Need help for NiMH bettery charging circuit

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
159
I am building charger circuit for NiMh battery charger circuit. but confuse after googling the topic.
which circuit can be work for this.
battery is of 7.2 volt that is 6 cell. current rating is 2500mah. input voltage is +12volt.
 

MikeML

Joined Oct 2, 2009
5,444
Buy a microprocessor based charger. The best ones look for the knee in the derivative of battery terminal Voltage vs time. Cheaper ones rely on battery temperature rise which occurs after the battery stop accepting charge.
 

BobTPH

Joined Jun 5, 2013
2,497
NiMH are the most difficult batteries to charge correctly.

You can trickle charge them at C/20 safely, but this will take a whole day to charge a completely empty one.

If you want to charge at higher rates, you must detect end of charge. There are two things that happen when end of charge is reached. The terminal voltage actually drops about 8mV per cell, and the batteries start heating rapidly. A good charger will look for both effects to avoid overcharging which will cause the batteries to vent and be permanently
damaged.

Your best bet is to get a commercial charger.

Bob
 

bertz

Joined Nov 11, 2013
327
I am building charger circuit for NiMh battery charger circuit. but confuse after googling the topic.
which circuit can be work for this.
battery is of 7.2 volt that is 6 cell. current rating is 2500mah. input voltage is +12volt.
NiMH batteries use a negative slope charging algorithm. You cant build a charger as cheaply as you can buy a commercial unit. However, if you must, google the datasheet for the MAX713 chip. It will give you all the information you need to build a linear or switch mode charger.
 

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
159
Yes there is risk and safety issues also.
I saw some of them charger online and also they are cheap but shipping cost itself is 50-100 $. Means battery price is of 5-6 $ and shipping is tremendously high.
Thats why it could be better if i got the solution at my end.
 

bertz

Joined Nov 11, 2013
327
Yes there is risk and safety issues also.
I saw some of them charger online and also they are cheap but shipping cost itself is 50-100 $. Means battery price is of 5-6 $ and shipping is tremendously high.
Thats why it could be better if i got the solution at my end.
Here is everything you need to know in order to build a charger. Let me know if you have trouble finding a chip.

http://datasheets.maximintegrated.com/en/ds/MAX712-MAX713.pdf

Good luck!
 

IcedFruits

Joined Jan 15, 2014
97
Yes there is risk and safety issues also.
I saw some of them charger online and also they are cheap but shipping cost itself is 50-100 $. Means battery price is of 5-6 $ and shipping is tremendously high.
Thats why it could be better if i got the solution at my end.
u can program an arduino to detect charge completion. continuously read battery voltage (10 times in a second perhaps) while charging with constant current, the reading should keep increasing, till there will be a little drop at charge completion.
u will have to shut down the charging when [current voltage] < [max voltage reached - (certain voltage)]. also, keep tracking the battery temp with a thermistor, if temp reaches 50 degrees C, u will need to shut down.
check the battery university site for exact parameter values, and don't forget to buffer analog inputs on arduino with lm324.

and most importantly, proceed with caution only after proper understanding.

but best choice should be to constant current trickle charge as people suggested, and as used in many of the cheapest commercial chargers.
 
Last edited:

BigD61

Joined Nov 8, 2014
20
If you are looking for fast charging, commercial product are the way to go. Lots of RC battery charger available from RC model shop & online stores. I build up a constant current source that outputs up to 500 mA. Could share the design, it is not simple. Has Bar Graph display for current set and buck regulator so I could source power from a 20V 1 amp wall brick. Still have to time the charge. I was using .2 C and timing for 150% of battery capacitor. It scheme work when battery are near full discharge (1.1 volt/cell). Today I use a Thunder Ace charger.
 
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