2 AA battery questions

Hey!
I have two questions about AA battery's


1) I want to build a circuit that charges 5 AA battery's in series with a zener diode, what voltage should i use and is there a way to make a led turn on when fully charged?

2) When charging these battery's, how much current are they gonna draw and how much resistants does my resistor with the Zener diode needs to have?

Regards Cecemel

1. If the Zener diode is used conventionally (positive supply to the cathode end) you add the Zener voltage to the battery charging voltage.

To make an LED charge indicator you probably want to monitor the voltage across the batteries,compare it against the voltage expected when they are fully charged and light the LED when it is equal or greater. You need a comparator to do this.

2. That depends on the supply voltage and type of battery, consult the manufacturers data for their recommended charge rate.

Please be aware that it isn't good practice to charge batteries in series as their charging characteristics tend to be different from each other, especially as they get older. Unless the batteries are matched, some will overcharge and some may undercharge. Explain why you want to use the Zener diode, it isn't normally done that way.

Brian.

Unless the batteries are matched < 5% state of charge, you can degrade the life expectancy from overcharging unless you monitor full charge on each cell and bypass the current. Parallel charging with a series current limiter is the better approach.

Charge profile and limit current depends on the Supplier specs, which you should not guess. Overheating will degrade life faster.

One approach is to use a metal oxide current limiter or several in parallel to select the charge current from a regulated voltage source. These are called Inrush Current Limiters at Digikey. They regulate at 85'C to keep constant current.

Ideally there are 3 stages. CC, CV then reduce voltage to Float Voltage.

Start with supplier battery recommendations. Get the specs.

AA batteries arte available as carbon-zinc, super heavy duty, and 1.5V Lithium all which are NOT rechargeable.
Rechargeable AA batteries are Ni-Cad and Ni-MH.
Which one are you asking about??

Energizer has a Ni-Cad Applications Manual and a Ni-MH one on their website. The manuals explain how to charge them.

A zener diode is not part of a complicated battery charger IC that does everything correctly.

Audioguru said:

AA batteries arte available as carbon-zinc, super heavy duty, and 1.5V Lithium all which are NOT rechargeable.
Rechargeable AA batteries are Ni-Cad and Ni-MH.
Which one are you asking about??

Energizer has a Ni-Cad Applications Manual and a Ni-MH one on their website. The manuals explain how to charge them.

A zener diode is not part of a complicated battery charger IC that does everything correctly.

Click to expand...

I will tell you what i actually want to do.
I want to make a device that has a USB in and output. I plug the input in a computer/my iPad charger and the aa battery's inside (uniross Ni-MH 2700 ma) start charging up.When fully charged, i take it out of the charger and i can plug any kind of USB device (like my iPad) in and charge it until the battery's are empty.

After checking my other thread about Zener diodes (https://www.edaboard.com/threads/320477/), i understand that it's better to use another method to charge my battery's. Do you now a simple circuit that i can use for this? I would like to have a led to indicate when the battery's are almost empty and one to indicate when they are fully charged. Because i want to charge 5 1,5v battery's (7,5v) and I only have 5v, i think it's the best to use a voltage doubler (5v to about 8,3v) which is, i think, enough to power a charger circuit.

For the output, i'am gonna use a voltage regulator with low dropout voltage (for if the battery's discharge) to bring the voltage down from 6v (5 x 1,2v) to 5,1v.

--------Edit--------
Checked it and the battery's ideal charge rate is 270 ma.

If you apply 7,5v 270ma with a battery charger to 5 aa battery's in series, they are gonna get 1,5v 270ma each right?


Regards Cecemel!

1. you will have difficulty with the current limit on a standard USB port, they are only rated to 500mA maximum and many can't even manage that. 5 x 270ma is more than 500mA!

2. A voltage doubler gives twice the voltage out but at the expense of puting twice the current in. The extra voltage has to come from somewhere! In real circuits which are less than 100% efficient, you have to put MORE than twice the current in to double the voltage.

3. If you apply 1.5V across a 1.5V battery, no current will flow, you need a little extra voltage to push current into the cell.

The only way you will get it to work reliably is to charge the cells individually. Basically, you need five parallel charger circuits, each running from the 5V USB supply and feeding one cell. If you limit the current to 100mA per cell (personally I would use less so it doesn't max the supply limit) it will take about three times longer to reach full charge but all will be equally charged and you don't need complicated voltage multiplier circuits. The cheap Chinese chargers use one diode then resistor in series with each cell and nothing more.

Brian.

betwixt said:

1. you will have difficulty with the current limit on a standard USB port, they are only rated to 500mA maximum and many can't even manage that. 5 x 270ma is more than 500mA!

2. A voltage doubler gives twice the voltage out but at the expense of puting twice the current in. The extra voltage has to come from somewhere! In real circuits which are less than 100% efficient, you have to put MORE than twice the current in to double the voltage.

3. If you apply 1.5V across a 1.5V battery, no current will flow, you need a little extra voltage to push current into the cell.

The only way you will get it to work reliably is to charge the cells individually. Basically, you need five parallel charger circuits, each running from the 5V USB supply and feeding one cell. If you limit the current to 100mA per cell (personally I would use less so it doesn't max the supply limit) it will take about three times longer to reach full charge but all will be equally charged and you don't need complicated voltage multiplier circuits. The cheap Chinese chargers use one diode then resistor in series with each cell and nothing more.

Brian.

Click to expand...

I don't think that i'am gonna use my computer to charge the battery's, i'am gonna use my apple charger for it (5,1v 2a) so after the voltage multiplier (i now the extra voltage has to come from somewhere) i will get about 8,3v and let's say 750mA which is (i think) more than enough to power the charger, not?:wink:

Cecemel

You have Ni-MH battery cells so you should read about how to correctly charge them. The Energizer manual will tell you and you can read about it at www.batteryuniversity.com .

1) They MUST NOT be over-charged. A battery charger IC will detect when they are fully charged then will disconnect the charger.
2) They must not have a trickle-charge unless the charger circuit detects when they are fully charged then it reduces the current to a very low amount.


Today on WagJag there is a power bank available that has a lithium 4400mAh rechargeable battery, a charger circuit that correctly charges its battery from USB and a voltage boost/regulator that provides 5V output.