question

Hello Yes, this can easily be done. Getting the cabling symmetrical is best, but not essential as a 50 and 100A will not deliver equal power anyway.

@Johannes Boonstra (Victron Energy Staff) Thanks for the reply. I've contacted a couple of distributors and they have very little practical experience with the buck boost product line. Sometimes it's difficult to get advice when you go outside the standard product line everyone is generally using.

I'd like to ask some follow-up questions if you had a moment but let me give you a bit of background on the setup.

Setup:
Truck and camper setup. Truck is 12v and camper battery system is 24v. 720ah@24v lifepo4 bank. Truck dual alternators that look to be temp compensating alternators per diagrams below. Max output of 377a.

Buck Boost Main 12v wiring:
12v +&- busbars in rear of truck cab.
4/0 between truck bus to a 12v+ busbar. This is due to distance.
4/0 between truck chassis to 12v- busbar. This is to match the size of the input lines.
1/0 between buck boost converters to 12v +/- busbars. I believe the max lug size is 1/0 on the buck boost converters.

Buck Boost Main 24v wiring:
24v +&- busbars in rear of truck cab
1/0 between buck boost converters to 24v +/- busbars.
1/0 between 24v +/- busbars to camper 24v +/- busbars that tie in the batteries.

Questions (sorry for all the follow-up questions):
1. Any special setting that should be set when I tie the 100a and 50a together?

2. Max lug size for the buck boost line is 1/0? This is not clear in the documentation.

2. Would you consider the alternators being temp compensating based upon the diagrams I attached?

3. Per documentation you need to factor in 20% of the output current for loss. So a 100a output will pull 120a input. Per documentation, you shouldn't exceed 60% of alternator output. I haven't determined what my alternator pully ratio is but I'm assuming 3000 rpm at engine idle. Taking the prior information into account, and using the worst temp output graph option, I'm 12a above the 60% max. Should I be concerned with this?

4. Any thoughts on having the solar and buck boost converters charge in parallel assuming the bulk charging voltages are set the same?

5. Since the buck boost is unable to detect voltage and could potentially over charge my battery what's the best option to control this? My thought was to use a Cyrix LI Charge and tie it off the BMV-712 to trigger the relay open (reversed) when at 97%+ SOC. Is there a better approach? I'm just wanting the alternators to help bulk charge my bank and not perform any absorption type cycle.

6. How does the buck boost handle any back voltage spike when the output is cut off?

Thanks for any feedback you can give!

1. Any special setting that should be set when I tie the 100a and 50a together?

A; No, As they work isolated from each other there is nothing needed. The controlling on/off could be done with a common plus signal (on PIN 1 )

2. Max lug size for the buck boost line is 1/0? This is not clear in the documentation.

A; the bolt there is M8 so that would be possible to use 95mm2 , not sure how that relates to 1/0 cabling

2. Would you consider the alternators being temp compensating based upon the diagrams I attached?

A; depends on the load you are putting on them, seems to be with these 50 and 100A converters they dont need any compensation as even on low rpm its fine.

3. Per documentation you need to factor in 20% of the output current for loss. So a 100a output will pull 120a input. Per documentation, you shouldn't exceed 60% of alternator output. I haven't determined what my alternator pully ratio is but I'm assuming 3000 rpm at engine idle. Taking the prior information into account, and using the worst temp output graph option, I'm 12a above the 60% max. Should I be concerned with this?

A; theoraticly perhaps, I would not be afraid to do this to be honest.

4. Any thoughts on having the solar and buck boost converters charge in parallel assuming the bulk charging voltages are set the same?

A; depends on what the battery can handle, but as you would be using LFP this should be fine,

5. Since the buck boost is unable to detect voltage and could potentially over charge my battery what's the best option to control this? My thought was to use a Cyrix LI Charge and tie it off the BMV-712 to trigger the relay open (reversed) when at 97%+ SOC. Is there a better approach? I'm just wanting the alternators to help bulk charge my bank and not perform any absorption type cycle.

A; You need to ise a dedicated BMS which is able to control the (dis)charge proces by switching of DcDc converters and inverters. That may never be don on an overall battery voltage. (ps the regular discharge is controlled by the BMV as i assume)

6. How does the buck boost handle any back voltage spike when the output is cut off?

A; The DCDC should not be switched off mechanically, but by using the remote control option (on PIN 1)

Wanted to provide an update to the community. 50a and 100a 12/24 buck boost converters installed. Tied to camper 24v bus with cyrix li cnt. Cyrix controlled via bmv712 soc relay setting. 100a tied to drivers battery and 50a tied to passenger battery (dual alternators).

Alternator charging at idle working nicely and solar parallel charging with alternator working as well.

Solar and alternator combined charging.

GX portal page shows negative DC power usage representing power coming in from alternator. I was running air conditioning at same time. Could power the 15k air con with just the alternators and still put 23a @24v into the bank without solar adding on.

The bank is 32 calb 180ah cells in a 4p8s configuration. Everything is working very well and the cyrix seems to be suppressing any disconnect back voltage spike. I'm thrilled with how this is working out. I'd like to wire the buck boost activation to the truck upfitter switch but haven't got to that point yet. Wanted to say THANKS to the Victron team for answering my questions.


Update 4/17. Tied pin 1 on each device to different upfitter switches in my truck. Now I can control when the devices are turned on and which ones I want to charge my bank with (100, 50 or both).