Backup battery and charging for circuit.

[MagSMG]

Hi guys, I see a few posts here regarding this - but don't understand them too well.
I need a backup battery in my circuit and also my mains power needs to be stepped down to 5V from 12V.
The battery needs to charge when necessary.



I have created this schematic for what I need. Please tell me if it could work or what I am missing.


Thanks a lot for your time!

 

[BradtheRad]

To charge 4 nimh cells you'll need to feed them more than 5V. Because at 1.25 V each they're only half charged.

It's important to stop charging NIMH batteries after they reach 'full' voltage.

A suitable maintenance level is 1.38 V each. That's what my nimh cells read a day after I charge them.

So I'd use a charging source of 5.6 V. This should be a safe level to both charge and maintain them. You want the slightest trickle charge since nimh lose their charge a bit quicker than other rechargeables.

There may be a way to trick the regulator into producing 5.6 V.
Or else you'll want an adjustable regulator.

And although your transistor configuration needs revising... You can omit the transistor if the batteries will never draw more about 1/2 amp from the regulator. That is if you never discharge the batteries low enough to draw a high charging current.

As a precaution it would be wise to put a 10 ohm resistor in place of the transistor.

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If you absolutely need a 5V supply, consider using a diode to drop .6 V from the 4 cells.

 

[andre_luis]

I thing that´s not so simple, due you must manage current flow during status charge.

+++

 

[MagSMG]

Ok, thanks for that info!

This device will be installed in a car and only needs the battery when the car battery is unplugged or flat. So basically this NiMH battery-set will constantly be on standby/charge for when ready for use.

You said I need more than 5V, can I just charge them with the 12V supply?

Also, regarding the

manage current flow during status charge

, can I just put a transistor connected to the battery, which toggles the input supply to the battery when the battery's volts drop below certain amount?

Another thing I'm not sure of: if one charges a battery, does one need to reverse the current?

What I could quickly assemble:

 

[BradtheRad]

You said I need more than 5V, can I just charge them with the 12V supply?

To make it safe you'll need to put the proper control devices between the battery and the 12V supply. It's crucial that charging stops when the batteries are full.

It's dangerous to apply 12V directly to that little 4 cell pack. High current will flow. The batteries will overheat and perhaps burst. The stuff that oozes out is bad for you and your clothes and the furniture.

You can insert a resistor in between to limit current. A suitable current to charge your 2400 mAH cells is 240 mA. But then you must disconnect the batteries when they are full. A full cell reads 1.44 to 1.5 V, depending on how fast you're charging it.

Disconnecting the batteries after charging is the correct way if you want to use a cheap and easy method to charge them.

Try measuring voltage on your batteries at various times when they are discharged, newly charged, etc. I've found that 1.38V is what my nimh type read while sitting in a box waiting to be used. So I think 1.38V should be a safe voltage to apply to a nimh cell constantly in standby mode.

can I just put a transistor connected to the battery, which toggles the input supply to the battery when the battery's volts drop below certain amount?

To do this it will require another component or two. It will be tricky to adjust carefully. If you adjust it wrong then you will ruin your batteries. Or else they won't charge fully.

This is a useful project to make. However it would be wise to consult a few more sources to discover the proper circuit to construct.

Another thing I'm not sure of: if one charges a battery, does one need to reverse the current?

Charging a battery means pushing current into the positive terminal.

Using the battery means current comes out of the positive terminal.

That's the simplified explanation.

 

[MagSMG]

Thanks Brad!

I will be looking into these thing now and hopefully solve the problem this way.

Since there is an ATmega328 in the project-device, I will just use that to measure/toggle the charging system.

Once again, thank you for your time!