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Old 02-12-2021, 10:57 AM   #1
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DC to DC Charger, why?

There have been a few threads on DC - DC charging and I didn't know much about it so I've gone and done some reading. I'm posting now to confirm I've understood, ask what I don't understand and get some opinions on it. From here out, don't take it as fact, but what I currently understand..

Why use DC -DC charging? You can get a much higher amp throughput to the house battery and charge 10x+ faster than using the 7pin hitch connector or just through charge controller in an A or C RV. The 'smart' alternators in todays vehicles will reduce output when the car battery is fully charged decreasing output to house battery. DC-DC chargers also have battery profiles so you can charge them at the right levels, normal, boost, maintain for lead acid, AGM, LiFPo4.

I've seen users connect the DC-DC charger to the car battery and also directly to the alternator. Which one is best? Why? I'd think you'd want to go directly to alternator so you are not passing through the car battery with high amps. Perhaps you need it connected to house battery though so the 'smart' alternator continues high output?

What controls the charge of the house battery from the alternator? What prevents over charging? I understand the newer Greyhawks will charge house battery when its running and when a trailer is connected via 7pin it will charge the house battery. I'm guessing the power always goes to the RV converter to the house battery same as if you were connected to shore power or running a gen?

If you are rarely without shore power to the unit and the batteries are never very low, you don't need a DC-DC charger as you don't need the high amps from an alternator to charge a drained house battery bank. Campers that rely on batteries (boondocking) should have one so in an emergency they can charge the batteries quickly from a vehicle alternator. True?
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Old 02-12-2021, 11:06 AM   #2
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It commonly comes up when people convert to lithium batteries because they require 14.4 to 14.6-Volts to fully charge, and that is more than a typical alternator will provide on its own. Basically, if you go lithium, your truck will never charge them more than about 70% capacity which defeats much of the purpose behind upgrading to lithium to begin with.

That said, my truck has no problem fully charging my normal lead-acid setup as far as I can tell anyway, it's just not that speedy of a process. I did option a 'heavy duty alternator' when I ordered my truck so that is probably helping me somewhat.

The lithium setups also require a modification to the existing converter. Not sure if you've seen this site, but more info here I think covers a lot of your questions:
https://www.progressivedyn.com/frequ...thium-charging
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Old 02-12-2021, 11:43 AM   #3
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Originally Posted by mini4mw View Post
There have been a few threads on DC - DC charging and I didn't know much about it so I've gone and done some reading. I'm posting now to confirm I've understood, ask what I don't understand and get some opinions on it. From here out, don't take it as fact, but what I currently understand..

Why use DC -DC charging? You can get a much higher amp throughput to the house battery and charge 10x+ faster than using the 7pin hitch connector or just through charge controller in an A or C RV. The 'smart' alternators in todays vehicles will reduce output when the car battery is fully charged decreasing output to house battery. DC-DC chargers also have battery profiles so you can charge them at the right levels, normal, boost, maintain for lead acid, AGM, LiFPo4.

Why use dc2dc charger? It is not that you get a higher charge amperage to the house battery, in fact in many cases it is so you can reduce the amperage going to the charge battery. The one I have from Renogy is the 40 amp model (input is slightly more than 40a and output is ~40a max). Think of it as a smart charger that instead of using 120v input it uses a 12v input. I would suggest that the most common use for one is when using a different chemistry type battery for the coach battery than the RV starting battery (the purpose for the multiple profiles) and especially when using a lithium (lifepo4) coach battery. For example, when using a lifepo4 coach battery, when that battery is low and you start your engine, your alternator will run at as high of its capabilities for an extended period of time trying to charge up the lifepo4 coach battery (not the case normally when using a lead acid coach battery), this causes two particular issues, one is that an alternator isn't designed to run at its max power for extended periods of time as that can overheat and damage it, and second is that on most systems like in a class C, the coach charge wiring isn't rated for more than 80a but the alternator can output more than that. Using the dc2dc charger prevents both of those issues while still providing for an optimal charge profile for the battery type chosen in the setup.

I've seen users connect the DC-DC charger to the car battery and also directly to the alternator. Which one is best? Why? I'd think you'd want to go directly to alternator so you are not passing through the car battery with high amps. Perhaps you need it connected to house battery though so the 'smart' alternator continues high output?
I don't think it really matters, the instructions on mine says to connect it to the battery, which on a class c is easier to do anyway. I haven't checked on mine, but I believe that mine as well as most alternators actually connect back to the battery directly anyway and regulated by an internal or external regulator (depending on the year, later models actually use a computer for managing the alternator output) .

What controls the charge of the house battery from the alternator? What prevents over charging? I understand the newer Greyhawks will charge house battery when its running and when a trailer is connected via 7pin it will charge the house battery. I'm guessing the power always goes to the RV converter to the house battery same as if you were connected to shore power or running a gen?
Without a dc2dc charger, the house battery charge voltage is the same as the starting battery is, and there is a solenoid that connects the two when the key is turned on. (not sure about the 7pin in this case as that is not part of the charge circuit on a class c, but on a truck pulling a trailer the 7pin 12v charge would be the same as the starting battery charge voltage) Over charging protection is the same as what protects the engine starting battery via the alternator's regulator.

If you are rarely without shore power to the unit and the batteries are never very low, you don't need a DC-DC charger as you don't need the high amps from an alternator to charge a drained house battery bank. Campers that rely on batteries (boondocking) should have one so in an emergency they can charge the batteries quickly from a vehicle alternator. True?
As mentioned above, the dc2dc charger isn't about providing higher amps, it is for providing a consistent charge profile and limiting amps so as not to overwhelm the alternator. The dc2dc charger really isn't there to help in emergencies either, as in the standard setup for our Class C (and many others) the system is setup to charge the coach batteries in a direct connected configuration (via a solenoid that connects when the key is on) which would be a faster charge, but if you had enough batteries or lifepo4 batteries, that amount of charge could cause the alternator to run at 100% which is what I was mentioning above which is not necessarily a good thing in most cases.
Responses above in red, and in addition, my advice is you don't need a dc2dc charger in the standard setup especially when using a lead acid coach battery, but one could be of value and is of value when using a different chemistry type coach battery such as lifepo4.
CA
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Old 02-12-2021, 01:20 PM   #4
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Good info. Thanks.

I'm gonna say dc-dc needed for lipo4 and helpful for boondocking (solar not charging batteries enough and need to charge from vehicle alternator).

I'm curious how the house battery is attached in my Greyhawk to charge. I wonder if they are just connected in parallel to alternator or if alternator connects to car battery and car battery attached to house with a solenoid between them to only charge house when car battery is charged.

I went through the electrical diagrams from Jayco but I didn't see anything about the house battery to car battery. It just has a single wire from house->battery disconnect solenoid->load center (12v fuse panel). There's got to me some more wires for the aux start and charge from car battery/alternator. I guess I'll just dig through the engine compartment next time I'm at storage and see what I can find.
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Old 02-12-2021, 01:35 PM   #5
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Originally Posted by mini4mw View Post
I went through the electrical diagrams from Jayco but I didn't see anything about the house battery to car battery. It just has a single wire from house->battery disconnect solenoid->load center (12v fuse panel). There's got to me some more wires for the aux start and charge from car battery/alternator. I guess I'll just dig through the engine compartment next time I'm at storage and see what I can find.
That should be the battery cable from the emergency start solenoid to the house battery. That solenoid should close when the ignition is on as well as when you push the emergency start button.
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Old 02-12-2021, 01:37 PM   #6
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Yes, there is a separate solenoid that is almost directly in front of the steering wheel but under the hood. That solenoid ties in the engine starting battery to the house battery and automatically turns on when the key is on so that the house battery can obtain a charge current from the engine starting battery, which is charged from the alternator. I wouldn't be overly concerned about exactly where the tie in occurs because it is on the same circuit but for reference, (if I remember correctly) there is an electrical block under the hood that the battery connects to, and that block is also where the solenoid connects to which in turn only connects the two batteries when either the key is on, or the aux start button is pushed. In essence, you are correct that the two batteries (engine starting and coach battery) are in parallel when the relay is engaged via the key switch or the aux start switch. It really makes no difference if the alternator is directly connected to the engine battery or if the alternator is connected to a common fuse block that the battery then connects to as ultimately the alternator is a direct connection to the battery and is a direct connection to the solenoid that allows for the same circuit to pass to the coach battery, but again only when the key is on or the aux start switch is pushed. ~CA
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Old 02-12-2021, 01:47 PM   #7
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BTW, to add to this, my comments and everything I mentioned is referencing the charge circuit from the engine alternator, the engine start battery, and the solenoid that connects that 12v power to the coach battery which in turn charges the coach battery from the engine alternator (assuming the engine is running). Nothing in my comments references the fact that you also have a converter in your RV that uses the 120v connection from the shore (RV park) power which also charges the house battery, but does not charge the engine battery as the solenoid is not engaged tying the coach and engine start battery together when the ignition key is off and the aux start button not pushed. ~CA
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Old 02-12-2021, 02:03 PM   #8
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but does not charge the engine battery as the solenoid is not engaged tying the coach and engine start battery together when the ignition key is off and the aux start button not pushed. ~CA
That has changed in recent years, from the owner's manual:

When the power cord is plugged into 120-volt shore power, or when the generator (if
so equipped) is operational, the inverter/charger functions as a battery charger and will
automatically charge the house batteries when required. A bi-directional charge relay
incorporated into the electrical system allows the inverter/charger to charge the chassis batteries once the house batteries are sufficiently charged.

The chassis alternator charging system supplies power to the house auxiliary batteries
when the engine is running and the chassis batteries are sufficiently charged
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Old 02-12-2021, 02:10 PM   #9
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That has changed in recent years, from the owner's manual:

When the power cord is plugged into 120-volt shore power, or when the generator (if
so equipped) is operational, the inverter/charger functions as a battery charger and will
automatically charge the house batteries when required. A bi-directional charge relay
incorporated into the electrical system allows the inverter/charger to charge the chassis batteries once the house batteries are sufficiently charged.

The chassis alternator charging system supplies power to the house auxiliary batteries
when the engine is running and the chassis batteries are sufficiently charged
Thanks Grumpy, That is good to know, do you know when this change occurred (what year)? I have plans to install a 120v trickle charger to the engine starting battery to overcome this shortcoming in mine. The change you mentioned sounds like a lot better system to keep the chassis battery charged when plugged into shore power than what Jayco provided earlier. ~CA
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Old 02-12-2021, 02:26 PM   #10
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Little reminder if you switch to lithium batteries and if you have just a existing voltage display in your RV to monitor voltage. Lithium batteries are great in that their voltage stays high for longer during discharge than lead acid but drop fast after a certain voltage so you need the proper monitoring when off shore power.
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Old 02-12-2021, 02:28 PM   #11
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Originally Posted by craigav View Post
Thanks Grumpy, That is good to know, do you know when this change occurred (what year)? I have plans to install a 120v trickle charger to the engine starting battery to overcome this shortcoming in mine. The change you mentioned sounds like a lot better system to keep the chassis battery charged when plugged into shore power than what Jayco provided earlier. ~CA
Unfortunately no I don't
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Old 02-12-2021, 04:03 PM   #12
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Originally Posted by mini4mw View Post
....
I went through the electrical diagrams from Jayco but I didn't see anything about the house battery to car battery. It just has a single wire from house->battery disconnect solenoid->load center (12v fuse panel). There's got to me some more wires for the aux start and charge from car battery/alternator......
I found a better drawing of the 12V circuits. I highlighted the chassis/house charging circuit in yellow. The wire to the dash-mounted pushbutton is highlighted in pink.
.
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Old 02-12-2021, 04:46 PM   #13
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Thanks Bullitt. I have 17 wiring PDFs from Jayco and I just triple checked and none had this detail. Based on this, I would think you could put a DC-DC charger after the 100a solenoid and utilize existing wiring?
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Old 02-12-2021, 04:55 PM   #14
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Thanks Bullitt. I have 17 wiring PDFs from Jayco and I just triple checked and none had this detail. Based on this, I would think you could put a DC-DC charger after the 100a solenoid and utilize existing wiring?
I have several projects planned, and while I have purchased a renogy 40a dc2dc charger I have yet to install it. My plan though is to leverage the existing wiring that connects to the solenoid but to keep the solenoid in place in order to maintain the ability to use the aux start switch. While it is not overly clear in the diagram, you will see below the 100a solenoid (see it in the diagram) that there is a OEM (IGN) Yellow\Pink wire that is used to trigger the solenoid when the key switch is on, I intend to disconnect that wire so the key does not trigger the solenoid and connect the DC2DC charger input to the battery side of the solenoid and the output to the coach battery side of the solenoid and likely will use a separate switch to turn on and off the DC2DC charger. The wiring instructions for the dc2dc charger say to use a power connection that is hot when the key is on (which would be the wire I am saying I will disconnect from the existing connection) however there may be times when I don't want the DC2DC charger on, even when the key is on. Haven't completely decided yet about using a separate switch though. ~CA
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Old 02-12-2021, 05:12 PM   #15
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Something like this is my plan. Hopefully the paint image is clear... ~CA
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Old 02-12-2021, 05:23 PM   #16
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Have you seen this from BIM?
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File Type: pdf BIM-LI225-Quick-Guide.pdf (250.8 KB, 12 views)
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Old 02-14-2021, 09:39 AM   #17
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Just checking my own manual and it says the same as Grumpy's note.

In section 6 electrical system/house batteries

##############
Unless a battery has been fully discharged, house auxiliary batteries are normally charged
in one of two ways:

When the power cord is plugged into 120-volt shore power, or when the generator (if
so equipped) is operational, the inverter/charger functions as a battery charger and will
automatically charge the house batteries when required. A bi-directional charge relay
incorporated into the electrical system allows the inverter/charger to charge the chassis
batteries once the house batteries are sufficiently charged.

The chassis alternator charging system supplies power to the house auxiliary batteries
when the engine is running and the chassis batteries are sufficiently charged.
###############
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Old 02-14-2021, 10:03 AM   #18
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Craig,

Looks like if gen or shore power is connected, it should charge your chassis battery. I was curious so I started going through all the manuals. The 2010 Greyhawk docs are here:
https://www.jayco.com/tools/archive/2010-greyhawk-m/

This is the language I found in different manuals. If it charges when generator is on, I'd think it would charge when plugged into shore power through the onboard charger/converter.

2016 Generator section, p81
When the generator is operating, it powers the
inverter/charger which in turn functions as a multi-stage battery charger to charge the house
auxiliary and chassis batteries

2015 Generator section, p6-6
When the engine is running, the alternator will automatically charge the house
batteries through the auxiliary start solenoid. When parked at a campsite and
plugged into shore power the power converter will automatically charge the
batteries.

2014 Generator section 6-6
When the generator is operating it will power the converter, which in turn will
charge the house and chassis batteries

2012
Generator section 5-7
When the generator is operating it will power
the converter, which in turn will charge the
house and chassis batteries.

2011
Generator section p48
When the generator is operating it will power
the converter, which in turn will charge the
house and chassis batteries

2010
Generator section p48
When the generator is operating it will power
the converter, which in turn will charge the
house and chassis batteries.
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Old 02-14-2021, 11:47 AM   #19
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For my 2016 Greyhawk, I agree that the manual states what you show, but in my case it is clearly wrong.
While on shore power or generator power, the converter does NOT charge the chassis battery. There is no chassis battery charge line shown on the schematics.
I verified with a voltmeter that the chassis battery is not being charged.


Checking your batteries with a voltmeter seems the best way to determine how your system actually operates.


(The manual also says I have a 120/240 volt diesel generator...)

.
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Old 02-14-2021, 12:30 PM   #20
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Interesting. I'll have to test mine out next time I pay her a visit in storage (gen) or have her at the house plugged in. I suppose if there is no secondary wires from charger/inverter direct to chassis battery there's no way to get power to it.
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