Hi there audioguru and protocol,
first audioguru:
I dont know if i would go that far, i think the risk is much less because we usually take the batteries off charge after a while anyway. That's actually the way i do mine. But i also monitor the current so i can tell when it gets low.
protocol:
That's a lot better. There are still some points to be considered carefully, and one as as audioguru pointed out, but i'll list here:
1. Charge voltage tolerance. The tolerance on this is that you can not go over 4.200 volts. You can always stop before this, but cant go over this. Due to voltage meter tolerances, it would be best to stop at 4.150v instead. I use a 50000 count volt meter to measure mine but most meters only have three digits at 4.2 volts, so your reading might be 4.20v, so keeping it down to 4.15v is a good idea. The absolute max is 4.250 volts but we dont want to go too close to that. Getting this kind of accuracy at room temperature isnt too hard, using a device like an LM317, but for a large ambient temperature range a better reference would be required.
2. Current limit. The current limit has to be set based on the manufacturer's data sheet. If each cell has a limit of 660ma then 660ma should be a good setting for three cells in parallel, because that means even if two cells are weaker and dont draw much current the third cell still only gets 660ma. So that should be a safe setting. Charge time for 2000mAHr cells maybe around 4 hours each, which comes out to about 12 hours for three in parallel (time approximate).
3. Low current cut out. Once the current gets down to about 1/20 fo the normal charge current, the charger should be shut off or cells removed.
4. Safety timer. The safety timer allows only a certain amount of time for a charge, no matter what else happens. In the case of 660ma and 2000mAHr cells, three in parallel, the timer would be set for around 12 hours. Some experimentation would be required to get this time right though. What kind of timer you use can vary, as long as you dont forget to check the cells.
5. Never leave the cells alone when charging. You should keep an eye on the cells as they charge, and never leave them alone.
6. Because there are so many cells in the pack, it would be a good idea to have a good fire extinguisher nearby ready to go. If you do three sets of three in parallel, you can keep the sets apart from each other too if possible.
If you follow the above you should not have any problems. The risk factor must be pretty low, and goes up as the current is increased because if one battery has to take all the charge then that one battery cell could get a current that is too high for it. A common mistake would be to try to charge three in parallel at three times the normal charge current reasoning that there are three cells and therefore three times the current is ok. But if two cells get weak then one cell gets three times the normal current...which is not good.
Other members might care to share their experiences too with this.
One last thing though, never feed them after midnight <chuckle>