15.1 Batteries & Energy Storage

15.1 Batteries & Energy Storage

Quick Answer: Primary cells are disposable; secondary cells recharge. Lead-acid cell ≈ 2.06 V full; NiCd ≈ 1.25 V (12 series ≈ 15 V). Capacity is ampere-hours (Ah); runtime ≈ (Ah × V) / watts. Charge flooded lead-acid → hydrogen gas. Undercharge → sulfation. Series adds volts; parallel adds Ah. Size marine emergency banks for TX + RX continuous load over the required hours.

Topic 3-I (Power Sources) opens with batteries—the reserve energy that keeps radiotelephone, DSC, and radar alive when the ship’s generators or aircraft alternator are offline. Key topics 060 (Batteries-1) and 061 (Batteries-2) mix chemistry voltages with the arithmetic GROL techs use every time they size or prove a battery bank.

Primary vs secondary cells

Primary cells convert chemical energy once; you replace them. Secondary cells reverse the chemical reaction under controlled charge current. Element 3 emphasizes storage batteries because compulsory marine radio and most aviation installations depend on rechargeable banks tied to chargers and the ship/aircraft electrical system.

Cell voltages the pool expects

Lead-acid

Average fully-charged voltage of a lead-acid storage cell: about 2.06 V. A classic 12 V flooded or AGM battery is six cells in series (~12.3–12.7 V open-circuit when healthy and full). Nominal “12 V” labels hide that resting voltage and charge voltage are higher than the rough 2.0 V/cell under moderate load.

Nickel–cadmium

A nickel-cadmium cell has an operating voltage of about 1.25 V (pool also uses ~1.2 V for pack math). Twelve NiCd cells in series total about 15 V (12 × 1.25 ≈ 15 V). That is why older avionics and portable packs often appear as “15 V NiCd” strings rather than “12 V lead-acid” geometry.

Other common secondary chemistries (service literacy)

Element 3 exam items lock onto lead-acid 2.06 V and NiCd ~1.25 V / 15 V for 12 series. Treat NiMH and Li-ion as modern service knowledge that still obeys the same series/parallel and Ah rules.

Capacity, ampere-hours, and runtime

Ampere-hour (Ah) capacity is the product of discharge current and time the battery can sustain that current under a stated test (e.g., 20-hour rate). Idealized energy:

[ \text{Wh} \approx \text{Ah} \times V_{\text{nominal}} ]

[ \text{Runtime (hours)} \approx \frac{\text{Ah} \times V}{P_{\text{load (W)}}} ]

or equivalently (t = C / I) when you first convert load watts to amperes: (I = P / V).

Pool-style emergency radio examples

TX 325 W + RX 50 W = 375 W on a 12.6 V, 55 Ah battery:

[ t = \frac{55 \times 12.6}{375} \approx 1.85 \approx \mathbf{1.8\ h} ]

RADAR 315 W + radio 50 W = 365 W on 50 Ah @ 12.6 V:

[ I = 365 / 12.6 \approx 29.0,\mathrm{A},\quad t = 50 / 29 \approx 1.72,\mathrm{h} \approx \mathbf{1\ h\ 43\ min} ]

RX 75 W + TX 325 W = 400 W on 50 Ah @ 12 V:

[ I = 400 / 12 \approx 33.3,\mathrm{A},\quad t = 50 / 33.3 = \mathbf{1.5\ h} ]

RX 50 W + RADAR 300 W = 350 W on 8 Ah @ 12.6 V:

[ I = 350 / 12.6 \approx 27.8,\mathrm{A},\quad t \approx 0.29,\mathrm{h} \approx \mathbf{17\ min\ (0.3\ h)} ]

These problems are pure (Ah × V) / W—memorize the pattern, not the story settings.

Sizing a bank for continuous TX load

Pool capacity problem: operate a transmitter for 6 hours with continuous load 70% of 40 A key-locked demand plus 1.5 A emergency lights:

[ I = 0.7 \times 40 + 1.5 = 29.5,\mathrm{A} ]

[ C = 29.5 \times 6 = \mathbf{177\ Ah} ]

Always include duty-factor (key-locked vs continuous) and parasitic DC loads when specifying marine reserve capacity.

Series and parallel connections

Series is how six lead-acid cells make 12 V and how twelve NiCd cells make ~15 V. Parallel multiplies Ah for longer runtime at the same system voltage. Never parallel dissimilar chemistries, mixed ages, or cells at different state of charge without isolation/BMS—circulating currents and thermal abuse follow.

Charge rates, trickle charge, and internal resistance

Resistor trickle charge from a DC bus

To trickle-charge a 12.5 V battery at 0.5 A from a 110 V DC line, put a series resistor that drops the excess voltage:

[ R = \frac{110 - 12.5}{0.5} = \mathbf{195\ \Omega} ]

Modern marine systems use regulated chargers; the exam still tests this (V_source − V_batt) / I_charge relationship.

Charge / discharge rates (C-rate language)

A 1C rate discharges the full Ah rating in one hour (a 50 Ah battery at 50 A). C/10 is a gentle 10-hour rate often used for bulk charge of flooded lead-acid. High discharge rates reduce effective Ah (Peukert effect on lead-acid). Radio TX peaks are short but high-amp—banks and cable gauge must support peak current without huge voltage sag.

Internal resistance load math

Batteries are ideal voltage sources in series with internal resistance (r_{int}). Example: 6 V battery, (r_{int} = 0.01\ \Omega), 3 W / 6 V lamp:

[ R_{lamp} = V^2 / P = 36 / 3 = 12\ \Omega ]

[ I = 6 / (12 + 0.01) = \mathbf{0.4995\ A} ]

Two 12 Ω lamps in parallel (6 Ω) on a 6 V battery with 1.2 Ω internal:

[ I = 6 / (6 + 1.2) = 6 / 7.2 = \mathbf{0.83\ A} ]

Voltage sag under load is (I \times r_{int})—old, cold, or sulfated banks show larger sag and shorter real runtime than the nameplate Ah suggests.

Lead-acid care, hydrogen, and sulfation

Hydrogen on charge

When a lead-acid storage battery is being charged, a harmful effect to humans is emission of hydrogen gas. Overcharge electrolyzes water; hydrogen + oxygen is an explosion hazard. Charge in ventilated spaces, keep sparks/flames away from cell tops, and follow vessel/aircraft ventilation rules. Do not confuse this with “sulfation under constant charging” as the human hazard answer—the pool’s human hazard is hydrogen.

Sulfation

Sulfation is hard lead-sulfate crystal growth on plates after prolonged partial state of charge or deep discharge without full recharge. Capacity falls, internal resistance rises, and chargers may never finish absorption. Prevention: keep float/charge systems healthy, avoid long idle discharge, and equalize flooded cells only per manufacturer procedure.

Marine / aviation care checklist

1. Secure batteries against vibration; inspect hold-downs and terminal corrosion.
2. Maintain electrolyte (flooded) and use correct temperature-compensated charge voltages.
3. Prove capacity periodically (load test / Ah discharge)—Element 1 reserve-power rules expect scheduled condition checks.
4. Size cables for TX peak current; fuse at the battery.
5. Isolate chemistry: never charge Li-ion on a flooded lead-acid profile or vice versa.

Memory effect and NiCd notes

Classic NiCd “memory effect” (voltage depression) appears when cells are repeatedly recharged after only shallow discharge at the same depth—the cell appears to “remember” a lower capacity. Full discharge/recondition cycles (where the equipment manufacturer allows) and modern smart chargers mitigate it. NiMH is less memory-prone but still hates chronic overheat. Li-ion does not use NiCd-style memory; its failure modes are over-voltage, over-current, and thermal runaway without a proper BMS.

Exam-day battery checklist (3-I 060–061)

1. Lead-acid full cell ≈ 2.06 V; NiCd ≈ 1.25 V; 12 NiCd series ≈ 15 V.
2. Runtime ≈ (Ah × V) / W; convert minutes carefully (0.3 h ≈ 18 min → pool “17 min”).
3. Capacity size: (0.7 × key-locked A + accessory A) × hours → e.g. 177 Ah.
4. Trickle resistor: (V_line − V_batt) / I → 195 Ω example.
5. Charge hazard: hydrogen gas; chronic abuse: sulfation; NiCd shallow cycles: memory/voltage depression.
6. Series → volts add; parallel → Ah add; include r_int in load current math.

Master Ah math and the two classic cell voltages and the 060–061 items become mechanical. Next section covers motors, generators, and regulated power supplies that charge and support these banks.