You'll not get caught with your charge down if you give your car battery reasonable care.
You slide under the wheel of the car one frosty winter morning, turn the key in the lock, and start to push down the accelerator to race the engine a little. Only the engine doesn't race; in fact it doesn't even walk.
"Oh, no, stuck with a dead battery!" you groan.
This won't happen to you if you consider the car's electrical system merely an extension of the house's power system, and give it the same type of attention and preventive maintenance.
Why does a car battery hold up perfectly well during the summer and poop out in the winter, just when you need it most? This can happen for two different reasons.
1) the oil in the crank case is stiffer in cold weather than in warm, and the starting motor has a harder job ploughing through it.
2) The output or effectiveness of a storage battery falls off markedly as the temperature drops. In other words, when the engine needs the most starting energy, the battery is least capable of providing it!
There is a simple solution to this nuisance: check the battery frequently with the arrival of cold weather, and give it an overnight charge with a charger occasionally to keep it to full strength. The more frequently you stop and start the engine, and the less you use the car in the winter, the less chance does the battery have to accumulate a good charge from the charging generator, which works only when the engine runs.
A test on a storage battery with a voltmeter doesn't mean very much. A three-cell battery can read pretty close to its maximum of 6.6 volts, and a six-cell battery to 13.2 volts, and still make very little impression on a current-hungry starting motor. Did you know that a three-cell battery has to deliver between 200 and 300 amperes to kick over a modern high-compression engine?
The only reliable test is made with a hydrometer. This is a syringe-like glass gadget with a squeeze bulb at one end; inside the glass is a shot-weighted float. To use it, you merely unscrew each cap of the battery in turn and suck up about half a syringeful of acid. The weight of the acid compared to that of water (its "specific gravity") is a direct indication of the state of charge.
The float is calibrated to show this specific gravity. Theoretical full charge is 1.3, or "1300," although with anything but a brand new battery a reading between 1200 and 1250 or 1275 is very good. When the reading falls below 1200 the battery needs a little shot in the arm.
A battery charger is a source of low-voltage direct current. It consists usually of a step-down transformer that reduces the 115-volt AC line to about 7 or 15 volts (for three- and six-cell batteries, respectively) after passing it through a rectifier that changes the AC to DC. Dual-voltage chargers are now common, to accommodate both types of batteries.
To do an overnight boosting job, a charger for three-cell automobile batteries should produce not less than about 5 amperes, although 10 is even better; for six-cell batteries, ratings of about 3 to 6 amperes are adequate. Always disconnect a charger before starting the engine.
Car vibration can loosen battery leads and sometimes make them jump off; inspect, clean, tighten. Several times a year, remove the connector lugs from battery and clean terminals with sandpaper.
Wipe battery often with cloth dampened with water; check center caps of the battery for fit. To minimize corrosion around terminals, caused by sulphuric acid vapor from cells, coat with Vaseline.
With these few simple techniques, you need never put up with a dead battery again.
Announcing: How To Fix Minor Electrical Repairs And Save Hundreds Of Dollars On Bills