DC House Batteries: The Epic, No-Nonsense Guide to Voltage, Chemistry, Amp-Hours, and Building a System That Just Works

Your “house” battery bank is the beating heart of an RV, boat, van, or off-grid cabin. It keeps fridges cold, laptops charged, fans humming, and lights bright when shore power is a memory. Done right, you get silent, reliable energy day after day. Done wrong, you get dim lights, tripped fuses, and short-lived batteries. This deep-dive unloads all the 4-1-1—voltage choices, chemistries, amp-hour math, wiring, fusing, charging, monitoring, sizing examples, tips, tricks, and a short FAQ—so you can design, install, and maintain a bank you can trust.

What Exactly Is a “DC House Battery”?

It’s the dedicated battery bank that powers your living systems—separate from an engine starter battery. The house bank runs DC loads directly (lights, fans, pumps, fridges) and feeds an inverter for AC loads (outlets, coffee makers, laptops). You size it for your daily energy use, your preferred depth of discharge, and how you’ll recharge (solar, alternator via DC-DC, generator/shore charger).

Voltage 101: 12V vs 24V vs 48V

• 12V: Ubiquitous, simple, tons of compatible devices. Best for small to mid-size systems, inverters up to ~2–3 kW, and short cable runs.
• 24V: Half the current for the same power, so smaller cables and happier inverters at 2–5 kW. Great for vans, larger RVs, and sailboats. Many DC loads still want 12V; use a quality 24→12V DC-DC converter for house circuits.
• 48V: Lowest current and cable sizes for big systems (5–8 kW+), common in cabins and serious off-grid rigs. You’ll convert down to 12V/24V for DC loads. Planning and parts availability matter more here.

Rule of thumb: If your inverter is ≤3 kW and cable runs are short, 12V is fine. Beyond that, consider 24V. At high continuous power (air-con, induction), 48V starts to shine.

Chemistry Deep Dive: Lead-Acid vs Lithium (LiFePO₄)

Lead-Acid Families

• Flooded (FLA): Affordable, robust, vented. Needs watering and ventilation. Typical usable DoD ~50% for good life. Sensitive to chronic undercharge; benefits from periodic equalization (per battery spec).
• AGM (Absorbent Glass Mat): Sealed, low maintenance, higher charge acceptance than FLA, better high-load performance, but still heavier/bulkier per Wh than lithium. Typical usable DoD ~50% for long life.
• Gel: Sealed, tolerant of deep discharge, lower charge rates; must use gel-appropriate charging profile. Less common in modern mobile systems.

Lithium Iron Phosphate (LiFePO₄)

• Pros: Light weight, high cycle life, ~80–90% usable DoD, flat voltage curve (steady power), fast charging, excellent round-trip efficiency.
• Cons: Higher upfront cost; requires a BMS (built-in on most drop-in batteries); do not charge below 0°C (32°F) unless the battery has a low-temp cutoff/heater.
• Use cases: Most mobile/off-grid users now choose LiFePO₄ for longevity and performance. It’s the standard for a 12V LiFePO4 house battery bank for RVs and boats.

Note: NMC/NCA lithium chemistries have higher energy density but are less common for DIY house banks due to safety and lifecycle preferences. LiFePO₄ wins on stability and cycle life.

Capacity: Amp-Hours vs Watt-Hours vs Usable Energy

Capacity labels can mislead if you don’t convert to a common unit. The key is watt-hours (Wh)—it lets you compare across voltages and chemistries.

• Formula: Wh = Volts × Amp-hours (Ah). For a 12V 100Ah battery, ≈ 12 × 100 = 1200Wh.
• Usable energy: Multiply Wh by usable DoD. Example: 12V 100Ah AGM (50% DoD) ≈ 600Wh usable; 12V 100Ah LiFePO₄ (80% DoD) ≈ 960Wh usable.

To compare two banks, convert to Wh and multiply by realistic DoD. That’s the energy you can count on between charges.

Sizing Your Bank (The Quick, Accurate Method)

1. List your loads: Fridge (60W avg × 24h = 1440Wh), lights (20W × 4h = 80Wh), fans (10W × 8h = 80Wh), laptop (60W × 3h = 180Wh), water pump (60W × 0.3h = 18Wh), misc. 200Wh. Total ≈ 1998Wh/day (call it 2.0 kWh).
2. Pick DoD by chemistry: AGM at 50% DoD, LiFePO₄ at 80–90% DoD.
3. Target autonomy: Days between charges. Many choose 1–2 days. For 1.5 days: 2.0 kWh × 1.5 = 3.0 kWh required usable energy.
4. Convert to Ah at system voltage: At 12V, 3.0 kWh = 3000 Wh ÷ 12V = 250 Ah usable. With LiFePO₄ (80% DoD), total bank ≈ 250 ÷ 0.8 = ~312Ah @ 12V. With AGM (50% DoD), ≈ 500Ah @ 12V.

Want 24V? Divide required Wh by 24V to get Ah at 24V. The energy is the same; current is half at 24V, allowing smaller cables and inverters that run cooler.

Series vs Parallel (and Why It Matters)

• Series: Increases voltage, Ah stays the same. Four 12V 100Ah in series = 48V 100Ah.
• Parallel: Increases Ah, voltage stays the same. Four 12V 100Ah in parallel = 12V 400Ah.
• Best practice: Keep parallel branch count low (1–2 strings) for easier balancing. If you need big energy, step up voltage (24/48V) rather than stacking many parallel strings.

Discharge Rates, Surge Loads, and the “C-Rate”

• C-rate = current as a fraction of capacity. 0.5C on a 100Ah battery = 50A.
• Lead-acid: High discharge reduces usable capacity (Peukert effect). Big inverters on small AGM banks disappoint.
• LiFePO₄: Handles higher C-rates with minimal voltage sag. Typical continuous 0.5–1.0C per battery (check spec). Great for coffee makers, microwaves, and short power tools bursts.
• Inverter surges: A 2000W inverter may surge to 4000W. At 12V, that’s 167–333A. Your bank and cables must tolerate it.

Charging: Solar, Alternator (DC-DC), and Shore

Solar (MPPT)

• Controller type: Use MPPT for efficiency, especially with higher-voltage panels.
• Setpoints: Always follow your battery’s spec sheet. Typical LiFePO₄ 12V absorb/bulk 14.2–14.6V; float 13.4V or disabled. AGM 14.4–14.8V absorb; float ~13.2–13.6V.
• Absorb time: LiFePO₄ needs minimal absorb (often 10–30 min once current tapers); AGM needs more time to reach full.

Alternator (via DC-DC Charger)

• Why DC-DC? Modern vehicles have smart alternators; a DC-DC charger provides correct voltage and isolates starter from house bank.
• Sizing: 30–60A DC-DC is common in vans; larger for RVs. Check alternator capacity and engine cooling.
• Wire & fusing: Long runs need heavier gauge to control voltage drop; fuse both ends near power sources.

Shore/Generator Charger

• Charger profile: Use a programmable charger with LiFePO₄ or AGM presets.
• Amperage: 10–20% of bank Ah is a reasonable charge rate (e.g., 30–60A for a 300Ah 12V LiFePO₄). Faster is possible—watch temps and cable ratings.

Temperature & Environment

• LiFePO₄ cold rule: Do not charge below 0°C (32°F) unless the pack has a low-temp cutoff or internal heater. Discharge is usually fine to ~-20°C (check spec).
• Lead-acid temp comp: Chargers should temperature-compensate (lower voltage when hot, higher when cold) to avoid over/under-charging.
• Ventilation: Flooded batteries produce hydrogen during absorb/equalize; provide venting. AGM/LiFePO₄ are sealed but still need general airflow for electronics.

Wiring, Fusing, and Hardware (Safety That Saves Your Day)

• Fuse the battery positive within ~7–12 inches (18–30 cm) of the post. Class-T or ANL for high-power inverters; MIDI/MEGA for mid-runs.
• Cable sizing: Size for current and voltage drop. Battery-to-inverter cables at 12V and 2000W are often 2/0 AWG (or 4/0 for longer runs). For 50A circuits over ~10–15 ft, 6–4 AWG is common. Verify with a voltage-drop calculator.
• Bus bars & shunts: Land all big cables on positive/negative bus bars. Put the shunt on the negative between the bank and the loads—everything except the chassis ground must be on the “load” side of the shunt.
• Lugs & crimps: Use tinned copper lugs, proper hex or hydraulic crimps, adhesive heat-shrink, and anti-oxidant paste on aluminum interfaces.
• Strain relief: Support heavy cables so battery posts aren’t load-bearing.

Monitoring: Know Your State of Charge (SOC)

• Shunt-based monitors: Count coulombs (amp-hours) in/out for accurate SOC—critical with LiFePO₄’s flat voltage curve.
• Voltage-only monitors: Acceptable for lead-acid at rest, unreliable for lithium under load/charge.
• Set capacity & Peukert: Configure your monitor for your bank’s Ah and chemistry; re-sync after a full charge.

Inverters & DC Loads: Matching Bank to Appliances

• Inverter size: Add up simultaneous AC loads. Coffee maker (900–1200W) + laptop (60W) + misc. (200W) → ~1.5 kW inverter suffices; microwave may need 2 kW.
• Surge: Pure sine inverters handle motor surges better. Check 2× surge ratings for fridges/pumps.
• DC loads first: Fridges and fans in DC save inverter losses. Use buck converters (24→12V or 48→12V) for stable DC house circuits.

Real-World Sizing Examples

Weekend Van (12V LiFePO₄)

• Daily use: 1.2 kWh (compressor fridge, lights, fans, laptops)
• Bank: 12V 200Ah LiFePO₄ (~2.5 kWh; ~2.0 kWh usable at 80% DoD)
• Charging: 200W solar + 30A DC-DC from alternator
• Inverter: 1200W

Live-aboard Sailboat (24V LiFePO₄)

• Daily use: 2.5 kWh (autopilot, nav, fridge/freezer, instruments)
• Bank: 24V 200Ah LiFePO₄ (~4.8 kWh; ~3.8 kWh usable)
• Charging: 600W solar (MPPT), 60A DC-DC from alternator
• Inverter/charger: 3000W

Off-Grid Cabin (48V LiFePO₄)

• Daily use: 6 kWh (fridge, lighting, devices, occasional microwave)
• Bank: 48V 300Ah LiFePO₄ (~14.4 kWh; ~11.5 kWh usable)
• Charging: 3 kW array, 60–100A 48V charger/generator backup
• Inverter: 6 kW split-phase

Charging Profiles (Typical Ranges—Always Verify Your Spec Sheet)

• AGM (12V): Bulk/absorb ≈ 14.4–14.8V, absorb time 2–4 hours depending on bank and charge rate, float ≈ 13.2–13.6V, optional equalize (as directed).
• LiFePO₄ (12V): Bulk/absorb ≈ 14.2–14.6V until current tapers; short absorb (often 10–30 min once taper occurs); float often 13.4V or disabled; no equalize.

Temperature compensation: AGM needs it. LiFePO₄ typically uses fixed voltages; many chargers disable temp comp for lithium but enforce low-temp charge cutoff.

Common Mistakes (and How to Avoid Them)

• Under-sized cables: Hot cables, voltage drop, inverter trips. Solution: calculate for current and length; upgrade to 2/0–4/0 for big 12V inverters.
• No main fuse: A dead short can melt metal. Solution: main Class-T/ANL fuse near battery positive.
• Mixing chemistries/ages: Parallel banks must be same type, capacity, age, and state of charge. Don’t parallel old AGM with new lithium; don’t mix sizes.
• Charging LiFePO₄ below 0°C: This can damage cells. Solution: heated batteries, battery warmers, or a charger with low-temp inhibit.
• Relying on voltage for SOC with lithium: It’s flat and misleading. Solution: use a shunt monitor.
• Chronic partial state of charge on AGM: Leads to sulfation. Solution: full absorb and periodic conditioning per spec.

Installation Tips, Tricks & Pro 4-1-1

• Layout: Keep batteries and bus bars physically close; short, fat cables beat long skinny ones.
• Balance parallel banks: Use diagonal take-off (pos from one end, neg from the other) so currents share evenly.
• Label everything: Heat-shrink labels on both ends of each cable; your future self will cheer.
• Service loops & strain relief: Leave slack at batteries and inverters; clamp cables so posts aren’t carrying weight.
• Chafe protection: Loom, grommets, and bulkhead bushings anywhere a cable crosses metal or a moving joint.
• Grounding: Bond negative to chassis/ground bus per best practice; keep AC and DC grounds appropriately bonded via your inverter/charger’s design.
• Fire safety: Mount an appropriate extinguisher within reach; use metal enclosures or battery boxes where needed.
• Spare parts kit: Extra fuses, a spare relay, lugs, a length of cable, and a quality crimper can save a trip.

Maintenance & Longevity

• LiFePO₄: Minimal maintenance; keep firmware (if applicable) and charger settings correct; avoid storage fully charged for months (store ~40–60% SOC).
• AGM/FLA: Regular full charges, equalize flooded per spec, check terminals for corrosion, and keep vents clear. Water flooded cells with distilled water as needed.
• All banks: Annual torque check on lugs, corrosion inspection, and shunt monitor calibration.

Glossary (Fast Reference)

• Ah (amp-hours): Charge capacity at a given voltage.
• Wh (watt-hours): Energy (Ah × V), best for comparing banks.
• DoD (Depth of Discharge): Fraction of capacity used. 80% DoD leaves 20% SOC.
• C-rate: Charge/discharge current relative to capacity (1C on 100Ah = 100A).
• MPPT: Solar controller that maximizes panel output.
• BMS: Battery management system that protects lithium cells.

Three Ready-to-Copy Long-Form Use Phrases (Woven Into This Guide)

We’ve spoken throughout about practical setups like a 12V LiFePO4 house battery bank for RVs, a 24V battery bank wiring diagram for boats, and off-grid solar battery sizing for cabins. Those reflect real-world needs and help you search for detailed build examples while staying focused on safety and reliability.

Quick Build Recipe (From Empty Compartment to Live Power)

1. Pick system voltage (12/24/48V) based on inverter size and cable lengths.
2. Choose chemistry (AGM vs LiFePO₄) by weight, budget, cold weather needs, and desired cycle life.
3. Size capacity in Wh from your daily load × days of autonomy ÷ usable DoD.
4. Design the charge stack: MPPT solar size, DC-DC alternator amperage, and shore charger profile.
5. Lay out components: batteries → main fuse → bus bars → shunt → loads/inverter.
6. Crimp lugs, install cables (short & thick), add chafe protection and labels.
7. Program chargers and monitor per battery spec; verify low-temp policies for lithium.
8. Bring online: charge full, confirm shunt calibration, load-test with real appliances.

Frequently Asked Questions

How big should my house bank be?
Sum daily Wh usage, multiply by your desired days of autonomy, then divide by usable DoD. Convert to Ah at your system voltage. It’s math, not magic.

12V or 24V?
If your inverter is ≤3 kW and runs are short, 12V is easier. For 3–5 kW or longer runs, 24V halves current and cable size. For large, stationary systems, 48V makes sense.

AGM or LiFePO₄?
AGM has lower upfront cost, tolerates cold charging with temp compensation, but is heavier with ~50% usable DoD. LiFePO₄ costs more but offers 80–90% usable DoD, fast charging, longer life, and lighter weight—just protect it from sub-freezing charge.

Can I mix battery ages or brands?
No. Keep parallel batteries identical in chemistry, capacity, brand, and age. Mixing creates imbalance and shortens life.

Is float charging needed for LiFePO₄?
Not typically. Many run a short absorb and then no float, or a low float around 13.4V for 12V packs. Follow your battery’s spec sheet.

How do I know my true state of charge?
Use a shunt-based monitor that counts amp-hours. Voltage alone is unreliable for lithium under load/charge.

What fuse type should I use?
For high-power inverter feeds, Class-T is a robust choice; ANL is common too. Place the main fuse close to the battery positive.

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

What about winter storage?
Store around 40–60% SOC in a cool, dry place. For LiFePO₄, avoid storing full for months; top up every few months. Don’t charge lithium below 0°C.

Final Word

Designing a DC house battery system isn’t complicated—it’s deliberate. Pick the right voltage, choose a chemistry that fits how you live, size capacity in watt-hours, charge it intelligently, and wire it like your safety depends on it (because it does). Do that, and you’ll enjoy silent, reliable power—morning coffee to midnight breeze—wherever the road, river, or ridge takes you.