This is probably the most least understood part of trobleshooting. Whenever the car won't start, it is usually the battery that is blamed, then replaced.
So how do we tell what is the real cause? We can divide the faults into three classes.
1. The starter struggles to turn the engine over, the starter keeps trying, but only turns slowly. Sometimes if it is left to rest for a while, it will start. This is a battery that is low or flat. It is this way from either a leak in the electrical system, the drain is too large(BIG stereos) or the charge rate is too low.
2. Every thing has been going good, till one day you turn the key on, all the dash lights come, every thing is normal till you hit the starter. Everything goes off, no starter, no dash lights, no nothing. This is usually a dirty battery terminal. It has been getting worst over time, but will still pass enough current. Eventually the area that is passing current gets too small for the large starter current and the small area burns off to leave no contact area at all. You can tell if this is the cause by turning the lights on, then useing a coin or screwdriver to bridge between the battery post and the terminal clamp, if it is the cause the lights will come on. it will usually be the positive terminal that will go bad. Just clean the terminals.
3. You notice that the starter does not spin the engine as fast as usually, it starts ok though. Then one day after starting the car, you drive down to the shop turn it off, come back, hit the starter and is does not turn over. The dash lights come on ok, but when you hit the starter, the lights dim out, but come back on when you let the key go, you may hear the starter try to turn, or just click. This is caused by a cell in the battery that has gone bad. The cell has gradually lost storage capacity,or has a bad internal connection. You can tell if this the problem, by removing the cell caps, and watching the cells while somebody turns the starter. A bad cell will bubble and fume, it will also emit a punget smell. If the specific gravity of the cells is tested with a hydrometer, it will show the bad cell. If the battery is sealed, a volt meter will show the voltage drop from 12 volts to 10 volts as the cell drops out.
There are many other symptions and causes, but in my experence, it will be one of the three.
Harvey.
