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Quick question about chargers

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2camjohn

Member
I am making a simple trickle charger for a NiMh battery.

It has a 12V DC power supply which is a cheap 'wall wart' type. It says it is regulated but I have heard these types of supplies are quite unsteady.

The circuit will simply be a resistor and diode,.



I am not sure whether charging a battery requires a very steady power supply.

Should I use a voltage regulator to step the supply voltage down to a steady 9V?

Or will it be fine to charge from the 12V source?

What, if anything can happen when charging a battery with an unsteady voltage?


Thanks
John
 

stevez

Active Member
Someone pointed to www.energizer.com as a great source of information on batteries. There's a PDF manual that goes into the details of charging. By reviewing that you'd conclude that some control, limiting or regulation is required - and you might be able to better define 'trickle charge' so you don't damage your batteries. You might identify the battery or cell size and other characteristics because someone may have already gone thru the exercise.

I don't think that power quality is a great issue except that you do need to limit or regulate the current and/or voltage.

If I had to throw together something quick and had no more info that you've provided I'd wire an LM317 as a current regulator to apply 10 ma (if the '317 will go that low) and see how that works. Look at National's site for a datasheet on the LM117/LM317 which shows a current regulator.
 

electronist

New Member
since your charger is not a standard one i recommend that you check the output voltage by a multimeter cause such cheap adapters normally give o/p voltages from 12V to 20V DC. In case the voltage is above 12 V and you are charging a 9V battery then do not do so as it may lead to an explosion. but 12 is sufficient t o charge a 9V battery.
Also extreme steady voltages are not necessary as long as the output is greater than the required minimum voltage.
A regulator would be better but is not necessary.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
2camjohn said:
I am making a simple trickle charger for a NiMh battery.

It has a 12V DC power supply which is a cheap 'wall wart' type. It says it is regulated but I have heard these types of supplies are quite unsteady.

The circuit will simply be a resistor and diode,.



I am not sure whether charging a battery requires a very steady power supply.

Should I use a voltage regulator to step the supply voltage down to a steady 9V?

Or will it be fine to charge from the 12V source?

You don't charge NiCd's or NiMh's with voltage, but with current - you just need sufficient voltage from the supply to pass enough current into the battery.

The simplest way is to use a resistor from a higher voltage supply.

As already suggested, measure the output of your supply - which is likely to be far higher than 12V - OFF LOAD. Calculate a suitable resistor for the charging current you require (using ohms law), then measure the current while it's charging. You will probably find it's less than calculated, because the supply will drop under load - you can then try adjusting the resistor value to give you roughly the current you want.
 

audioguru

Well-Known Member
Most Helpful Member
Hi John,
Are you aware that a "trickle charger" will take many, many hours to charge a fully discharged battery? How will you know when the battery is fully charged?
You probably need a "regular overnight charge current" circuit that automatically switches to a lower "trickle charge current" by a timer after about 14 hours. Then the battery can remain in the charger indefinately.

Let's start at the beginning.
1) What is the voltage and ma/hr rating of your battery? We can't think about a charger unless you tell us that most important information.
2) Measure the output voltage of your power supply without a load and tell us what it is.
3) What is the output current rating of your power supply? Connect a load so that the power supply is operating with its rated output current, measure its output voltage across that load and tell us what it is.

When we know the above information then we can calculate the overnight charging current, the trickle charging current, determine whether the power supply has enough output voltage and current capability and calculate suitable current-limiting resistors. Can you design a timer?

If you want to use an "idiot" diode to prevent charging a battery that is plugged-in backwards, you might as well use a full-wave bridge (4 diodes) so that the battery is fine either way.

BTW, you can charge from a 1000V or more supply, if you avoid the shock hazzard and limit the charging current to the same value as from a much lower voltage supply.
 

2camjohn

Member
Hi Guys

I already worked out my charge current, and the resistor to limit the current.

I am using a PIC with an ADC to measure the status of the battery.

Cheers for the tip on the bridge audioguru, I shall do that!!!




What I really wanted to know was whether an unstable power supply would cause any problems when charging.

Cheers
 

audioguru

Well-Known Member
Most Helpful Member
Hi John,
Is your PIC going to detect the slight peak of voltage that occurs when the battery is nearly fully charged, or the temperature rise of the battery, or both?

Don't forget to allow for the voltage that will be lost across the diode bridge. It will reduce the charging current.

With your simple resistor limiting the charging current, any change of the power supply voltage (unstable?) will reflect on the charging current. If you use a constant-current-source circuit (LM317 plus one resistor) then the current won't change.

You still haven't told us the voltage and ma/hr ratings of your battery, and now how much charging current and duration that you have chosen. Please let us know.
 

2camjohn

Member
Thanks for taking the time to help...


Its 6V 1.2A output.

1.2MaH


I am not interested in the speed of the charging, so I decided to charge at 0.1A.




All I am measuring with the PIC is the voltage using an ADC. I have measured the voltages at various stages of battery use, so I think I can get a good output using a tri colour LED (Green is fine, Red means it really needs charging, orange for somewhere in the middle)


The PIC will also have the ability to switch the battery to a discharge circuit to empty the battery before charging. This is when the PIC will measure and output voltage of the battery.
 
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