Where to connect DC-DC Charger

[BLDanielsJr]

Does anyone see any issue with my proposed location for a DC-DC Charger that I need to protect my alternator from my 300ah lithium house battery bank.

If I put it where I have it shown, the chassis battery will be limited to no more than 30amps of charging current as well as the house batteries.

What is not shown is that I also have 1000w of solar that helps charge my house batteries while I am driving.

If you have a better idea as to where the charger should go, please let me know. The Big Boy makes it hard to tie the Charger only to the house batteries since the current can flow both ways through the Big Boy relay.

Thanks for any help you can provide!!

Edited May 13 by BLDanielsJr

[JDCrow]

Not sure I did mine correctly, but I ditched the big boy. My understanding was that combining chassis (lead) with house (lithium)  wasn’t a good idea. 
 

There is a combiner out, James on YouTube (AZ Expert) did an install with one. 
 

Like I mentioned, I just eliminated the Big Boy. Always have a tow rig to jump if I ever need to

Edited May 13 by JDCrow

[timaz996]

You can run your DC/DC charger output directly to your House batteries. Then you will put 30amps to the house and what ever your chassis batteries needs from the alternator. 

[BLDanielsJr]

 

4 minutes ago, timaz996 said:

You can run your DC/DC charger output directly to your House batteries. Then you will put 30amps to the house and what ever your chassis batteries needs from the alternator. 

That would make it in parallel with the Big Boy. Isn't the Big Boy the path of unlimited current being requested by the house batteries?

20 minutes ago, JDCrow said:

Not sure I did mine correctly, but I ditched the big boy. My understanding was that combining chassis (lead) with house (lithium)  was a good idea. 
 

There is a combiner out, James on YouTube (AZ Expert) did an install with one. 
 

Like I mentioned, I just eliminated the Big Boy. Always have a tow rig to jump if I ever need to

Am I understanding correctly that your DC-DC charger is wired where the Big Boy used to be and ends up being the only bridge between the chassis and house?

One aspect of the system that would go away is the house helping to maintain the chassis battery when parked.

Edited May 13 by BLDanielsJr

[Seward G]

I made the change to LiFeP batteries two years ago.  The motorhome is a gas Monaco Monarch, with a 170 amp alternator.  I thought the alternator was about 230 amps and installed a 60 amp DC/DC converter.  The converter is fused at both on the alternator (125 amp) and LiFeP battery side (75 amp).  I have been concerned about overheating the alternator; no problems so far with about 15,000 miles of travel.  Power from the ignition switch turns on and off the converter.  A problem early on was that the toad battery, when towing and connected, would supply 12 volts to the ignition circuit, even with we were stopped and the RV engine was off.   This kept the converter operating and that quickly discharged the RV chassis battery.  This got fixed by adding a diode, then later a relay to the toad charging circuit.  The RV has a residential refrigerator and 1,100 watts of installed solar panels.  Even with the 60 amps charging when driving (five hours) and the solar panels the house batteries are barely recharged enough for overnight boondocking. 

[JDCrow]

1 hour ago, BLDanielsJr said:

 

That would make it in parallel with the Big Boy. Isn't the Big Boy the path of unlimited current being requested by the house batteries?

Am I understanding correctly that your DC-DC charger is wired where the Big Boy used to be and ends up being the only bridge between the chassis and house?

One aspect of the system that would go away is the house helping to maintain the chassis battery when parked.

The dc/dc comes off the positive alternator and goes to the house batteries.
 

I added a new Victron Multiplus 2 which has 4 amp trickle charger built in to maintain chassis batteries when the coach is plugged in.

Edited May 13 by JDCrow
Added for clarity

[timaz996]

"That would make it in paral30 amplel with the Big Boy. Isn't the Big Boy the path of unlimited current being requested by the house batteries?"

The DC/DC is your limiter. it will limit the current from the Alt to a little over 30 amps.

[BLDanielsJr]

4 minutes ago, timaz996 said:

The DC/DC is your limiter. it will limit the current from the Alt to a little over 30 amps.

Is your Big Boy still connected?

19 minutes ago, JDCrow said:

I added a new Victron Multiplus 2 which has 4 amp trickle charger built in to maintain chassis batteries when the coach is plugged in.

I have the Multiplus II and while I have not seen the trickle charge connector, it must be there. Do you know if this trickle charge works when the Multiplus is in Inverter mode and not just charging mode when you have shore power or the generator? When I am boondocking, my primary charging is done through solar and the Inverter is inverting. I would want the trickle charge to be present while boondocking.

[BLDanielsJr]

41 minutes ago, Seward G said:

Even with the 60 amps charging when driving (five hours) and the solar panels the house batteries are barely recharged enough for overnight boondocking. 

This seems odd to me. I have roughly the same size solar array and on a reasonably good day I get 3kw+. This ought to be at least twice what a typical fridge needs per day. The solar surplus plus the alternator should be more than enough to recharge a typical battery bank. What is the size of your battery array and what is your typical percentage available when you get up in the morning?

[Seward G]

Clarifying my earlier post, the Monarch has a relay similar to a Big Boy to tie together the chassis and coach systems when the engine is running and when the battery boost switch is pressed.  I disabled this relay.  

[JDCrow]

30 minutes ago, BLDanielsJr said:

Is your Big Boy still connected?

I have the Multiplus II and while I have not seen the trickle charge connector, it must be there. Do you know if this trickle charge works when the Multiplus is in Inverter mode and not just charging mode when you have shore power or the generator? When I am boondocking, my primary charging is done through solar and the Inverter is inverting. I would want the trickle charge to be present while boondocking.

I do not know off the top of my head.

I found this video, have not watched it yet

 

[Seward G]

Responding to BLDanielsJr, there are three 200 amp batteries in parallel, for a total of about 7,800 watt-hours. I am puzzled too about the overall charging system not keeping up with use.  We have all LED lighting. We use a lot of appliances when boondocking (microwave, hot water kettle for coffee, electric blanket on cold nights, etc.).

[BLDanielsJr]

36 minutes ago, BLDanielsJr said:

Do you know if this trickle charge works when the Multiplus is in Inverter mode and not just charging mode when you have shore power or the generator? When I am boondocking, my primary charging is done through solar and the Inverter is inverting. I would want the trickle charge to be present while boondocking.

I found a discussion on Victron Community regarding the trickle charger and, YES, it does function regardless of the mode of the Multiplus, including OFF.

Looks like I can eliminate the Big Boy, put the DC-DC Charger in its place and wrap the trickle charger back around to maintain the chassis battery. The only big boy function that would not be available is the house assist for the starter motor which should not be a problem if the chassis battery is being maintained and a jumper can tie the banks together if necessary.

10 minutes ago, Seward G said:

Responding to BLDanielsJr, there are three 200 amp batteries in parallel, for a total of about 7,800 watt-hours. I am puzzled too about the overall charging system not keeping up with use.  We have all LED lighting. We use a lot of appliances when boondocking (microwave, hot water kettle for coffee, electric blanket on cold nights, etc.).

My rough calculations would be that your solar array on a reasonable day could charge about 30% of your battery bank and five hours at 60amps from your alternator might get 35-40%. That would mean that you should be able to recover during the day as long as you start the day with 35-40% SOC and your consumption through the day is not excessive.