Battery Charger

[Screech]

I bought a battery charge to charge up AA batteries for my digtal camera.


It charges 2 banks of 2 AA batteries. (4 batteries).

Each bank seams to be outputing 5.8 volts and 300 milliamps when I replace the batteries with my multimeter.

or,

It can charge 2 banks of 9 volt batteries.
Problem is when I checked the output voltage of the charger with my mutli-meter,(no batteries connected) it reads 5.8 volts. It also gave out 80 milliamps of current :!:

the specs on the back of the charger says
2 x(2.8Vdc 170-190 mA) - size AA
2 x(9.0Vdc 14-17 mA) - size 9V

Whats going on, especially with the 9volt part?
Is this charger faulty?
Thanks

 

[audioguru]

Hi Screech,
You should measure the voltage and current of the charger when it is doing its job. It probably has a voltage that is higher than necessary and a resistor to limit the charging current. When you replace the battery with a current measuring meter you are shorting the charger and of course the current will be higher than if it was charging a battery that has voltage across it.
Maybe the voltage reads low without a 9V battery because the charger outputs 12V pulses that your meter reads as the 5.8V average. Again, the 12V puses probably feed through a current limiting resistor and of course the current will measure too high when it is shorted by your current measuring meter.

 

[Screech]

Thanks Audiguru.
I don't have a 9 volt rechargable battery to test with.
but I did connect an led and 330 ohm resistor to the 9 volt terminals and connected my multimeter to the terminals too, which then read less than 5 volts.

I dont think my charger sends pulses because it's one of those cheap Dumb ones. But I'm only guessing.
It has an inbuilt 15 hour timer, then switches off.
It's the Energiser brand that came with two 2300mA AA size NiMH batteries.

 

[theimperia]

In my opinion, if it's cheap and dumb, it DOES output 12V pulses, since it's much easier and cheaper, than putting a big transformer inside to provide a constant DC current. They'd rather pulse it, to avoid high currents.

Just an opinion though

 

[Agent 009]

i think i have a similar charger, Model CHM4AA, but i thought it "senses" if the batteries r charged up and turns off, not a timer of 15hrs... And i think the timer just works with Ni-MH cells, not NiCd... why?

 

[kitedude]

chargers are generally current limited, as the multi meter is drawn no load you will, read a higher voltage.

when you are drawn a current the voltage will drop to provide a constant current

 

[audioguru]

Energizer's website has lots of chargers. They are all fairly heavy so probably have a transformer. A few are "smart" and charge very quickly. The "dumb" ones just use a timer and probably don't know or care if the battery to be charged for 15hrs is already charged.
The dumb ones certainly won't know if the cells are Ni-Cad or Ni-MH, a Ni-Cad is overcharged a little and a Ni-MH is undercharged a little.
Most of the chargers have "charge control" which I think is just a different charging current determined by whichever contact is reached by the cell size.

 

[adamthole]

I am working on a project that requires me to solder 4 AAA rechargable NIMH batteries together in series. I would also like to build my own charger, because buying one and soldering the connections in series would look terrible (it's for a buisness).

I am curious as to whether you have any tips on soldering batteries without solder tabs together, and more importantly a charger for this circuit. I did some looking on google and all of the smart charger ciruits I saw were fairly complicated, as I have no PIC experience (yet), so I think I will just go the generic way...unless someone has an easy schematic? Or an easier schematic, I plan to learn PIC soon anyway... So either a simpler smart schematic, or info (or schematic) on how to do it the cheap way would be helpful.

Thanks!

 

[audioguru]

Hi Adam,
You are probably going to ruin the battery cells by soldering them. They have low-melting temperature insulation inside. You can purchase battery packs at hobby stores that are joined by the manufacturer with flash-welding, not soldering. My Cobra 1W walkie-talkies (FRS?) have 3 AAA rechargeable cells welded together.

Maxim-IC have some "smart" charger IC's that work well in a simple circuit. Battery manufacturers recommend redundant timer and temperature sensor circuits to halt the charging if the "smart" circuit goofs, to avoid an explosion.

 

[Roboticinfo]

reply

Well in my opinion it is not a good Idea to make your own charger unless it is for a seald-lead-acid battery but that is my opinion.

 

[adamthole]

I chose 4AAA cells because I am using them for a reciever that has a max of 5.5 V and a min of 4.5 I figured 4.8 was good enough. I just need a battery that is in those voltage ranges and is approximately the size of 4 AAA batteries arranged as a parallelogram. I've searched on google and haven't had any luck looking for anything that fits my description, besides 4 AAA's, and I haven't found any AAA's with solder tabs.

Anyone have any suggestions on where to look, or ways to join the AAA's together that will not ruin them?

 

[audioguru]

Energizer's 9V Ni-MH battery (they don't make Ni-Cad's anymore) has 6 AAAA cells inside, probably joined with flash-welded straps. Maybe you can use them.

 

[Agent 009]

Joining many Cells

Y they join many smaller cells to make big ones, instead of just making a big one?

 

[Nigel Goodwin]

Re: Joining many Cells

Y they join many smaller cells to make big ones, instead of just making a big one?

Because a single cell is only 1.2V - so making a bigger one only gives you more current capacity, but still only 1.2V.

You need to use multiple cells in series in order to increase the voltage, all batteries work in this way - including car batteries, which use 6 cells in series to give 12V (2V per cell).

 

[audioguru]

Remember 90V carbon-zinc batteries? Lots of cells!
Long ago, I potted one in clear plastic with a neon bulb chaser circuit and it ran for about 6 months before it blew-up! I forgot to allow the battery to vent its gasses. He, he. :lol:

 

[Nigel Goodwin]

Remember 90V carbon-zinc batteries? Lots of cells!
Long ago, I potted one in clear plastic with a neon bulb chaser circuit and it ran for about 6 months before it blew-up! I forgot to allow the battery to vent its gasses. He, he. :lol:

I remember them well, although they were really a little before my time, although I did used to get an occasional battery valve radio for repair.