battery recharge ???

Discussion in 'General Electronics Chat' started by Mathematics!, Jan 3, 2010.

1. Mathematics! Thread Starter Senior Member

Jul 21, 2008
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Ok , I can pretty much make circuits to recharge a rechargeable battery but I want to beable to make a rechargeable circuit for batteries which indicates when the battery is 50% , 75% , 100% charged.... (using different color LED's)

Right now I have to mathematically approximate the time it takes for a battery to be charged... I do this by the mAH and the rate at which the circuit is charging the battery ... this is only an approximation and usually it is not linearly charge so I let it go for any extra hour or so to be sure... (their must be a better way of getting more exact since what happens if I couldn't find out the mAH ratings then what ?)

I would like a way on how to determine when it is fully recharged and when it is 50 , 75 , ...etc % charged...

Don't know how to do it with a circuit but first I would just like to understand how I would determine this mathematically before think on how to rig up a circuit for it (don't want to copy a schematic anybody can do this)....

Question when a battery is fully charged will current stop flowing in my circuit ... or could it possible keep charging until it explodes the battery ....
I am using a 9volt to charge a 9volt rechargeable battery so I am assuming once the 9volt battery is fully charged the KVL would imply the voltages would cancel each other out and no current would flow because no net work is being done at this point....?

Input would be great since I am just learning about rechargeing batteries
I am alittle shaky on using a source that is alittle higher voltage then the thing being charged since I would think the KVL would imply if I used a 10volt source to charge a 9volt battery we would have an extra volt being drop somewhere else ..... Maybe somewhere inside the battery potential damaging it ....duno...

Thanks again

Last edited: Jan 3, 2010
2. jpanhalt AAC Fanatic!

Jan 18, 2008
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912
I hope you have reviewed basics on battery chemistries and charging. If not, here are two good sources:

http://www.batteryuniversity.com/partone.htm
http://www.batteryuniversity.com/parttwo.htm
http://www.powerstream.com/BatteryFAQ.html

For most purposes, I have found Battery University to be the better of the two.

You will see that there are different methods to detect state of charge depending on the battery type. That is particularly important with fast charging and/or with certain chemistries (e.g., LiPo). Doing an integration of time and current; however, will get you in the ball park for NiCd and NiMH (and probably others), IF you know your starting point and you are not converting a lot of the charging energy into heat. Some older NiCd and NiMH chargers did just that. Even current ones that use other methods to determine the endpoint will give you a read out of the "capacity" based on that integral.

John

3. Audioguru AAC Fanatic!

Dec 20, 2007
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I have two old "9V" Ni-Cad batteries. They are actually only 7.2V because they have six cells. They have printed on them: "Charge at 10mA for 14 hours".

I have two new "9V" Ni-MH batteries. They are actually 8.4V because they have seven cells. They are charged at 15mA for 14 hours.

Each cell reaches 1.4V to 1.5V when fully charged so my Ni-Cad batteries are 8.4V to 9.0V and my Ni-MH batteries are 9.8V to 10.5V when fully charged but are over-charging.

At these low charging rates, Ni-Cad batteries get a little cooler when charging and Ni-MH batteries get a little warmer. They both get warmer when over-charging.

4. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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Ok , so I guess all rechargeable batteries have a chargeing current and how many hours to charge listed on them somewhere....

So are all rechargeable battery circuits specific to one type of battery...

Like it would be pretty easy to create a circuit that delievers 10mA for 14hours and then after that swtiches off and turns on a LED indicating it is done charging....

Also what voltage should I charge a specific rechargable battery at ?
Do they have that listed somewhere or is it just important to have it 10mA , for 14hours regardless of how much power your using?

Forgetting for a moment indicating the %'s I am just interested in indicating when it is finished with a LED....
Though I am curious how they make the battery checkers where it indicates bad , good , very good ,...etc this would have to be based on the percentages....

Also I got 2 6volt 50mA solar cells and I was wondering if I could create a simple circuit to recharge a 9 volt , 500mA rechargeable battery,... or at least store enough energy in capcitors to power the camera through the night because the battery only lasts for 3 to 5 hours

Curious if I can't chrage the battery efficiently then would it be possible to run the camera on the stored solar charge thru out the day basically this is an infrared camera (night vision) that would only be used when the sun is not present so in the day when it is sunny the camera circuit would just be in a storing state... wondering if this is possible to do to get me thru the night with camera not dieing out?

Last edited: Jan 3, 2010
5. Audioguru AAC Fanatic!

Dec 20, 2007
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A timer controlling a battery charger is a stupid method.
The battery will be extremely over-charged if it is charged by the timer when it is not completely dead.

6. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
So I guess the question is how do you know when a battery is fully charged?

I can easily make a circuit to supply 10mA or whatever current I want at what ever voltage I want but I don't know how to indictate when it is done charging and stop overcharging it?

And still curious on how much voltage I should use, their is no rating I can see on the rechargeable battery to charge at... it only say's 10mA for 14hours or so... (basically I would need a power rate to charge it at or voltage rate also)

Also still looking for away that they figure out a bad , good , great battery on those battery testers ...
I would think this would led into resolving when to stop charging the battery....

Thanks

7. Audioguru AAC Fanatic!

Dec 20, 2007
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A battery charger IC is smart. It knows that a battery is not dead so it charges it only long enough to make it fully charged without over-charging. It detects when the battery is fully charged.

A battery charger IC will also shut down if it detects a shorted battery. A stupid circuit will set your house on fire.

8. spinnaker AAC Fanatic!

Oct 29, 2009
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And apparently there are also Fuel Gauge chips.

Mathematics!,

I would be very interested in what you come up with as I will be working on a similar project.

9. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok , do you have any battery charger IC in mind?
And the specs on the IC they will probably have the schematic of the circuit of the IC and if not to complicated I will just build the equivalent....

I did built the LM386 from scratch onces by using the spec's
However I get the feeling this IC will have to much components or components you can't make with just resistors , caps , transistors ,...etc

Curious though

Also will this IC work for recharging different types of rechargeable batteries or are these IC your talking about only good for a specific battery....

I still thing their should be a simple circuit or some circuit you could build without IC to recharge a battery an not overcharge it.....

Also I have converted 120 VAC down into 12.6vdc pulsating dc awhile ago and I was wondering if I could charge my car battery with it.... I would think yes but my question is now how will I know when it is done charging and not overcharge/explode it....? <-- this question gets back to the main question

Thanks for any help understanding this
Is their any mathematical way of looking at a rechargeable battery and based on some varibles of the battery calculating and approximate time it will take to charge it at a specific current ?

10. spinnaker AAC Fanatic!

Oct 29, 2009
5,671
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I'm just a noob so maybe I am just naive but they do not appear to be overly complicated to configure.

The problem is knowing HOW to configure them for your needs. I know there are several projects on the internet that use a variety of these chips for charging batteries. You might want to do soem searching.

11. Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
OK , so I guess what I want to know is if their is any battery charging circuit that can be made using only cap , resistors , transistors , inductors , diodes ,...etc to charge a battery fully based on some math and knowing the amount of current/voltage supplied and time...etc

I.E no IC allowed ....

Also even if I used an IC what mathematics are they using to sense that the battery is already fully charged ....I mean the IC has to mathematically know when to stop chargeing in some way based on something?

Thanks for any help

Nov 23, 2009
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