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Slightly off topic.......

I have an Arduino developed AVR circuit inside a project box that is going to be battery powered (DSLR intervalometer) and i want to put 4 x AA rechargeable batteries (2700mAh or something like that) inside the box to power the AVR and the LCD.

I am looking for a schematic for a simple battery charger circuit that I can use to plug into my project box, via a jack plug, to charge the batteries inside.

Can anyone help please?
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It depends on if your batteries are nickel cadmium or lithium ion. The latter are much harder to charge in circuit due to the problem of over charging. Nickel cadmium are much easer to trickle charge at about one tenth of their rating, you just need a current limited supply.
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I'll be using standard NiMH rechargable AA batteries.

So it it simply that I connect up a power supply of say 10% of the battery rating, i.e. for a 1000mAh 1.5v battery I put in 1.5v at 100mA and charge for 10 hours?
« Last Edit: January 14, 2009, 10:05:04 am by zooto68 » Logged

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Almost. You will be charging using a constant current (100ma) so the actual voltage will be changing as the battery charges.

You can either use a constant current circuit, the LM317 is a popular choice (google lm317 batter charger) or use a power supply a few volts higher voltage then the battery is rated and use a series resistor to provide the 100ma)
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Thanks. Searching for LM317 brings up lots of options.   smiley
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Quote
I have an Arduino developed AVR circuit inside a project box that is going to be battery powered (DSLR intervalometer) and i want to put 4 x AA rechargeable batteries (2700mAh or something like that) inside the box to power the AVR and the LCD.

I am looking for a schematic for a simple battery charger circuit that I can use to plug into my project box, via a jack plug, to charge the batteries inside.


Simple question, before worrying about recharging those batteries - have you successfully run the project assembly from 4x rechargeable AAs ?
Generally 4x NiMH AAs are likely to deliver less than 5v ...


Recently, I was reading ladyada's description of getting a regulated 5v supply out of 2x AAs. For more battery duration, I suppose you could do that stuff with 2x2 AAs (two parallel stacks of two batteries in series). I like the way she brings her unit to a usb socket, making it easy to power (or recharge) anything that expects to plug into usb. Hmmm.
She describes the whys and wherefores here http://www.ladyada.net/make/mintyboost/process.html

As an alternative to AAs, though Mike justifiably says Lithiums have been difficult, I believe there are now fairly economical ICs that will look after their charging process. (And Lithium cells do provide more stored energy in less weight.
Lithium charger chip example http://www.sparkfun.com/commerce/product_info.php?products_id=674
And breakout board http://www.sparkfun.com/commerce/product_info.php?products_id=726

And there is even a compact Arduino-compatible board that has the facility already on-board. Just directly plug in the (lithium) battery. Recharge via usb or a charger that looks like usb... And using such a board could make economic sense rather than adding a separate charger.
http://www.sparkfun.com/commerce/product_info.php?products_id=8957
« Last Edit: January 14, 2009, 01:02:08 pm by wdh » Logged

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Quote
I'll be using standard NiMH rechargable AA batteries.

So it it simply that I connect up a power supply of say 10% of the battery rating, i.e. for a 1000mAh 1.5v battery I put in 1.5v at 100mA and charge for 10 hours?

No, at least, never ever count on that. You don't take the capacity of a battery and take 10% to find the trickle charge. The trickle charge is 10% of the high charge rate. In other words, if the high charge for a battery is 100ma then the trickle is 10ma. I'm building a 12V lead acid charger that is 400ma normally and 40ma trickle charge. I really do not think that you want to charge little 1.5V AA batteries at 100ma constantly. Also, you lose power due to resistance (which is dissipated as heat) so you cannot just take the charge rate and multiply.

So, be sure to find out what the proper charging rate for the battery you are charging is.
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Typical NiMH batteries can be charged at the 1C rate, so 10% of the maximum charge is the same as 10% of the rated capacity. But you are correct to point out that most NiMH cells will start getting warm after reaching full charge at the 10% rate (anything up to 24 hours) and are happier if the charging rate is dropped down to 3% to 5 %  if left on charge after that.

also bear in mind that the ratings marked on the batteires sometimes need to be taken with a pinch of salt.
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Sounds like a good project for a micro controller !! smiley

Get the charger to charge at the specified rate for 10-14 hours then drop down to a lower charge after that.

I have already ordered the Ladyada PCB for their 2 AA - 5v convertor (after unsuccessfully trying to get it to work on a breadboard). I will give 4 x AA's a go and leave it running to see how long it lasts. If it is no good I will look into an alternative. I was thinking maybe a li-ion battery from a digital camera maybe.
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