Specs for charging nicads

It depends on the charger. It probably is one of those cheap almost-constant current chargers. If so, you will have shortened the life of the battery ever so slightly. If you do it hundreds of times, the battery will take noticeable damage.

NiCd is getting hard to find. You could replace them with NiMH, which would give you more capacity. They are more sensitive than to overcharge than NiCd, but they are still pretty tough.

The manufacturer's data sheet for the battery is the final word when determining the best charging strategy. A good charger lets you select charge current and ends the charge automatically when the battery is full.

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RoRo

Thanks for the info.

Andy

Cordless 14.4V tools usually have a simple charger into which you insert the battery pack. Most likely, the only charger that is made for that pack is the one that came with the tool, so you cannot buy a "better" one. Sometimes, you can improve that charger that came with the tool by adding a cuttoff circuit to stop the charging when the battery voltage reaches a predetermined level. That may be worth doing.

If you are really ambitious, you can design and build what you consider a "perfect charger", remove the original circuit, and substitute your own design, using the molded plastic "charging stand" that came with the tool. That's not worth it just for the sake of making the battery pack last longer, but it might be worthwhile for the learning experience.

For your first DIY NiCd charger, I recommend a trickle rate with automatic termination (or charge rate reduction, if not complete charge termination), such as the schematic I posted to you in response to your other post asking about a step down transformer as a battery charger. The trickle rate is C/10, where C is the mAH rating of the pack. The target termination voltage is just a bit less than 1.43 volts per cell. For a 14.4 volt pack, that's 12 cells at 1.43 per cell, or a bit less than 17.16 volts. Note that the charge rate reduction method in the schematic is intended only for a constant current circuit such as is shown in the schematic. With the simple chargers that come with some power tools, a different method is needed - I'd recommend complete charge termination - ie automatically disconnecting the charger from the battery.

Ed

NiCd cells can be found in RC shops. They have a lower internal resistance compared to NiMh cells which makes them more suitable for motors and stuff drawing high current spikes.

NiMh have also a less pronounced DeltaV voltage drop so that some chargers could ignore it and countinue charging the cell. Usually a charger not specifically intended also for NiMh's should be tested before being let charging NiMh cells unattended.

I may do some experiments where I see how long it takes for the battery to get to 17.16 volts.

I would like to establish a max time. Then I might be able to attach my charger to a timer.

I guess I would need an ammeter to check the charging rate.

This link has some interesting info.

Would be interested in your thoughts.

Andy

e

Thanks for the info.

The RC shops also have a gel cell.

I was told that the early ones would explode if charged too long.

Andy

Experimenting is a good idea. The length of time will of course depend upon the state of the battery when you start to charge it. Obviously, it will take longer to charge a battery that is almost completely discharged than one that is only haly way discharged.

Generally speaking, the max time occurs when you use the so called "14 hour" charger. Different chargers can have different rates. For battery packs that tend to get put into a charger and forgoten, which seems to be what you were talking about in your post, a "14 hour" charger is desireable (because it won't kill the NiCd pack quickly), but much less desireable than an automatic charger that terminates or reduces the charge at the proper point.

You can do that. But it does not prevent the scenario of putting a battery on the charger when the battery is only discharged a little bit. Say the battery becomes fully charged after 15 minutes in your charger, but your timer is set for 6 hours. You get

5 hours and 45 minutes of charging that you did not need.

Not if you use the schematic I posted, and select the proper resistor value for the resistor that connects between the LM317 Vout and adj pins (shown as [15R] in the schematic). To determine the value, use the formula R = 1.25/I where I is the charging current you want to use. For example, say you want to charge at 500 mA. R = 1.25/.500 = 2.5 ohms.

If you want to check the charging rate of a different charger, an ammeter may mislead you. The charging rate may not be constant, depending on what type of charger is being used.

He's got the terminal voltage wrong where he says it's 1.5 or 1.6 volts per cell: "Under full charge they require about 1.5V to 1.6V" However, his method of detecting full charge does not depend on that voltage, so that error may be irrelevant to his design. A link to the source code for the PIC program is not posted, so there's no way to comment on that. The schematic shows 4300 ohms connected across the battery, which would draw about 3.3 mA from the 14.4V battery if it was installed in the charger and the power was off. That's not objectionable, unless you consider a situation where the battery sat in an unpowered charger for an extended period. It would kill a 14.4V, 1300 mAH pack in 16 days. I don't know why he didn't stuff a FET in there to protect against that. I don't have the charger, so my remarks are based soley on what is in the article.

Ed

For NiCd and NiMH batteries, voltage is a very unreliable way of detecting full charge. The voltage changes with temperature and also as the battery ages. In addition, different makes and models will have slightly varying voltages, so you'll have to calibrate the charger for each battery.

For lithium-ion and lead-acid, on the other hand, voltage is the proper way of detecting full charge. A current limit will normally also be needed to avoid charging too fast.

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RoRo

I posted a pic of the circuit board for the 14.4 Volt Nicad battery charger.

Andy

t=3Dcharger.jpg

I think you may have missed the context: It's a first DIY NiCd _trickle_ charger. I'll quote what you snipped from:

"For your first DIY NiCd charger, I recommend a trickle rate with automatic termination (or charge rate reduction, if not complete charge termination), such as the schematic I posted to you in response to your other post asking about a step down transformer as a battery charger. The trickle rate is C/10, where C is the mAH rating of the pack. The target termination voltage is just a bit less than 1.43 volts per cell. For a 14.4 volt pack, that's 12 cells at 1.43 per cell, or a bit less than 17.16 volts."

The charger being discussed was a trickle charger, charging at C/10. Powerstream indicates full charge of at least 1.41 per cell at 20 degrees C.

Adding a charge termination circuit that operates at a bit less than

1.43V per cell to the trickle charger to either reduce the charge rate greatly (the specific example given in the prior post was C/80), or to terminate it entirely, is better than leaving the pack charging beyond 1.43V per cell. Thus the recommendation of that charge termination scheme for that charger.

Nope. You set the termination voltage based on the number of cells in the pack, and set the charge rate to the mAH rating of the pack divided by ten. Brand name/model has _nothing_ to do with it. Remember, we're talking about a first time, do it yourself, NiCd trickle charger, not the "perfect charger" I mentioned in the paragraph preceeding the single line you quoted.

Let's get back to the NiCd the OP asked about. If you still maintain that termination (or charge rate reduction) at a bit less than 1.43V per cell is not the way to go for the trickle charger proposed, what alternative termination method do you propose?

Ed

Thanks for everyone's input.

My current charger charges to it's max voltage in about 2.5 hrs.

After 3 hrs, the voltage is still the same.

My battery uses special screws with a protrusion in the center, so I can't take it apart right now.

It may have 12 cells at 1.2 Volts each.

When I have the funds, I will look at building a better charger than the Craftsman.

Andy