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Old 07-05-2015, 06:20 PM   #1
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Charging batteries from generator

I was doing some boon docking over the holiday. My house batteries are two 210ah batteries. I would run them to about 12.1 or 12.2 volts. I would run the generator to recharge. I measured the voltage as they were charging from the generator and was only seeing 13.1 to 13.3 volts on boost mode. I have a PD9245C converter/Charger. When I'm on shore power I can get over 14 volts on boost mode. I drove about 2 hours home today with a starting voltage of 12.1, let the battery's sit for 6 hours and tested, they tested at 12.7 fully charged. The generator is able to power the 2 AC units at a time. Do I have an issue with my generator to converter or is this to be expected when charging from the generator. If so I should just get a battery charger that can out put 30A and 14.7 volts. Any insight on this.
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Old 07-05-2015, 06:38 PM   #2
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When you start charging low batteries, the volts don't just go up to 14.6.


As they charge up, in bulk mode, the volts climb up to 14.6. This could take a long time if you are using a small charger and have large battery capacity.


Your charger puts out 45 amps in bulk mode. Your 420 Amp Hour, batteries are down 210 amp hours, or so. At 45 amps output it will take, over 4 hours, to get to full. ( 45 amps X 4.5 hours = 202 AH )


Near the end of that time, your batteries will reach around 80 to 90 % charged and the volts will drop back to around 14.1, ( Absorbsion Voltage )


If you want faster recharge, add another smart charger or up grade to a 95 amp one.
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Old 07-06-2015, 05:41 PM   #3
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Your saying 4.5 hours to fully charge. I was able to fully charge while driving for only 2 hours. I can also charge that fast on shore power, just not when running generator. I ran down batteries today to 12.1, was able to charge up to 12.7 in under 3 hours. Just not sure why with generator so slow, if other charging is so quick.
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Old 07-06-2015, 05:55 PM   #4
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The converter doesn't know anything about the power source. Generator or shore power are all the same. It's just 120vac.

I seriously doubt if your batteries were fully charged in just two hours of driving.. If they were, the voltage would have read more like 13.3-13.6. While 12.6 is nominally a "full charge", that's for a battery at rest after a brief load, not one that has just come off an active charger.
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Old 07-06-2015, 07:27 PM   #5
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Quote:
Originally Posted by JWLSC View Post
Your saying 4.5 hours to fully charge. I was able to fully charge while driving for only 2 hours. I can also charge that fast on shore power, just not when running generator. I ran down batteries today to 12.1, was able to charge up to 12.7 in under 3 hours. Just not sure why with generator so slow, if other charging is so quick.
Your alternator is probably around 100 amp output. That is twice the output of your charger.

Have you checked the generator voltage output ?
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Old 07-06-2015, 07:49 PM   #6
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I have not checked the generator output. What is the best way to check the generator output.
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Old 07-06-2015, 08:03 PM   #7
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Add to the problem that every 12 V load you have on detracts from the 40 Amps the converter can deliver. IF you are drawing 10 Amps for lights, fans and entertainment you get 30 left to charge the batteries with. If you are drawing 30 Amps for the toys you get 10 to charge with. With a bit of careful planning you could load the system enough to discharge the batteries while plugged in. ;-)

BTW I would not be surprised to find your generator is rated somewhere around 150 Amps when running the engine. Most of it is probably available to charge the batteries unless you are running the AC full blast driving after dark while the kids in back are watching TV and cranking up the stereo.
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