Fast Nicad Charger

[Kingpin094]

Quiestions, questions, questions...... why can't manufacturers fully explain
example circuits in there datasheets.

I am trying to create a fast nicad charger using the MC33340 from ON semiconductor.
The datasheet has a nice circuit that looks like it will work the problem is there are some
holes that need to be filled in. Batterys being charged are 850mAH NiCD cells.
There are 6 cells in the pack for a total of 7.2 volts.

All question will refer to infomation that is in or not in the datasheet as the case may be.
Circuit in question is figure 14 on page 9 of the datacheet.

1) What does Vin need to be for the cells in question?
2) If a carge rate of C/2 is used how long should the timeout be set for?
3) When they say Vbatt is that discharged, fully charged....?

That is about all thanks in advance.

Datasheet can be found at:
https://www.electro-tech-online.com/custompdfs/2004/03/MC33340-DPDF.pdf

 

[mozikluv]

charger

hi,

the recommended voltage input is 18v.

the C/2 referred to is the battery capacity charge rate. C/2 means that it will charge your battery (whatever battery capacity you have) say 800maH, it will be charge in 2 hrs. C/3 in 3 hrs. C/10 in 10 hrs

have not read totally the datasheet so i cant say what Vbatt means.. however Vbatt could probably mean the battery voltage of 1.2v.

 

[Johnson777717]

the C/2 referred to is the battery capacity charge rate. C/2 means that it will charge your battery (whatever battery capacity you have) say 800maH, it will be charge in 2 hrs. C/3 in 3 hrs. C/10 in 10 hrs

Another way to look at this is literally. "C" meaning the capacity or how many amps the battery can deliver in one hour. "C/2" would mean the capacity of your battery divided by 2. Thus, if we use a 800mAh battery, "C/2" would mean that 400mAh is being delivered to that battery, and theoretically would take 2 hours to charge the battery. It's all the same, it just depends on how you wish to think of it.

I would say that the datasheet is assuming a fully discharged battery. Any point in between fully discharged and charged is too difficult to arrange a circuit around. Thus, fully discharged becomes the common point, or reference point, especially when organizing a datasheet.

 

[Kingpin094]

I was under the assumtion that the input voltage was a function of the number off cells being
charged. Maybe that is an invalid assumption.

For overnight charging you use C/10 for 12 to 14 hours.
Does it work the same way with fast charging? Meaning C/2 for 2 to 2.5 hours?

For NiCD cells there nominal voltage is 1.2V. It can range from ~1.45V fully charged to
~1.1V discnarged. So for a 6 cell pack it should be charged until it is 1.4V *6 or ~8.4V, right?.
So do I need to worry about the power source voltage when fast charging?

 

[Johnson777717]

I was under the assumtion that the input voltage was a function of the number off cells being
charged. Maybe that is an invalid assumption.

In a discrete circuit, this assumption may be true. In this case, the IC is taking care of some voltage comparisons, so it is best to stick with what the datasheet explains for the input voltage: