How can you tell if the cell has gas recombination capability?
I don't know of any way to tell without asking the manufacturer. If you know of a way, please tell me: I'm curious. That's not something I've looked into. Behaviour at end of charge may well provide some clues.
Anyway, as for traditional NiMH batteries:
The advice from most major NiMH suppliers is that traditional NiMH batteries over about 1800–2000 mAh should not be trickle charged at all. A few suppliers say you may trickle charge at low mA for a time after charging—but only for hours, not days.
A few suppliers tell you to trickle charge at C/10. Run away fast from their products.
As for LSD NiMH batteries:
- LSD cells are tricky. They have lower capacity per size, but this does not prove they're safe to trickle charge. I'd guess that, at rates well below C/10, Eneloop Lite cells are OK to trickle charge, Eneloop standard may be OK, and that Eneloop XX are not OK to trickle charge. For cells which are safe to trickle charge, C/40 sounds like it's probably an OK figure.
It sounds like you may have a handle on your question already, but I would like to add a couple of points about NIMH vs Alkaline.
You don't say how many watts or how many amps or milliamps your megaphone will be demanding, but the NiMH have a much lower internal resistance and so they can provide a much higher current without dropping their voltage as much as an Alkaline will. At higher loads, a Nimh will provide more power than a Alkaline; at very low loads, an Alkaline will provide more power (a very very loose generalization, but for example, an Alkaline will last forever in a remote control while the NiMH will not last as long - of course there is also the self discharge of many of the NiMH).
Also, most NiMH have a NOMINAL volatage of 1.2, but fully charged they start out at closer to 1.4 (I have measured some NiMH fully charged at 1.5) volts, and as mentioned already, hold to the 1.2 volts for most of their discharge.
The Alkalines often start closer to 1.6 volts, but quickly lose voltage as they discharge so that their average voltage through their life is about 1.2 Volts! Of course this depends on what the cut off voltage of your Megaphone is.
Don't forget that most NiMH have a very high self discharge rate and can lose 10-20% of their capacity in the first day, and 1% of their capacity per day just sitting there and are essentially completely discharged in 3 months or less! There are newer NiMH that hold 85% of their capacity over a year.
Check out http://batteryuniversity.com/learn/article/Nickel_based_batteries for more info.
As far as your 4 options, option 2 sounds like the best one. Options 3 and 4 have been discussed by others as to why they offer such poor performance.
Best Answer
Yes. However, make sure they have the same voltage before you connect them in parallel. Either charge/discharge to the same voltage, or connect them with a resistor to limit the current while they equalise, before connecting properly.
Given the high current capability of NiMH, it would be very wise to include a fuse in series with each battery. If one pack suffered a short in some of the cells, the other 9 could gang up on it to drive a fire-starting current through it.
Apply a charging current from some source.
That's the nominal voltage of the batteries, you'll need more than 24v to drive current through them.
While individual NiMH cells don't get damaged even if abused down to 0v, a battery of series connected cells can reverse any single cell that discharges first, so sending it below 0v. This can happen even with an apparently adequate voltage on the battery pack. Unless you monitor the voltage of individual cells, you will need to stop discharging at a very conservative threshhold, at least 1 V per cell, to reduce the possibility of this sort of failure.
The other way to reduce this sort of failure is to balance the cells with a proper overcharge, 16 hours at 0.1C. This is why we only hear about cell balancing with LiPos, as it happens easily and automatically with lead and nickel chemistries with a simple overcharge, not possible with LiPo!