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The Problem of Battery Charging

8th April 1938, Page 62
8th April 1938
Page 62
Page 63
Page 62, 8th April 1938 — The Problem of Battery Charging
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Which of the following most accurately describes the problem?

CE of the many advantages of electric vehicles for local delivery of milk, bread, groceries, etc., is the cheap running cost, which is low even when compared with that of a horse and van. This is largely due to the encouraging attitude of the electricity supply authorities, who benefit by establishing a reliable and often substantial off-peak load, tariffs as low as id. per unit being offered in some districts.

The power consumption for refuelling a vehicle running a matter of 25-30 miles with a pay-load of 8-10 cwt. is then a matter of 6d. or less, that is, about £7 10s. a year.

There is little difficulty in charging batteries of electric vehicles—it is a much more simple and cleaner job than with a petrol vehicle, which, after all, requires an external charge only when something goes wrong. The charging can be left entirely in the hands of the driver or roundsman, a semi-skilled man inspecting the batteries at regular , intervals, topping up, taking " gravity readings and so forth.

The vehicle driver, on reaching the depot, connects a plug, attached to the charger by a length of flexible cable, to a socket on the vehicle, and in the morning he merely disconnects the plug and drives away. A timeswitch is provided, which closes at the beginning of the cheap-tariff period, and the charger immediately comes into operation, until the battery is fairy charged, when the charger is automatically switched out of circuit, and remains so until the driver collects the vehicle in the morning.

When the power supply is a.c., some means for converting this to d.c. will be required, taking the form of a motor generator, a valve rectifier or a Westinghouse metal rectifier. Each of these requires some device for regulating the charging current to reduce it to a low value towards the end of the charge, to prevent damage to the battery plates. A motor generator is of use only where a fleet of large vehicles is maintained and a man is in attendance at night— engaged in cleaning, minor adjustments and repairs—who can keep an eye on the charging plant and regulate its output as the battery voltage rises.

Battery chargers employing a thertnionic or mercury-vapour valve as a rectifier have advantages over a Motor generator, in that they have no rotating machinery and are more simple to operate. Thermionic valves occasionally have a limited life, failing without any warning, with the result that the electric vehicle may be powerless the following day.

Valve Failure Undetected.

A spare valve may be kept and can be fitted in a few minutes, but the fact of a valve failure is not discovered, as a rule, until the driver finds the battery empty when he starts work in the morning. In some chargers of this type, however, if one valve fails others will take over the load.

Mercury-vapour valves have a much greater life, but are more costly, whilst their reduced efficiencies at low voltages rule them out for some purposes. The mechanism for starting up a mercuryarc rectifier bulb is fairly complicated and must be of substantial construction.

An interesting development is the charger employing a Westinghouse metal rectifier which is a robust unit with an unlimited life and of high efficiency. . This type of charging plant appears to be most popular in spite of its higher initial cost, and is used by many of the manufacturers of electric vehicles.

Where the power supply is direct current, a resistance bank or a motor generator is required. If a fleet of vehicles be kept, the batteries may be connected in series to build up a voltage nearly equal to that of the mains, when the losses will be reduced to a minimum. For economy, -it is important that all the batteries should be discharged to about the same extent, as the power consumption remains unchanged until all the batteries are charged, because each is switched out when fully charged and replaced by a resistance so that the current flow is not interrupted.

If there be only one or two vehicles having 60-volt batteries, the efficiency of the charging system will be about 35-40 per cent.; accordingly. a motor generator may be justified if current be expensive. '

The system of charging requires a full investigation where a fleet of vehicles is involved. A motor generator is occasionally found economical, but only where the batteries are very large, and a man is in attendance during the night, as with a corporation's fleet of refuse-collecting vans.

Multi-circuit chargers, incorporating valve rectifiers of some type, are a considerable improvement on the single-circuit valve chargers, as there is a number of valves in parallel, a failure of one merely reducing the current. The efficiency is low, however, clue to the large resistances necessary in each circuit to produce the falling off of current as the battery becomes more charged. Tests made on a number of these chargers show an efficiency through the period of charge of about 55-60 per cent., whilst the valves have tct be replaced eventually.

A group charger using a Westinghouse rectifier instead of a valve could be used, but the efficiency again would be low due to the regulating resistances. Westinghouse single-circuit chargers seem to be a solution, as they show particularly high efficiency, about 75 per cent. overall, whilst their maintenance is negligible.

Independent Charging Preferable.

This is the experience of retany large operators of electric _vehicles who run fleets of anything up to 270. each with a separate charger which is of the Westinghouse type. A separate charger for each vehicle as the advantage that any trouble occurring restricts the effect to only one vehicle, whilst, with a group charger, the whole fleet may be crippled by the blowing of a mains fuse.

In spite of the many advantages of the electric delivery vehicle, we must not shut our eyes to the high initial cost. Such vehicles are not bought on the score of price; why, therefore, are they becoming so increasingly popular? The answer is obvious— because they can be relied upon to have a long useful life-12-15 years— and to give practically trouble-free service during this period. To obtain such service there must be no weak link in the chain and the necessity of employing chargers which will also give long and trouble-free service is readily apparent.

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