The changing of the batteries takes three minutes with proper appliances.  One set of cells is drawn out by means of a small winch and a freshly charged set is put in.  It takes the same time to charge the battery as it does to discharge it in the working of the cars, so one reserve set would be sufficient to keep the car continually moving.

The loss of energy from standing about is probably nothing.  If a battery were to stand charged for three months in a dry case, the loss of energy might be in three months 10 per cent.  I purposely had a set of cells standing for two years charged and never used them.  After two years there was still a small amount of energy left.  So as regards the loss of energy in a battery standing idle, it is practically nothing, because no one would think of charging a battery and letting it stand for three months or a year.

I have had them stand three or four months and I could hardly appreciate the loss going on, provided always that the cells are standing on a dry floor.  If the exterior of the box be moist, or if it stands on a moist floor, there will naturally be a surface leakage going on:  but where there is no surface leakage the mere local action between the oxides and metallic lead will not discharge the battery for a very considerable time.

I have made experiments in London with a loaded car pulled by two horses.  I put a dynamometer between the attachment of the horse and the car, so as to ascertain exactly the amount of pull, measured in pounds multiplied by the distance traversed in a minute.  You will be surprised to know that two horses, when doing their easiest work, drawing a loaded car on a perfectly level road, exert from two to three horse power.  I have mentioned a car in Philadelphia where we use between two and twelve horse power.  A horse is capable of exerting eight horse power for a few minutes, and when a car is being driven up grades, such as I see in Boston, for instance, pulling a load of passengers up these grades, the horses must be exerting from 12 to 16 horse power, mechanical horse power.  That is the reason that street car horses cannot run more than three or four hours out of the twenty-four.  If they were to run longer, they would be dead in a few weeks.  If they run two hours a day, they will last three or four years.

The life of the cells must be expressed upon the principle of ampere hours or the amount of energy given off by them.  Street car service requires that the cells work their hardest for fifteen or sixteen hours a day.  The life of the cells has to be divided; first, into the life of the box which contains the plates.  This box, if appropriately constructed of the best materials, will last many years, because there is no actual wear on it.  The life of the negative plates will be very considerable, because no chemical action is going on in the negative plate.  The negative plate consists almost entirely of spongy lead, and the hydrogen is mechanically occluded in