Running a battery charger off my generator

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Dooger54

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I have a Champion 3000 watt generator that produces right at 25 continuos amps of power.  I am thinking of running my stand-alone battery charger directly from the generator. It has the ability to put out a 40 amp charge to do a quick charge for a battery.  My stupid question is - if my generator only puts out 25 amps, will the charger be able to put out 40 amps?  I assume that a battery charger that can put out that high of a charge doesn?t actually ?use? 40 amps of power?
 
40 amps at 12V will be about 4 amps of 120 volts. I say about because losses in the charging circuit will mean you actually don't get quite that full 10 to 1 ratio, but for our purposes, using 1/10th amperage on 120 other than 12 volts is close enough. Of course that means going the other way is 10X multiplication. For example if you want to run your 3 amp 120V refrigerator from an inverter, it will take 30amps at 12 volts.
 
What is the charging output of your onboard battery charger?
 
Sun2Retire said:
What is the charging output of your onboard battery charger?
Click to expand...

I believe it?s 55 amp.  It?s a replacement converter I bought from best converter.com.

What I am considering is the following -

I use a CPAP machine and that consumes a fair amount of power overnight (I use the 12v converter).  So my challenge is replenishing the batteries the next day.  What I am thinking about doing is using a separate battery I have and hooking the CPAP to that with alligator clips instead of running it off my RV batteries ( 2 Trojan 105s).  Then as we use the generator in the morning (coffee maker, microwave, tv) I would also run my separate battery charger off the generator to charge the separate CPAP battery.

So I want to make sure the generator will handle the RV needs in the morning while also powering the separate battery charger.  I can see running the generator each morning for 2-3 hours. 
 
That should work fine. As others have said the separate charger will not draw much
 
Most of those 40 amp "quick charge" boxes have a very limited duty cycle, sometimes only a few minutes.  Or they use pulse modulation, e.g. a couple seconds at 40A followed by a couple seconds at zero amps, so that the average charge current is much lower.  And all too often, the charger brochure  or manual doesn't say much about that or explain its net effect when using for more than a quick jump start.  A true (constant) 40A charger is pretty expensive.

Usually the onboard charging system is going to be more effective, especially over a 30-60 minute period. If its a multi-stage charger, it is definitely more effective.
 
Gary RV_Wizard said:
Usually the onboard charging system is going to be more effective, especially over a 30-60 minute period. If its a multi-stage charger, it is definitely more effective.
Click to expand...


Right Gary, but the OP is talking about charging a separate, standalone battery with this other charger. I considered suggesting jumping the standalone battery to the house batteries so as to use the onboard charger, but with disparate sizes and charge conditions didn?t think that was a good idea.
 
Sun2Retire said:
I considered suggesting jumping the standalone battery to the house batteries so as to use the onboard charger, but with disparate sizes and charge conditions didn?t think that was a good idea.
Click to expand...

Actually, that's likely the most effective solution.

A battery's rate of charge is determined by the voltage delivered to the battery terminals.  Then the battery "drinks" in as much current as it needs depending on it's state of charge.

If the house and CPAP batteries are different sizes, or have different states of charge, they'll each "drink" from the voltage at different rates, each one tapering off independently as they approach full charge.  Neither will interfere with the other's charging unless the combination draws enough current to max out the converter.  Then the voltage sags until the batteries gain enough charge to draw less current.  Once the batteries are less thirsty the voltage returns to normal.

Terms like multi-stage charging, bulk vs. absorption, etc. just further explain the above principle.  Problems with mixing battery types are more related to getting dissimilar batteries to discharge equally, not with the charging itself.

If you have a battery amp-hour monitor, attach the CPAP battery's negative lead to the load side of the current shunt, not the battery side, so the additional current draw doesn't flow through the shunt and mess up the amp-hour indication.

Just clip the CPAP battery across the house batteries and let the house converter/charger do it's thing.
 
Interesting Lou. I was thinking that, let's say in the case of a heavily discharged CPAP battery, there would be a high current from the house batteries, actually pulling them down until the whole "bank" (now including the CPAP battery) had a roughly equivalent state of charge. And there you have it, one more thing I didn't know that I didn't know
 
Sun2Retire said:
Interesting Lou. I was thinking that, let's say in the case of a heavily discharged CPAP battery, there would be a high current from the house batteries, actually pulling them down until the whole "bank" (now including the CPAP battery) had a roughly equivalent state of charge. And there you have it, one more thing I didn't know that I didn't know
Click to expand...

That won't happen because batteries need more voltage to push charging current into them than they deliver to a load.  It's called hysteresis.  Two batteries connected together won't self-level, there's not enough voltage difference to pump current from the first battery into the second battery.  But if you put a load on them, the first battery will supply the majority of the load current until it declines to the level of the second battery.

You might get some current flow for a minute or two if one of the batteries is very deeply discharged, but it will stop once the discharged battery gains a surface charge.  You won't get deeper charging until the voltage is raised enough for both batteries to charge.
 
Your generator puts out 25 amps at 120 or 120/240 volts. 25 amps at 120 volts is like 225 amps at 12 volt (you loose about 10% in transformation which is where the other 25 went)

So no problem

A better solution however exists..

First: How much battery do you have?  For a single Group 24, 27 or 29 40 amps on a DEEP CYCLE is a bit...fast.. On a starting battery perhaps. but for best life you need to keep it below 25-30 amps depending on the size

For GC-2 pairs. 40 amps is just fine (you can go 60-70) but  read on.

My RV came with a Progressive Dynamics 9180+ wizard. a bit big for a pair of GC_2 (60 is proper here) but with teh addition of the starting battery (I hav e a bi-directional isolator) the 80 is good.

I'd suggest a Progressive Dynamics 9260 (Wizard built in) plug in model as a stand alone charger for a pair of GC_ for a PAIR of 24/27/29 the 9245

Full charge takes six hours, no such thing as a fast charge.

Easiest way to do this.. Might be the Progressive Dynamics 4600 series. it replacxes the "Guts' of your on board converter.. all the features of the 9200 save the power cord.
 
John From Detroit said:
Your generator puts out 25 amps at 120 or 120/240 volts. 25 amps at 120 volts is like 225 amps at 12 volt (you loose about 10% in transformation which is where the other 25 went)

So no problem

A better solution however exists..

First: How much battery do you have?  For a single Group 24, 27 or 29 40 amps on a DEEP CYCLE is a bit...fast.. On a starting battery perhaps. but for best life you need to keep it below 25-30 amps depending on the size

For GC-2 pairs. 40 amps is just fine (you can go 60-70) but  read on.

My RV came with a Progressive Dynamics 9180+ wizard. a bit big for a pair of GC_2 (60 is proper here) but with teh addition of the starting battery (I hav e a bi-directional isolator) the 80 is good.

I'd suggest a Progressive Dynamics 9260 (Wizard built in) plug in model as a stand alone charger for a pair of GC_ for a PAIR of 24/27/29 the 9245

Full charge takes six hours, no such thing as a fast charge.

Easiest way to do this.. Might be the Progressive Dynamics 4600 series. it replacxes the "Guts' of your on board converter.. all the features of the 9200 save the power cord.
Click to expand...

My RV batteries are two 6 volt Trojan T105?s.  My separate Cpap battery is a standard 12 volt Interstate battery with about 70 amp hours capacity.
 
Lou Schneider said:
Actually, that's likely the most effective solution.

A battery's rate of charge is determined by the voltage delivered to the battery terminals.  Then the battery "drinks" in as much current as it needs depending on it's state of charge.

If the house and CPAP batteries are different sizes, or have different states of charge, they'll each "drink" from the voltage at different rates, each one tapering off independently as they approach full charge.  Neither will interfere with the other's charging unless the combination draws enough current to max out the converter.  Then the voltage sags until the batteries gain enough charge to draw less current.  Once the batteries are less thirsty the voltage returns to normal.



Terms like multi-stage charging, bulk vs. absorption, etc. just further explain the above principle.  Problems with mixing battery types are more related to getting dissimilar batteries to discharge equally, not with the charging itself.

If you have a battery amp-hour monitor, attach the CPAP battery's negative lead to the load side of the current shunt, not the battery side, so the additional current draw doesn't flow through the shunt and mess up the amp-hour indication.

Just clip the CPAP battery across the house batteries and let the house converter/charger do it's thing.
Click to expand...

Interesting idea which I obviously never thought of. I do have a Trimetric monitor with shunt.  Can you provide a simple diagram of how to hook this up?  I?m not sure if I understand the above explanation.  Thanks!
 
Ok so if you parallel them a 9280 would be the ideal charger.. Like my 9180 with Wizard (Basically the same thing,, operationally identical less I push a button that does not exist on the 9200 line,,, but can be added)
 
Dooger54 said:
Interesting idea which I obviously never thought of. I do have a Trimetric monitor with shunt.  Can you provide a simple diagram of how to hook this up?  I?m not sure if I understand the above explanation.  Thanks!
Click to expand...

Sure, here you go.  Just connect the negative side of the CPAP battery to the side of the shunt going to the house wires and charger, not the battery side, so the shunt doesn't measure the current going into the CPAP battery.

Disconnect it when you shut down the generator so it doesn't drain back into the house loads.
 

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Lou Schneider said:
Sure, here you go.  Just connect the negative side of the CPAP battery to the side of the shunt going to the house wires and charger, not the battery side, so the shunt doesn't measure the current going into the CPAP battery.

Disconnect it when you shut down the generator so it doesn't drain back into the house loads.
Click to expand...

Thanks so much for that!  Looks like that is my best and least expensive option. 
 
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