Car Battery 101: Surface Charging Is Not Enough for Flat Batteries
A flat or dead battery won’t have any juice left to start a car. When you have this problem, the easy solution would be to bring in another car, connect jumper cables to the both batteries, and jumpstart the flat battery to life. Once the car has started, you've probably been taught to leave the car idling or go for a short drive, thinking that these actions will recharge the battery. Unfortunately, these 'surface charging' methods do more harm than good.
The fact of the matter is, you'll never fully recharge a flat battery by idling the engine, going for a drive, or running a short stop-start trip. Sure, these surface charging techniques can give your battery enough power to start the next time around, but then your battery remains undercharged. This is because your vehicle's alternator isn't designed to completely recharge a flat battery.
Rechargeable batteries need to achieve full charge to maintain their capacity. Surface charging--which is the same as continuously undercharging your battery--will lower the capacity of the battery in the long run, and shorten its lifespan. If your battery is still under warranty, you risk voiding it by not recharging it correctly.
The only way to fully recharge a flat battery is to use an applicable 'multi-stage' or 'smart' battery charger. Quality chargers generally cost somewhere between PHP1,500 to PHP2,500, which is practically half of the price of a good battery, so it's smart to keep one handy in your garage.