Grid technology and silver additive influence on VRLA-AGM batteries performances for electric vehicle application

With the purpose to develop improved positive grids for electric vehicles (EVs), tests on valve-regulated lead acid (VRLA)-AGM prototype batteries have been carried out. Batteries were made using positive grids from gravity cast technology or rolled expanded technology, with and without silver additive at 0.025% and 0.05%. Batteries were submitted to self-discharge tests, deep discharge tests and TC69 cycling. The effect of silver after the self-discharge test of batteries is not significant, or very small, for both technologies; the differences observed between grid technologies are small, but give a better behaviour of batteries using rolled expanded grids, taking into account the lower weight of such grids for the EV application. The effect of silver in deep discharge test of batteries is only significant for gravity cast technology, but even then is limited. No effect of silver is observed for the rolled technology. The differences observed between grid technologies show a better electrical behaviour of batteries using gravity cast grids. However, the voltage limitation should be adjusted to higher values for better rechargeability of the rolled expanded technology according to EV application. The evolution of specific energy of batteries with TC69 cycling shows an important difference between the specific energy of the two battery technologies without over significant effect of silver. The batteries using rolled expanded technology give better results in specific energy and better stability during the TC69 test. However, due to the softening failure mode, it is very difficult to relate directly the role of grid alloy composition and grid technology to the behaviour of batteries in the cycling test. Other parameters like positive active mass elaboration and battery design play a major role in such test.