It's called a battery charger. There's a few NiCd/NiMH charger circuits you can find via google. You can also learn about charging rates that way. You'll find some stuff in the R/C modeler's sites - even articles on discharging. I forget some of it, but if you're under the impression that you have to discharge batteries often to prevent mythical memory effects, you'll shorten their life.
There's some info here, but I doubt you want to deal with the PIC.
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A FAQ:
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This one wasn't as easy as I thought to find on the net, but it might help:
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It has a temperature monitor. The thing I like is that it works like the charger for cordless drill batts which have built in thermistors to signal the charger that it's time to quit. Contrast that with an auto batt charger which supplies a voltage and you know it's charged when the charging current goes to zero or close to it.
If you need help modifying a circuit to meet your needs, we can help.
I think the enclosure for one of these projects will make it more expensive than an off the shelf solution, but I don't know your motivtion for wanting to build.
Does anyone know of a pre-made device or simple circuit that opens a set of contacts when a certain voltage is met? I want to use it mostly to charge and discharge batteries.
Building this would probably be easy, except the tricky part would be the ability to change the trigger voltage. This would need to be programmable, or I could settle with presets for every voltage between 1 and 15 in half volt increments.
But seriously - what is the software, and what's it running on? It seems that'd be the most elegant way to do this - just fix the software, so it can give an indication of some kind, or if it's in a control loop it could do the switching.
Failing that, just a comparator and sonalert will do what you want.
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I'm too lazy to draw an ASCII circuit, but basically it's a voltage divider to one input, a settable reference to the other, a little positive feedback for hysteresis, and a beeper.
look in to using a voltage comparator, a simple common Op-amp for example. use the + input to set the peak voltage you need via a voltage divider or pot, use the - input from the charging cell area's the OP-AMp will drive a NPN transistor to keep relay contacts closed until the voltage on the - input of the op-amp reaches the set point of where the voltage on the + input is. you may want to use a little feed back from the OP-amp output to the
input. this will give you a little window, once turned off the battery level will need to reach below a lower point to turn on the relay once again.. if you need more detailed info on this little scheme, i am sure there are tons here willing to feed you ascii prints all day long. i my self like to give simple examples so that others interested can follow along. for your application, the op-amp of choice is not critical. 741, LM324, TL082 etc...
I doubt you find an off the shelf unit. If you'd have read the links I gave you, you'd have found the Max712/Max713 fast/trickle charge chips. Anything wrong with those? 1 - 16 cells. No discharge feature mentioned, but that could be a separate circuit.
The temperature controlled charger or at least that part of the circuit, is a good idea. It'll shut off if your charging rate causes overheating. For fast charging a comparator with voltage reference would do the job.
You said programmable from 1 - 15 in .5 volt steps. What kind of programming? PIC based? You could use an LM317 regulator and pot for the reference and meter the trip point. Or use a PIC with ADC. PICs also come with comparitors, so there's lots of options here.
All you need is a comparator and a relay. Only you know your application.
BUT. Think about better than half volt resolution. Half a volt at 1V is BIG. Even on a 10-cell pack, half a volt can mean the difference between OK and smoked battery. Then you need to figure out the value to set.
FWIW, I've been plotting charge curves on various kinds of batteries. Depending on age, history, size, type, the peak voltage of the charge curve varies all over the map. The only cells I've had terminate charge on overvoltage were those seriously worn out.
It's much safer and less stressful on the cells if you use some kind of slope termination with a secondary safety based on temperature. mike
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I own a number of battery chargers as well as voltage logging software. Unfortunately the software does not have an alarm or alert.
All chargers/dischargers have shortcomings, like the ability to change parameters like the rate of charge/discharge, discharge depth, etc.. So sometimes I do it manually.
When doing it manually sometimes I forget to watch it. That's why I'm seeking a device to either set off an alarm or kill the process when a certain voltage is met.
If I can't find one manufactured then I'll have to build it instead.
swap the comparator inputs with a up/down switch. mike
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When you're done, re-read my posts and see that that's one of the solutions I mentioned, but I suggested a regulator to set the trip point - might not need that. I figured you knew what a comparator was and that you knew you could control a relay with it.
A voltage comparator and a relay. I like that idea. Off the same regulated power supply that's running the charge or discharge circuit I could make a voltage divider with a rheostat.
But what if the voltage is going up (charge) or down (discharge)? Guess I should read-up on voltage comparators.
What I meant was... Depending on the cell chemistry, terminate when dV/dt is appropriate with safety termination on temperature limit. mike
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I meant that I didn't understand what you meant :)
Oh! *That* slope.
I'd think that when a cell of any chemistry is almost done charging, that a continued constant current charge would stress it and isn't that why it heats up? IIRC the internal resistance of the cell drops as it's voltage increases. I think that is what leads to the temp rise.
Suggest you do some more research. In my experience, a properly charged cell with dV/dt termination does NOT heat much. The only reason for the temperature cutoff is SAFETY when something goes wrong. With -dV/dt cutoff, the most likely combination of factors is 1) low charge current, 2)mismatched initial state of charge so they peak at different times and the total curve of the sum of the series string never goes negative. THEN it gets hot.
Another problem situation is with a poorly designed charger that depends on none of the cells being shorted. mike
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I plan to whenever I get around to that deep cycle charger I'll need. No need to worry about NiCds now.
I'm going to put each separate thought in it's own paragraph so you don't misunderstand me. Read them as if the others weren't there.
Nothing I said was intended to suggest that it would. I opined that a constant current all the way up to the end might heat it up.
I said that IIRC the internal resistance of a cell decreases as it reached full charge.
I said I think that a current through a small valued resistor causes more heat (heat = work = energy = P*T) than the same current through a larger value resistor.
I don't have any charge curves handy AFAIK but I'm guessing (hell, I can almost picture curves I've seen in the past) that when it's almost up to full charge, dV/dt is less than when it started charging.
I take it you mean it's always under constant current charge for the dV/dt in question.
My cordless drill batts have a thermistor inside. I thought that was the *only* thing that stops the charge.
Now I have a minus sign to ponder.
Is that the -dV/dt above? The cell or string starts uncharging?
I'd like to know how to detect and/or deal with that situation. I'd guess that I'd have to determine that the string never reached it's expected voltage at the expected time.
Mr. Ohm and those starting from his equations are all turning over in their graves about now.
I'm not guessing, my battery charger plots the curves every time it charges. For NiCds dV/dt peaks before it levels off then turns negative.
That's a reasonable assumption.
It is. There are many cheap chargers that do just that. They charge the thing until it gets hot enough to turn off. Great way to cook batteries to death. I've rebuilt about a dozen drill packs. Usually, all the cells have vented. FWIW, this technique works reasonably if you fully discharge the pack...but not too fully to reverse a cell...before charging. During the full recharge time, there's enough heat built up to bring the thermal mass up to cutoff without serious damage. Problem is when you put a 70% charged pack on charge. The internal temperature comes up too fast for the external sensor to trip. Cell vents.
A key phrase seems to have been snipped from my earlier post" "In my experience, a properly charged cell..." Temperature only cutoff is not a properly designed charger. Today, you'd use that only if you cared about the lowest possible product sales price, and you made a bundle off replacement batteries.
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
Nope, still charging. There's lots of stuff on the web on proper charging of NiCd cells. Google 0-delta-v or -delta-v or some permutation of spaces and dashes. Or start at the cadex website.
No problem. Make the charge current independent of pack voltage. Charge at the proper current relative to cell capacity. (there's argument over exactly what this is. I usually use C" Terminate on voltage slope rather than voltage. Have three safety cutoffs
1)over temperature
2)over voltage
3)over charge I x T > 1.2 x pack capacity And apply a trickle charge if the pack voltage is below some minimum voltage. Only start fast charge if the pack voltage is within some min/max range and the temperature is within some min/max range.
The only reason my charger needs to know the voltage is so it can properly normalize the graph so every pack looks the same on screen and I can immediately recognize a problem.
mike
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I respect your modesty. Commo's a bitch sometimes.
And yet again my heads up my ass[-]backwards! :( Caffeine, sugar and a long day or two? Day/night - whatever it was. "A V across..." That kills that hypothesis.
Got some returns on dV/dt and dT/dt. It sounds similar to a slight peak at the cutoff of a LP filter. From the recombination. I probably saw that in the curves from the enerhell (sic) quidbook, wherever that is at the moment. Last I checked, the online thing sucked.
"With the NiMH battery the voltage depression is smaller, and harder to detect than with the NiCad battery"
"with dV/dt termination does NOT heat much."
It's up there below my first comment. We'd have lost nothing if it were snipped, I got it the first time. Didn't mean to suggest otherwise.
I read only a bit of what I've stumbled across years ago re: R/C.
The quote is not a typo?
If the day comes that I need to really get into it, I can add that logging into one of my progs. How many uses for a DSP/FIR/IIR with time domain/freq domain and plotting prog?
What is this? :
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seach find: "Type ANSI" (no quotes)
What's this C5maH ? 5*C in maH ?
Hey! I think we did well on commo this time. And with me wired on caffeine.
It's right under the chart: Note that this is the capacity when the battery is discharged over 5 hours time period.
Remember that cell capacities are 3-4X what they were when that was written. mike
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Oh. I read the lines below, duh. At any discharge rate? Is it a permanent loss of capacity?
About that dV/dt thing:
"Another reason memory effect is a myth since all the consumer charger's I've seen actually overcharge until there is a slight voltage drop (due to an increase in resistance from the formation of larger cadmium hydroxide particules that cause contact loss). It's because consumer chargers actually overcharge that you have to give the battery a deep discharge from time to time. It has nothing to do with memory."
Does this mean that they go further into voltage depression than you would allow them using the technique you recommend?
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