Battery charging regime for low charge rate?

Discussion in 'Power Electronics' started by eem2am, Nov 8, 2012.

1. eem2am

429
0
Aug 3, 2009
Hello,

We are charging four series NiCd (SAFT VNT D U ) cells at C/10 (~450mA) .

Our charger is just a 450mA current source which can be turned on and off, it cant be reduced or increased.

Since our charge rate is so slow, does this mean that we can use a very simple charging regime, whereby we simply monitor the voltage across the cells, and when its got up to 5.6V, we simply stop charging it?

SAFT VNT D U cell
http://www.saftbatteries.com/doc/Doc...d6a0ff6b3f.pdf

2. GreenGiant

842
7
Feb 9, 2012
ideally you would want some sort of constant voltage (CV) hold where you trickle off the current over time, but at C/10 you should get a pretty full charge, at the company I work for we typically do a 1C charge to full voltage then CV hold to anywhere from C/20 to C/100 but that is also for Li-Ion batteries

3. eem2am

429
0
Aug 3, 2009
Thanks, but our charger current source is from an inductive coupler, and its 450mA only...this cannot be reduced or increased......but just switched on and off.

We cant do any of that trickle charging, unless there is some scheme where you can do PWM charging in a high frequency pulse current sort of way.........so do you think that for us, we simply set 0.1C, and wait for the battery voltage to get to 5.6V, then turn off the charging current?

4. GreenGiant

842
7
Feb 9, 2012
I understand that's what you have, I would say that unless you could use PWM which would theoretically work though I have never used it for charging batteries before, just go with the charge until it reaches 5.6V then turn it off, you could let it sit for a little longer than when it reaches exactly, but too long and you're looking at burning the battery so that's up to you

5. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
653
May 8, 2012
Are you desirous of an automatic system that stops the charge @ Vtrip?

Chris

7,682
1,689
Jan 5, 2010
7. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
653
May 8, 2012
When the NiCd becomes a blip in history I will miss them about as much as the germanium transistor.

Chris

8. JMW

90
3
Jan 30, 2012
Use thermistor circuit to disable the charger, is the way to go. Use a comparator, such as 2 or 3 degrees F above ambient is a good point. Make this so the circuit is latching and won't reset until power down. NiCd, Li ion increase in temperature when fully charged.

9. eem2am

429
0
Aug 3, 2009
we have 18 hrs to charge the battery so we can do C/10 charging, and we wont suffer any heating problems etc....i hope not anyway?

...is lithium polymer a good battery type?

10. BobK

7,682
1,689
Jan 5, 2010
Charging at .1C for 18 hours would probably be okay, assuming they are nearly exhausted when you start.

Bob

11. CDRIVEHauling 10' pipe on a Trek Shift3

4,960
653
May 8, 2012
An Nicad expert I'm not. I do know what keeps them going though, and that's daily use and recharge cycles. They will give years of use when cycled daily. I'm happy to say that I've never heard of them termed as "emergency batteries" because that would be an oxymoron.

Chris

12. BobK

7,682
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Jan 5, 2010
Yep, they will not hold a charge for a year even.

Bob

13. eem2am

429
0
Aug 3, 2009
can we just do C/10 trickle charging continuously, and just keep the battery voltage below 6.4V (1.6V/cell)?

...or will a constant C/10 current flow cause the battery to overheat?

14. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
At C/10 they will tolerate an extended charge, but not indefinitely.

From vague memory C/200 is the rate that can be used to prevent discharge, but don't take that from me without verifying it.