You are giving the alternator's voltage regulator, which controls the output of the alternator, far too much credit in terms of what it is trying to do, and how it decides.

Perhaps a few modern vehicles measure the field current sent to alternator to control how much amperage it makes to seek the desired voltage, and a few might also have have a Hall effect sensor on a battery cable seeing just how mucch amperage is entering or exiting the battery, ,but no vehicle previous to the mid 2000s consider the variables into how long to seek and maintain a specific voltage.

Having the engine battery and house battery being similar make model and design is also not accurate.


With the engine running each battery will acccept whatever amput of amperage it wants at the voltage reaching the battery terminals. The depleted house battery at 13.8v might accept 12 amps,m the fully charged startng battery might accept 0.6 amps. They take what they want at the voltage allowed by the vehicle and the wiring to and from the batteries from alternator, incvluding the usually well undersized ground path.

What one does NOT want to occur is leaving the dissimilar batteries in parallel( connected together) with NO charging source present.

Voltage is electrical pressure and batteries accept whatever they want at the pressure reaching the battery terminals, upto the maximum output of the charging source.

Problem with house batteries and the alternator as the only charging source is it takes no less than 3.5 hours, at 14.5 volts, to get an 80% charged battery to 100%. Yet one can discharge a 100% charged battery to 80% in a few minutes of blasting their well amplified stereo.

Since No vehicle allows 14.5v for that amount of time, the actual time reqired to return a depleted battery to full takes mucch much longer, and every human out there seems to think that the alternator is somehow able to defy physics and instantly and fully charge the battery.

When a lead acid battery lives less than fully charged its lifespan is dramatically reduced. If one wants respectable lifespan from a house battery it must be regularly fully charged or be allowed to live at a high average state of charge.

If a House battery is deeply cycled eacch night then recharging it to 98% is good, ,but only half as good as recharging to a true 100%, and this last 2% can require 2 more hours being held at 14.5v or 10 more hours at 13.8v.

No one is going to drive that long just to get a 98% charged battery to 100%, and manipulating the vehicles voltage to seek and hold high 14v range is not easy to accomplish. Most will drop to the 13.8v range once hot, no matter if the house battery is still well drained andable to accept huge amperages. In fact most of this voltage dropping is related to the heat the alternator, or voltage regulator if not within the alternator, is subjected to, and the more output the alternator makes the hotter it gets and lowers the system voltage sought. So it is more likely the depleted house battery asking for everything the alternator can make will have it lower the voltage sought and greatly slow the actual recharging of the depleted house battery.


The care and feeding of lead acid batteries is grossly misunderstood by most, and there is much incorrect folklore out there concerning this topic.

Having a house battery charged by the alternator when driving is not rocket science, but when one cannot control the voltage sought by the alternator, then the house battery is at the whim of the voltage reaching its battery terminals, and 99% of the time this is not ideal for quickly nor fully recharging of the battery.

When one gets tired of trying to warranty prematurely sulfated house batteries, or has that warranty denied, one then learns how to better treat the battery so it does not prematurely fail.

This basically requires alternate charging sources applied to take the battery from whatever the alternator left it at when the engine was shut off, to much closer to 100%. Solar or a plug in charger.

Also, most 'smart' chargers these days are also poor at actually fully charging a well depleted battery. These too, for fear of overcharging a slightly depleted battery, will not allow the mid 14 volt range to be held for long enough. they will drop to float/maintenance voltages well before the battery is full, and flash the green light.

Most smart chargers will drop out of the 14.5v range to the 13.2 to 13.6v range, in the 92 to 95% charged range. If the battery is sulfated, then it really really wanted the 100% recharge, and a sulfated battery needs significantly more time than a healthy battery to get from 95% to 100% state of charge.

But Lead acid batteries are only rented. The length of the rental contract is directly related to how well they are recharged. How well they are recharged is determined by the voltage the charging source is seeking, and how long it holds that higher voltage. Even the best marketed smart chargers, get this wrong, and this is verifiable with a hydrometer on a flooded battery with removable caps, or wqith an ammeter on an AGM battery when it is held at 14.5ish volts.

Almost NO vehicle out there will seek and hold the mid 14 volt range when the battery is less than 100% fully charged. Except mine, as I have bypassed the insane voltage regulator inside my engine computer, and hooked up an external adjustable unit with a dial on my dashboard, next to my digital ammeters and voltmeters, and I choose 14.7v whenever the battery is not fully charged, and 13.6v when it is. Pretty rare that i dial it down to 13.6v, and I have 200 watts of solar on the roof and a plug in charging source capable of 40 amps output at anywhere from 13.12 to 19.23volts.