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).
Battery voltage drop, starter motor performance, and transmission efficiency in cold weather.
| Identifier | ExFiles\Box 50\2\ Scan094 | |
| Date | 16th July 1921 guessed | |
| Oy1 - G 16721 Sheet #2. the United States, to have curves plotted on an ampere base. (3) With regard to the voltage drop mentioned by Mr. Clarke of 1 volt per 50 amps, we would ask him to refer to our letter Oy4-G8821 to the attention of Mr. Hives, on the subject of "Batteries and Starter Cables", and to the accompanying chart of terminal volts, copies of which are attached. It will be seen from this chart that the S.A.E. standard drop of terminal volts is 1 volt per 100 amps, starting at 12, and that all the curves we have sent apply to these terminal volts. Also, it will be seen that the Exide LXR battery (which drops 1.5 volts per 100 amps) and our light starter cable (24 feet of which drop .69 volts per 100 amps), militate seriously against the performance of the starter motor at the higher discharge rates, that is under winter conditions. For this reason, we are fitting, with the Bijur starter motor, No.0 S.A.E. cable consisting of 133 wires .0285 diam with an external diameter of .600". The drop in this case is only .22 volts per 100 amps per 24 ft. Also, we are obtaining a small shipment of Willard batteries, SJRN-28, which give a voltage drop of about 1 volt per 100 amps, and are expecting early shipment of a sample Exide battery of the new type 6-XXC-13-1, which gives a drop of only .8 volts per 100 amps. (In size and quality both these batteries are similar to the LXR). The result with the 6-XXC battery will be that our terminal volts will closely approximate to the S.A.E. standard, as shown by the chart. (4) With regard to the efficiency of the transmission which Mr. Clarke assumes, namely 87%, we have no accurate data on this point. From the observed fact, however, that in winter conditions (say freezing point) the R-R starter motor with LXR battery turns the engine at about 30 r.p.m., we think that the motor torque under cold conditions must be around 80 or 90 lbs. ins. We think the efficiency of the transmission under these conditions is not much more than 50%, largely because of sticky oil in the gear box. We should thus get an engine torque with 50% efficiency of 1050 lbs. ins. or 87 ft. lbs. Although we were not able to take accurate records of current consumption, all our experiences on cold cars last | ||