Which battery for a Triumph Stag?

The factory specification is 12 volts, negative earth, 56 ampere hours at the twenty hour rate. Rimmer Bros list a type 071 as the original equipment specification; a current 071 runs to 12 volts, 70 Ah, around 700 cold cranking amps, in a case of roughly 269 by 173 by 198 mm. LDpart gives the case as 265 by 175 by 206 mm and names type 089 as an alternative.

A modern battery of the right size will exceed the original 56 Ah, which is a gain rather than a mismatch. The battery page gives the full factory background and the lead change at LD 20001.

LDpart says the case length should not be exceeded because the space at the end of the tray is needed for the hold-down rod, and warns that not every maker of an 071 or 089 keeps to the same dimensions. Several online battery finders return a type 069 for a Stag, which is a longer battery.

Lift the old battery out and look at the tray before buying. It is a recognised rust trap, and a new battery in a corroded tray will not stay secure. Fit the clamp properly: a loose battery chafes and can short.

Ask for the terminal layout of a type 071, so the leads reach without strain. The Stag is negative earth throughout production. Connecting a battery the wrong way round on a car with an alternator can destroy the alternator's diodes, so check which terminal the earth strap goes to before connecting anything.

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 on carburettors. Slow cranking with a good battery usually points to voltage drop in the leads, earths or starter circuit, which the electrical guide shows how to measure.

A conventional wet battery has free liquid electrolyte. An AGM battery holds its electrolyte in a fine glass fibre mat; VARTA describe the design as sealed, leak proof, maintenance free and resistant to vibration. That suits a car that sits for long periods or is stored indoors.

Battery makers stress that AGM and wet batteries need different charging, and that a charger may need resetting when you change type. The archive has no source confirming how the Stag's original alternator and regulator suit an AGM battery, so check with the battery maker before fitting one, and use a conditioner with the matching setting. A wet battery needs its electrolyte level checking if it has removable caps.

Persistent undercharging, not age alone, is what kills batteries in cars that stand. Over a lay-up either keep the battery on an intelligent conditioner, which monitors it and only tops it up, or disconnect it and store it on a dry bench and charge it periodically. An old style trickle charger pushes a constant current and is not the same thing. The storage guide covers the rest of a winter lay-up.

If a charged battery goes flat while the car stands, something is drawing current. Work in good light, keep sparks away from the battery, and do not crank the engine with a meter in circuit.

1. Charge the battery fully, close the doors, boot and glovebox, and switch everything off.
2. Disconnect the earth lead from the battery.
3. Set a multimeter to its highest current range and connect it in series, between the battery's earth terminal and the earth lead.
4. Read the current. A small draw, such as the clock, is expected. The archive has no verified figure for a normal draw on a Stag, so compare readings rather than chase a number.
5. Pull fuses one at a time and watch the meter. The circuit whose fuse makes the reading drop is the one to trace.
6. If no fuse changes it, disconnect the alternator and watch again. Owners point to failing alternator diodes as a cause of drain.
7. Check the boot and glovebox lamps go out when closed.

The fault finder entry for a battery that goes flat gives the same tests in its usual format.