For conventional cars without emission-saving technologies, buying a battery that has either a greater capacity (Ah), or cold crank start (CCA) ability is likely to be more expensive, but it can offer longer-term savings by lasting longer and reducing the risk of leaving you stranded. While upgrading is a good idea, especially if your car covers many short distances, underspecifying a battery is a false economy.
As an example, a low specification flooded lead-acid battery can provide up to 20,000 starts during its lifetime, while a premium battery will give up to 50,000. EFB and AGM batteries offer a significantly higher number of starts: up to 270,000 with EFB and 360,000 with AGM. So, upgrading a flooded lead-acid battery to an EFB is viable and economically sound, despite the higher purchase price. However, downgrading from an AGM to an EFB should never be considered, because of potential damage being caused to vehicle systems – the stop-start function might not work and the battery is likely to fail within a few months.
For cars equipped with regenerative braking and smart alternators, fitting a higher specification AGM battery might be possible, but consult your manufacturer, or a reputable aftermarket battery company, for advice on specifications and if any extra programming is necessary.