When Must You Replace an Aircraft Lead-Acid Battery? The 80% Capacity Rule in AC 43.13-2B
Under AC 43.13-2B, paragraph 1049b, a lead-acid aircraft battery "is considered airworthy if it meets 80 percent of its C1 (1 hour) capacity rating." The AC recommends returning batteries to service only above that minimum: 85 percent, or 51 minutes to end-point voltage. A battery that still fails after a second discharge test is replaced. Separately, the AC requires that after a complete loss of the primary generating system, the battery can power essential loads for at least 30 minutes.
Batteries are easy to judge by how well the engine cranks, and that's a poor test. AC 43.13-2B, Chapter 10 (Aircraft Battery Installations, dated 3/3/08), gives a measurable one. This article walks through it and the surrounding rules on installation and records. All figures come from the FAA documents indexed in the Kopik AC 43.13 base.
The 80% and 85% thresholds, in minutes
C1 is the battery's one-hour capacity rating: the test discharges it at the C1 rate and times how long it takes to reach the end-point voltage. Because the reference is one hour, the percentages translate directly into minutes:
AC 43.13-2B paragraph 1049b(6) thresholds
| Threshold | Capacity | Time to end-point voltage at C1 rate | Status |
|---|---|---|---|
| Airworthiness minimum | 80 percent of C1 | 48 minutes (0.80 x 60 min, our calculation) | Considered airworthy |
| Recommended return-to-service minimum | 85 percent | 51 minutes (stated in the AC) | Recommended |
| Below 80 percent after retest | Replace the battery (1049b(8)) |
Why the 5-point margin? The AC simply recommends returning batteries to service "when their capacity is above the minimum." A battery at 81 percent passes today but leaves little room before the next test.
The lead-acid capacity test, step by step
Paragraph 1049b sets out the procedure:
- Check for proper battery installation per the STC or manufacturer's ICA, at annual and 100-hour inspections and when replacing the battery after a capacity test.
- Stabilize the battery at 15°C (59°F) or higher for at least 24 hours.
- Remove the battery and charge it per the recommended charging instructions, then let it stand on open circuit for 1 hour.
- Connect the fully charged battery to a capacity tester with a load resistance, ammeter, voltmeter and timer.
- Discharge at the C1 rate to 1.75 volts per cell: that is 10 volts for a 12-volt battery and 20 volts for a 24-volt battery. Note the discharge time.
- Compare with the thresholds: 80 percent of C1 is airworthy; 85 percent (51 minutes) is the recommended return-to-service level.
- If it fails the minimum runtime, continue using the constant current C1 method in the manufacturer's ICA, let it stand on open circuit for 1 hour, and repeat the discharge test.
- If the failure persists, replace the battery.
- If the battery is airworthy, recharge it with constant potential (CP) before reinstalling it.
How often?
AC 43.13-2B does not set a fixed calendar interval for the capacity test. It says the battery should be removed and capacity tested, and to follow the battery manufacturer's ICA to determine service periods (1049a(1)). The same paragraph asks for inspection of electrolyte levels and the sump jar every 100 flight hours.
The 30-minute rule: what it says and where it sits
The often-quoted 30-minute figure appears in paragraph 1032b, in Section 3 on nickel-cadmium battery installations: "In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continue safe flights and landing." The 30 minutes include the time pilots need to recognize the loss of generated power and shed load.
For lead-acid installations, the battery installation checklist in Section 4 (paragraph 1043) asks a related question: if a battery is the only source of electrical power, determine whether it has sufficient capacity for dependable operation of all electrical equipment essential to safe operation. The AC does not restate a 30-minute figure in the lead-acid section, so check the aircraft's own certification basis and the manufacturer's data rather than assuming it.
Replacing the battery: who, how, and what to record
- Who: "Replacing and servicing batteries" is on the preventive maintenance list of 14 CFR Part 43 Appendix A (c)(24), which a certificated pilot may perform on an aircraft they own or operate that is not used under parts 121, 129 or 135 (§ 43.3(g)).
- Removal: master switch OFF and tagged, external power disconnected, then disconnect the ground cable first; on installation, install the ground cable last (1008, 1015).
- Inspection before fitting: cracks in metal or plastic containers are not permitted; dents in metal containers that impinge on the interior plastic container are not acceptable (1008b).
- Hold-downs and terminals: torque and safety-wire hold-down hardware per the airframe manufacturer's manual; torque terminal bolts as the manufacturer recommends; don't overtighten terminal nuts, which can fracture the posts (1008b, 1015).
- After installation: operational test, update weight and balance if necessary, update the equipment list if applicable, and make a logbook entry with the battery serial number and date of installation (1008b).
On weight and balance, paragraph 1021 says to recompute if the replacement battery's weight or location differs from the original. AC 43.13-1B Chapter 10 defines a negligible weight change as one pound or less for aircraft with an empty weight under 5,000 lb.
Swapping battery chemistry is not a battery change
When installing lead-acid batteries in place of nickel-cadmium, some airframes require deactivating or removing the temperature monitoring system, generally with a flight manual supplement. AC 43.13-2B notes that FAA field approval or an STC is required for this kind of alteration (1019d).
Installation details that keep a good battery good
- Battery box: open drain in case of electrolyte overflow, and venting to prevent hydrogen accumulation (1014).
- Sump jar, if installed: about one pint capacity, with a 1/2-inch pad saturated with a 5-percent sodium bicarbonate solution or about 3/8 inch of dry sodium bicarbonate; the inlet tube extends about 1 inch from the lid (1016).
- Drains at least 1/2 inch in diameter, with a positive slope and no traps (1018).
- Electrolyte kept just over the plates; top up with distilled or demineralized water (1049a(3)).
- Terminals: coat with paralketone, heavy grease or similar protective coating (1015).
- Shop safety: never service flooded lead-acid and nickel-cadmium batteries in the same shop area, and always pour acid into water, never water into acid (1001).
Lead-acid vs nickel-cadmium: how the AC compares them
Characteristics described in AC 43.13-2B Sections 2 and 3
| Point | Lead-acid (1013 onward) | Nickel-cadmium (1025 onward) |
|---|---|---|
| Voltage behavior | Voltage slowly drops | Constant voltage |
| Cost and upkeep | Typically less expensive; virtually no maintenance; no temperature sensor monitoring | Generally more expensive to purchase |
| End of life | Removed and replaced when unable to meet the manufacturer's capacity requirements | Capacity test per the manufacturer's recommendations (1050b) |
| Monitoring | Not required | Charge-rate control, over-temperature warning or battery failure sensing system required (1032a); sensor functional test at least once each calendar year (1050a) |
| Ventilation | Vented box; sump jar often fitted | Minimum airflow of 0.040 CFM (1.13 lpm); hydrogen mixture below 4 percent for viewport/louver cases (1030) |
Nickel-cadmium batteries also deliver their rated capacity in an ambient range of 70°F to 90°F, and the AC suggests replacement batteries with increased capacity to offset reduced capacity when cold soaked (1026b(2)).
Ask the base, for example: "After a battery capacity test, at what percentage of rated capacity is a lead-acid aircraft battery still considered airworthy?" The source document is AC 43.13-2B on the FAA website.
Battery, wiring and weight-and-balance answers, cited
The Kopik base covers AC 43.13-2B Chapter 10, AC 43.13-1B electrical and weight-and-balance chapters, and 14 CFR Part 43. Ask in plain English and check the paragraph it cites.
Frequently asked questions
At what capacity is a lead-acid aircraft battery no longer airworthy?
Per AC 43.13-2B paragraph 1049b(6), it is considered airworthy if it meets 80 percent of its C1 (1 hour) capacity rating. Below that, retest using the manufacturer's constant current C1 method; if it still fails, replace it.
What end-point voltage is used in the capacity test?
1.75 volts per cell: 10 volts for a 12-volt battery and 20 volts for a 24-volt battery, discharging at the C1 rate.
Can an aircraft owner replace the battery?
Replacing and servicing batteries is listed as preventive maintenance in 14 CFR Part 43 Appendix A (c)(24), which certificated pilots may perform on aircraft they own or operate, outside parts 121, 129 and 135, and then approve for return to service.
Where does the 30-minute battery requirement come from?
AC 43.13-2B paragraph 1032b, in the nickel-cadmium installation section: after complete loss of the primary generating system, the battery must provide at least 30 minutes of power to loads essential for continued safe flight and landing, including recognition and load-shedding time.
Do I need to redo weight and balance after a battery change?
AC 43.13-2B paragraph 1021 says to recompute it if the replacement battery's weight or location differs from the original.
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