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Designing a battery charger to automatically switch over from 12V source to battery

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it_boy

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I am designing a battery charger to charge a 9V, 8-cell, 1500mAH NiMH battery from an input of 12V DC.
The battery is supposed to act as a backup power source for the rest of my design. So, the input power is supposed to source both battery and the rest of the design. The design is supposed to switch over automatically to battery power when input power is down.
I am trying to use OnSemi MC33340 as the charger is very cheap.

Has anybody used this earlier? Did you face any problems with this device?

I have still not figured out how to design the battery charger to automatically switch over from 12V source to battery when 12V is switched off and again switch over from battery to 12V when 12V is on. the main constraint is cost of the design.

Any suggestions?
 

laktronics

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Re: battery charger

Hi,
Automatic switchover is easy, just connect a properly rated diode from the battery (anode to battery) to the load, to which input power is also connected. The diode will not conduct unless the input power falls below 0.5V from the battery voltage.
Regards,
Laktronics
 

it_boy

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Re: battery charger

Thanks for the input. I was thinking if a mosfet would be better as a diode would drop around 0.7 to 1 volt which decreases the efficiency of the battery.

What should be the charging voltage of a 9.6V NiMH battery?
 

rogerynt

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Re: battery charger

You can consider the followings :

1.) Detect 12V charging voltage to see whether it's fail or not.

2.) If 12V charging voltage is there then use a relay to switch to charging mode.

3.) If the 12V charging voltage not available, let the relay to pull back to it's original position and switch back to battery( 9V ) supply mode

4.) You can also monitor the battery voltage at some point so that when the battery voltage exist certain level then the charging circuit is disable. This will help to extend battery life time.

rogerynt
 

it_boy

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Re: battery charger

Thanks rogerynt. The battery charge controller MC33340 is supposed to do all the functions indicated by you.
Unfortunately I found that it is not possible to charge a 9.6V battery from a 12V source with a switching regulator.
 

oddbudman

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battery charger

You will struggle to find a good way to charge 9.6V of batteries with a 12V source, as the battery voltage is going to be close to 12V when fully charged.

Don't suppose you can change the operating dc voltage of the application or the cell voltage of the battery? (remember you can boost the battery voltage when in battery mode)

You should also consider using a microcontroller for this application.

How fast do you need to charge the battery?

oddbudman
 

it_boy

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battery charger

Time for charging the battery is not a real constraint.
I cannot change the battert though. So, I am stuck with a 9.6V battery.
Should I consider boosting the input from 12V tp 15 or 16V for charging the battery?
 

oddbudman

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battery charger

Yes, it certainally is an option.

A simple way of doing it could be use a microcontroller with PWM function to act as a boost converter, then simply use a RESISTOR/DIODE combination to give the battery charge current.

If you use a microcontroller you could time the charge and also implement extra battery temperature sensing or other smarts.

oddbudman
 

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