Which battery should I buy?
QI need to buy a new battery for my 1958 Morris Oxford. It has a retrofitted Lucas ACR alternator. I've seen two suitable batteries, one of 51Ah and another of 58Ah capacity. Which would be better, on balance? Jonathan Prideaux, Whimple
Ed says
AWe'll generalise here. In principle, a 50Ah (Amp hour) capacity battery can supply 50A for one hour, 5A for 10 hours or 0.5A for 100 hours. So, it's a measure of how much energy the battery can deliver before it goes flat. A petrol engine that starts easily doesn't take much out of it. Most alternators produced since the late Seventies (including your Lucas ACR) produce a decent amount of voltage and current, even at low revs, and so will make up the lost charge quite quickly.
The other main power draw is the headlights and again, your alternator should be able to supply enough to power the lights and still make up for lost charge. Any battery that physically fits, can be secured in place and has the terminals the right way round will be suitable. The most cost effective is often a type 063 – typically around 45Ah. It is relatively small, which means it's versatile enough to rotate around several vehicles if you have a collection. For normal motoring, this should be fine for almost any petrol car.
Early alternators, often those with external regulators, are less effective and especially so at low revs – if additional driving lights are
heavily-used.
Dynamos struggle when headlights are on and revs are low. If not enough charge is supplied, the car has to use the reserve stored in the battery for much of its running time. In this case, buying a higher-capacity battery means it will last longer before an overnight charge is needed – but it'll still be needed.
If in doubt, fit a voltmeter to monitor the system voltage. If it spends most of its time between 13.5V and 14.4V, you're probably winning. If not, give the battery an overnight charge once a week or whenever the starter seems unenthusiastic. Once it gets to this stage, the generator alone will never be able to make up the lost charge. One more point. Although they may offer a tempting cost saving, secondhand batteries are a lottery: you don't know if they've been left half-flat for ages, which ruins them.