Size your DC-DC charger (alternator to house battery). Enter your alternator output, battery size, and driving habits to get the correct charger size, cable sizing, and alternator headroom check.
Why DC-DC charger sizing matters
VSRs destroy lithium batteries
A simple voltage-sensitive relay (VSR) dumps unregulated alternator voltage into LiFePO4 cells. They need a proper 3-stage charge profile — a DC-DC charger provides this.
Alternators overheat at >80% load
Running your alternator at full output for hours causes heat damage. We size the DC-DC to leave at least 20% headroom so your alternator lasts.
Dual-input = two sources, one cable run
Dual-input DC-DC chargers (Redarc BCDC, Renogy DCC) accept both alternator and solar — so you only need one cable run to the battery.
System Settings
Charging Scenario
Buy a 50A DC-DC charger
~128Ah per 3h drive
Charger Output
50A
Daily Charge
128Ah
Input Cable
16mm² / 6 AWG
Input Fuse
75A
Output Cable
10mm² / 8 AWG
Output Fuse
65A
Alternator load
Other loads: 15A + DC-DC: 50A = 65A of 80A alternator
LiFePO4 · 12V · 100Ah
Total alternator load (81%) is high. Running above 80% continuously can overheat the alternator. Consider a smaller DC-DC or reducing other loads.
At 50A output, you'll add ~128Ah per 3h drive — enough to reach target SOC in one drive.
LiFePO4 batteries accept high charge rates (up to 0.5C) — a larger DC-DC charger is worth it to maximise driving time.
Recommended DC-DC Chargers
Renogy DCC50S 50A
Dual-input (alternator + solar), good value
~$220–$280 AUD
50A output
DC-DC charger rules
A DC-DC charger (DCDC) is a must for LiFePO4 batteries — a simple VSR (voltage sensitive relay) won't charge lithium properly and can damage the battery.
Size the DC-DC to 70% of alternator headroom — never load the alternator above 80% continuously or it will overheat.
Dual-input DC-DC chargers (Redarc BCDC, Renogy DCC) accept both alternator and solar — one device, two charge sources, one cable run to the battery.
Always fuse both the input (at the starter battery) and output (at the house battery) of the DC-DC charger. Use the correct cable size for each side.
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