thanks MrAl
just confirming Mike's observation about using a timer has been more than worth the post to me.
you obviously know your way around a circuit board. Any thoughts on a circuit that would shut off at full charge, then come back on after a drop to a certain threshold (for a laptop)? Or taking it a step further and a different one that discharges completely upon dropping to the threshold, then fully charging (occasionally used item like a wine bottle opener)?
Why, you may say? Its my understanding that fully discharging NiCD's makes them last even longer, and reduces the memory effect. Or so I am told.
Any enlightenment on any or all of the above would be great.
I guess I need to learn some electronics for hire someone.
Hi again,
Well, i havent had to discharge mine ever so im not sure how much this really matters. Supposedly it helps so you may want to try it.
It's not really recommended to use the battery voltage itself as an indicator of the state of charge except for indicating extremely charged or extremely discharged. Either of those states we seldom if ever really care about, so it's a good idea just to forget about the voltage except about the way it *changes* when we are charging the cells. Even then though it becomes a bit of a problem when we start having to charge a bunch of cells in series rather than one individual cell. There are methods such as "Minus Delta V" which charge a cell until the voltage starts to dip and then it turn off, but that technique is designed for a single cell not really a pack of cells in series which is common in portable tools. This means we cant even rely on that to work for us.
The only thing that seems to be left is to determine what does work for packs of cells in series and try to use that. We know that voltage measurement does not work except in the extreme charge states, so we can not use that, but we know that if we supply a current through the cells they eventually charge after a certain time period and some finite charge period is considered long enough to charge but not do too much damage, and we also know that the cells all have a self discharge that has to be replenished on a regular basis, so we use this information to design a charger.
Based on the above the scheme then is to do a full timed charge after we use the pack, and do a partial short timed charge on a daily basis. This way we dont do too much damage to the pack with regular use plus we keep the self discharge replenished so the pack is ready for use after a longish period of non use.
As i posted previously, we use a timer that clocks out 24 hours to charge fully (pass transistor turned on the whole time) and another to keep the charger turned on for only 30 to 60 minutes each day. We use a push switch to initiate the 'full' charge cycle, and make the daily timer automatic.
If you want to discharge the cells periodically too, then all we need is another switch to initiate a 'discharge' cycle where we can choose to discharge after some time like a month or two or whatever you think would be appropriate. We need a more heavy duty transistor for this and some load resistance, but even 20 amp MOSFETs are pretty cheap these days so that wont be too expensive either.
If you like, using a PIC (or similar) chip we could program in delays such as two months or more to automatically initiate a discharge cycle, but im not sure how this would work out in the real world because what if a cycle starts just before we go to use the drill and we pick it up and it's almost discharged? Not too nice
I think i rather initiate it manually when i think it's appropriate.
Anyway, the project really calls for a microcontroller, but it may be possible to do it with a few timer chips. I've never had to create a delay as long as 24 hours with a timer chip though, but it may be possible with a few of them or maybe with an added divider chip. That would give us the 24 hour period we need for both a full charge and for a daily charge. We only need another timer then to time the 30 to 60 minute daily charge period, and perhaps another timer to time the discharge period.
What sounds best for you, or anyone else reading this? Other ideas for the timing?