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 page comparing the construction and performance of two different batteries, specifically focusing on plate elements and capacity.
| Identifier | ExFiles\Box 164\3\ img158 | |
| Date | 5th May 1938 guessed | |
| -2- Both batteries have 15 plate elements, consisting of 7 positives and 8 negatives per cell. It will be noted that the Delco positive plates are of the same dimensions as our own, but their negative plates, separators and container are smaller. In the case of our battery the negative plates are of the same dimensions as our positive plates, this being more or less standard practice for starter batteries. When the positive and negative plates are of equal dimensions the effect is to provide excess negative capacity, and the battery capacity is limited by the positive plates, so that in case of over-discharge the excess capacity on the negatives represents a reserve against the harmful effects of over-running the negatives. This is important in connection with getting a good life from the battery, but if life is not considered it may be argued that a thin negative can be fitted, as Delco have done, without affecting the capacity to any extent. This would be true if sufficient electrolyte were provided, but Delco have curtailed the acid by fitting a 15-plate element in what is actually a 13-plate container, and so restricting the available acid volume. Evidence of this is afforded by the final specific gravity on the discharge test, the figure being 1.105 - 1.115 for an output of 65.5 ampere hours. With our standard GX15 on its proper container the final specific gravity is about 1.100 at the end of a discharge of the full rated capacity of 98 ampere hours. Any restriction in the acid volume involves a lower final specific gravity and specific gravities lower than 1.100 directly affect the activity of the plates, and so reduce the available capacity. Taking these facts into consideration we find it difficult to believe that the capacity of the Delco battery when in good condition is much greater than 85 ampere hours at the 20 hour discharge rate. The present faulty condition of the battery may be due to some extent to it being used on test work and not getting regular service conditions, but the high voltage of .25 - .35 between the negative plates and the cadmium electrode at the end of our test discharge shows that the negative plates have suffered from being too small for the positives, and from the effects of insufficient electrolyte volume. For high rate discharge work the surface area of the plates is the main factor, and the starting performance of the Delco battery would be practically equivalent to that of our 3-GX15. | ||