Generator DC Charging question

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kportra

kportra

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Jun 12, 2017
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I know generators have been talked about to death on this forum.  I am just trying to reconcile some things I've read here with what the user manual and dealer told us.  We have a Firman Inverter style generator and two 12 volt batteries.  Using the generator to charge the batteries through the shore power cord takes forever.  Not literally but I hate the generator sound!  The book and the dealer both say we can charge the batteries using the DC cable and hook directly to one of the batteries.  We've done this and it does charge the batteries a bit faster not going through the shore power converter.

I think I've read here (or elsewhere) that charging that way causes damage to the batteries.  The dealer says that the voltage is controlled by the inverter so that is not an issue.  I'm truly just a battery power dummy and can't figure this out.  Wouldn't the DC cable method be similar to a portable solar setup?

Here is the generator:  https://www.shopfirman.com/p/firman-2000-watt-gas-inverter-generator-parallel-capable/FIRSPS2000I

All thoughts are welcome.  Thanks.
 
Modern converters produce a steady voltage regardless of how much or how little current they're putting out.  The battery charges until it reaches the same voltage as the converter.  As it approaches that voltage, the charging current tapers towards zero and then the battery just sits there,  not drawing any more current until it is called on to power a load.

The same thing happens with a solar setup.  The solar charge controller monitors the battery voltage and shuts off the charging current when the battery reaches a full charge.

The problem with using a 12 volt charging cord is the generator doesn't have that logic.  The 12 volt charging outlet is just connected through a rectifier to a 12 volt tap on the generator windings, so it keeps on sending current into the battery the entire time the generator is running..

The converter will shut off it's current flow when battery reaches full charge, but the charging cord will continue to pump electricity into the battery.  This makes the battery heat up and use water, as the excess current breaks down the electrolyte into it's component elements of hydrogen, oxygen and sulfuric acid vapor.
 
Lou,
So if we charge using this method but stop before approaching full charge (and switch to the shore power cord) we should'nt have to worry about damaging the batteries?  That is what we did last summer - but just by accident.  No logic involved  ;D

We are also going to get a 100 W portable solar setup but with hubby's cpap draw I don't think we can actually top off with just solar most days.  So with what you've said, we could run the generator for a few hours in the AM with the dc cord and then switch to solar the rest of the day and our batteries should not be damaged.  And our peace and quiet should be mostly undisturbed.
 
I think you are kidding yourself if you think that direct charging from the genset DC outlet is faster.  A genset that size rarely produces more than about 8A at the DC port, and I see that nowhere does Firman quote a DC output voltage or amperage. If it was better at DC than a Honda or Yamaha, you can bet they would be touting the number.  The converter/charger in your Heartland is surely capable of 2x or 3x that much. The problem is that the battery will only accept so much amperage at a time, no matter what the source.

With a limit of maybe 8A, you don't really need to worry about overcharging. Even if that outlet is putting out 14v (charging requires at least 13 to achieve anything at all).  As long as the genset doesn't run for 18 hours straight, there is not going to be any damage.

The advantage of using the RV's converter/charger is that it can potentially charge with more amps when the battery is heavily discharged, plus it will automatically "float" charge as the battery nears full charge.
 
Thank you Gary.  That was my next question...Did it just seem faster or was it really faster?  I think adding the minimal 100W solar will help us more than anything.  Plus I'm searching out a few more plug in sites for this summer and our reservations for non-plug in ones will be more thought out.  I am looking for sites not in the middle of the loop or sites by a creek or more isolated in general to give us (actually our neighbors) more peace of mind while we charge up.

Leaving the CPAP at home is just not an option since I actually do love my hubby.  :D

Kass
 
The typical converters in TT's are on the low end of performance for charging of batteries.  They are pretty lame.  I like using Xantrex inverter/chargers.  Best of both worlds.  Inverter for when on Battery power and needs some AC power for some stuff.  Acts like a charger for the batteries when shore power or generator is hooked up.  Most Xantrex inverter chargers provide 55 amps of charging to the batteries.  The typical converter in a TT doesn't offer more than 10-15 amps.  The Xantrex units have multi stage charging to do the job right.  Hook up the genset to the TT 30A plug and it runs right to the Xantrex and offers maximum charging to the batteries.  I can have 4 12v Group 31 batteries fully charged in 2-3 hours using a Yamaha EF 2000 gen set.  Would be the same if using shore power, maybe a little less time.
 
Unless you want a reason to buy another inverter (assuming you have one, it sounds like you might only have the converter that came stock?) then what you're looking for is an appropriately-sized battery charger like one from Iota engineering. Their DLS series, while the older of the series they carry, are pretty bomb-proof and provide 4-stage automatic charging (and a warranty just in case). Choose the output that matches your battery bank's size and generator output capacity.

https://www.iotaengineering.com/dls.htm

We run a 90-amp Iota into a bank of 8 parallelled 12V/125amp-hour AGM batteries and the charger, powered by a 3500W Champion dual-fuel generator, can take them from flat (12.3 is about as low as I ever let them go) to float in a few hours. It can run simultaneously with our solar panels and MPPT solar charge controller, too, so no need to choose which is sending power to the batteries or run it for longer than needed. That charger was chosen based on output to the battery bank (a good portion of what it can take without shortening battery life) while keeping our generator running at it's peak-efficiency output, about 70-75% load.

Some math to do, but you've only got to do it once!
 
kportra said:
We are also going to get a 100 W portable solar setup but with hubby's cpap draw I don't think we can actually top off with just solar most days.  So with what you've said, we could run the generator for a few hours in the AM with the dc cord and then switch to solar the rest of the day and our batteries should not be damaged.  And our peace and quiet should be mostly undisturbed.
Click to expand...


As this is in the boondocking section, might you want to consider more solar? Costs on panels are way down from years ago.  I found that the cost of a 2 panel setup was not that much lower (percentage wise) than a 4 or 6 panel setup.  Most of the money goes into the controller and labour.  Screwing down a couple more panels does not add a lot of labour.


Now, you are looking at a portable solar setup, I know.  However, to some extent, the same principles apply up to the point that it is a nuisance to handle many panels for setup / teardown.  And, if your husband is even moderately handy, installing a permanent solar setup is not rocket science.  Then you avoid the setup/teardown, and you get a good solar charge when traveling on the highway as well.  :)


Admittedly, you will get the most power for the buck from a generator, but as has already been noted, it is not so much the amount of power as the length of time that it has to be applied to bring lead-acid batteries up to a full charge.


Frank.
 
kportra said:
Thank you Gary.  That was my next question...Did it just seem faster or was it really faster?  I think adding the minimal 100W solar will help us more than anything.  Plus I'm searching out a few more plug in sites for this summer and our reservations for non-plug in ones will be more thought out.  I am looking for sites not in the middle of the loop or sites by a creek or more isolated in general to give us (actually our neighbors) more peace of mind while we charge up.

Leaving the CPAP at home is just not an option since I actually do love my hubby.  :D

Kass
Click to expand...
Most CPAP's have a humidifier with a heater to warm the water so more humidity gets into the air going to the mask.  Some CPAP's have a heated hose.  If you will turn the heater(s) off you will find the CPAP doesn't pull that much power.  Just the blower and internal circuits. 

My wife's Respronics (sp??) with the heater off, only pulls about 1.4 amps, or about 12 amp hours per night.  The CPAP is also a 12V unit, so I just tapped into the reading lamp wires at the head of the bed and added a 12V power plug to power the CPAP. 

If the CPAP is a 24V model you can buy a small 100 watt inverter and tap into the reading light wires to power the inverter.  Note that the reading light wires are probably too small to handle the full 8-9 amps of 12V DC power that a 100 watt inverter CAN pull.  However as long as the heater portion of the CPAP is off, the inverter should only have to pull 2-3 amps of 12V power and the wires should be able to handle that current.

100 watt inverters are not very expensive so you can just buy one and test it to see if it works.
 
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