The Savvy Aviator #27: Battery TLC
Aircraft batteries are sensitive and fragile creatures, especially compared to their automotive brethren. Treat them with care and respect and they'll be there when you need them.
Batteries are the Rodney Dangerfields of aviation: They get no respect. We let them sit unused for weeks at a time and then expect them to crank our engine. We deep-discharge them by forgetting to turn off the master switch and then jump-start our airplane to go flying, subjecting the battery to a punishing rate of charge. We fail to check our aircraft bus voltage regularly, and allow it to drift too high or too low. Perhaps we check the battery's electrolyte level once a year at annual (if we don't forget); between annuals, it's out of sight and out of mind.
Then, after five or six years of faithful service, we curse them when they refuse to start the engine on a brisk, winter, Sunday morning in Cold-As-Hell, N.D., when there's not a mechanic or battery cart anywhere on the field.
We learned most of these bad habits from our experience with automobiles. Automotive batteries are big, heavy, hell-for-stout brutes that can take this kind of licking and keep on ticking.
But aircraft batteries aren't Die-Hards. They're built to be lightweight and compact. Their capacity is quite low compared to automotive batteries. Their plates are comparatively thin, fragile and closely spaced. They simply can't stand the kind of abuse -- either physical or electrical -- that car batteries seem to shrug off without even noticing.
If you treat it right, your aircraft battery should provide three to five years of reliable service (and some owners do even better). If you don't, it'll leave you stranded in the worst possible place at the worst possible time. Count on it.
Aircraft batteries are exquisitely sensitive to bus voltage and charging rate. A 12-volt battery requires a bus voltage of approximately 14 volts to reach and remain in fully charged condition. A 24-volt battery needs about 28 volts.
If the bus voltage is too low, the battery will not charge to its rated capacity. That might not be such a good thing if we have an alternator failure in IMC.
But if the bus voltage is too high, even worse things will happen: The battery will overcharge and overheat, often causing electrolyte to be lost and plates to warp. Invariably this drastically reduces the life of the battery, and in extreme cases the battery fails catastrophically.
Recommended Bus Voltage
|Temp.||12V Battery||24V Battery|
|120°F||13.8 volts||27.5 volts|
|90°F||14.0 volts||28.0 volts|
|60°F||14.3 volts||28.5 volts|
|30°F||14.5 volts||29.0 volts|
|<0°F||14.8 volts||29.5 volts|
|Source: Teledyne Battery Products|
The optimum bus voltage varies a little with ambient temperature, as shown in the table at right.
The solid-state voltage regulators installed in most of our aircraft are capable of holding bus voltage constant within a few tenths of a volt, and of varying the voltage slightly to compensate for temperature. But they need to be adjusted properly, and to be checked at every annual inspection (and readjusted if necessary) to ensure that the bus voltage doesn't creep out of spec.
Unfortunately, many mechanics neglect to perform an accurate bus-voltage check during inspections, and we frequently see airplanes with bus voltages that are significantly higher or lower than they should be. This is as serious a problem for battery life as over- or under-inflation is for tire life.
Owners would be wise to keep an eye on bus voltage themselves, and to bring it to the attention of their A&P if the voltage is not right. Any discrepancy of more than a few tenths of a volt is worth correcting. Adjusting the regulator is a very simple operation that only takes a few minutes to accomplish.
Nowadays, more and more of our airplanes are equipped with a digital engine monitor that provides an accurate digital readout of bus voltage. Some other digital instruments (such as 900- and 1000-series Stormscopes) also provide a digital voltage display. If your plane doesn't have a digital voltmeter on the panel, you can buy one for around $300 from Davtron, Electronics International and various other sources. Or you can use a standard digital multimeter hooked to your cigarette-lighter socket.
During periods of disuse, the battery will gradually lose its charge. The rate of such self-discharge is highly dependent on temperature. At an ambient temperature of 77°F, a fully-charged battery will lose approximately 1/4 of its charge every 30 days. For every 18°F increase in temperature, the self-discharge rate doubles! At 95°F the battery will lose 1/4 of its charge every two weeks, and at 113°F it will lose 1/4 of its charge every week.
Consequently, any time the airplane will be inactive for more than a couple of weeks, it's a good idea to put the battery on a trickle charger to maintain it at a fully charged state. This is especially important during hot weather. If the battery is ever allowed to discharge deeply (to 11.4 volts or 22.8 volts), it can sustain permanent damage.
There are a number of relatively inexpensive, microprocessor-controlled, trickle chargers now available that do a good job of maintaining a battery at full charge during periods of disuse, and that are "smart" enough that they can be used to trickle-charge the battery indefinitely without fear of overcharging. Brands that have worked well for a number of Cessna Pilots Association members include the "BatteryMINDer" and the "Deltran Battery Tender." These are available from such vendors as Battery Mart and Battery Stuff and come in both 12- and 24-volt versions.
If by some chance you do let your battery run down to the point that it won't start your engine, do not start the airplane with auxiliary power (APU) or by hand-propping and try to recharge the ship's battery with the aircraft alternator or generator. Doing so will subject the battery to a punishingly high rate of charge (generally, far in excess of 10 amps). If it doesn't cause a catastrophic battery failure, it will certainly take years off the life of the battery. If you find yourself in this situation, the smart thing to do is to hook the battery to a suitable charger and then go have a leisurely meal while it charges.
Some owners use an ordinary, automotive battery charger to charge their aircraft battery. This may or may not be a mistake, depending on the charger used. Many automotive chargers will charge at a rate that can be damaging to an aircraft battery.
Aircraft batteries should never, ever be charged at more than a 10-amp rate, and even that is pushing things. A 3-amp charge rate is just about ideal. Naturally, this means that the charging process will take some time. If your aircraft battery is rated at 35 ampere-hours and it is fully discharged, it will take about 12 hours to charge it to full capacity at a 3-amp charge rate. Patience is a virtue here: Charging the battery at a substantially faster rate may be hazardous to its health.
Most automotive chargers are "constant-voltage chargers" that apply a fixed voltage to the battery as it charges, similar to what the aircraft electrical system does in flight. With a constant-voltage charger, the charging current starts out relatively high and gradually tapers down toward zero as the battery becomes fully charged.
For initial charging, however, both Teledyne/Gill and Concorde recommend that their aircraft batteries be charged using a "constant-current" charger that gradually increases its charging voltage as necessary to maintain a constant charging current. A constant-current charger will charge the battery faster and more completely than a constant-voltage charger. The downside is that such a charger can easily overcharge and damage a battery if it is left connected for too long, so it's essential to monitor the battery's charge state (either with a hygrometer or a voltmeter) and disconnect the charger once the battery reaches full charge.
Teledyne/Gill sells two very nice, constant-current chargers, but at about $300 and $600 they cost more than most owners are willing to spend. Fortunately, there are a number of excellent 12- and 24-volt constant-current chargers available from suppliers of golf carts, motorized wheelchairs, and mobility scooters that cost a lot less and work quite well for aircraft batteries. A good choice is a 3-amp charger with an automatic three- or four-phase charge cycle, such as the 3-amp "Mobility Charger" and 3-amp "Deltran Battery Tender," both available from Battery Stuff for under $100. (Resist the temptation to buy a 5- or 8-amp charger.) These three-phase-cycle chargers work in constant-current mode to begin with, then automatically switch to constant-voltage mode as the battery approaches full charge so that they won't overcharge.
Both Teledyne/Gill and Concorde require periodic inspection and maintenance of their aircraft batteries. Gill requires an initial maintenance cycle at 800 hours or 12 months (whichever comes first), and subsequent maintenance cycles every 400 hours or six months thereafter. Concorde specifies the initial cycle at 600 hours or 12 months, and subsequent cycles every 200 hours or 12 months thereafter. (Most operators simply do this at each annual inspection.)
Both manufacturers require a four-step maintenance cycle:
- Check the electrolyte level and add distilled water as necessary to bring each cell up to the bottom of the split ring;
- Charge the battery to full capacity;
- Perform a capacity test; and
- Charge the battery once again to full capacity and return it to service.
Virtually every mechanic performs steps 1 and 2 at every annual, but few do steps 3 and 4. The reason for this is that not many shops have access to a capacity tester, and the ones for 24-volt batteries are rather pricey.
The capacity test simply consists of placing a specified load on the fully-charged battery and then measuring the time it takes for the battery to be drawn down to a specified voltage (10 volts for a 12-volt battery, 20 volts for a 24-volt battery).
For example, if your battery's rated capacity is 25 ampere-hours, then it should be able to deliver 25 amps for 60 minutes or 42 amps for 30 minutes. (Notice the discharge time does not vary linearly with discharge current.) If the capacity test reveals that the battery has at least 80% of its rated capacity (in this example, at least 48 minutes at 25 amps or 24 minutes at 42 amps), then it's considered airworthy and can be returned to service. On the other hand, if it has less than 80% of rated capacity, it should be replaced.
DIY Capacity Test
Even if you can't lay your hands on a capacity test box, it's not difficult to do a capacity test. All you really need to do is find a way to put the desired current load on the battery and then measure how long it takes for it to become depleted.
For example, my Cessna T310R uses a Teledyne/Gill G-246 battery rated at 24 volts and 19 ampere-hours. According to Gill's specs for that battery, it should be able to provide 19 amps for one hour or 32 amps for 30 minutes before the battery is fully discharged. Full discharge is defined as the point where the battery voltage decreases to 20 volts.
T310R Equipment Rated Load
|Prop Deice||15.0 amps|
|L. Landing Light||9.0 amps|
|R. Landing Light||9.0 amps|
|Pitot Heat||3.9 amps|
|Taxi Light||3.6 amps|
If I refer to the electrical loading chart in my T310R service manual and look for the things that draw the most current on a continuous-duty basis, I come up with the table at right.
These load figures are based on normal bus voltage (about 28 volts), but during the capacity test the battery voltage will average about 22.5 volts, so the loads really need to be adjusted downward by 20% to be accurate. Therefore, if I turn on prop deice, both landing lights, pitot heat and the taxi light, the total load should be about 80% of 40.5 amps, or a total of 32.4 amps. According to Gill's battery performance chart, my battery should be able to deliver 32.4 amps for just under 30 minutes. If it doesn't last at least 80% that long (i.e., just under 24 minutes), then it flunks the capacity test and needs to be replaced.
This is the sort of test that you can easily do yourself without A&P supervision, so long as you have access to a battery charger so you can fully charge the battery before and after the capacity test. You can look up the specs for whatever battery you use in your aircraft by going to the following websites: Gill Batteries and Concorde Battery.
If you or your A&P performs such a capacity test once a year, you can be pretty confident that your battery is in good shape and won't leave you stranded on a cold Sunday morning.
There's no hard-and-fast rule about how long an aircraft battery should last, but most owners seem to get three to five years out of them. If your battery doesn't last three years, chances are that it was abused by being deep-discharged and/or overcharged, or that your regulator is misadjusted and your bus voltage is too high.
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.