I’m a novice, so please bear with my ignorance. I want to build a device that will sense when a rechargeable battery is fully charged and then stop charging it. The charger that I have is the common type small 110 volt transformer which plugs directly to the device to be charged. I have several items in need of such a device , and the batteries range in output voltages from 1.5 to 19 VDC.

Many years ago I built (but not designed) some electronic stuff using tubes, but, although I understand resistors and capacitors, I am not knowledgeable with regard to solid state circuitry.

 

Your better off buying a dedicated charger for the rechargeable battery type, as there are several types of rechargeable batteries.

 

Let me push a bit harder on that. There are several recent types of batteries that are VERY picky about their charging methods. You can't do a universal charger any more. You are going to have to decide whether to limit the types of batteries you wish to maintain and/or build several different charger designs, many of them based on an integrated chip that was designed for THAT battery type.

 

Let me push a bit harder on that. There are several recent types of batteries that are VERY picky about their charging methods. You can't do a universal charger any more. You are going to have to decide whether to limit the types of batteries you wish to maintain and/or build several different charger designs, many of them based on an integrated chip that was designed for THAT battery type.

Maybe I misstated my question. I don't need to build a charger, I only need something that goes between the charger and the device being charged. I'm hoping that that "something" will sense when the battery is fully charged and then stop the charge.

 

Yes that is exactly what chargers do, stop when its full

 

Yes that is exactly what chargers do, stop when its full

I am a little confused. Let me explain why. I have an electric shaver for example whose user manual says that if the charger is allowed to continue after the LED begins to blink, the battery life will be shortened. This admonition also appears in a toy RC helicopter's manual. Please explain.

 

I am a little confused. Let me explain why. I have an electric shaver for example whose user manual says that if the charger is allowed to continue after the LED begins to blink, the battery life will be shortened. This admonition also appears in a toy RC helicopter's manual. Please explain.

What kind of charger are you using ????
Personal I would buy a better charger and use that..

 

I am a little confused. Let me explain why. I have an electric shaver for example whose user manual says that if the charger is allowed to continue after the LED begins to blink, the battery life will be shortened. This admonition also appears in a toy RC helicopter's manual. Please explain.

What this probably means is that the recharger is not intelligent , in the sense that It doesnt cut-off the supply to the battery when it senses that it's full .
It's about cost reduction.
I'm guessing you are using a cheap shaver and a toy Rc helicopter speaks for itself.

 

Maybe I misstated my question. I don't need to build a charger, I only need something that goes between the charger and the device being charged. I'm hoping that that "something" will sense when the battery is fully charged and then stop the charge.

It's not that simple.
The charger has to be digitally programmable and has to be 'told' what battery is being connected and how many cells they consist of .
You can make a charger that charges a specific say 'Nimh 2 cells' but that's not going to be easy either if you are a novice .


Get a good universal charger for any type of chemisty and cell count . You can get decent ones for pretty cheap 30-70 bucks if i'm not mistaken .
Check out hobbyking dot com . They sell everything for Rc models and you will find chargers there

 

Maybe I misstated my question. I don't need to build a charger, I only need something that goes between the charger and the device being charged. I'm hoping that that "something" will sense when the battery is fully charged and then stop the charge.

I think that you can't do it as simply as that

 

I think that you can't do it as simply as that

Is there some way to make something that detects a balance such as when the charging current equals the battery's fully charged state? Could such a thing then discontinue the charging?

 

Is there some way to make something that detects a balance such as when the charging current equals the battery's fully charged state? Could such a thing then discontinue the charging?

Yes, but the devil is in your "battery's fully charged state". That determination depends on different criteria for every battery, it's chemistry (lead vs. ni-cad vs LiIon, etc.), its capacity, even its temperature. So unless you have a smart human to match a charger to the right battery, you need a complex smart charger that can run experiments on the attached battery to attempt to determine how to best charge that battery. That's not so easy.