Batteries

You cannot really predict how much capacity is left in an alkaline battery by measuring the voltage.  However, the attached curve shows some typical discharge curves at two different temperatures.  If you measure the loaded voltage (for example, an Aquadopp operating for 5 minutes while collecting data at 1-s measurement interval), the curves can give you some sense of how much capacity is left.  It is not accurate to 5 or 10%, but it can tell you if a battery is "almost new" " />'> or "almost depleted" " />'>  

- Atle Lohrmann

We are very cautious in the use of batteries.
It is just not worth taking any risk with possible non-collection or loss of data or any instument failure.
Consider the cost of deployment and recovery of an instrument and place that against the cost of a fresh battery pack.

As standard practice we always install a fresh battery pack before every instrument deployment, whatever the instrument, whether a data collecting instrument such as a currentmeter, or an acoustic release to go in an oceanographic mooring.

When we supply an acoustic release to a client from our rental pool, we install fresh batteries just before despatch. Even if the unit has not be used during the rental, we still replace the batteries before it goes out again. And we recommend that batteries are replaced every time an instrument goes in the water.

We buy battery cells i bulk direct from the manufacturers, check the voltage on each cell before dropping in place, and only use cells from the same batch in a pack.
For good contacts we clean the contacts with a glassfibre pen

A general equipment design point:
I do not like the concept of special battery packs for instruments. When you buy a ghetto blaster or Walkman you are not obliged to go backto Mr Sony for a special battery pack. You just pop into the nearest shop and buy the appropriate size cells, drop them in, and get on with using the equipment. iXSea Oceano instruments have embodied this design concept since 1977. This has saved several operations in remote locations such as West Africa where shipments of spares from Europe have not arrived. Field operators have been able to go to the nearest bicycle shop, buy every simple zinc-carbon battery in the store and use them to keep the show on the road (or at sea).

And we have never used rechargeable batteries - it is very difficult to gauge remaining capacity, and techies still forget to switch the charger on!