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Discussion Starter #1
I have a question for the experts

If I have the 220A alternator,
and 2x 100A AGM house battery (with Batt Doc),

and the house batteries are down to 50% (ie max draw down).

How long does it take to bring the charge back up?

ie. how much driving do I need to do?

Can I idle the engine to charge the battery?

if so, what is the longest time I can idle the engine?

I want to see if I really need a genset?
or 2nd alternator?
and/or how much solar do I need.

Premium Member
8,809 Posts
There are way too many factors to make a reasonable guess. Experience is probably best. I expect the alternator will provide less than 80 amps and less still as the battery set fills due to it’s internal resistance. An initial surge much higher might be expected though. If that were so, and your 200 AH battery set requires 100 AH to fill I expect you need several hours of charging. Remember that AGM has a recommended charge rate and it is well below the 80 amps we are talking about. Probably about 20 Amps.
This is why I like solar. You can buy enough to get the charge rate recommended for your batteries, MPPT controllers customize for your battery type, and you don’t have to run the van to do it. I have used the alternator about 4 times in a year camping for 2 months of that time in 2 week intervals when it was real rainy.
Enough solar to produce 20 Amps/12 volts (about 250 watts- err on the 300 watt side?)
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Alternators are rated as if they are physically cool and dumping into about zero resistance; that is, an extremely large battery that is totally dead, and that they are running about 6000 RPM. (Pulley ratios are generally such that the alternator turns about twice engine speed.) Output drops off quickly as the alternator get physically hot.

My vast experience with carefully metered and monitored boat systems is that a fast idle engine speed of 1100/1200 RPM gets out amps about as fast as the internal resistance of the typical house battery can allow acceptance.

My PM has the 220 amp alternator and an 80 amp fuse towards the house battery a 220 amp hour AGM. MY engine idle is at about 800 RPM and I have no provision for fast idle. At regular idle I have never blown that fuse even while drawing well over 80 amps out of the battery via the inverter running the microwave oven while charging! I did blow a 50 amp fuse used previously which indicates that the max charge rate at idle is somewhere between 50 and 80 amps. When driving I don't think that the charge rate is much higher based on experience of time it takes to charge a low battery.

I have a smart shore power charger rated at only 20 amps and it seems to bring the battery up to full charge at only about twice the time of the engine alternator! It peaks out at about 14.1 volts, the same as the alternator.

Why are alternators rated so high on modern vehicles? I can only guess that it is because they are now regulated differently these days by smart computers. Instead of gradual regulation of the field current they sometimes seem to be turned alternately completely off then completely on. (This probably saves a tad of fuel as in many driving conditions with no lights and no heater fan it can be off more than on?) When the battery is full up I sometimes heat hot water that draws about 80 amps while the engine is idling. Both engine and house batteries are combined. When the voltage is at 14.1 it seems as if the alternator shuts off as it drops to about 13.5. Apparently the computer senses the continuation of the heavy load and the alternator appears to suddenly turn on and the voltage is instantly back to 14.1 for several seconds. Does anyone else observe this action?

Based on post 2 above I agree that charge rate probably never exceeds 80 amps.

How do I know if my battery is truly fully charged? I believe that in my case being at 14.1 for just a few minutes does the job. My smart shore power charger drops off of 14.1 to float at 13.3 when the amps going in has reduced to 4 amps and is dropping.

Long term idle does not seem to hurt gas engines, we have had recent threads discussing this.
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