Re: Alkaline vs NiMH batteries in electronic products

You have pointed out the two major considerations in substituting one 
battery for another; voltage and storage capacity.  The only other 
consideration is the amount of current that each battery can supply.

Handheld devices, unlike the starter motor on your car or the photoflash 
on you camera (two devices that need a lot of current) generally have low 
current demands.  So any battery should be able to meet the current 
demands of the device.  That takes us to the major consideration; voltage.

As batteries discharge their voltage tends to drop.  Most electronic 
devices that use semiconductors really aren't very happy about operating 
with anything but a fixed voltage.  Therefore, in many devices (like a 
PDA), an inverter circuit will be included to step up the battery voltage 
to say a constant 3.3 volts.  Some people with really sensitive hearing 
can actually hear the inverter in operation.  The frequency of the 
inverter is determined by the amount of power the hand held device is 
drawing.  When the PDA is turned off, you might hear an 
occasional "click."  When it is turned on, the inverter might emit a low 
hum.  Then when you activate the backlight, an second inverter provides a 
higher voltage to the devices providing the backlight, and you might hear 
a louder hum, particularly since the backlight uses more power.

The inverter is designed to operate down to a certain supply voltage; 
below that voltage, it just can't keep supplying the 3.3 volts to maintain 
memory and other operations of the PDA.  There is a little program called 
battery.prc that I found at:  http://ftp.northstar.k12.ak.us/palm/Palm_Utilities/battery%20Folder/ that 
you can load on a Palm type PDA that will actually tell you the current 
voltage of the batteries in your PDA. (There may be other locations you 
can find the same or similar programs; I just did a quick search on 
Google.com).  On my wife's Palm III, that program tells me that critical 
battery voltage for that device is 1.76 volts, with initial warnings 
coming at 2.05 volts.  So, in that device, if I use rechargeable batteries 
that supply 2.4 volts when completely charged, the device should work 
fine, but for not as long as with the alkaline batteries.  I suspect, 
however that the combined battery voltage will drop below 2.0 volts before 
all of that 1800mAh has been used.

So, with that being said, the first answer to your question is I don't 
know; it depends on how the circuit was designed and what the circuit 
designers expected the batteries to provide in terms of voltage and 
current.  Second, I believe that too little voltage will not damage your 
device.  Your device may not perform as well as you want; it might have to 
be recharged frequently, and, if you don't recharge as frequently as 
needed, you may occasionally have the voltage drop so low that you lose 
your data.  But it shouldn't cause your device to go up in smoke.

Too much voltage, on the other hand, is a bad thing.  Stick 3 volt lithium 
batteries in the device and you may well see smoke come out of it.

One last thought; there is at least one company selling rechargeable 
alkaline batteries.  In your search for batteries and a recharger, you 
might want to make sure that your recharger could charge both nickel metal 
hydride as well as the rechargeable alkaline batteries.