# Constant current NIMH charging. What about Voltage?

Discussion in 'The Projects Forum' started by doug08, Aug 18, 2011.

1. ### doug08 Thread Starter Member

Jan 30, 2011
153
2
I have a constant current charger for my 9V 300Mah batteries. It uses approximately 40ma of current. I use a wall transformer designed for 12Vdc output. If I charge the 9v battery from dead at a constant current rate of 40ma, I usually do it for between 9 and 10 hrs. The full Mah of the battery rating plus 25%. My question is, what about the voltage of the charging supply? Currently the 12Vdc wall transformer has a no load voltage of 17.5Vdc. Does it make any difference? I know if the input voltage is too low, then the constant current source cannot maintain the constant current. Can the input voltage be too high? I would think so.

Thanks

2. ### #12 Expert

Nov 30, 2010
16,705
7,354
NiMh batteries are rather picky about charging voltages and currents. I got my best information from manufacturers websites, and general information from www.batteryuniversity. I think you are doing something wrong.

Go forth and browse.

3. ### doug08 Thread Starter Member

Jan 30, 2011
153
2
Was hoping someone who knows could just tell me. The constant current charging definitely works, I'm just concerned about the high voltage dc source. Thanks

4. ### wayneh Expert

Sep 9, 2010
12,394
3,246
If you were to check the voltage across the battery poles while charging, you'd see a much lower value, probably not much over 9. As long as you're limiting current, and not overcharging, the power supply voltage really doesn't matter within reason.

BUT, the possibility to overcharge is almost a guarantee if you don't watch things carefully. I'd consider a 9V regulator so that the current would be minimal once the battery was charged. If you forget about it, or accidentally put a charged battery into the charger, it won't be a disaster.

5. ### doug08 Thread Starter Member

Jan 30, 2011
153
2
That output 9.6V. When I took the output from the regulator circuit and fed it into the lm317t constant current circuit....it would not work. No current came out of the constant current circuit. I had to remove the voltage regulation circuit that worked, in order to allow the constant current circuit to operate properly. I can always feel the battery to see if it gets very warm, then disconnect the charger.

6. ### #12 Expert

Nov 30, 2010
16,705
7,354
You can add an LM317 or 7805 chip as a voltage regulator. That way, you have both voltage and current limits engaged. You have to design the circuit so the voltage limiter only sees the battery voltage for its control circuit.

Edit. I see you already tried that. Try again. I've done it and it worked.

7. ### wayneh Expert

Sep 9, 2010
12,394
3,246
Really? You must have more time on your hands than I do and/or more focus on your charging task. I always tend to forget about mine.

But anyway, once the temp rises, you're already into overcharge territory. Unless you're holding it in your hand to catch it quickly, it's not a great strategy. Worked better for NiCads, not as good for NiMHs.

8. ### #12 Expert

Nov 30, 2010
16,705
7,354
This is how I did it. Add capacitors as required.

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9. ### doug08 Thread Starter Member

Jan 30, 2011
153
2
I found this one online.....Look 3/4 the way down on the webpage. I made it already. It works perfectly! I set the max voltage at 9.75, and the output is constant current at 40ma.

10. ### #12 Expert

Nov 30, 2010
16,705
7,354
Good to know I didn't waste my time because you wanted somebody who knows to just tell you, like you couldn't go look it up yourself.

11. ### Audioguru New Member

Dec 20, 2007
9,411
896
A "9V" Ni-MH battery can have 6 cells (nominally 7.2V) or 7 cells (nominally 8.4V).
Each cell charges to about 1.45V so the 6 cells battery will be 8.7V and the 7 cells battery will be 10.15V when fully charged.

An LM317 or LM338 current regulator needs 1.5V for the dropout voltage plus 1.25V for the sensing resistor which requires at least 2.75V more than the max battery voltage.

12. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
Typically the max charge current for 9V battery is C/10, in other words,for your battery, the current should be a max of 30mA for about 15 hrs. the charging voltage for 9V batteries per cell is a bit higher than single cell batteries such as AA's or AAA's and is sometimes as much as 1.55V/cell. I'd keep it lower at about 1.5V/cell x 7 = 10.5V. After 15 hours you have the chance of overcharging the battery, especially MiMH, not so bat with NiCads. So some mechanism to reduce the voltage down to 9.8V would help. It would significantly reduce the force at which the supply would attempt to stuff more current into the battery. If you stick with 12v or 17.5V I would reduce the current limit to near 10mA.

Last edited: Aug 18, 2011
13. ### doug08 Thread Starter Member

Jan 30, 2011
153
2
I looked earlier, but had no luck. The circuit I found is exactly what I wanted. BTW,the purpose of my thread was to ask if the high dc voltage was a problem in my constant current charger. You answered my question, said yes it was, so I searched and found a circuit.

14. ### Pasqual New Member

Aug 14, 2011
1
0
the process of recharging batteries, as a matter of common sense, should whenever possible, since a constant current proportional to the capacity of the set of cells, you already contemplated this question by using the charger. When the process reach the end, the voltage between the poles of the battery must meet at threshold voltage determined by elements of the cell. If the voltage regulator to "float" near the limit for the cell under load, the circuit is characterized self-regulating, otherwise must improve the project by adding a voltage limiter circuit sensor.