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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).
Report comparing the performance of Exide and P&R batteries with a dynamo, detailing voltage, amperage, and temperature results.

Identifier  ExFiles\Box 54\1\  Scan063
Date  2nd December 1925 guessed
  
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which we used various batteries in succession. The temperature rises, the dynamo terminal voltages, and minimum outputs, were observed, and we have made a copy of the table of results which we are attaching for you to see. The figures underlined in green are all for Exide cells, whereas those underlined with red are for P & R cells, and you will notice that no Exide cell is higher than 2.70 volts, whereas no P & R cell is lower than 2.83 volts, in one particular case, and 2.88 volts in general. You will see the effect on the volts and ampere output of the same machine, namely that the voltage of the system is 1.5 volts higher with the P & R batteries, and the ampere output is 1.2 higher. The figures not underlined are for cells of other makes of uniform size. Of the figures in the temperature rise column, the first is the ambient temperature, the second the rise, and the third the carcase temperature measured in the usual way. (All °C.)

You will also note that apart from difference in the output, that the output of a machine which is representative of standard is only 12.3 amperes on the P & R battery at 19-volts, and as previously stated the highest figure we have yet seen or recorded is 13.6, so the fact remains that I have not sufficient evidence to warrant a reduction in the output curve.

I do not know whether the battery we have tested is one of the lower resistance. However, I am ordering up for this purpose one of the latest batteries, fully charged, and we will try that as soon as it comes to hand.

In regard to letting you have a different dynamo, we will see if the movement which we have on the control brush will permit of cutting down to a peak of only 8 amps. At any rate we will cut it down as low as convenient and let you have a curve of it running on the wires on the new P & R battery when warmed up, and then you can say if you would still like to try it. In answer to your question "Why don't we try one as well ?", unless we run a car specially for the purpose, we do not ever get the conditions in our experimental cars at all satisfactorily. for this experiment.

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