The Triumph Stag battery

What the factory specified

The Repair Operation Manual data for the Stag gives the battery as 12-volt, negative earth, with a capacity of 56 ampere-hours at the twenty-hour rate. It is charged by a Lucas alternator rated at 540 watts, driven by a vee belt, and the starter is a Lucas M418G pre-engaged unit.

Fifty-six ampere-hours sounds modest by modern standards, and it is. Batteries have improved considerably in the same physical case, so a modern replacement of the correct size will comfortably exceed the original figure. That is a gain, not a mismatch, and there is no reason to hunt for a 56 Ah battery to be correct.

What to buy today

Rimmer Bros, the largest Stag parts supplier, lists type 071 as the original equipment specification. A current 071 runs to 12 volts, 70 Ah at the twenty-hour rate, around 700 cold cranking amps, in a case of roughly 269 by 173 by 198 mm.

Be careful with online battery-finder tools. Several of them return type 069 for a Stag, which is a physically longer battery. The marque specialist's recommendation and a tape measure across your own tray are better authorities than a lookup box, and a battery that does not sit properly in its cradle will chafe, move and eventually short.

Cold cranking amps were not quoted in the same form in 1970, so there is no factory figure to match. Anything at or above the 700 CCA of a current 071 is ample for a 3.0-litre V8 that starts on carburettors.

Negative earth, and why it matters

The Stag is negative earth throughout its production. This is worth stating plainly because a good many British classics of the preceding decade were positive earth, and owners moving between cars get caught.

Connecting a battery the wrong way round on a car with an alternator will destroy the alternator's diodes, usually instantly. If you are unsure which way your car is wired, check before you connect anything: the earth strap runs from one terminal to the body or engine, and on a Stag that is the negative one.

The battery leads changed at LD20001

This one catches people out. The positive battery lead is different between early and later cars, and the change falls at commission number LD20001, which is the start of the 1973 model year and the beginning of what enthusiasts call the Mk2.

Early cars, up to LD20000, use a two-piece positive lead: an upper lead and clamp, and a separate lower lead. Cars from LD20001 use a single positive lead and clamp, and that part is currently listed as no longer available. The negative lead and clamp are common to all models.

The practical consequence is that a later car needing a new positive lead may have to be made up rather than bought off the shelf. Check the commission number on the plate before ordering, rather than after.

The tray underneath

The battery tray is one of the Stag's recognised rust traps, and this archive's buying guide treats it as such for a reason. Trapped rainwater and spilled acid collect in it, and the result is either a perforated tray or a hurriedly patched one.

Lift the battery out and look at what it has been sitting on. A sound tray with clean paint says a good deal about how the car has been kept; a fresh-looking repair says something else. A battery cradle and a clamp kit are both available and are worth fitting properly, because a loose battery in a corroded tray is how the tray got that way.

A car that sits

Most Stags are not daily drivers, and a battery that spends weeks unused will not last. Persistent undercharging is what kills batteries in classic cars, not age alone. If the car is laid up, the archive's storage guide covers the wider routine, and the electrical guide covers the earths and connectors that account for most Stag electrical faults, both of which matter more than the battery itself once the correct one is fitted.