Designing an comparator from op-amps like lm386 or ua741

[Michael Weaser]

Your charger voltage will automatically drop to battery voltage which is too low. Therefore you need to draw from your main power source, and connect that where my schematic says 3V. My schematic only shows the concept of operation. I don't know the voltage of your power source. You need to ensure that volt levels are safe for components. For instance, adjust the value of the safety resistor so the led does not get overmuch current.

i am using a 9 volt power supply and not the 1.5v battery as the main voltage?, i wasn't using the pulsating voltage as the main power for the 1.5 volt detection system, or a 1.5 volt battery. So after the main power supply goes through the 555 chip to make the 5v pulsating signal i need, that 5 volts will drop to the voltage to the battery or what do you mean?

 

[Audioguru]

I think if you pulse current into an alkaline battery cell that is partially discharged to 1.2V then the pulses will be maybe 1.7V high if they have low current and might be 2V or more if the pulses have high current.
Maybe the pulse voltage decreases and becomes closer to 1.5V or 1.6V as the battery gets "charged". Since a non-rechargeable battery is not supposed to be re-charged anyway then how do you know then it is done? When it gets hot? Leaks? Explodes?

A 555 with a 9V supply has a peak output voltage of about 6.5V if you limit the output current to 200mA. If you limit the current with a resistor then it will have a voltage drop. 6.5V - 2V= 4.5V. Then if the battery limits its charging voltage to 2V and the current of the pulses is 200mA then the resistor value is 4.5V/200mA= 22.5 ohms but the 2V is just a guess.

 

[BradtheRad]

i am using a 9 volt power supply and not the 1.5v battery as the main voltage?, i wasn't using the pulsating voltage as the main power for the 1.5 volt detection system, or a 1.5 volt battery. So after the main power supply goes through the 555 chip to make the 5v pulsating signal i need, that 5 volts will drop to the voltage to the battery or what do you mean?

The transistor can turn on the led even if the supply is a different 9V supply from the supply that charges the battery. They are all referenced to ground, as long as you hook up all their ground wires.

In regard to turning on the led, AudioGuru's post #22 is enlightening, because it may be sufficient merely to connect the led across the battery (and add a safety resistor). The pulses of current will cause battery voltage to rise momentarily, to a high enough voltage to light the led.

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In regard to charging alkaline batteries, I echo what AudioGuru says. I have given up the effort. Years ago I tried it with new ones which got depleted to 1.3V or so. I had some success. I charged them at a very low rate. They returned to 1.5V. I said to myself 'You beat the system.'

However over the years I had a few cases with older cells, where they leak a clear fluid. It's stuff that could be corrosive to humans and equipment.

So I purchased several nicads and nimh cells, mail order, cheap.

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I remember the Renewal alkalines came on the market 20 years ago. I tried a pack. I didn't buy the expensive charger specially made for them. I charged them myself, at a very low rate. At that time I was inexperienced with rechargeables, so my methods might have been unhealthy. The Renewal type started holding a charge only a short time, as I recall.

 

[betwixt]

Before going further - a warning that some batteries can explode if you do that. They can liberate hydrogen but unlike a rechargeable cell, they may not be vented to allow it to escape.

You are right that pulsing is the way to do it but there is more to it than simply turning the charger on and off repeatedly. What you need to assess is how much charge the battery holds, not what it's terminal voltage is.

To understand the principle, you first have to look at what happens when a cell discharges. The voltage it produces does not necessarily drop below nominial, at least until it is almost 'empty'. What happens is it's ability to maintain terminal voltage while under load is reduced. For example, a new 1.5V cell will measure 1.5V with no load and maybe 1.4V under heavy load. The same battery after some use may still measure 1.5V with no load but only 1V under the same heavy load. It follows that simply measuring the voltage doesn't tell you how good the battery is. We visualize a battery as a perfect (never changing) voltage source with a resistor in series with it. The resistor is not a physical component, it's actually a reluctance for the battery chemicals to react efficienctly, but for sake of calculation it behaves like a real one. Like a resistor, it drops a voltage when current flows though it and in the battery model we see it as a low value resistor (low voltage drop for a given load) which gradually increases (more voltage drop) as the battery wears out. We call it 'internal resistance' or 'equivalent series resistance' (ESR).

To test how well the battery has charged, instead of measuring the voltage, you need to measure the ESR. It is relatively easy to do because it causes a predictable drop in battery voltage when a load is placed across it. So the pulse charge system isn't just cyclically turning the charge on and off, it's checking how much the voltage drops when a load is applied in the off period. For a given load, the less the voltage drops, the better the battery is at producing power.

Brian.

 

[Audioguru]

Exactly, Betwixt.
A smoke detector is hardly ever used and has a 9V alkaline battery. It detects "low battery voltage" during the flashing of its LED that loads the battery during the voltage measurement. The flashing LED also shows you that the battery is still good. But after one year the battery is supposed to be replaced because it might not have enough voltage when loaded by its screaming sounder. Take the "dead' battery from a smoke detector and measure its voltage with a high input resistance meter. It will still show close to 9V but when loaded it falls flat on its face.

NOBODY should charge the 9V alkaline primary battery in a smoke detector because the charging is poor and does not last long. In fact, NOBODY should ever charge an alkaline primary battery.

 

[D.A.(Tony)Stewart]

NOBODY should charge the 9V alkaline primary battery in a smoke detector because the charging is poor and does not last long. In fact, NOBODY should ever charge an alkaline primary battery.

I take exception but agree if you do not understand what you are doing. Anything that generates excess heat or sealed flammable gasses in a battery can also create a very unsafe condition. Be aware of how it works.

A "spent" primary battery effectively has very little mAh remaining which translates the a significant drop in time constant T=RC with the chemical voltage threshold ( which does not change unless the chemistry changes drastically), electrolytic capacitance ( which drops as much as 90%) and effective series resistance which rises as much as 100x .

That being said there are some types of batteries which do not degrade as severely and a a slow float charge can and does restore about 10% or more "capacity" in mAh from this process.
This is a measureable parameter which I have done on occasions in the past and verified to my satisfaction.

However if you want a reliable source for security alarms for the home, using a recharged spent battery for long periods in a fire alarm is a bad choice, although I have done it on occasions until I had time to buy a Lithium 9.0V cells which cost and last about 5x longer.

 

[Michael Weaser]

what voltage should you be charging standard alkaline batteries? i know that the voltage needs to be a little bit more than the battery voltage, so if the battery is 1.5 volts what should be the voltage charging it? what is the minimum and maximum voltage that the battery will give a charge without blowing it up?

 

[Audioguru]

The cover of my Energizer AA alkaline battery cells says, "Warning, do not install backwards, charge or put in fire - may explode or leak".
For rechargeable batteries I use Nickel-Metal Halide (Ni-MH) and Lithium-Polymer (Li-Po).

 

[betwixt]

And don't charge at a voltage, charge at a current. If you really are sure you want to do this, set a voltage limit of say 10% above nominal battery voltage but limit the current to about 10% of it's AH rating. Those are not offical figures (there are none) but should be safe to try. It means you set a ceiling voltage for safety but if the battery tries to draw too much current it will pull the voltage from the charger down to a safe level. As the battery charges, the supplied voltage will rise with it up to the 10% overvoltage limit.

So the pulse charger works like this:
1. Current limit at 10% of AH rating and voltage limit at 10% above nominal voltage.
2. Connect charger to battery for pulse period (~ 2 seconds?)
3. Disconnect charger.
4. Measure Voltage (call it V1)
5. Apply load of about 10% of AH rating.
6. Measure Voltage (call it V2)
7. Remove the load.
8. If V1-V2 is small enough, the battery is 'renewed', otherwise go back to step 2.

Brian.

 

[Michael Weaser]

i am just trying to connect different circuits together, like for i have the main charging circuit which is the pulsating voltage, than i have a 1.5 volt detector circuit i want to use that turns on an led that i might use or i could use an comparator, i just need help putting the circuitry together, so i need away to attach a voltage detection system for my alkaline battery charger, so it can turn the charger off and tell me when the alkaline battery has been renewed

 

[BradtheRad]

what is the minimum and maximum voltage that the battery will give a charge without blowing it up?

This is an area where we would need to know a lot more about battery chemistry. And also do a great deal of testing.

Battery charge rate has to do with how quickly, or how easily, the internal electrodes and electrolyte can absorb current, and turn it into stored electrical energy.

An alkaline can 'sort of' do this, but there are many variables. I have never heard of a method to find out its internal condition of the electrodes and electrolyte, during a charge.

What is a safe charge rate? Probably not greater than C/20, since alkalines are not designed to be recharged.

How hot should we let it get? When the battery starts warming up, it implies that it is done converting electricity to electro-chemical potential. Personally I would be nervous about letting an alkaline's temperature rise 5 degrees. You never know what the excess charge will cause inside the cell.

What is the highest and lowest safe volt level? Alkalines start out at 1.6V new. Typical 'empty' voltage is 1 V.

I guess what all this means is that your method might be as good as any. Few people would like to do all the testing required to produce meaningful figures. Manufacturers state that we should not recharge alkalines. In view of this they are not likely to publish minimum and maximum figures for the best method to do so.

 

[Michael Weaser]

i think i finally found a circuit that might work for im im using , i found a circuit that turns off the battery when it gets to around 1.5 volts,
https://i.stack.imgur.com/oN4mg.png

if i use the pulsing signal as the main voltage source will it still work?

 

[betwixt]

Yes, but why bother. The pulsing principle is to test how much the battery has 'absorbed' by seeing what it does under load. If you are just using a constant voltage charger there is no point in pulsing it. All it will do is slow the process down. Consider what that circuit will do if the battery is removed, what voltage will you measure across the output?

Brian.

 

[D.A.(Tony)Stewart]

what voltage should you be charging standard alkaline batteries? i know that the voltage needs to be a little bit more than the battery voltage, so if the battery is 1.5 volts what should be the voltage charging it? what is the minimum and maximum voltage that the battery will give a charge without blowing it up?

I don't have any handy precise standard for primary alkaline.

But the safety depends on the absence of generation of combustible gas and pressure buildup inside.
If the leakage current is low at full charge, then self-heating is prevented and thus more gas generation. Also the seal can rupture causing chemical leakage, which can occur over time when drained or rapidly when over charged.

If leakage current is low and you can monitor this, then it is fully charged at 1.5V ~1.6V for max voltage safely.
You can also measure ESR by a pulse load with a circuit design to measure it, or waste more power using the thermal resistance colour film sometimes provided with certain brands. The resistance film heats up the film and changes its colour with more heat meaning better low ESR.

A better way is a pulsed load with sample and hold on the current at 1V similar to a car cold crank test for CCA at 7.2V on a 12V lead acid at ~2/3 Voc.
In the old days we just used a 1A analog current meter to pulse a short circuit and see if it pegged the needle or only drifted to 100mA or something in between for a rough gauge on ESR.

Used batteries recharged again cannot be expected to have more than 20% of original capacity and more like 10~15% depending on quality.
Panasonic batteries are among the best.

Unlike Supercaps, the efficiency of charge ratio out/in or discharge /charge in terms of mAh is said to be poor for most secondary batteries unless you use a very low charge current rate like C20 ( 20 hr) I use numbers like 50% for lead acid and 80% for LiPo but the variance from quality is huge.

Obviously Primary batteries can be expected to be worse for charge ratio effiicency as the chemistry is not designed for re-use but rather high density single use only.
But unlike Supercaps the minimal SOC voltage is still useful at 2/3 of the 100% SoC voltage. Whereas a "supercap" has to be drained to 0V to get all of its capacity out which is well below the useful voltage range so it's capacity is limited by the practical voltage range and how we measure energy storage density is sometimes misleading.

 

[nitin96]

u can stop using ur 555 kind thing use a MUC that support analog systems. all u need to do is to charge and monitor the voltage simultaneously. Now you need to code ur MCU such that it use a single pin as both input and output. All u need to do is for certain number of clocks gives pulses on that pin which charges ur battery and (say 10) and then use that to check the voltage on that pin compare that with the required value (1.5 V) and display it on a digital pin using led.

 

[betwixt]

That method won't work Nitin96. If you charge at 1.5V (or even a little higher) and limit the current to a level that won't cause pyrotechnics, then remove the cell from the charger, your reading will still say it's fully charged, even though it isn't physically there any longer!

The best chance is to pulse charge but as we keep saying, between the pulses, apply a load and note the voltage drop. The aim is lowest ESR, not highest voltage. Plenty of dead cells will still measure 1.5V while in a charger but not work at all when removed from it. I've even got a dead 12V sealed lead acid battery beside me that measures 12V across it's terminals but can barely light an LED and draws about 1mA from a charger.

I gave the best, although not recommended, method back in post #29. ESR = (voltage when pulse removed - voltage after load applied) / load current. You need to aim for lowest ESR = least voltage drop under load, not any particular voltage.

Brian.