Datatek
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You still need to check that the alternator is producing at least 14.4 volts at 2K rpm after starting.
P38A 12.5 volts and not enough to start engine.
- Thread starter Radar1946
- Start date
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...with all full beam, A/C & heater heated seats etc. etc. on too?
Datatek
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If the alternator is OK the voltage should stay the same, so worth checking with a heavy load. The voltage will fold back to below 14 volts as the state of charge of the battery nears full charge.
Radar1946
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Cheers I will check.. Just like to say an extra thanks to Grr for taking time to write all the info in plain english, must have taken a while to write.
Springelec
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Just for reference, a 12V x 90Ah lead acid battery should be charged at 14V4 and a manufacturer will typically advise ten hours, known as the '10Hr rate' for a full charge. Therefore, a 90Ah battery should be charged with 9A for ten hours. Thereafter, it should be 'float' charged at 13V74. The same 12V battery is considered fully discharged at 10V5, although it will retain quite a lot of charge even with a terminal voltage of only 10V5. If a 90Ah battery offers a CCA (Cold Cranking Current (Amps)) of 550A, this suggests an internal resistance of 5.9m Ohm, where 0R0059 = (13V74 - 10V5)/550A and batteries fail to crank because they have no capacity or the cranking circuit exhibits high resistance. A typical cranking cycle can draw 400A from your battery, with diesel engines demanding more current. A new and fully charged battery will exhibit an internal resistance of about 6m Ohm (0R006) and drop the battery's internal terminal voltage by about 2V4 when delivering 400A (2V4 = 400A x 0R006). Therefore, an o/c measurement of 13V will drop to 10V6 as soon as 400A cranking current is demanded. Between the battery and the starter motor, there are the battery terminals (x2), cable (x2) and starter contacts (x2). Depending upon the car, each component will also cause voltage to be lost across each junction and total circuit resistance needs to be less than 30m Ohm (0R03) to allow 400A to flow from a 12V battery. For this example, if cranking circuit resistance is higher than 30m Ohm (0R03) due to poor contact cleanliness and contact area, corroded +ve and chassis return cables, corroded battery terminals, full cranking current and therefore, cranking torque will not be achieved, which explains why cranking some cars is sluggishly or impossible.
Springelec
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I forgot to add that a lead acid battery is also limited by a finite number of charge/discharge cycles, typically 3500 if depth of discharge is below 20% and higher depth of discharge shortens battery life. It's electrical capacity decreases with age and temperature, whilst its internal resistance increases when cold and with age. And that statement assumes nothing else changes, e.g., the quality and volume of the electrolyte, which adversely affects battery performance. In summary, age and cold reduces battery capacity but, increases internal resistance. Hence cranking ability falls with battery age and battery temperature and could fail if the battery is very old or very cold or both.
Datatek
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Lead Calcium batteries generally charge at 14.8 volts in my experience and from 12.8 volts my 120AH 900CCA battery drops to 11.2volts whilst cranking the M51 diesel.
