From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical explanation of vehicle electrical system voltage, battery charging characteristics, and their effect on lamp filaments.
| Identifier | ExFiles\Box 70\3\ scan0164 | |
| Date | 1st June 1926 | |
| Contd. -3- EFC1/T1.6.26. With regard to your impression that the voltage of the system will not rise much above 15, I am afraid this is not correct, and I see no reason to doubt any of the observations recorded in Mr. Sidney's son's letter. Quoting from a report which I recently made out in connection with some burning out of side lamps which had recently occurred, (this having turned out subsequently to have been due to a consignment of lamps with wrong filaments having left Messrs. Lucas') I said :- "It may not be generally realised that a so-called 12 volt battery may, whilst fully charged and charging, have a true E.M.F. of 16.8 volts (we have actually observed this figure across the terminals of a battery by switching off the charge completely and taking the figure immediately). When we add to this the drop of potential due to ohmic resistance in the battery and leads thereto, it is clear that the voltage of the system when the battery is fully charged and charging at the high rate, may, and in fact often does, reach and exceed 18.0 volts. Insofar as the battery itself can be expected to reach 16.8 volts it is clear that it is not altogether a question of the generator output, and that the lamps should be capable of standing 17 volts at least, and up to 18, without material diminution of life." As recorded by Mr. Sidney's son, there is not a great deal of difference between one make of battery and another in this respect, and temporarily higher E.M.F. is due to the presence of gasses, and though it may be slightly greater in the case of our own battery in which we have the additional ebonite separation, there is not much difference. There is, however, a difference in the additional voltage required to overcome the ohmic resistance of the battery, insofar as the ohmic resistance of our own battery, due to | ||