Battery Basics

Lead-Acid (Flooded)

• Pros: Cheapest per Wh, proven technology, recyclable, forgiving of overcharge
• Cons: Heavy, require watering, vent hydrogen gas, 50% max depth of discharge
• Life: 3-7 years with good care
• Best for: Budget systems, stationary installations, cold climates

DIY Value: Golf cart batteries (6V 200Ah) wired in series are the best value. Two in series = 12V. Four = 24V.

Sealed Lead-Acid (AGM/Gel)

• Pros: No maintenance, no venting required, can mount in any orientation, spill-proof
• Cons: More expensive, sensitive to overcharge, still 50% depth of discharge
• Life: 4-8 years
• Best for: Vehicles, portable systems, locations where venting is difficult

AGM = Absorbent Glass Mat. Gel = gelled electrolyte. AGM handles higher currents, Gel handles deeper cycles but charges slower.

Lithium Iron Phosphate (LiFePO4)

• Pros: 80% depth of discharge, 10-15 year life, lightweight, no maintenance, built-in BMS
• Cons: Expensive upfront, sensitive to freezing while charging, requires BMS
• Life: 3000-5000 cycles (10-15 years typical)
• Best for: Long-term investment, mobile applications, limited space

LiFePO4 is NOT the same as laptop/phone lithium. It won't catch fire. It's the safe lithium chemistry for solar.

Nickel-Iron (NiFe / Edison)

• Pros: 20-50+ year life, nearly indestructible, handles abuse, works in extreme temperatures
• Cons: Very expensive, low efficiency (65% vs 90%+ for others), high self-discharge, hydrogen gas
• Life: Decades
• Best for: Permanent off-grid installations for generations

Edison invented these. They're still made. For when you want batteries your grandchildren will use.

The 50% Rule for Lead-Acid

Lead-acid batteries should never be discharged below 50%. Going deeper shortens life dramatically.

Usable capacity = Rated capacity × 0.5

Example: 100Ah battery = only 50Ah usable. If you need 100Ah usable, buy 200Ah rated.

The 80% Rule for Lithium

LiFePO4 can safely use 80% of capacity. Some go to 90% but 80% extends life.

Usable capacity = Rated capacity × 0.8

Example: 100Ah lithium = 80Ah usable. More efficient use of space and weight.

Three-Stage Charging

1. Bulk: Constant current at max rate until ~80% full. Fastest phase.
2. Absorption: Voltage held constant, current tapers. Finishes charge to 100%.
3. Float: Lower voltage to maintain full charge without overcharging. Holds battery at 100%.

Solar charge controllers and quality battery chargers do this automatically. Cheap chargers just push current until you unplug them.

Charging Voltages (12V System)

For 24V systems, multiply by 2. For 48V systems, multiply by 4.

Storage Temperature

• Ideal: 50-77°F (10-25°C) - Best life and performance
• Cold (32-50°F): Reduced capacity (can lose 30-50% at freezing), but OK for storage
• Very Cold (below 32°F): Capacity cut in half or worse. Don't charge lithium below freezing!
• Hot (80-100°F): Faster degradation. Every 15°F above 77°F cuts life roughly in half.
• Very Hot (100°F+): Rapid degradation. Batteries die young.

Batteries in a cool basement outlast batteries in a hot garage by years. If garage gets over 90°F, consider burying battery box or ventilating.

Temperature Compensation

Charge voltage must adjust with temperature. Colder = higher voltage needed. Hotter = lower voltage.

Compensation: ~0.03V per cell per 10°F change from 77°F

Good charge controllers have temperature sensors. Install the sensor on or near the battery.

Step-by-Step Calculation

Step 1: Calculate daily energy need in Wh

Total Wh/day = Sum of (Device Watts × Hours used)

Step 2: Convert to Ah at system voltage

Ah/day = Wh/day ÷ System Voltage

Step 3: Add days of autonomy

Ah needed = Ah/day × Days of autonomy

Step 4: Adjust for depth of discharge

Bank capacity = Ah needed ÷ DoD (0.5 for lead-acid, 0.8 for lithium)

Example: 2000Wh/day, 12V system, 2 days autonomy, lead-acid
Ah/day = 2000 ÷ 12 = 167Ah
With autonomy = 167 × 2 = 334Ah
Bank capacity = 334 ÷ 0.5 = 668Ah
Practical: 4 × 6V 200Ah golf cart batteries in series-parallel = 400Ah at 12V (close enough with conservation)

Flooded Lead-Acid (Monthly)

• Check water level in each cell. Add distilled water to cover plates.
• Clean terminals with baking soda and water solution. Rinse and dry.
• Check specific gravity with hydrometer (should be 1.265-1.285 when full).
• Inspect for corrosion, cracks, bulging cases.
• Equalize charge every 1-3 months (controlled overcharge to mix electrolyte).

Sealed & Lithium (Quarterly)

• Clean terminals and check connections.
• Inspect for damage, swelling, leaks.
• Verify charge controller settings match battery type.
• Check that BMS (lithium) is functioning.

Most "maintenance" is just looking for problems before they become failures.

Safety Rules

• Always disconnect negative terminal first, connect last.
• Use insulated tools or wrap wrenches in tape.
• Don't wear jewelry when working on batteries.
• Have baking soda and water ready for acid spills.
• Eye wash station or running water nearby.
• Vent the battery enclosure.
• Fuse or breaker on every circuit connected to batteries.

• Undersizing: Bank too small = constant deep discharge = short life
• Mixing ages/types: New battery with old = old drags down new. Same with different types.
• No temperature compensation: Undercharges cold, overcharges hot
• Ignoring water levels: Plates exposed = permanent damage
• Parallel without balance: Batteries in parallel need equal cable lengths or one works harder
• No monitoring: If you don't know state of charge, you'll kill them

Battery voltage dropping overnight

1. Check for phantom loads (disconnect everything, measure current).
2. Verify bank capacity matches load calculation.
3. Test individual battery health with load tester or hydrometer.
4. One bad cell can drain the whole bank. Isolate and test each battery.

Batteries won't reach full charge

1. Check solar input - panels may be shaded or dirty.
2. Verify charge controller settings match battery type.
3. Look for sulfation (chronic undercharging).
4. Bank may be oversized for available charging capacity.

Swollen or bulging battery

1. Disconnect immediately.
2. Do not attempt to charge.
3. Ventilate area.
4. Plan replacement - battery is failing internally.
5. Check charging system for overcharging.