Originally posted by 120D4D
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Optimas can have an OPEN CIRCUT State of Charge voltage reading of up to 13.1v. The problem is that OPEN CIRCUIT voltage readings of ANY battery does NOT reflect the actual CAPACITY of a battery.
You can have a battery with an OPEN CIRCUIT voltage reading indicating the battery is fully charged, but place even a small load on the battery and it can go flat in minutes.
In your case, with the SC80 in STORAGE Mode, when you turn off your motor, because your cranking battery is always going to be slightly lower ( or much lower with out an SC80 ) than the auxiliary battery, the auxiliary battery will reverse charge into the cranking battery.
But a short time after turning the motor off, the cranking battery will have pulled the COMMON voltage of both batteries down to below the Cut-Out threshold of the SC80 when in STORAGE Mode.
At this point the SC80 turns off and both batteries then begin to settle independent of one another.
Again because the cranking is not fully charged, it will continue to settle down, while the Optima will settle up and not necessarily because it is fully charged, the Optima has a much high fully charged OPEN CIRCUIT voltage reading.
So even when not fully charged, an Optima will still settle up higher than the cranking battery, once all loads are removed from the Optima.
The TURN ON ( Cut-In ) voltage level of the SC80 is 13.2v and this is deliberately set to be just above the maximum SETTLED voltage level of any battery, like Optimas, that have a high SETTLED voltage level.
The highest SETTLED voltage of some lead acid battery is 13.1v, but most will have a maximum SETTLED voltage level of between 12.7v to 12.85v.
In any case, no lead acid battery will settle up high enough to turn the SC80 back on again.
Only starting the motor will raise the voltage high enough to cause the SC80 to turn on ( Cut-In ) again.
And again, this is done deliberately ( by most VSR type isolators ) to stop the isolator oscillating on and off, once the motor is turned off and the SC80 has Cut-Out, and batteries like Optimas, settle up once any form of load is removed from the battery.
NOTE the only way to gauge the true CAPACITY of a battery, is to carry out a controlled load test, and this can be something as simple as placing a known current load on the battery.
Preferably, for best accuracy, a load somewhere between 1% and 2% of a battery’s total Ah. For a Optima D34 ( 55Ah ), a load no larger than 1 amp would give you a very accurate indication of the TRUE capacity of a battery.
You simple monitor the battery voltage WHILE THE LOAD IS CONNECTED, and see how long it takes for the voltage to get down to 12.0v.
Theoretically, with a 1 amp load, it should take about 20 to 22 hours for an Optima D34 ( 55Ah ) battery, in good condition, to discharge down to 12.0v
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