Battery Charging & Electrical Supply
Ammeters were dropped in favour of voltmeters for the very good reason that ALL the car’s circuitry save only the high amp starter circuit had to run through the gauge, this was fine on older cars with only a handful of circuits but putting a modern on this system would be crazy. Plus if the ammeter packs up (as it had on a TR6 I was working on recently) EVERYTHING stops working!
It is undoubtably very inconvenient if an ammeter fails and becomes open-circuit, but one can readily bypass an ammeter if this situation arises. It is similarly inconvenient, if either one or both of a Triumph’s two fuses (direct battery supply & ignition-controlled supply) fail for whatever reason.
The voltmeter and ammeter complement each other, so both are desirable for monitoring the electrical system.
The principal purpose of an ammeter, is to show whether the battery is being charged or discharged rather than indicate the total current being supplied by the generator (i.e. dynamo or alternator) and/or battery to the electrical system (discounting the starter motor). This is why automotive ammeters have a scale encompassing both positive & negative readings!
The electrical system is prone to faults, which might cause a breakdown, less than optimum performance and more besides; about which, little if any information is provided, by the standard ignition warning light.
Even when the ignition warning light is extinguished, one cannot be sure that the battery is being charged; only that the difference in voltage between the battery and the generator (dynamo or alternator), is insufficient to cause a 12 V bulb to glow. In fact, the battery may even be discharging! To satisfactorily monitor the behaviour of the electrical system, one needs to measure both voltage and current flow; necessitating a voltmeter (sometimes known as a battery condition meter) and an ammeter.
In order to maintain the health of a lead-acid car battery, it is important that it should be fully recharged; ideally requiring a generator voltage of about 13•9~14•3 V (see Tony Tranter, "Automobile Electrical Manual", Haynes Publishing Group, 1983, Page 57)
. If the applied voltage is too low, full charge capacity will never be restored, resulting in the battery charge capacity and maximum discharge rate, progressively deteriorating, owing to sulphation of the plates. The principal purpose of the voltmeter, is to ensure that the generator's voltage regulator, is functioning correctly.
Some batteries are marked with the warning, that the external charging voltage, applied across the battery terminals, by the generator or mains charger, should not exceed 14•4 V. Severe overcharging, causes the battery to overheat, boiling off the sulphuric acid and possibly buckling the plates which may lead to an internal short, in one or more cells; rendering the battery unserviceable. Sulphuric acid vapour can also result in extensive corrosion, in surrounding body panels, as some owners have found to their cost.
Even if the optimum charging voltage is applied, it is not certain that the battery is being adequately recharged. This may arise if there are significant voltage drops in the cables or connections (very likely if vehicles are more than a few years old and haven't been adequately maintained or the cable connectors are merely crimped and not soldered), or the battery is nearing the end of its useful life, owing to sulphation or an internal short.
Under some conditions, the battery may even be discharging; supplementing the current provided by the generator, to meet high electrical loads, which typically occur, when driving in slow moving traffic, in cold, damp and dark conditions. The only way to be sure that the battery is being charged, is to have an ammeter in the circuit, directly between the generator and the battery, which will indicate the instantaneous rate of charging or discharging, but not the total quantity of charge stored.
A remote-shunt ammeter (made by VDO, Lucas and others), which requires only light-duty cable, is the most appropriate (see Austin Jack Smith, "Ampere and Volt Meters", Library Section, type2.com)
to the rear-engined VW, but the more widely available, conventional internal-shunt ammeter, may also be used, albeit in conjunction with several metres of ultra-heavy-duty cable.
As a 12V battery approaches a fully charged state, the voltage across its terminals, may rise to as much as 15•0~16•2V, dependent upon the applied voltage of the charging source. When the charging current ceases (i.e. the generator or battery charger has ceased operating), the battery terminal voltage quickly falls to about 12•6V. As the battery discharges at a nominal rate (effect of high current load is discussed later, in relation to starter motor operation), the terminal voltage falls rapidly from 12•6V to 12•0V, at which it remains for most of the discharge period, until close to the fully discharged condition, it rapidly diminishes to about 11•8V.
Hence, a voltmeter only gives any indication of the charge stored by a battery, when it is either close to the fully charged or fully discharged conditions. The voltage values cited in the preceding paragraph, assume a battery temperature of 25°C; the voltage diminishing as temperature decreases (a topic in electro-chemistry, covered as part of B.Sc. Chemistry). Before starting their engines in winter, Russian motorists allegedly turn on their headlamps for a short period, so that the current draw, warms the battery, thus increasing the voltage and current available to the starter motor; or so I've been told. It sounds plausible, but I have yet to try it myself!
From the preceding discussion, it is apparent that under most circumstances, a voltmeter and ammeter, only give an instantaneous view of prevailing conditions in the electrical system; any assessment of the state of battery charge, being reliant upon one's recall, of the history of charging and discharging, during the journey, which inevitably tends to be rather subjective. To measure the extent of total battery charge, would require some electronic circuitry, to integrate the charge flow rate with respect to time (i.e. electronics applied to integral calculus), which is readily achievable these days. Such instruments are available from some boat chandlers and are commonly advertised in magazines such as Practical Boat Owner.
Battery suppliers assess battery condition, when fully charged, by conducting a high-rate discharge test, simulating the high current discharge of circa 100 A, which occurs when running a typical starter motor. If one conducts a similar test, by observing the decrease in voltage reading, as the engine is turned over on the starter motor (typically 0•5~0•6 kW rating - circa 42~50 A @ 12 V, for a 1968~79 VW Type 2), one might reasonably expect a reading of about 11V with a new battery, which will progressively diminish with age, as the battery's internal resistance increases, owing to sulphation and other processes.
If a reading as low as 9 V is observed, then the battery is nearing the end of its useful service life. Sulphation may be inhibited by adding to each battery cell (preferably when new), about one heaped teaspoon, of an analytical chemical reagent, known as EDTA (i.e. Ethylene Diamine Tetra-acetic Acid or its Di-sodium salt), which I have done since the mid-1980s, at the recommendation of an analytical chemist, writing about this procedure, in the Motorcaravanners' Club magazine. My family's car batteries typically last about 6~8 years. The battery of my father's Ford Sierra XR4x4, lasted an incredible 13 years!
Whilst on holiday in Sweden in 1982, with my family's 1973 VW 1600 Type 2, we suffered two breakdowns, owing to failure of the dynamo and voltage regulator, on two separate occasions. In the first case, when the dynamo failed, I was warned that something was amiss, by sudden illumination of the ignition warning light, which after inspection of the V-belt, prompted us to visit a Volkswagen agent in Västervik, the next town, where the dynamo was replaced.
When the voltage regulator burned out some days later, the ignition warning light did NOT illuminate at any stage, despite a functional bulb. The first hint of this problem, was when the engine began to judder, in a way which was reminiscent of a faulty cut-out type rotor arm, shorting the HT ignition pulses to earth, at low engine revs, which I had experienced on holiday in Belgium, some years earlier. Having a burnt out voltage regulator, the battery wasn't being charged, so the ignition system and the headlamps (daytime use is compulsory in Sweden, unless one has special daytime running lights), were running on the battery's reserve capacity, leading to a drop in supply voltage.
Having stopped the vehicle on a steep uphill gradient (not the best of places!) and found the rotor arm to be okay, I attempted to restart the engine, but discovered that the battery was virtually flat. Not possessing a starting handle facility in those days (later fitted, owing to sporadic episodes, of the dreaded, 'dead starter motor syndrome'), I was obliged to undertake a rolling start, downhill, in reverse gear; a procedure which I would not generally recommend.
By driving with the headlamps switched off, there was sufficient battery voltage available, to run the ignition system, for several more miles, until we reached Vetlanda, the next large town, where there was a Volkswagen agent. Had we been driving in a country where daytime headlamp use was not required, we would have been stranded on the hill!
Another failure mode of voltage regulators, results in a battery being charged at an excessively high voltage, causing it to overheat, leading to the corrosive sulphuric acid electrolyte being boiled off, and in extreme cases, buckling of the battery's internal electrode plates, which renders it unserviceable. This is something which Ken Brimson (the then VWT2OC membership secretary) experienced in July 2013, with his 1968 VW 1600 Type 2 "Nelly", when returning home from a VW show, at Dettling, Kent, to his home in Winchester, Hampshire.
Not having a dashboard-mounted voltmeter, Ken was oblivious to his electrical-charging-system failure, until he got home. Using a hand-held meter, he discovered that the alternator (not a dynamo, as normally fitted to 1968~73 model-years) was charging at 14½ volts, at engine idle, and circa 16½~17 volts, when the engine was revved to RPM levels, typical of normal driving conditions. In 1987, Ralph Pettitt (1996~2001 Transporter Talk editor), suffered similar alternator voltage-regulator failure, which "destroyed" both batteries of his early-1980s VW 1600 Type 25 diesel campervan, whilst in Croatia (then part of north west Yugoslavia); prompting him to somehow limp home to England, with no on-board battery power.
Given that the output voltage of an alternator, is determined by both the rotor's rotational speed and the current in the magnetic-field coils, the output voltage would be regulated by interrupting or otherwise restricting, the mean current flow to the coils. Hence, with an inoperative voltage regulator, I envisage that the output voltage would increase in proportion to engine speed (hence the defunct batteries!), so one would be obliged to maintain the engine revs below a certain threshold. This could be achieved most readily, by having a dashboard-mounted voltmeter, but knowing the crankshaft to alternator pulley ratio, and something of the alternator's output characteristics, it could also be done indirectly after some calculations, using either an engine tachometer or the speedometer.