Friday, Sep 16, 2016 at 13:02
The "hypothesis most certainly does hold up ....... the cut in and cut out voltages vary from manufacturer to manufacturer AND it must be considered that the behaviour of the battery differs considerably when there is a source of charge compered to when there is none AND the behavoir will be strongly influenced by the various very low measured resistances in the system.
When a battery that is at a low state of charge .... it will have a low resting terminal voltage ...... but as soon as a source of charge is applied its terminal voltage will rapidly increase, the terminal voltage will rise way ahead of the stste of charge ....... the internal resistance of the battery has much influence in this situation. ...... an older battery will have a much higher internal resistance, lower charge acceptance and its terminal voltage will rise very raidly ... but its state of charge may rise very slowly.
Both the VSRs and the DC to DC chargers are designed to detect that the battery A/ has a source of charge or B/ is in a resting state. ... no more.
My VSR, if both batteries are fully charged and there is no load on either system can take 10 to 15 minutes to drop out after the engine is stopped and the source of charge is removed ...... it takes the battereis this long to return to a resting state and drop below the low threshold.
neither VSRs nor DC to DC chargers can or do detect that the primary battery is at full or even approaching full charge ...... or for that matter any state of charge.
IF the device was detecting full charge or anything like, it would have a threshold
well above 13.5 volts ( more like 13.8 to 14.5 volts, what is full charge is an endless argument) ... and in reality would have to monitor current flow.
They detect that the terminal voltage has risen ...... above somewhere around 13.xx ish Volts ( depending non the brand), even a fairly flat battery will rise to a 13.5 ish Volts within minutes of the engine starting ......... now remember hysteresis ....... once the engine has started and the main battery has reached threshold it WILL NOT disconnect till it drops below the bottom 12.5 ish Volt threshold.
In a modern car with a 100- 120 amp alternator even at low RPM it will be capable of kicking out 50+ amps ... enough to keep the main battery and the now connected aux battery above the bottom threshold .......
The primary battery will still have a higher terminal voltage than the AUX battery .... thus current MUST flow toward the AUX battery from the main battery ...... IN ADDITION ..... the majority of the current supplied by the alternator MUST flow to the battery with the lower state of charge.
The alternator holds both batteries above the lower threshold while the AUX battery takes the majority of the current from the alternator and current is shunted from the main battery till both equalise ....... the result in the short term is a nett discharge of the main battery, until the point of equalisation then both will charge.
So you now have around 160 to 200 AH of battery charging from an alternator that can provide 120 is amps MAX ....... so 2 hours at highway speed to even get both batteries close to a reasonable state of charge and considering current tapering 6 to 8 hours ( perhaps more) to fully charge.
Current shunting is something a lot of people simply do not want to acknoweledge happens ....... but it must.
cheers
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