VSR rating, cable size & fuses — with safety checks
Size the VSR to your alternator\u2019s max output, then size the cable to the current and run length (mm², 3% drop), and fuse both battery ends. A VSR won\u2019t work with a smart alternator or lithium battery — use a DC-DC charger instead.
Voltage-sensitive relay
120A
Cable size
70mm²
1.97% drop · current-governed
Fuse (BOTH ends)
125A MIDI
One at each battery terminal
Design current
120A
At 120A the 125%-fuse / 80%-cable rule can't be met on a single 70mm² cable. Run the cable in parallel or fit a DC-DC charger; any fuse shown only protects the cable.
Affiliate disclosure: shop links are Amazon Associates links; we may earn a small commission at no extra cost to you. Guidance only — confirm against your VSR, battery and alternator manufacturer specs before installing.
Size the VSR to your alternator’s maximum output (plus any other charger on the same battery). A deeply discharged leisure bank can briefly pull the full alternator current across the relay, so the relay’s continuous rating must exceed it. Common UK stock ratings are 120A, 140A and 230A.
No. Euro 5/6 smart alternators (roughly vehicles from 2015 onwards) drop charging voltage below the VSR’s engage threshold once the starter battery is satisfied, so the relay never closes. You need a DC-DC (B2B) charger instead.
No. Lithium (LiFePO4) batteries need regulated, current-limited charging that a VSR cannot provide, and can pull very high current that stresses the alternator. Use a DC-DC charger sized for lithium.
It depends on current and cable run length. The cable must both carry the current and keep voltage drop low over the round-trip run. This tool sizes it in mm² using the ISO 6722 ratings and a 3% (or BS EN 1648-2 0.8V) drop limit.
Yes. Fit a fuse at both battery terminals, close to each positive post. The leisure battery is also a power source and must be protected independently — a single starter-side fuse leaves the leisure cable unprotected.
Many kits pair a high-rated relay (e.g. 140A) with cable rated far lower. The cable becomes the limiting part and causes voltage drop and slow charging. Size the cable to the current and run length, not to the relay rating.