How much time will an alternator take to charge two batteries? 1

[prasansurana]

I have approx 1500 watt RMS load in my truck. I have two batteries, one is 80ah and other is 180ah (auxiliary battery). The batteries are isolated with the help of a 150 amp battery isolator. The alternator capacity is 90 amps. Considering that the alternator is giving output 80% of its peak capacity at any given time, is it correct to assume that the alternator is supplying 36 amps to a battery (=90*0.8/2)? Will the alternator supply half of the current it is producing to each battery? Also, what is the minimum time required by the alternator to fully charge both the batteries when each has a reserve of only 50% of capacity (half discharged state)
Truck is 12vDC.


Thanks.

 

[LiteYear]

Will the alternator supply half of the current it is producing to each battery?

No. Is the isolator just a switch or is it a smarter device? Often dual battery isolators restrict charging current to the aux until the primary is full. If yours is one of those, then there's your answer.

If the isolator is just a switch, and it is closed, then the answer is that the batteries will share the current depending on their state of charge. And given that no two batteries are the same (certainly not a 80Ah and a 180Ah), their state of charge will not rise in unison.

The usual way to figure it out is a bit simplistic but pretty useful for a first approximation: the batteries are modelled as a voltage source whose voltage rises somewhat linearly with state of charge, in series with a resistance that is fairly small (say a tenth of an ohm) and reasonably constant (actually varies with temperature and other things). As the batteries absorb charge their internal voltage will rise at different rates. Because the voltage across the batteries has to stay the same (since they're wired in parallel), they will draw dissimilar currents.

In practice, the larger capacity battery will likely have a lower internal resistance so will draw more current to begin with. As its internal voltage rises, it will start to draw less. As both batteries approach 100% state of charge, they will continue to draw current according to their respective internal voltages. If the 100% charged voltage of one battery is lower than the 90% charged voltage of the other battery, the charged battery will continue to draw current, wasting it as heat. That's why dual battery systems with dissimilar batteries should have a smart isolator that only charges one battery at a time.

 

[prasansurana]

This is the isolator that we are using and it is wired in the following fashion.

 

[waross]

The alternator output rises with RPM. At a fairly low speed it reaches 13.8 Volts or 100% output. As speed increases the regulator holds the Voltage at 13.8 Volts. As the battery reaches full charge, the current drops off quite sharply.
The internal impedance of the alternator combined with the internal resistance of the battery limit the current to a safe level, but trying to charge too much battery capacity may damage the alternator. A classic case is boosting a motorcycle with an automotive battery in a poor state of charge and not immediately disconnecting the boost battery when the motorcycle starts. Run the engine at high revs with the boost battery still connected and the alternator may smoke.
If your second battery is much larger than the factory installed battery you may be entering the danger zone.

Bill
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"Why not the best?"
Jimmy Carter