Recently I saw a neighbour looking angrily at the dead battery in his car. The battery looked well past its best and although I managed to get the car mobile with a jump start, I urged him to get the battery tested and reckoned it was time to buy a new one.
I would have suggested testing the battery before opting for a replacement, but the disgruntled owner did not seem in the mood for helpful suggestions, so after getting his car mobile, I left it to him to decide what to do.
The DIY way is pretty straightforward; when a battery goes flat, remove it from the car and give it a full charge. If it charges very quickly from dead that is bad news, contrary to what you may think, because it normally means the plates are well past their best and a new battery is needed.
Conversely, if after 24 or even 48 hours on the charger it is still not fully charged, there must be a short inside. In effect the short will be fighting the charge rate, hence the long time on the charger with no improvement.
If the battery charges with no problems, but still tends to go flat, you must find out what is causing the problem. Connect the car’s earth to the negative terminal of the meter and the meter’s positive terminal to battery positive.
Place the meter so it may be read with the bonnet closed. Set it to amps and activate the alarm/immobiliser. The meter will then show the current draw when parked. Check the manual for permissible current draw. If this is excessive, check the cause.
No problem? Connect the battery and try cranking the engine. It should spin easily. If it doesn’t, the battery, or a starter or earth problem, could be to blame. Connect to another running car via jump leads and try again. If cranking speed proves okay, the battery is duff. No improvement means the car cannot make use of any extra current.
To check the charge rate, run the engine for a couple of minutes, then with meter set to volts and connected across battery posts, switch on all the electrical accessories – lamps, blowers etc. At 2000rpm voltage should be 13.5 to 15.5. If it is less, suspect a charging problem.
If the battery continues to lose charge, it is still possible that there is a small internal short that did not show up in the initial test. So take the battery off again and charge it fully again. Then let it stand for a couple of hours. Put it somewhere such as a heated garage where the temperature will be even for 24 hours or so. Take note of the voltage and do the same after 24 hours. A drop of more than 0.1 shows an excessive self-discharge.
If you leave a battery with less than 60 percent charge for more than a week or so, sulphation (the formation of insoluble white sulphates of lead) will form on the battery plates.
It naturally forms on the plates when charge level drops, but during charging will normally dissolve back into the acid solution with no detrimental effects. -Star Motoring