Aircraft Lead-Acid Battery Capacity Testing: The FAA's 80 Percent Rule in AC 43.13-2B, Explained
The FAA's AC 43.13-2B (paragraph 1049b) treats a lead-acid aircraft battery as airworthy if a capacity test shows at least 80 percent of its C1 (one-hour) rating, and recommends returning it to service only at 85 percent or more, i.e. 51 minutes to end-point voltage. The test discharges a fully charged battery, stabilised at 15°C or above, at the C1 rate down to 1.75 volts per cell. It is FAA guidance; the base doesn't cover UK CAA or EASA requirements.
A weak battery can go unnoticed until the day it matters. AC 43.13-2B, Chapter 10 (Aircraft Battery Installations, March 2008), replaces guesswork with a timed discharge test and a pass mark. Below is that test, the related installation rules, and the limits of the document. The circular is indexed, alongside AC 43.13-1B and 14 CFR Part 43, in the Kopik AC 43.13 base.
An FAA document, read from the UK
AC 43.13-2B contains methods acceptable to the FAA, for non-pressurised areas of civil aircraft of 12,500 lb gross weight or less. It does not replace the battery or airframe manufacturer's instructions, and it says nothing about UK CAA or EASA rules. Whether its test is accepted for your aircraft, and who may carry it out and certify it, are questions for your maintenance organisation or the competent authority.
The pass mark, translated into minutes
Because the C1 rating is a one-hour figure, percentages read directly as minutes of discharge at the C1 rate:
- 80 percent of C1, the airworthiness floor = 0.80 x 60 minutes = 48 minutes (our arithmetic from the AC's percentage).
- 85 percent, the AC's recommended minimum for return to service = 51 minutes (the AC states this figure).
- Anything under the floor goes to a second test, and then to replacement if it still fails.
Running the test in the workshop
Paragraph 1049b lists nine steps. In practice they fall into three stages.
Preparation
- Check the battery installation against the STC or manufacturer's ICA (the AC ties this to the FAA's annual and 100-hour inspections and to refitting after a capacity test).
- Stabilise the battery at 15°C (59°F) or higher for at least 24 hours.
- Remove it from the aircraft, charge it to the recommended instructions, then leave it on open circuit for 1 hour.
Discharge
- Connect it to a capacity tester with load resistance, ammeter, voltmeter and timer.
- Discharge at the C1 rate to 1.75 volts per cell: 10 volts for a 12-volt battery, 20 volts for a 24-volt battery.
- Note the time and compare it with the 80 and 85 percent marks.
Outcome
- Failed: continue with the constant current C1 method in the manufacturer's ICA, stand on open circuit for 1 hour, then repeat the discharge test.
- Still failing: replace the battery.
- Passed: recharge with constant potential (CP) before refitting.
The AC does not fix a test interval. It tells you to follow the battery manufacturer's ICA to determine service periods, and separately asks for the electrolyte level and sump jar to be inspected every 100 flight hours (1049a).
Putting the 30-minute requirement in context
You will often see "30 minutes" quoted alongside battery capacity. In AC 43.13-2B it sits in paragraph 1032b, under nickel-cadmium battery installations: after a complete loss of the primary electrical power generating system, the battery must supply at least 30 minutes of power to the loads essential for continued safe flight and landing, and that period includes the time for the pilots to recognise the loss and shed load.
The lead-acid material doesn't repeat the figure. What it does ask, in the installation checklist (paragraph 1043), is whether a battery that is the only source of electrical power has enough capacity for all equipment essential to safe operation. So a lead-acid battery passing at 80 percent has met the AC's capacity test; whether it meets an endurance requirement for your aircraft depends on that aircraft's own certification basis and data, which the base doesn't contain.
Fitting a replacement: the AC's checklist
Key points from AC 43.13-2B paragraphs 1008 to 1021
| Stage | What the AC asks |
|---|---|
| Isolation | Master switch OFF and tagged; external power disconnected and tagged |
| Cables | Disconnect the earth (ground) cable first, connect it last |
| Inspection | No cracks in metal or plastic containers; no dents in metal containers impinging on the inner plastic container |
| Tray | Clean and dry; treat and paint any corrosion |
| Hold-down | Torque and safety-wire per the airframe manufacturer's manual |
| Terminals | Torque as the manufacturer recommends; do not overtighten (posts can fracture); protect with paralketone or heavy grease |
| Afterwards | Operational test; weight and balance and equipment list updated if needed; logbook entry with battery serial number and date |
Weight and balance must be recomputed if the new battery's weight or its location differs from the original (1021). And changing chemistry is a different matter: fitting lead-acid in place of nickel-cadmium may require removing temperature monitoring and a flight manual supplement, and the AC states that FAA field approval or an STC is required for that alteration (1019d).
In the US system, "replacing and servicing batteries" is listed as preventive maintenance in 14 CFR Part 43 Appendix A (c)(24). That is a US rule for aircraft within Part 43's scope; the base does not say who may change a battery on a UK- or EASA-registered aircraft.
Before it goes in: delivery inspection and connectors
A new battery is not automatically a good one. Paragraph 1007 asks for a delivery inspection before it enters service:
- Damage: look for liquid spilled into the shipping container, which may indicate a damaged cell; check for dented, cracked or discoloured areas on the case sides and bottom, and for cracked cell cases or covers. A damaged battery is not placed into service.
- Shorting straps: some nickel-cadmium batteries are shipped with shorting devices across the main power receptacle terminals, which must be removed before the battery is put into electrical service.
- Electrical connections: test all terminal hardware for tightness, since poor contact can reduce discharge voltage, cause local overheating and damage the battery.
- Read the manufacturer's charging procedure before the first charge.
The quick-disconnect connector has its own replacement criteria (1020): replace the terminal pins for excessive pitting or corrosion that cannot be removed, signs of burning or arcing, a cracked part or housing, excessive wear on contact pins, socket lock pins or worm screw, large deposits on contacts or discoloured plastic, or an excessively loose handle and locking assembly.
Box, vent and sump: protecting the airframe
- The battery box needs an open drain for overflow and venting against hydrogen build-up (1014).
- A sump jar, where fitted, holds about one pint, with a 1/2-inch pad soaked in 5-percent sodium bicarbonate solution or about 3/8 inch of dry bicarbonate (1016).
- Drains at least 1/2 inch in diameter, sloping without traps (1018).
- Asphaltic or rubber-based paint on the structure adjacent to and below the battery box helps protect it (1003e).
- Keep lead-acid and nickel-cadmium servicing in separate shop areas, and always add acid to water, never the reverse (1001).
To check a figure quickly, ask the base: "After a battery capacity test, at what percentage of rated capacity is a lead-acid aircraft battery still considered airworthy?" The circular itself is on the FAA site: AC 43.13-2B.
The FAA battery chapter, searchable
Query AC 43.13-2B Chapter 10 with the rest of the AC 43.13 series and 14 CFR Part 43, and get answers that cite the paragraph.
Frequently asked questions
What does C1 mean on an aircraft battery?
AC 43.13-2B refers to the battery's C1 (1 hour) capacity rating. The capacity test discharges the battery at the C1 rate to 1.75 volts per cell and compares the time with that one-hour rating.
Is 80 percent capacity good enough?
AC 43.13-2B considers a lead-acid battery airworthy at 80 percent of its C1 rating, but recommends returning batteries to service at 85 percent minimum, or 51 minutes to end-point voltage.
At what temperature should the test be done?
Paragraph 1049b(2) says to stabilise the battery at 15°C (59°F) or higher for at least 24 hours before the test.
Does the 30-minute rule apply to lead-acid batteries?
In AC 43.13-2B, the 30-minute requirement appears in paragraph 1032b, in the nickel-cadmium installation section. The lead-acid section does not restate it; check your aircraft's certification basis and manufacturer's data.
Are nickel-cadmium batteries tested the same way?
No. AC 43.13-2B paragraph 1050b says the nickel-cadmium capacity test must be performed in accordance with the manufacturer's recommendations.
Can lead-acid and nickel-cadmium batteries be serviced in the same workshop?
AC 43.13-2B paragraph 1001e says not to service flooded or vented lead-acid and nickel-cadmium batteries in the same shop area, because acid and alkaline electrolytes may cross-contaminate.
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