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300mA AA NiMh charger design needed

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rparigoris

Joined: 24 Nov 2009
Posts: 797

Posted: Wed Sep 28, 2011 12:10 pm Post subject: 300mA AA NiMh charger design needed

Hi Group



I need a design for an easy to build, low parts count and inexpensive constant current circuit to charge 3,000mA AA NiMh batteries (~300mA rate C/10 for 14 to 16 hours) from a 12 volt lead acid battery that could have the input voltage ranging anywhere from ~ 12.2 to 14.5volts.



I would prefer to have as many cells in series as possible when charging.



I have the charge stick figured out to hold cells in series by using 1/2" PVC water pipe that's a pretty good fit for AA cells.



Thanking you in advance.

Ron Parigoris



BTW I just ordered 50 AA NiMh supposedly 3,000mA batteries from E-bay including shipping for $25.85 (~$.52 each) Buy it now. I will report on self discharge rate, ability to dump amps and capacity once I get them.

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Sep 28, 2011 8:47 pm Post subject: 300mA AA NiMh charger design needed

At 03:10 PM 9/28/2011, you wrote:

 	  Quote: 	
		  

<rparigor(at)suffolk.lib.ny.us>



Hi Group



I need a design for an easy to build, low parts count and 

inexpensive constant current circuit to charge 3,000mA AA NiMh 

batteries (~300mA rate C/10 for 14 to 16 hours) from a 12 volt lead 

acid battery that could have the input voltage ranging anywhere from 

~ 12.2 to 14.5volts.



I would prefer to have as many cells in series as possible when charging.


	


   The simplest is an LM317 wired as shown here



http://diyparadise.com/yhlmccs.html



   The resistor would be 1.25/.3 or 39 ohms.

   The CC Gen will need at least 3 volts

   of 'headroom' so 12.5 - 3.0 leaves

   9.5 volts worth of cells. This means

   you can charge 6 cells in series.



   This charge rate will heat the cells.

   Plastic pipe may not be the best holder

   for allowing the string of cells to

   reject heat.



   If you like, I could take a few of those

   cells and discharge them on the system I

   used to produce this article:



http://aeroelectric.com/articles/AA_Bat_Test.pdf

   Bob . . .

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rparigoris

Joined: 24 Nov 2009
Posts: 797

Posted: Thu Sep 29, 2011 1:13 am Post subject: Re: 300mA AA NiMh charger design needed

Hi Bob



Thx. for the reply.



** First off if you don't mind I can send you some AA NiMh cells to test once they arrive. How many would you like for testing? 



** I like the simplicity of the circuit you pointed me to. The first question is would Allied part number 248-0718 LM317P be OK to use? This is not a National Semiconductor part, I like the TO220 style and didn't see one offered by National.



** Instead of a LM317, would Allied 288-0963 LM1117T-ADJ Low dropout be an OK choice with only a 1.2 volt drop out? This should allow a cell count increase?



** You mention that putting cells in a PVC plastic pipe may not be a good idea because it can aid in heat build up. You mention that this charger will heat the cells. I am questioning this because in my past experience charging NiMh cells at a one tenth capacity rate for 14 to 16 hours doesn't seem to develop too much heat. Has your experience proved in the past that a 1/10C charge rate for 14 to 16 hours warms up the cells? The cells are AA with a 3,000mA capacity, so 300mAs is 1/10 capacity.



** I would love for this simple circuit to accomplish the task of providing a constant 300mA charge rate. I don't have too much experience working with LM317s. I can kind of see where if you have a constant supply voltage and a constant load how the circuit should be OK. In my application where the supply voltage may be as low as 12.2 volts, or if the motor is running then up to 14.5 volts do you think it will still maintain a constant 300mAs?

Also we know that when NiMh cells when discharged have a lower resistance. As the cells begin to charge, the resistance begins to go up thus you need to begin increasing the voltage in order to maintain a constant 300ma charge rate. Then when the cells become fully charged the resistance begins to go down again, thus you need to begin lowering the voltage to maintain a constant current. If you put too much current into a fully charged cell this is where you can get them warm or downright hot (IE 1C or more).

Anyway do you think this circuit could do an OK job of maintaining 300mA charge rate with input voltage fluctuating between 12.2 and 14.5 volts, and the NiMh cells changing their internal resistance?



** I kinda had an eyeball on this circuit #2:

http://talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

But they are using a constant current 500mA charger that is kinda regulated (because of  transformer coupling?) and wasn't sure how it would work with a source that could provide a gazillion mAs of current.

I was thinking of trying both the regulators I mentioned above and since Allied doesn't sell BC338 NPN transistors (Digikey BC338ND), I was thinking of giving a Allied TIP122G 568-0402 a try. I would rather keep it simple, but if just the regulator and resistor proves unacceptable results, do you think this would work OK?



Thx.

Ron Parigoris

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Sep 29, 2011 3:39 am Post subject: 300mA AA NiMh charger design needed

At 04:13 AM 9/29/2011, you wrote:


<rparigor(at)suffolk.lib.ny.us>



Hi Bob



Thx. for the reply.



** First off if you don't mind I can send you some AA NiMh cells to 

test once they arrive. How many would you like for testing?



    Four.  No hurry, that fixture is packed away

    somewhere from the move. I'll have to dig it

    out and dust it off. I've been thinking about

    repeating some of the tests done earlier and

    adding a few more 'house brands' of AA-alkalines

    to the study. But I'm up to my eyeballs in a

    project for Hawker-Beech and it will be a

    couple months before I can 'play'.



** I like the simplicity of the circuit you pointed me to. The first 

question is would Allied part number 248-0718 LM317P be OK to use? 

This is not a National Semiconductor part, I like the TO220 style and 

didn't see one offered by National.



** Instead of a LM317, would Allied 288-0963 LM1117T-ADJ Low dropout 

be an OK choice with only a 1.2 volt drop out? This should allow a 

cell count increase?



    ANY of the 117/217/317 series parts will do.

    You just need to make sure it's heat-sinked

    well enough to dissipate the power. The low

    dropout part is only a portion of the voltage

    drop . . . the 39 ohm resistor has 1.25 volts

    across it too which boosts your head-room

    requirements.

** You mention that putting cells in a PVC plastic pipe may not be a 

good idea because it can aid in heat build up. You mention that this 

charger will heat the cells. I am questioning this because in my past 

experience charging NiMh cells at a one tenth capacity rate for 14 to 

16 hours doesn't seem to develop too much heat. Has your experience 

proved in the past that a 1/10C charge rate for 14 to 16 hours warms 

up the cells? The cells are AA with a 3,000mA capacity, so 300mAs is 

1/10 capacity.



    'Too much heat' is non-quantified. Further,

    small surface area parts wrapped in an

    low heat transfer media will experience

    a greater temperature rise than when

    operated in still air. Just a point to

    ponder and explore.



** I would love for this simple circuit to accomplish the task of 

providing a constant 300mA charge rate. I don't have too much 

experience working with LM317s. I can kind of see where if you have a 

constant supply voltage and a constant load how the circuit should be 

OK. In my application where the supply voltage may be as low as 12.2 

volts, or if the motor is running then up to 14.5 volts do you think 

it will still maintain a constant 300mAs?



    The LMx17 series devices are a fixed calibration

    voltage regulator with a factory adjustment of

    1.25 volts. Higher voltages can be achieved by

    addition of the voltage divider shown in the

    data sheets. But when wired with the single resistor

    as suggested, the device works to maintain a

    constant voltage of 1.25 volts across the series

    resistor. 1.25 ohms yields 1 amp, 125 ohms yields

    10 milliamps, etc. So yes, this is a true constant

    current generator with an upper limit equal to the

    design point. It will fall off only if the device

    looses operating head-room discussed above.



Also we know that when NiMh cells when discharged have a lower 

resistance. As the cells begin to charge, the resistance begins to go 

up thus you need to begin increasing the voltage in order to maintain 

a constant 300ma charge rate. Then when the cells become fully 

charged the resistance begins to go down again, thus you need to 

begin lowering the voltage to maintain a constant current. If you put 

too much current into a fully charged cell this is where you can get 

them warm or downright hot (IE 1C or more).

Anyway do you think this circuit could do an OK job of maintaining 

300mA charge rate with input voltage fluctuating between 12.2 and 

14.5 volts, and the NiMh cells changing their internal resistance?



     Absolutely.

** I kinda had an eyeball on this circuit #2:

http://talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

But they are using a constant current 500mA charger that is kinda 

regulated (because of  transformer coupling?) and wasn't sure how it 

would work with a source that could provide a gazillion mAs of current.



     The circuit depicted are variations on a theme and

     all will probably function as advertised.



I was thinking of trying both the regulators I mentioned above and 

since Allied doesn't sell BC338 NPN transistors (Digikey BC338ND), I 

was thinking of giving a Allied TIP122G 568-0402 a try. I would 

rather keep it simple, but if just the regulator and resistor proves 

unacceptable results, do you think this would work OK?



     Yes. The transistor style is not critical. Go get

     an LM317 and TIP31 at Radio Shack and do the

     experiments.



     LM317

http://www.radioshack.com/product/index.jsp?productId 62601&numProdsPerPage=60



     TIP31



http://www.radioshack.com/product/index.jsp?productId 62610&numProdsPerPage=60



     This is the simplest approach. If you want something

     'fancier' with lower heat dissipation, some sort

     of active constant current generator like the LED

     drivers we offer



https://matronics.com/aeroelectric/Catalog/AEC/9051/9051-700A.pdf



     can be used. With a single resistor to adjust

     output current, you can set your charging level

     at any value up to 1000 mA. Further, this particular

     device has a boost output capability. I think it would

     charge a longer string of cells.



   Bob . . .

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retasker(at)optonline.net
Guest

Posted: Thu Sep 29, 2011 7:41 pm Post subject: 300mA AA NiMh charger design needed

Bob,  your calculations are off by a factor of ten relative to the resistor needed to get 300 mA.

The 317 series has a reference voltage of 1.25V as you noted.  But to get 300 mA you would need 1.25V/0.3A = 4.17ohms.  A value of 3.9 ohms is the closest 5% resistor value and it would give you 320 

mA - likely close enough.  You would need at least a 1/2 W resistor for this application and a 1W would keep the temperature rise in the resistor more reasonable.

Other than that everything else you say is fine.

It should be noted that using a charger like this is fine as long as one removes it shortly after the batteries are fully charged.  Otherwise, the overcharging will shorten their useful lives.

Dick Tasker

Robert L. Nuckolls, III wrote:

 	  Quote: 	

 At 03:10 PM 9/28/2011, you wrote:

> 

>

> Hi Group

>

> I need a design for an easy to build, low parts count and inexpensive constant current circuit to charge 3,000mA AA NiMh batteries (~300mA rate C/10 for 14 to 16 hours) from a 12 volt lead acid 

> battery that could have the input voltage ranging anywhere from ~ 12.2 to 14.5volts.

>

> I would prefer to have as many cells in series as possible when charging.

   The simplest is an LM317 wired as shown here

 http://diyparadise.com/yhlmccs.html

   The resistor would be 1.25/.3 or 39 ohms.

   The CC Gen will need at least 3 volts

   of 'headroom' so 12.5 - 3.0 leaves

   9.5 volts worth of cells. This means

   you can charge 6 cells in series.

   This charge rate will heat the cells.

   Plastic pipe may not be the best holder

   for allowing the string of cells to

   reject heat.

   If you like, I could take a few of those

   cells and discharge them on the system I

   used to produce this article:

 http://aeroelectric.com/articles/AA_Bat_Test.pdf

   Bob . . .

-- 

Please Note:

No trees were destroyed in the sending of this message.  We do concede, however,

that a significant number of electrons may have been temporarily inconvenienced.

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Sep 30, 2011 6:10 am Post subject: 300mA AA NiMh charger design needed

At 10:38 PM 9/29/2011, you wrote:

 	  Quote: 	
		  

<retasker(at)optonline.net>



Bob,  your calculations are off by a factor of ten relative to the 

resistor needed to get 300 mA.



The 317 series has a reference voltage of 1.25V as you noted.  But 

to get 300 mA you would need 1.25V/0.3A = 4.17ohms.  A value of 3.9 

ohms is the closest 5% resistor value and it would give you 320 mA - 

likely close enough.  You would need at least a 1/2 W resistor for 

this application and a 1W would keep the temperature rise in the 

resistor more reasonable.



Other than that everything else you say is fine.



It should be noted that using a charger like this is fine as long as 

one removes it shortly after the batteries are fully 

charged.  Otherwise, the overcharging will shorten their useful lives.


	


    Yes! Thank you my friend!

   Bob . . .

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tonybabb(at)alejandra.net
Guest

Posted: Sat Oct 01, 2011 9:58 pm Post subject: 300mA AA NiMh charger design needed

Hi Bob,



Snip

  " The resistor would be 1.25/.3 or 39 ohms..."



I'm no electron herder but...isn't 1.25/.3 about 4 ohms?



Still learning about all this stuff...



Tony Babb.

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