How many amps does an alternator put into a house battery?

With a direct connection, roughly 30-40% of alternator output is available for the house battery after the vehicle's own loads. A 150A alternator typically delivers 40-55A to the house bank directly. With a B2B (DC-DC) charger, the output is capped at the charger's rated amps (e.g. 40A) but is more controlled and efficient.

Do I need a B2B charger for a LiFePO4 battery in a van?

Yes — strongly recommended. LiFePO4 batteries have very low internal resistance and can pull maximum current from an alternator, potentially overheating it. When the LFP battery's BMS disconnects at full charge, the voltage spike can damage some alternators. A B2B (DC-DC) charger like the Victron Orion-Tr Smart isolates the starter battery, provides a proper multi-stage charge profile, and protects both battery and alternator.

How long does it take to charge a 200Ah battery from the alternator?

With a 40A B2B charger and a 200Ah LiFePO4 battery at 50% SoC: 100Ah needed ÷ 40A = 2.5 hours of driving. Lead-acid takes longer because the acceptance rate tapers above 80% SoC — effectively 4-6 hours for a full charge even with a larger alternator.

What size B2B charger should I buy?

30A for small vans with 100Ah batteries. 40A for most full-time vans with 200Ah batteries — the most popular size. 60A for motorhomes and large builds with 300-400Ah. 100A for skoolies. Buy one size up from your estimate — a larger unit runs cooler and charges faster during longer drives.

Alternator Charging Calculator

How much does driving charge your battery? Enter your alternator output and battery size — get daily Ah added and hours to full charge.

Vehicle & charging details

Alternator output?

amps

House battery capacity?

Daily driving time?

hrs/day

Starting battery SoC?

B2B (DC-DC) charger?

Battery chemistry?

Alternator charging results

160 Ah added per day

Effective charging rate40A to house bank

Available from alternator40A (via B2B charger)

Energy added per drive day1,920 Wh (160 Ah)

Battery SoC after driving100%

Hours to full charge2.5 hrs driving

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How to Use This Calculator

Find your alternator output

Your vehicle's alternator rated output is usually printed on the alternator itself or listed in the owner's manual under "electrical system." Most modern vans: 120-180A. Heavy-duty trucks: 150-270A. Older vehicles: 60-100A. The alternator doesn't dedicate all output to your house battery — the vehicle's own loads (engine computer, lights, starter) take priority.

B2B charger selection

A DC-DC battery-to-battery (B2B) charger like the Victron Orion-Tr Smart or Renogy DC-DC is strongly recommended, especially for LiFePO4 batteries. Without a B2B charger, the starter battery and house battery are directly connected — potentially discharging your starter battery. A B2B charger isolates them and ensures proper charging profiles. Choose 40A for most vans, 60-100A for motorhomes and larger systems.

Starting state of charge

The starting SoC matters more for lead-acid batteries, which slow their charging acceptance above 80% SoC. LiFePO4 accepts full charge current until about 95% SoC, making it far more effective at using alternator charging time.

The Formula

Alt available for house bank: With B2B = min(B2B amps, Alternator × 0.40) Without B2B = Alternator × 0.35 Effective charging amps: LiFePO4 = Available amps × 1.0 (accepts full current) Lead acid = Available amps × (1 - SoC × 0.7) (tapers above 80%) Ah added = Effective amps × Drive hours New SoC = (Starting Ah + Ah added) ÷ Battery capacity × 100% Hours to full = (Empty Ah) ÷ Effective amps

Example: Weekend Van Road Trip

Transit van with 200Ah LiFePO4, 4-hour drive day

A Ford Transit campervan with a 150A alternator, 40A Victron Orion B2B charger, 200Ah LiFePO4 house battery, starting at 50% SoC. Driving 4 hours/day.

Alternator output150A

B2B charger40A Victron Orion

Battery200Ah LiFePO4

Starting SoC50%

Drive time4 hours

Result

Effective charging rate40A (full B2B output)

Ah added160 Ah

Energy added1,920 Wh

New SoC100% (from 50%)

Hours to full2.5 hrs driving

A 4-hour drive fully recharges a 200Ah LiFePO4 battery from 50% SoC via a 40A B2B charger. Combined with even 200W of solar, this van can run indefinitely without shore power in most locations.

FAQ

Will alternator charging damage my lithium (LiFePO4) battery? ▼

Without a B2B charger: potentially yes. Vehicle alternators charge at 13.8-14.4V, which is within the LiFePO4 acceptance range, but the BMS in LiFePO4 batteries can disconnect when fully charged, which can spike voltage and damage some older alternators. A B2B charger isolates this problem, provides a proper charging profile, and protects both the alternator and battery. The Victron Orion-Tr Smart is the gold standard — worth the $100-150 investment.

What size B2B charger do I need? ▼

30A B2B: small vans with 100-150Ah batteries and short drives. Adds ~360 Wh/hour. 40A B2B: most full-time vans and weekend rigs with 200Ah batteries. Adds ~480 Wh/hour. 60A B2B: motorhomes and large vans with 300-400Ah batteries. Adds ~720 Wh/hour. 100A B2B: skoolies and large builds. Adds ~1,200 Wh/hour. Buy one size up from what you think you need — heat buildup is the main limitation, and a larger unit runs cooler.

Can I skip a B2B charger with a VSR (voltage-sensing relay)? ▼

A VSR (or battery isolator) connects the batteries when alternator voltage is detected and disconnects them otherwise. This works with lead acid but is problematic with LiFePO4: the LFP battery's low internal resistance can cause it to pull maximum current from the alternator (potentially overheating it), and the sudden BMS disconnect when the LFP is full can spike voltage. VSRs are fine for lead-to-lead setups. For any LiFePO4 house battery, use a B2B charger.

How much does alternator charging add per day of driving? ▼

With a 40A B2B charger driving 4 hours: 40A × 4h × 12V = 1,920 Wh (160 Ah). With a 60A B2B driving 4 hours: 2,880 Wh (240 Ah). Most van builds need 800-1,500 Wh/day, so 3-4 hours of driving with a 40A B2B covers most daily usage. For heavy users (AC, induction cooking), driving alone won't cover it — solar becomes essential for stationary days.