# 9V battery charger

Discussion in 'The Projects Forum' started by davidGG, Nov 2, 2013.

1. ### davidGG Thread Starter Member

Dec 22, 2012
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0
Hello.
I have purchased a couple of 9V batteries from Energizer.
Data sheet: http://data.energizer.com/PDFs/HR22-175_EU.pdf

I want to make my own charger circuit.

If I simply get a 9V source and a diode, will this be sufficient to safely charge the battery?

In the data sheet, it does not say at what current should I charge the battery or for how long. I assume that it will automatically draw the correct current given a 9V source is charging it.

(I tried using the search function first but as soon as I hit "search now" i get stuck with a blank page)

2. ### MrChips Moderator

Oct 2, 2009
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3. ### THE_RB AAC Fanatic!

Feb 11, 2008
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9v rechargeables are normally trickle charged in appliances (very common in things like clock radios, answering machines etc) and are safe to trickle charge at about 1/20th of C capacity continually.

I think the ones I use are 165 mAh so assuming your battery is the same (I didn't check your PDF) you can trickle charge at 165mA / 20 = approx 8mA. Charge time will be approx 24 hours, or can be left at 8mA indefinitely.

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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Use a constant current charger, set to 10% of the max value of your battery, so in your case max current is 175mA , 10% is approx 18mA and charge it for 10 hours.

or 36mA for 5 hours, the longer the charger at the lower rate the safer.

5. ### davidGG Thread Starter Member

Dec 22, 2012
51
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Thank you guys for your replies. So if I used the trickle charge method I just leave it at that current and let it charge? I do not have to turn the charger off and on to let it cool?

TO limit the current I must use a resistor, but do I have to account for the internal resistance of the battery? Because V=IR, R=series resistance.
So if I were to charge at 30mA I would just simply use a 300Ω resistor without taking account the resistance of the battery?

I also have to take into account the voltage losses at the resistor and diode right? So my source would need to be a little over 9V.
Thanks

6. ### THE_RB AAC Fanatic!

Feb 11, 2008
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9v rechargable batteries have a very high internal resistance already (by battery standards) as the internal cells are tiny like button cells.

What was the mAh rating of your battery? Does the PDF show recommended max mA charge current?

30mA sounds high for "trickle charging" a battery that might be 165mAh. That's still ok but you should use a fixed max voltage, which you can do with a simple LM317 IC. So you could use a LM317 set to the fixed voltage (end of charge voltage) and a simple series resistor should be enough to limit max current into a flat battery.