Electronic – getting more from batteries via series connection

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.

You can see from the comments that your only potential option is from the lithium chemistry. The lead acid is just way too heavy and probably too bulky as well.

265Wh/Kg is quoted for lithium. This is correct, but also more of an accepted 'potential' value and not necessarily what you will get given your specific needs. For example your need for a relatively high current might require a lithium battery that is constructed slightly differently giving you a lower capacity per kilogram.

Let's look at what you can get in the real world. The LiMn battery that powers the Nissan Leaf will handle your compressor. With a little rearrangement of the cells you could make it into a 12.6 volt 60 Ah battery that weighs exactly 12 lbs (5.5kg). It would give a nominal 684 Wh and can supply as high as 240 amps! Note that this lithium battery doesn't come close to the example given by Russel McMahon!

Depending on your compressor's duty cycle, this might work for the 10 hours, or it might only give you 3 hours. You will easily get the 100 cycles you asked for even if your depth of discharge is 90%. .

This battery could be purchased for about $160 or so.