Winter flight operations bring additional challenges above our warmer weather adventures. Ice, snow, winds and extreme cold temperatures can hamper even the most seasoned General Aviation pilot as he/she tries to pilot from Point A to point B. Add in a cold soaked aircraft battery in an icebox tin hangar and your odds of cranking up your favorite Lycoming or Continental beast go down by a predetermined factor should your Gill or Concord battery not be up to the task. Poor or neglected periodic battery maintenance increase your chances of that dreaded dead battery and lowering that dispatch reliability that transforms your smiling passengers to one of disappointed frowns.
Most of us are handy and probably own or have access to a digital multi meter and know how to jump across the (+) positive and (-) negative terminals occasionally and get a reading close to either 12 or 24 volts and think all is right with the world. True, it is one of the indications you need to consider in hoping you're going to get airborne when the temperature begins to plummet, but a steady state voltage reading isn’t the only test that can be run to get a snapshot of your battery’s health.
Testing your battery’s electrolyte by getting a specific gravity reading is a maintenance function you or your mechanic should perform routinely to help develop a trend analysis of the overall health of the battery. A specific gravity reading is a ratio of the weight of a solution (battery electrolyte) to the weight of an equal volume of water. Specific gravity is used to give an indication of the state of charge of a cell of the battery. A batter hydrometer is the tool used to make the reading and is used to draw up battery electrolyte up from each cell and read a corresponding measurement. A fully charged battery will have a specific gravity reading of 1.265 at 80 degrees F. Most hydrometers have a temperature scale which will allow the user to either add or subtract from that reading depending upon ambient temperature conditions. The more dense the electrolyte, the higher the reading and the higher the state of charge.
I like doing a specific gravity measurement on my batteries because it gives me an indication on a cell by cell basis what is going on with the health of the battery. Connecting a multi-meter across the battery paints an overall picture of what the total voltage looks like but can’t pinpoint an individual cell that isn’t pulling its weight. There are many variables that influence the specific battery of a cell, temperature, state of charge, etc. but comparing the specific gravity indications of either 6 or 12 cells of your battery is another piece of the puzzle. Knowing your battery is healthy helps ensure your prop spins in a manner you're accustomed to next time you jump in the cockpit for that trip to the beach or just around the pattern a few times to add to your currency.
Next time you talk with your favorite mechanic ask them if they are routinely performing this test on your battery either at Annual Inspection or more often when taking a look at the battery electrolyte level from time to time. Its a check you can use to keep an eye on the health of the battery which in the long run will help ensure you're not canceling a flight due to a failing battery. As always, feel free to give us a call and we would be glad to share some of our knowledge on battery maintenance and give you some tips on this and other maintenance functions.
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