Rechargeable Batteries

Part of Energy Storage & Batteries

Rechargeable batteries store energy from generators and release it on demand. They are the bridge between intermittent power sources (wind, water, pedal) and continuous electrical service.

Why Rechargeable Batteries Are Essential

Primary cells are consumed as they discharge — the zinc dissolves, the chemicals deplete, and you need fresh materials to build new ones. Rechargeable batteries reverse their chemical reactions when you push current backward through them, restoring the original chemistry. This means a single set of electrodes can be charged and discharged hundreds or thousands of times.

In a rebuilding scenario, your power sources are intermittent: wind blows sometimes, streams flow unevenly, and human-pedal generators run only when someone is pedaling. Rechargeable batteries absorb energy when it is available and deliver it when you need it. Without them, you have electricity only when the generator is turning.

Lead-Acid Batteries

The lead-acid battery (1859) is the most practical rechargeable battery you can build from available materials. It uses lead plates in sulfuric acid and produces 2.1V per cell.

Materials Required

Component	Material	Source
Positive plate	Lead dioxide (PbO2)	Formed from lead
Negative plate	Sponge lead (Pb)	Cast lead sheet
Electrolyte	Sulfuric acid (H2SO4) ~30%	From sulfur + water process
Separator	Rubber, wood, or glass mat	Carved thin strips
Container	Glass, ceramic, or lead-lined wood	Hand-blown or pottery

Plate Construction

The key to a good lead-acid cell is maximizing plate surface area:

1. Cast lead sheets 3-5mm thick using a flat stone or iron mold
2. Score a grid pattern into both faces (3mm deep grooves, 5mm spacing) to increase surface area
3. Alternatively, cast lead into a grid frame and press lead oxide paste into the openings
4. For the paste method: mix litharge (PbO) with dilute sulfuric acid to form a stiff paste, press into grid openings, and let cure for 24-48 hours

Grid Casting

Cast lead grids by pouring molten lead into a flat mold with raised ridges (carved from soapstone or sandstone). The ridges leave channels in the casting, creating a grid pattern. This grid holds paste better than solid sheet and has 5-10 times more effective surface area.

Forming the Plates

Fresh lead plates must be “formed” — converted to their active materials by repeated charging:

1. Assemble positive and negative plates in dilute sulfuric acid (1.15 specific gravity)
2. Connect to a DC source at approximately 2.5V per cell
3. Charge for 24-48 hours at low current (C/20 rate)
4. The positive plate turns dark brown (lead dioxide) and the negative turns grey (sponge lead)
5. Discharge through a resistor, then recharge
6. Repeat this cycle 3-5 times until full capacity develops

Electrolyte Preparation

Sulfuric Acid Safety

Concentrated sulfuric acid causes immediate, severe burns. ALWAYS add acid to water, never water to acid. Adding water to acid causes explosive boiling. Wear eye protection and keep a bucket of clean water nearby.

Mix sulfuric acid with distilled water to achieve 1.26-1.28 specific gravity (approximately 30% acid by weight) for a fully charged cell. Measure with a hydrometer — a float in a glass tube that reads specific gravity from calibrated markings.

Charging and Discharging

Parameter	Value
Full charge voltage	2.10V per cell (12.6V for 6 cells)
Discharged voltage	1.75V per cell (10.5V for 6 cells)
Charging voltage	2.3-2.5V per cell
Maximum charge rate	C/5 (capacity divided by 5 hours)
Float charge voltage	2.15-2.20V per cell
Cycle life	200-500 cycles
Electrolyte SG charged	1.26-1.28
Electrolyte SG discharged	1.12-1.15

Never Deep-Discharge Lead-Acid

Discharging below 1.75V per cell causes lead sulfate crystals to harden on the plates (sulfation). These crystals resist re-dissolving during charging, permanently reducing capacity. Stop discharging when voltage reaches 1.75V per cell.

Nickel-Iron (Edison) Batteries

The nickel-iron battery (1901) is extraordinarily durable — some original Edison cells are still functional after 100+ years. It tolerates abuse that would destroy lead-acid batteries.

Materials

Component	Material	Notes
Positive plate	Nickel hydroxide + nickel flake	Packed in perforated nickel-plated steel tubes
Negative plate	Iron oxide + iron powder	Packed in perforated pockets
Electrolyte	Potassium hydroxide (KOH) 21%	Caustic — handle with care
Container	Nickel-plated steel	Welded or soldered

Why Choose Nickel-Iron

Property	Lead-Acid	Nickel-Iron
Cycle life	200-500	2,000-5,000+
Abuse tolerance	Low	Very high
Self-discharge	3-5%/month	20-30%/month
Energy density	Higher	Lower
Voltage per cell	2.1V	1.2V
Material cost	Moderate	Higher
Maintenance	Moderate	Lower

The nickel-iron battery’s weakness is high self-discharge and lower energy density. It excels in stationary applications where it is regularly cycled and longevity matters more than portability.

Construction

Building a nickel-iron cell requires more metalworking than lead-acid:

1. Make positive electrode tubes: roll thin perforated nickel-plated steel into tubes 6-8mm diameter
2. Fill tubes with alternating layers of nickel hydroxide and nickel flake (for conductivity)
3. Make negative electrode pockets: stamp or perforate flat steel, fill with iron oxide mixed with iron powder
4. Assemble plates with rubber or plastic separators
5. Place in a nickel-plated steel container
6. Fill with 21% KOH solution with a small addition of lithium hydroxide (improves capacity)

Simplified Nickel-Iron Cell

For a crude but functional cell: use a nickel-plated steel container as the positive electrode (coat the inside with nickel hydroxide paste), and suspend a porous iron block as the negative electrode. Fill with KOH solution. Capacity will be low but the cell will work and last decades.

Charging Systems

Connecting generators to batteries requires a charging system that prevents overcharging and reverse current flow.

Basic Charging Rules

1. Charging voltage must exceed battery voltage by 10-20% (e.g., 2.3-2.5V per cell for lead-acid)
2. Limit current to prevent overheating — C/10 rate is safe for most batteries
3. Block reverse current: When the generator stops, the battery will try to discharge through it. Use a blocking diode or mechanical switch
4. Monitor temperature: If the battery case becomes hot to touch, reduce charge rate immediately
5. Watch for gassing: Lead-acid cells produce hydrogen when fully charged. Ventilate the charging area — hydrogen is explosive

Constant-Voltage Charging

The simplest practical method for lead-acid batteries:

1. Set generator output to 2.35V per cell (14.1V for a 6-cell battery)
2. Connect through a current-limiting resistor sized for C/10 maximum current
3. As the battery charges, it draws less current automatically
4. The battery is fully charged when current drops below C/50

Trickle Charging

For maintaining a full battery from a continuous low-power source:

1. Set voltage to 2.17V per cell (13.0V for a 6-cell battery)
2. Current will be very low (milliamps)
3. This compensates for self-discharge without overcharging
4. Ideal for solar panels or small water turbines

Common Mistakes

1. Overcharging lead-acid: Excessive charging voltage boils off water from the electrolyte and corrodes the positive plates. Never exceed 2.5V per cell.
2. Mixing battery types in a bank: Different battery chemistries have different voltage curves. Never connect lead-acid and nickel-iron in series or parallel.
3. Ignoring hydrogen gas: Both lead-acid and nickel-iron cells produce hydrogen during charging. A single spark in a closed room with charging batteries can cause an explosion. Always ventilate.
4. Using tap water for electrolyte: Minerals in tap water contaminate the electrolyte and reduce battery life. Use distilled water (condensed steam) only.
5. Storing batteries discharged: Lead-acid batteries sulfate when left discharged. Always store fully charged and top up monthly.

Summary

Rechargeable Batteries -- At a Glance

• Lead-acid: 2.1V/cell, 200-500 cycles, most practical to build — requires lead, sulfuric acid, and careful forming
• Nickel-iron (Edison): 1.2V/cell, 2,000-5,000+ cycles, nearly indestructible but higher self-discharge and harder to build
• Plate surface area determines capacity — use grid casting with paste for maximum performance
• Always charge at controlled rates (C/10 or less) and never deep-discharge lead-acid below 1.75V/cell
• Ventilate charging areas — hydrogen gas is explosive
• Block reverse current with a diode or switch to prevent generator damage