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Bob UK
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Posted on Monday, 15 December, 2014 - 09:14 am:   Edit PostDelete PostView Post/Check IP

Batteries have internal resistance due to manufacturing tolerances it varies between two identical cells by as much as 10%. This means that when a battery is tested for resistance the figures are not accurate. This thus means that shops that use a resistance meter to test batteries are likely to fail a good battery. The different resistance of the cells also mean that one cell will charge better than another which leads to cell imbalance. It left like this the weaker cell(s) will not be fully charged and will sulphate.bug

To equalize load battery with head lights for 15 secs and then recharge at a terminal voltage of 14 volts.

If a battery is deep discharged then and then charged the out of balance will be bigger.

Fortunately when a battery is fitted the cars electrics load the battery and then the car starts and the alternator recharges. This will equalise the battery. If the battery is not in circuit then no electrons flow and the imbalance remains. Even an electric clock will help.

To overcome the resistance the voltage at the terminals must be more than the open circuit voltage.

Alternators are set to 14.2 to 14.4. The alternator senses loads and increases the amps to keep the voltage correct. The alternator sees the battery as another load like the lights. If the battery not fully charge the potential difference between battery voltage and alternator voltage will be greater than fully charged which cause the load to be bigger so the alternator gives more amps. Once the battery voltage starts to rise the load becomes less and the amps drop back.

Alternators like all machines like gentle frequent not a once a week fight to charge a battery that never got fully charge in the previous weeks fight. The brushes wear quicker. Plus the battery will fail in two years.

Mains battery chargers. A lot of the automatic small chargers when connected have a terminal voltage of under 13.5v. Because float voltage is 13.92v these chargers can never fully charge a battery. This makes them very safe for the people who know nothing about batteries. I have one of these and charges to 12.5 open circuit battery volts (24hours).

My big charger at full power is 23v un loaded and 18v with a battery in circuit being charged. This on a 078 55 amp hour starter battery is 50 amps. This is a lot and I only use for 5 mins to fizz up the battery. Then down to 15amps for 1 hour and then it's digital volt meter time and below 14.4v for 24hours.

Warning about cheapo chargers. Ripple voltage. Mains is ac and inside the charger is a rectifier. If the rectifier is cheap 1/2 wave then the ripple voltage will be at 50/60 Hz. If the rectifier is a bridge and full wave then the frequency is double 100/120 Hz. Batteries don't like ripple voltages. 100Hz is not good and 50 is very bad. So good chargers are at least full wave. At lower charge rates the 100 ripple is not enough to cause damage but a high rates it is. My big charger has a capacitor to smooth ripples. A cheap 1/2 wave with no capacitor is likely to damage a battery over time.

Alternators have at least 3 phases ( some have nine) and full wave rectifiers so the ripple is very very small plus they run at different rpms so the Hz varies a lot from high to even higher.

If an alternator is rated at 50 amps then it will maintain 14.2 to 14.4v up to 50 amps any more and the volts drops. Once it falls below 13.92 the battery will start to discharge.

Note in general once an engine is running at 1000 rpm the alternator will give maximum if required. Running faster doesn't give more. Also a 50 amp alternator cannot put 50 amps into a battery at best 30 amps for about 5 mins and then the battery voltage will rise and the amps drop. If all the lights and heater are going then probably only 5 or so gets in the battery.

Important when ever alternators are tested the battery open circuit voltage must be 12.5 and over. If not then the readings, which are still an indication that the alternator is at least working, won't match the makers specification.

Electrolyte. Is about 70 water and sulfuric acid. When a battery discharges the sulphur goes from the electrolyte to the lead plate to lead sulphate on the plate. Charging reverses this process.

But every time this happens, some of the soft lead sulphate crystallizes. Which charging won't reverse and the battery fails. One method of recovery is to reverse charge the battery then charge the correct way. This is violent to the battery and liable to bend plates. Sometimes it works and battery last another month and sometimes it completely scraps the battery.
My big charger will do this a small 5 amp jobbie won't and the fuse will blow. The neg plate makes hydrogen the positive oxygen, this is Brown's gas and is explosive. So no flashing of leads to see if current is flowing. Plus acid.

And you thought battery charging was simple.

(Message approved by david_gore)

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