery cradle to my battery charger.

 

[Rob (IL)]

[attachment 154026 2010-02-27_180152.jpg]

The thermistor sets the temperature of the batteries at which they quit charging. Changing the position of the thermistor relative to the pack will affect the maximum voltage. Rob

 

[slumpshooter]

This was very helpful Rob, thanks !
Part is on order, so should have a new pack to charge-up this time next week.
Thanks to all for the info...

 

[chris poteet]

Thanks for the great info. I built two packs they charge up around 11.44 volts. It saves alot of batteries from being thrown away.go green!

 

[slumpshooter]

I built one using info on this string, charged to 11.47 v my original NiMH pack charges to 11.64 v. A good way to save cost over a season of swinging.

 

[chris poteet]

I was out at the ballfeild swinging today,using one of the packs i modified ,got a good dime vdi went into pinpoint and my v3 made a load deep sound then shut off. just prior to dime reading i checked the battery had 10.44 volts after shut off i turn my v3 back on it was showing 4.2 volts any body got any ideas about this.

 

[slumpshooter]

It sounds like you could have had a cell (or two) go bad, for the voltage to drop that fast. I have been using my modified pack for several hours now after its first charge and all still seems to be working okay. I am not sure if NiMH cells reverse like NiCADS could do. This would happen when all the cell where not matched in regard to mAH rating or capacity. The lower rated cell would reverse current when it was exhausted, which caused a very quick voltage drop like you have described. It could be you just have a bum battery in the pack. You could use a VOM to check each of the cells voltage, you should be able to find the bad cell, (noticeably lower surface voltage) from the others.

 

[Rigit]

Excellent instrunctions many thanks for your hard work


Rigit