A battery is a voltage with an apology.
Every battery is two numbers, not one: an open-circuit voltage that falls as the charge drains, and an internal resistance that subtracts voltage in proportion to the current you draw. The label says 12 V; the instruments see the label value minus every one of those subtractions — chemistry, state of charge, temperature, and the wiring between.
The two chemistries in glider service apologize very differently. Sealed lead-acid starts near 12.8 V and slopes down all day, its internal resistance growing as it empties — the sag gets worse exactly when the reserve gets small. LiFePO4 holds a flat 13 V plateau across most of its capacity with a fraction of the resistance, then falls off a cliff near empty. Flat and firm beats high and sagging in every way but one: lead announces its death for hours, lithium gives you minutes — which is why a lithium pack must be flown on its gauge or a voltage alarm, not on intuition trained by lead.
The wire is part of the circuit.
Between the battery and your instruments sit two metres of wire, a fuse holder, a switch and three connectors — and every element has resistance. A thin 0.5 mm² run with tired crimps can easily total 0.3 Ω. At the panel's quiet 1 A that is a harmless 0.3 V; the moment the radio transmits and the total draw jumps to 3 A, it is a full volt — gone, in the wiring alone, at the worst possible moment. Transmit is the load that finds every weak crimp in the aircraft.
This is why the honest measurement is bus voltage at the instrument, during a transmit, near the end of a flying day — and why the classic mystery reboot ("only happens when I call downwind, only in the afternoon") is not a mystery at all. It is a voltage drop with a schedule.
Drain the battery. Key the radio.
Solid lines are the quiet panel; dashed is the same battery during a radio transmit. Now do the afternoon experiment: set the wiring to 0.4 Ω — one tired fuse holder is enough — and watch the lead battery's dashed line cross the red limit with three-quarters of its capacity still in the box. The battery was fine. The installation spent it.
Amp-hours for a racing day.
Capacity planning is one multiplication and one margin. Add up the panel: a modern computer and vario around an amp together, FLARM a quarter, transponder half when it is interrogated, radio a hundred milliamps listening and two amps only while you talk. Call it 1.5 A quiet for a full panel. An eight-hour competition day is then 12 Ah of demand — against a nominal 12 Ah lead battery that only delivers ten usable on its best day, aged toward seven. The arithmetic explains the fleet-wide tradition of the second battery better than any failure story.
The practice that follows: two batteries, switched, never paralleled — paralleling batteries of different age lets the strong one pump current into the weak one, and a changeover switch turns a fading supply from an emergency into a shrug. Budget honestly, switch at half-time, and land with reserve in both.
Fuses protect wires, not instruments.
One mental correction pays for the whole article: a fuse's job is to stop a shorted wire from becoming a fire, which is why it sits at the battery end of every run and is sized for the wire, not the load. An unfused metre of cable to the "temporary" phone charger is a metre of cable that can weld itself to the airframe. And the instruments protect themselves — modern LX units ride out dips and recover in seconds — but a recovery is still thirty seconds without a vario, a final glide computed from a cold start, and a FLARM rebooting in traffic. The goal is a system where they never have to.
Wire it once, measure it under transmit, alarm it at eleven volts — and the electrical system disappears from your flying, which is the highest compliment an installation can earn.
Power over Ethernet for the whole panel — one clean supply story instead of a loom of separate feeds.
The recorder draws milliamps and keeps logging — the last instrument standing on a fading battery.
Standalone vario with a wide supply range and a supply-voltage readout on the panel where you will actually see it.