Understanding DC-DC charging speed helps you plan around driving time and decide between a 30A and 50A unit. Here are the real numbers.
Charge rate by DC-DC charger size
| DC-DC charger | Charge per hour driving | Energy per hour (Wh) |
|---|---|---|
| 20A | 20Ah | ~256Wh |
| 30A | 30Ah | ~384Wh |
| 40A (Renogy DCC50S) | 40Ah | ~512Wh |
| 50A (Victron Orion XS) | 50Ah | ~640Wh |
These are the output amps to the house battery during bulk charging. In the absorption phase (battery above ~80%), charge current tapers off as the battery approaches full — total charge time is a bit longer than bulk math suggests.
Time to charge from various starting points (LiFePO4)
200Ah LiFePO4 bank
| Starting SoC | Target SoC | 30A charger | 50A charger |
|---|---|---|---|
| 20% (40Ah) | 80% (160Ah) | ~4 hours | ~2.5 hours |
| 50% (100Ah) | 90% (180Ah) | ~2.7 hours | ~1.6 hours |
| 60% (120Ah) | 100% (200Ah) | ~2.7 hours + absorption | ~1.6 hours + absorption |
100Ah LiFePO4 bank
| Starting SoC | Target SoC | 20A charger | 30A charger |
|---|---|---|---|
| 30% | 90% | ~3 hours | ~2 hours |
| 50% | 90% | ~2 hours | ~1.3 hours |
What affects charging speed
Battery starting state: The deeper the discharge, the longer the charge. LiFePO4 accepts full current until ~80–90% SoC, then tapers in absorption.
Alternator load: If the vehicle's other loads (headlights, AC blower, heated seats) are drawing heavily, the alternator may reduce available current to the DC-DC charger slightly. Most units self-regulate.
Simultaneous solar: Solar and DC-DC charge the battery at the same time. On a sunny travel day, a 300W solar array adds another 15–20A on top of the DC-DC output — a 50A DC-DC + 18A solar = 68A total, significantly faster than either alone.
Temperature: LiFePO4 BMS units block charging below 32°F (0°C). The DC-DC charger will show it's running but no current flows into a frozen battery. Self-heating battery models (Battle Born Heated, LiTime self-heating) solve this.
30A vs 50A: does it matter in practice?
For a 100–150Ah bank: a 30A charger typically refills the battery in 1–2 hours of highway driving — often enough that the extra cost of 50A isn't worth it.
For a 200Ah+ bank: a 50A charger meaningfully shortens charge time. Two hours of driving refills ~100Ah vs 60Ah with a 30A. If you drive relatively short legs between camps, the 50A makes a real difference.
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