DC House Batteries: How to Choose, Size, Charge, and Install Safely

DC house batteries supply lights, fans, pumps, fridges, and inverters on the “home” side of RVs, boats, vans, and cabins—separate from the engine starter battery. This guide shows how to pick voltage and chemistry, size capacity in watt-hours, charge correctly, and wire with proper protection.

What Is a House Battery Bank?

A dedicated battery bank that powers DC loads directly (lights, fans, pumps, 12V fridges) and feeds an inverter for AC loads (outlets, microwave, coffee maker). It is sized around your daily energy use, depth-of-discharge (DoD), and how you recharge (solar, alternator via DC-DC, or shore/generator).

When to Choose It (and When Not To)

• Use if: You camp or cruise off-grid, want quiet power for fridges/electronics, or need short AC bursts via an inverter.
• Avoid if: You run continuous heavy AC loads (air conditioning, induction cooking) without enough battery and charging capacity—consider higher voltage (48V) or a generator-hybrid approach.
• Alternatives: A portable power station for light weekend duty, or shore/generator-only setups for occasional stops.

Select System Voltage

• 12V: Easiest and most compatible. Best up to ~2–3 kW inverters and short cable runs.
• 24V: Half the current for the same power, smaller cables, happier 2–5 kW inverters. Use a quality 24→12V converter for 12V house loads.
• 48V: For large systems (5–8 kW+). Convert down to 12/24V for DC loads; plan component selection carefully.

Rule of thumb: ≤3 kW inverter and compact wiring runs → 12V. Above that, step to 24V; at continuous high power, consider 48V.

Battery Chemistry: What to Pick and Why

Size Your Bank (Fast Method)

1. List loads (Wh/day): Example: Fridge 60W avg × 24h = 1440Wh; lights 20W × 4h = 80Wh; fans 10W × 8h = 80Wh; laptop 60W × 3h = 180Wh; misc. 200Wh → Total ≈ 2.0 kWh/day.
2. Pick autonomy: Days between charges (typical 1–2 days). For 1.5 days → 2.0 × 1.5 = 3.0 kWh usable.
3. Account for DoD: LiFePO4 80% DoD → 3.0 ÷ 0.8 ≈ 3.75 kWh bank. AGM 50% DoD → 3.0 ÷ 0.5 = 6.0 kWh bank.
4. Convert to Ah by voltage: At 12V, 3.75 kWh = 3750Wh ÷ 12V ≈ 312Ah LiFePO4. AGM would be ≈ 500Ah usable ÷ 0.5 = 1000Ah total at 12V.

Discharge Rates and Surges

• C-rate: Current as a fraction of capacity (0.5C on 100Ah = 50A).
• Lead-acid: High discharge reduces effective capacity (Peukert effect). Large inverters on small AGM banks underperform.
• LiFePO4: Typically supports 0.5–1.0C continuous per battery. Check your battery’s BMS specs.
• Inverter surge: A 2000W inverter may surge to ~4000W. At 12V, that’s ~167–333A. Size bank and cables accordingly.

Charging Options (Set Correct Profiles)

Solar (MPPT)

• Use an MPPT controller, especially with higher-voltage panels.
• Typical 12V LiFePO4: bulk/absorb 14.2–14.6V; short absorb after taper; float 13.4V or disabled.
• Typical 12V AGM: bulk/absorb 14.4–14.8V; float 13.2–13.6V; equalize only if specified.
• Always follow your battery’s spec sheet for exact setpoints and absorb times.

Alternator via DC-DC

• Use a DC-DC charger to manage smart alternators and protect the starter battery.
• Common sizes: 30–60A for vans; verify alternator capacity and cabling heat.
• Fuse both ends near power sources; size cables for voltage drop on long runs.

Shore/Generator Charger

• Use a programmable charger with LiFePO4 or AGM profiles.
• Charge rate guideline: 10–20% of bank Ah (e.g., 30–60A for 300Ah at 12V).

Wiring and Protection (Safety First)

• Main fuse: Install a Class-T or ANL fuse on battery positive within ~7–12 inches (18–30 cm).
• Cable size: Match current and length. 12V/2000W inverter cables are often 2/0 AWG (4/0 for longer runs). Confirm with a voltage-drop calculator.
• Bus bars & shunt: Land large cables on bus bars. Place the shunt on the negative between the bank and all loads/chargers.
• Terminations: Use tinned copper lugs, proper crimp tools, adhesive heat-shrink, and strain relief.

Temperature and Storage

• LiFePO4 cold rule: Do not charge below 32°F (0°C) unless the battery has a low-temp cutoff or internal heater. Discharge limits are lower (often to −4°F/−20°C). Check spec.
• Lead-acid: Use temperature-compensated charging to avoid over/under-charge.
• Storage: Store cool and dry. LiFePO4 around 40–60% SOC for long-term storage; top up every few months.

Compatibility Notes

• Most RV/boat DC loads expect 12V. If your bank is 24V or 48V, use a DC-DC converter to supply a stable 12V bus.
• Pure sine inverters minimize motor/electronics issues; check continuous and surge ratings.
• Keep parallel strings to a minimum; move up in voltage before stacking many parallels.

Quick Build Checklist

1. Select system voltage based on inverter size and cable runs.
2. Choose chemistry (AGM vs LiFePO4) by weight, climate, cost, and cycle life.
3. Size capacity from daily Wh × autonomy ÷ usable DoD; convert to Ah at your voltage.
4. Plan charging: MPPT solar size, DC-DC amperage, shore charger settings.
5. Lay out: batteries → main fuse → bus bars → shunt → loads/inverter and chargers.
6. Crimp/terminate cables, add chafe protection, label both ends, and secure for strain.
7. Program charge profiles; verify low-temp policies for lithium.
8. Commission: full charge, calibrate shunt, and test typical loads/surge events.

Common Mistakes to Avoid

• Undersized cables causing voltage drop and inverter faults.
• No main fuse at the battery positive.
• Mixing battery ages or chemistries in parallel.
• Charging LiFePO4 below 32°F (0°C).
• Relying on voltage-only SOC readings with lithium—use a shunt monitor.

FAQ

How big should my house bank be?
Sum daily Wh, multiply by days of autonomy, then divide by usable DoD. Convert Wh to Ah using system voltage (Ah = Wh ÷ V).

Should I choose 12V, 24V, or 48V?
Up to ~3 kW and short runs → 12V. 3–5 kW or longer runs → 24V. Large, continuous loads or stationary systems → 48V.

AGM or LiFePO4?
AGM costs less upfront but offers ~50% usable DoD and more weight. LiFePO4 costs more but provides 80–90% usable DoD, faster charging, and longer cycle life.

Can I charge from my vehicle alternator?
Yes. Use a DC-DC charger to manage voltage and protect the alternator/starter battery. Size for your alternator and wire gauge.

Do I need to float charge LiFePO4?
Often not. Many use a short absorb then no float or a low float around 13.4V (12V packs). Follow your battery’s spec sheet.