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).
Letter to Messrs. McNeill & Co. discussing the limitations and usefulness of battery capacity meters.
| Identifier | ExFiles\Box 54\2\ Scan241 | |
| Date | 9th July 1929 | |
| X. 4457. c. Mailing. EFCl/AD. Messrs. McNeill & Co., Queen St. {Capt. P. R. Strong} , Broughty Ferry. 9th July, 1929. Dear Sirs, Yours of July 6th re Battery Capacity Meter. We know of these instruments, and have investigated two different makes. They are purely moving coil voltmeters with special scales. The P.D. across a battery or across two points connected to its terminals, is dependent upon so many circumstances other than that of the state of charge, that it is impossible for these instruments to be really more than rough indicators of the two extreme conditions, i.e. when to switch off charge when charging, and when to recharge after discharging. Also there is no such exact thing as the "state of charge" of a battery in the sense of a level of water in a tank. A battery in many ways, is comparable to a human being rather than to a machine. In so far as the reading of the indicator does not continue to increase under other constant conditions, it would be useful on the one hand, if its range goes high enough, as a definite indication when to switch off charge. (N.B. Every battery should occasionally be charged up to its full final P.D. on normal rate). On the other hand, it would be useful on discharge, to tell us when the voltage of a battery approaches 1.8 per cell (i.e. usually about zero on these instruments), and therefore when to recharge, in cases where such circumstances obtain on a car, e.g. parking, but it would not be found as satisfactory to rely on supposed ampere hour readings as on density of electrolyte to judge the remaining ampere hour capacity for standing lighting. | ||