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 internal resistance, charging characteristics, and their impact on dynamo performance.
| Identifier | ExFiles\Box 54\1\ Scan042 | |
| Date | 21th November 1925 guessed | |
| -3- Contd. Now on the high rate discharge our limit of battery internal resistance is .015 ohm, and that is the effective internal resistance of the battery when it is putting out 104 amperes through an invariable external resistance of .1 ohm. So you see that the resistance of the battery on charge is something more than 16 1/2 times what it is on discharge, whereas that of the Exide battery on charge is perhaps only two, three, or four times as much. It is a point I have not myself appreciated so fully as I do now, and that is that the act of charging a battery puts up the internal resistance considerably. (Quite apart from any rise of true E.M.F., that effect makes the dynamo charge at a higher voltage and amperage and still further aggravates the effect, so that it is essential so to construct the battery that this rise of resistance on charge is as small as possible.) Really we have got down to this, that the excessive charging rate, where it does occur, is much more a variation in the battery which is connected to the system, than in the dynamo. Our dynamos now-a-days, tested on a scheduled volt-ampere characteristic, are pretty consistent in their output, and I am afraid we can only ascribe these big variations of output (and there are certainly big variations from chassis to chassis from all accounts, because I am very definitely informed that a large number of chassis are O.K., but so far as I can make out to date, it has not been particularly noticed by the Test Dept. whether the cases of large output have been P & R batteries more particularly, though I myself think it must be so) to variations in the battery. You will think I am, as it were, having my own back upon you, but I am really not doing this. I have really and truly arrived at the definite conclusions which I have stated, that the cause of the variation lies more in the battery than in the system, and that it is an important thing in making batteries which have to run with these so-called inherently controlled dynamos, that the battery shall be made in such a way that its internal resistance cannot increase abnormally when on charge. I imagine what happens is that the gases that are produced cannot readily get away to the surface and that they actually reduce the current paths through the electrolyte sufficiently to put up the resistance to the extent observed; then of course the high resistance means a bigger generation of heat and a greater amount of evaporation takes place. | ||