Charging 9.6v Ni-MH battery with 12v charger?

[amando96]

I have made a small temperature monitoring thing, it will beep if the temperature of the battery goes over 35ºC.
it isn't a fast charger, i remember the other 12v battery it was made for took something like 8 hours to charge, my single cell quick charger charges them in less than an hour.

 

[Dragon Tamer]

Now your starting to get more complicated. Now I'm compelled to tell you that what you should do is buy a battery charging IC. There are a lot of them out there, just take your pick. If you're sure you just want to make a temp alarm (batteries can easily go over 35 degrees while charging), a simple thermistor and comparator circuit should work just fine.

 

[amando96]

I used a ds1820 i had laying around and an atmega.

I will invest in a proper charger, or simply get an IC soon.

 

[Dragon Tamer]

The MAX712 is the battery charger that I'm most familiar with.

 

[nginmu]

As a kid I used to charge up non rechargeable batteries and heat them up on the fire to get some more life out of them! Worst that ever happened was a leak

I did too, with the same results - except once at the dinner table when I was about 7 or 8. I tried to charge some kind of button cell with a 9V PP3.. it exploded in front of me and sent a shard in my eye. Better off being cautious I guess.

 

[MikeMl]

The correct way to charge NiCd or NiMh is to use a constant-current source; not a constant-voltage source (like a fixed output voltage power supply). If y0u connect NiCd or NiMh to a fixed-voltage power supply, you better have a ballasting resistor (lamp, power resistor) connected between the supply and the battery to limit the initial current.

When using a constant-current supply, you need to terminate the charging cycle either by time (e.g. 14h at a 0.1C rate), or by detecting a temperature rise in the batteries and shutting off the charger when the batteries get warm to hot... This is how most power-tool chargers do it...

Unlike other battery chemistries, NiCd or NiMh do not have a detectable, reliable voltage rise upon reaching full-charge, so you cannot reliably terminate charging when the batteries reach full charge based on voltage alone.

Leaving NiCd or NiMh on a dumb charger much beyond where they feel warm to the touch screws them up royally in very short time because the internal vent opens to relieve the rising pressure inside the cell. Having vented, you might as well throw the cell away...

Most of the advice offered previously in this thread is just plain wrong!

ps: Oops, got sucked into an old thread...

 

[audioguru]

Battery charger ICs detect the voltage hiccup that occurs when a Ni-Cad or Ni-MH battery is fully charged. Some detect the peak and others detect the drop in voltage after the peak.
I have a Rayovac Ni-MH charger that limits the current and never stops. It takes all night at a low current so an overcharge is not a serious problem. My Energizer charger has a current limiter and a timer that is stupid because it will overcharge a battery that is already charged. When the electricity fails when it is charging then the timer starts from the beginning when the electricity returns.

On Black Friday I bought 8 Duracell AA Ni-MH batteries on sale and they came with a free "Fastest SMART" charger. This charger is pretty quick. It lights red LEDs when charging and blinks green LEDs "When they are ready" but it seems that the batteries are fully charged which is supposed to make the LEDs steady green (which happens sometimes), not blinking green. The red LEDs blink "Error/Bad Battery" frequently. The reviews talk about, "What is the Error" and Duracell replies asking for them to talk to customer service about it (maybe it is built wrong and will be replaced?).

Have you ever charged Ni-Cad and Ni-MH at the same time? The Ni-Cads actually get cooler while charging but the Ni-MHs get warmer. They both heat when fully charged.

 

[audioguru]

Deleted double post.

 

[spec]

Hi, i got a 9.6v 1300 mAh Ni-HM battery pack, it has 8 cells, i don't have a 9.6v charger, but i do have a 12v and 250mA charger, can i charge it with that? it was originally made for a 10 cell 12v pack, but is it possible to charge with that? or get a 7809, and a diode, and do it from there? thanks.

Hi amando,

Here is a quote from the battery University web site regarding the charging of NMH batteries:

'It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold.'
**broken link removed**

In short, NMH batteries are the most difficult batteries to charge and to make matters worse, MMH batteries do not like being overcharged either, because it radically shortens their life.

The only way to charge a NMH battery is to 'fast charge' and detect the very small voltage drop when the cell is fully charged coupled with a detection of a temperature increase, both of which are problematic. Even the most expensive battery chargers make errors when charging NMH batteries.

But your NMH batteries are in a pack, which makes the situation much worse. so your only option is to do a timed charge of your NMH battery pack- sorry.

spec