Battery charging time?......Coulombs in = Coulombs out?

[treez]

Hello,

I have a battery of NiCd's which is 6V nominal.

I wish it to supply 0.714 Amps for 3 hours.
-this is 3*60*60*0.714 Coulombs = 7714 Coulombs.

If i am to charge this battery up for this service, and i have a 19 hour charge time
then would i be right in saying that i need a constant charger current of 112mA?

 

[MarkChristensen]

Another way of looking at it is that you want a nicad that can provide a curtain number of milliamp-hours (0.714 * 3 = 2.142 mAh). There are different ways of charging, but one of the easiest is an overnight (16 hour) charge at the rated value divided by 10. Let's say you had a battery rated at 2.5 mAh, you would charge it for 16 hours at .25 mA.

I found an article that talks about some of the ways to charge NiCds - https://www.powerstream.com/NiCd.htm

Hope this helps...

 

[ark5230]

This is right except for Ah in place of mAh at few places.

 

[MarkChristensen]

Ooops - you're right! Sorry about that. (What's a few decimal places here and there?? LOL)

 

[Darktrax]

OK - here is the whole deal on fast charging NiCads. Forgive the length, there is lots NiCads do.

First, don't take silly risks! Fast charge carefully. Detect all the wrong conditions the battery can get into, and have the charge cut off safely. Know that you may be (carefully) going outside the manufaturer expectation of cell usage.

You can mess with the charging time in a major way.. provided you clearly understand how NiCads behave, the cell capacity condition at the time, and some other tricks.

The small slow chargers, 12, 14 hours or longer, usually only use a resistor to set the current at a value low enough not to badly affect the cell chemistry by continuing to charge beyond the point where the cell is fully charged. This value is usually somewhere between C/10 and C/20.

Several things that are related are ..
1. How NiCad capacity becomes diminished when it is repeatedly partially discharged, then recharged on slow and left overcharging at the "safe" rate.

2. How NiCad capacity is changed when used in series with several cells, and one or more cells become exhausted early because of unequal capacity, and end up being having a reverse current forced through them until the user decides to charge. The "full charge" point of the cells diverges.

3. The effect of nickel crystal growth in the cell, until even it goes short-circuit.

etc.. but we try to stay with charge time...

Provided the "full charge" point can be reliably detected, and you use the temperature detect (diode), and you don't use a current so abusively high that the cell overheats and takes internal damage, you can charge at a rate you choose. The charge efficiency of NiCads is around 2/3, or 66%. This means you add on time to charge the last third.
This may vary a bit depending on the type of cell. Some NiCads have a "sintered" internal feature that greatly increases the cell electrode area, which makes them more suitable for high current fast charge use.

I did once put about 2A charge into a AA cell that was rated to charge at 50mA. It charged in about 30 minutes, and was hot, but not uncomfortably so. I seemed to operate fine, and was abused in this way until I lost count.

When the "full" point is reached during faster charging, the chemistry changes, and the temperature immediately gets going upward rapidly. Even though some fast chargers use this to switch off, it is not a good thing! Only over-charge rates like C/10 avoid destruction!

Aside from keeping tabs on the temperature, to shut down if things go wrong, the way of telling the cell has arrived at the full charge is the delta-V/dt turnover point. Unlike lead-acid batterys, the cell voltage of NiCads changes very little during charge, so voltage is not a reliable indicator of the charge state. It is the rate of change of V while the cell is discharging that is meaningful.

Clever chargers use pulse currents that charge and discharge, with the charge time duty cycle over 90%, and measuring the cell voltage during the discharge pulse. At the full charge point, the cell voltage starts to DECREASE, and the temperature starts to go crazy. I stress that this is what happens per cell. The effect can be masked by the other cells in series. Chargers for portable drills use this, but unless you have access to both terminals of all the cells (like in laptop Li-Ion packs), then uneven charging will happen, and the time to reach the deteriorated capacity shortens. The full-charge voltage also seems to get higher on deteriorated cells.

So - provided you charge smart, and can detect when full, you can charge as fast as is safe. There is lots of information on the internet going into much greater detail about NiCads. For me, NiCads are so expensive, short-lived, and polluting, I prefer other types.

 

[treez]

our charger current is fixed (as it is supplied by a resonant inductive coupler), and we have 19 hours to charge up as its an emergency light. So can we just keep a constant current of C/10 flowing into the battery and just keep it like that permanently?....or will it overheat after several weeks of being trickle charged at C/10?

 

[Darktrax]

Provided you stay at C/20, or C/15, you can leave a NiCad on charge well beyond the time it would take to charge from empty. Several days would not bring direct harm.

The norm is to have a timed constant current C/20 charger that guarantees the charge gets to full, and beyond, without keeping it up forever. Some chargers discharge to empty before putting in the charge. This is good for NiCads, but you would need to have alternate 2-battery system.

BUT..

This is not to say it is "good" for the battery. NiCads work best when they are kept for longer periods (if you must!) with only a small charge present, and then get charged and discharged fully when in use. Better they are worked often. Keeping NiCads in a state where nickel crystal growth starts massively reducing the capacity, maybe even shorting out some of the cells in the series. Folk who keep NiCad-powered portable tools unused in the garage know this.

NiCad is a very unsuitable choice for a emergency light. There are small lead-acid gel batteries that are made for the purpose, (tiny, tough, invertable, any position, no spill, etc), and these can be kept in excellent operating condition for years by a "maintainance" charger that delivers a little pulse now and then, mediated by the straightforward battery voltage. It effect, it just replaces the self-discharge. Eventually they need to be checked/replaced for safety, but they don't just die in place like NiCads. Burglar alarm kit uses these.

Chargers with maintain-type features are much used by the camping/leisure/boating folk who want to keep their batteries in excellent condition while standing unused for long periiods. This stuff is available.
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In-passing, the capacity of Laptop Li-Ion types if kept constantly fully charged, reduces by about 20% per year. If you leave a laptop for days with the charge less than 15%, and only charge it before you need it, the baterry degradation is only about 4% per year!