recharging NiMH 9V battery. Help!

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MathGeek

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I have 2 battery chargers, but none supports 9V (in real 8.4V). To buy another charger seems like a waste of money. So I would like to charge 9V with help of wallwart and some wires.

The question is how to charge it. Should I just hook up the 9V wallwart and a couple of resistors? Is more intricate microcontroller circuit required to "smart" charge this 9V battery?

Help me get started.

Thanks.
 
You should check the battery manufacturers website for charging directions. As I remember temperature is monitored during charging to decide when to stop. If I were you I'd just buy a charger and be done with it.
 
Look near the camera dept. at your local Walmart store. they usually have chargers for around $15.00, and include AA batteries. Noticed once there was also a built in 9 v connector. Also, Big Lots gets them cheap often. Unless this is a learning project, more than a desperate need... I'd just buy a ready built unit. Mostly likely to save money, better performance, and a little safer. Your 9 v battery wasn't cheap, will last longer if you take care of it.
 
A "9V" Ni-MH battery has 6 skinny AAAA cells inside. Its voltage during discharge is about 7.2V and it is about 8.4V to 8.7V when still charging and fully charged.
My Sound Level Indicator project has one and it is charged continuously with a 390 ohm resistor from a 9V/300mA wall-wart.
https://www.electronics-lab.com/projects/motor_light/009/index.html
 

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You are right. I might as well buy a charger (ebay has them cheap).

But I am still interested in learning the charging methods for any general NiMH drycell (battery).
 
MathGeek said:
I am still interested in learning the charging methods for any general NiMH drycell (battery).
Click to expand...
Go to www.energizer.com and click on Technical Info near the top of the page. Then select the battery type Ni-MH Rechargable and below the list of datasheets is a very good Applications Manual which shows the details of charging and discharging their cells. They have an older manual about Ni-Cads but they don't make them anymore.
 
You don't want to fast charge the tiny cells inside a 9V Ni-MH battery. They will internally melt long before the case gets warm.
 
It seemed like some of the ones I saw were 7-cell 8.4v versions and some were 6-cell 7.2v versions.
 
7 cell is more sommon and they usally aren't AAAA cells, they're more often a stack of square shaped button cells with rounded corners.
 
I haven't cut one open to see, but Energizer's 9V Ni-MH battery has the low voltage and other spec's of 6 AAAA cells. I remember the rectangular cells in carbon-zinc 9V batteries but Energizer's 9V alkaline batteries have 6 AAAA cells inside.
 

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From what I remember, NiMH were one of the trickier batteries to charge. They aren't "supposed" to be trickle charged, and the only other methods of charge detection were temperature based and voltage based. The problem was that the "delta V" effect was much smaller than NiCd cells, so temperature measurement was the easiest way of terminating charge.
 
Yeah, NiMH is a bit weird. You can either have voltage peak detection or thermal peak detection to signal the charge cycle is complete, however I have been lead to believe that the temperature increase of the NiMH is responsible for the peak in voltage. My charger manual says (and I have read a few places) that charging NiMH with a very low current will cause trouble in detecting the peak because at such a low charge current there is not a noticeable increase in temperature. But the charger has no temperature probe for the batteries so I can only assume it uses voltage peak detection (also says so in the manual). So I think the temperature increase is required for the voltage peak to occur. Does anyone know more about the relationship between NiMH temperature and voltage peaking?
 
dknguyen said:
Does anyone know more about the relationship between NiMH temperature and voltage peaking?
Click to expand...
It is detailed in Energizer's Applications Manual.
 

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