simple nimh charger

I'm making rechargeable juggling toys out of recycled plastics and we need a design for a simple charger for there batteries. because of the design they need to be charged by a plug in there base and they run 3 nimh cells in series. what is the simplest design for a charger?

I am using a voltage divider using resistors at the moment but i understand there is a technique using zenner diodes to stop the charging when the cell internal voltage gets up to the value of the zenner.

any suggestions would be greatly appreciated and very helpful.

Reply to
nicholaslealand
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Reply to
bungalow_steve

This will just slowly cook the batteries. The correct charging current is 1C, or 2.6A for a 2.6Ahr battery. NuMH does not like slow chargings.

It's easier to do it with a smps and a micro. Overcharging is also fatal to NiMH.

Reply to
linnix

In message , dated Thu, 7 Sep 2006, linnix writes

Good job you didn't hit the 'p' instead!

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Reply to
John Woodgate

Hope you don't mind if I ask a related question.

I recently resoldered a NiMH in place of the single cell NiCad in my electric toothbrush. The performance is 100% better. The unit has a cool noncontact charger the toothbrush drops into. I doubt it's sophisticated enough (or able) to measure voltage let alone temperature. Would you guess that the NiMH is at risk of overcharging? If necessary, maybe I'll just make the battery removable and charge it in the Duracell charger.

Thank you.

Reply to
John Doe

They are slow charger, so expect a short life for the battery.

Not a problem if you brush your teeth everyday. I would not leave it on the charger on vacation.

Reply to
linnix

According to Powerstream that's not a problem.

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"The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours. "

Here's a simple C/10 circuit - 3 parts:

-----

+9 ---+---Vin|LM317|Vout---+ | ----- | | Adj [R] [C1] | | | +----------+---> To batt (+) | Gnd --+------------------------> To batt (-)

C1 is .1 uF; R = 1.25/(C/10) where C is the capacity of the cell in Ah. For example, say your cells are rated 1000 mAh - that's

1 Ah. C/10 is .1, so R = 1.25/.1 or 12.5 ohms.

If the computed resistor value is not a standard value, use the next higher standard size. The resistor will need to dissipate P = (C/10)^2*R watts - use one with a higher wattage than P.

Mount the LM317 on a heatsink. The input voltage can be 9 or 12 or something higher, but the higher the voltage, the more heat in the LM317. The LM317 will dissipate about (Vin - 3)*(C/10) worst case with 3 cells.

Powerstream recommends a timer so that you don't leave the cells charging forever.

Ed

Reply to
ehsjr

Here is a different opinion:

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Many battery manufacturers do not recommend long term ( months at a time) trickle charging. If trickle charging is used then the charge rate should be very low or only intermittent. The best smart chargers will only send an occasional pulse charge to the battery once it is charged. They do not apply a continuous low rate of charge. Some battery resellers state that applying a continuous trickle charge of about 1/10th the battery's capacity is not harmful. However, we have not seen any battery manufacturer condone the practice.

That is fine as the trickle charge part. But for fast charging, you would need burn 20W on the Lm317 and 10W on the resistor.

as well as a termination method. They also say that they can't find a good enough termination method based on dV/dt. The best method is based on dT/dt. So, you need to add a thermosistor to a micro with A2D as well.

They recommended tricke charging to bring up a fully discharged battery, then switch to 1C charging and terminate with temperature detection. They are not recommeding trickle charging as the default method.

Reply to
linnix

This is from Energizer's Battery Application Manual for nickel metal hydride (NiMH) batteries.

Page 19: For charging schemes that then rely on a timed "topping' charge to ensure complete charge, a rate of 0.1C appears to balance adequate charge input with minimum adverse effects in overcharge.

Page 20: Because there is no sensing of the cell's transition into overcharge, the charge rate must be kept low (0.1C) to minimize overcharge-related impact on cell performance and life.

Page 21: The intermediate charge normally consists of a 0.1C charge for a timed duration selected based on battery pack configuration.

Page 23: However, to avoid adverse effects on cell life and performance, charging rates must be limited to 0.1C, which constrains time-based charging to those products where overnight return of charge is acceptable.

Page 24: Charge rates must also be reduced at low temperatures. An upper limit of 0.1C is recommended below 15°C.

Hopefully that's not written by the same engineers that designed Energizer's one-hour AA/AAA CH30MN charger that wouldn't charge AAA batteries.

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Reply to
John Doe

Who the hell is talking about leaving the battery in the charger months at a time? Powerstream says, in the part of the quote you snipped " To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours. "

If trickle charging is used then the charge

You want to talk fast charging, post a circuit - don't pervert mine, which is intended for slow charging: C/10.

Helloooooo ... the timer *is* the termination method. Powerstream does *not* recommend a timer "as well as" (meaning in addition to) a termination method.

They also say that they can't find a

Irrelevant. Nothing to do with slow charging. You claimed NiMh cells don't like slow charging. That's just plain baloney. Both Powerstream and Greenbatteries recognize it as a viable method. Neither claims NiMh cells don't like slow charging. Both talk about improper charging being the culprit - however it is arrived at. If you charge improperly, whether it is a fast charge or a slow charge, you can damage batteries.

The best method is

The op did not ask for the "best" method. He asked for the simplest: "what is the simplest design for a charger?" His resistor divider is likely simplest - but as you point out is likely to cook his batteries. A light bulb in series would be a candidate for simplest, but requires a regulated voltage which is not as simple as the LM317, cap and resistor. And those three parts work whether the supply is regulated or not, and don't require a light bulb. You want to add analog to digital conversion, a thermistor, a micro and who knows what else. That takes it far away from simplest.

Your ideas belong under another heading - perhaps "best NiMh charger". And the idea that NiMh cells don't like slow charging - well, it's hard to say where that idea belongs.

Ed

Reply to
ehsjr

I hope we are reading the same document. The full context of this statement is within the sentenses before it.

... fast-charge rates serve to accentuate the slope changes used to trigger both the temperature and voltage-related charge terminations.

(i.e. slow charging would never detect the termination condition.

A charge rate of 1C is recommended for restoring a discharge cell to full capacity.

(i.e. 1C until termination condition reached, then the below for timed charging)

Reply to
linnix

Since the "powerstream" quote was written by me maybe I can clarify a little. There are lots of ways to charge a battery. There are batteries that can only be charged quickly, but NiCads and NiMH are not among them. They can be charged slowly.

There is a kind of no-man's-land that is tough, however. If you are charging fast enough that pressure will build up in the battery when it is full then you need to charge fast enough that the end-of-charge signal can be easlily detected. We consider this to be a charge rate less than 3 hours.

Best regards mark

Reply to
lasf13

Powerstream does not make batteries, so we can't design based on what they think. The Energizer's Battery Application Manual says charge with 1C until termination, then 0.1C for timed topping charge. If our system failed, we can blame Energizer, not Powerstream.

Reply to
linnix

linnix wrote: >

Now that's just a dumb thing to say. None of the semiconductor manufacturers that make charging components make batteries either - so they can't be trusted either?

If Powerstream is making a commercial product, they're having to deal with battery brands, consumers, lawyers, suppliers etc. I suspect that to get that far, they have a pretty good idea how things work and certainly don't deserve to be dismissed out of hand.

GG

Reply to
Glenn Gundlach

OK, show me a link to the chip charging NiMH as stated. If they follow the spec, they can be trusted. Energizer (and others) specs their NiMH batteries for 1C charging. If you don't follow the manufacturer's spec, you are guessing.

Reply to
linnix

A single power resistor is the simplest. You can just touch the battery terminals with some wires and feel the temperature with your finger. As long as you choice a current fo 1C (2 to 3 Amps). You can charge a battery this way in an hour, just don't blame me for burning down your house or office.

And here it is, for less than $20:

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Reply to
linnix

Here is the stuff you snipped, from Energizer's Battery Application Manual for nickel metal hydride (NiMH) batteries.

Page 19: For charging schemes that then rely on a timed "topping' charge to ensure complete charge, a rate of 0.1C appears to balance adequate charge input with minimum adverse effects in overcharge.

Page 20: Because there is no sensing of the cell's transition into overcharge, the charge rate must be kept low (0.1C) to minimize overcharge-related impact on cell performance and life.

Page 21: The intermediate charge normally consists of a 0.1C charge for a timed duration selected based on battery pack configuration.

Page 23: However, to avoid adverse effects on cell life and performance, charging rates must be limited to 0.1C, which constrains time-based charging to those products where overnight return of charge is acceptable.

Page 24: Charge rates must also be reduced at low temperatures. An upper limit of 0.1C is recommended below 15°C.

As lasf13 clearly explained, detecting a termination condition is only necessary when the charge rate is fast enough to build up pressure when charging is complete.

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Reply to
John Doe

The only experiences I've had with NiMHs has been bad.

For one, there's no such thing as a "simple" NiMH charger. For another, NiMHs have a shelf life of days when they're new, and when they get to be about a year or so old, they don't even hold a charge at all, even with a "smart" charger.

I just tossed about $15.00 purchase price (I don't say "worth," because they're essentially worth squat) for two sets of four NiMHs. I still have the charger, but have no idea why I'd want to keep it - probably because it's touted as being able to do NiCds as well. :-)

But my camera explicitly says "Do Not Use NiCds", (I initially bought the NiMHs specifically for the camera - got a second set when I found out how often I had to recharge the worthless things), so I'll be getting some alkalines for it. A couple bucks every few years is worth the cost, vs. the PITA that NiMHs turned out to be.

Design them out of consumer items; stick with NiCds, which will last for years, even if they're a little bit abused, as consumers tend to do. Or even alkalines, which have to be replaced, but seem to last forever.

Good Luck! Rich

Reply to
Rich Grise

In message , dated Mon, 11 Sep 2006, Rich Grise writes

Not possible; they are being withdrawn because of the toxicity of cadmium.

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Reply to
John Woodgate

Isn't cadmium a common plating element?

Reply to
Homer J Simpson

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