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12v Dual Inverter System review

D-Wreck

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Aug 2, 2021
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14
Hello,

I purchased a class A motorhome with an existing Solar and Inverter system, but no effective battery bank. The house batteries are connected but they are about 15 feet away and using a 4awg wire, so they can run the AC refrigerator but not much more.

I am finally ready to install some12v 100 Ah LiFePo4 batteries (2p) but I wanted to see if anyone had any thoughts or pointers before getting started.

The wiring diagram is attached , hopefully it makes sense. Cable lengths are in the table. (Solar cable from MPPT should be 3’, not 5). I didn’t draw the negative cables exiting the Lynx power in because it got crowded, but rest assured they will be installed in real life. This battery bank would be isolated from the existing house battery bank and alternator.

I would like to add 2 more 12v batteries in another year as fund’s permit. this would give me 4 parellel batteries at 12v, 400 Ah, 400 amp max continuous current….correct? I would also upgrade the T class fuse to 500 amps.

I didn’t include a shut off switch, not sure how much I would use it vs just unbolting the T fuse. If I were to use one, I would use the Blue Sea heavy duty switch rated at 600amps continuous load. Thoughts?

could I add another 4p 12v battery bank at some time in the future? This would have its own T series fuse, shunt, and equal length cables the first battery bank. It would attach to the opposite side of the lynx power in.

To be honest, a 12v 400 or 800 amp system seems a little scary. I would much prefer a 24v or 48v system, but the Victron Multiplus that are already installed are 12v. Not much I can do about that other than start over fresh.

thanks for looking!
 

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The wiring diagram is attached , hopefully it makes sense. Cable lengths are in the table. (Solar cable from MPPT should be 3’, not 5). I didn’t draw the negative cables exiting the Lynx power in because it got crowded, but rest assured they will be installed in real life. This battery bank would be isolated from the existing house battery bank and alternator.

2awg to the solar may be overkill, but if you got it, use it.

I would like to add 2 more 12v batteries in another year as fund’s permit. this would give me 4 parellel batteries at 12v, 400 Ah, 400 amp max continuous current….correct? I would also upgrade the T class fuse to 500 amps.

You should only upgrade the fuse to 500A if the wire can handle 400A. Fuses are there to protect the wires. Wires must be sized to handle the current and voltage drop.

Peak draw by inverters:
6000W/12V/.85 = 588A

Fuse: 588A*1.25 = 735A

With the 250A fuse, you're limited to:

250A/1.25*.85*12V = 2040W

You are likely limited to a little less than 200A total because two batteries in parallel will never have perfect balance.

I didn’t include a shut off switch, not sure how much I would use it vs just unbolting the T fuse. If I were to use one, I would use the Blue Sea heavy duty switch rated at 600amps continuous load. Thoughts?

Do it.

could I add another 4p 12v battery bank at some time in the future? This would have its own T series fuse, shunt, and equal length cables the first battery bank. It would attach to the opposite side of the lynx power in.

To be honest, a 12v 400 or 800 amp system seems a little scary.

Good. You're getting it. ;)
 
Are you going to convert the Lynx Power In to accept fuses? If not, you will want a Lynx Distributor. And if you get a Lynx Distributor, you can do a $5 mod to enable the LEDs.

200A for solar? Holy crap. What SCC(s) are you using?

Add a cutoff switch. I used the same one you are considering.

What BMS are you using? Those inverters will want 300A each to get to full output. Will help determine how much the two batteries can handle.

Welcome to 12V high power inverting. Can't wait for the 24 volt version of the MultiPlus II 2x120.
 
Thanks for the replies! I’m concerned enough to start looking at alternatives now.

existing solar is 4 Canadian solar CS6U-340M 340 watt panels, wired in 2s2p…I think? 2 panels wired in series, parallel with the other 2 other panels. This feeds a combiner box then goes to a Victron MPPT 150/100 -TR. I have enough roof area to double the solar but I would have to buy another MPPT. Is that fuse overkill for this dieting solar system?

One thought I’m having to salvage my existing inverters..what if I split the system up so the inverters are completely independent? 2 individual 12v / 3000w /300 amp systems sounds more a little more sane. I would lose my cool Victron display, and I’m not sure how to handle the incoming solar, but I might sleep better.
 
As far as BMS, I haven’t bought anything yet. But I was thinking about Amperetime 12v 100Ah battery with 100 amp continuous output. I know these don’t have a low temp cutoff, so I was hoping to use that functionality in the Victron inverters and MPPT.
 
Thanks for the replies! I’m concerned enough to start looking at alternatives now.

existing solar is 4 Canadian solar CS6U-340M 340 watt panels, wired in 2s2p…I think? 2 panels wired in series, parallel with the other 2 other panels. This feeds a combiner box then goes to a Victron MPPT 150/100 -TR. I have enough roof area to double the solar but I would have to buy another MPPT. Is that fuse overkill for this dieting solar system?

125A fuse is all that's NEEDED based on the anticipated current. Your wire needs to support 100A and an acceptable voltage drop.

One thought I’m having to salvage my existing inverters..what if I split the system up so the inverters are completely independent? 2 individual 12v / 3000w /300 amp systems sounds more a little more sane. I would lose my cool Victron display, and I’m not sure how to handle the incoming solar, but I might sleep better.

That would create way more headaches than it fixes.

As far as BMS, I haven’t bought anything yet. But I was thinking about Amperetime 12v 100Ah battery with 100 amp continuous output. I know these don’t have a low temp cutoff, so I was hoping to use that functionality in the Victron inverters and MPPT.

Victron inverters DO NOT have native low temp charge protection, but they might be able to through virtual switch or assistants. MPPT does, but it requires a BMV/smartshunt or smart battery sense to provide life temp data. A GX device MIGHT enable the inverter inverter to have low temp charging protection as well.
 
Thanks for the replies! I’m concerned enough to start looking at alternatives now.

existing solar is 4 Canadian solar CS6U-340M 340 watt panels, wired in 2s2p…I think? 2 panels wired in series, parallel with the other 2 other panels. This feeds a combiner box then goes to a Victron MPPT 150/100 -TR. I have enough roof area to double the solar but I would have to buy another MPPT. Is that fuse overkill for this dieting solar system?

One thought I’m having to salvage my existing inverters..what if I split the system up so the inverters are completely independent? 2 individual 12v / 3000w /300 amp systems sounds more a little more sane. I would lose my cool Victron display, and I’m not sure how to handle the incoming solar, but I might sleep better.
your defiantly going to need more battery capacity with those multi's, I wouldn't make separate/isolated systems though. way more headache then its worth. You could make several parallel groups with independent kill switches. large 12v isn't scary, just needs to be balanced and use sized proper wire
 
Thank you again for all the replies, it’s very helpful. I’ll avoid wiring up independent circuits, and I’ll add th switch to the battery bank. I’m still trying to work out everything else, starting with figuring out the actual use case of this all. I can’t imagine I will use 6000w at once when not hooked up to shore power, so I will have to figure that out. more questions though:

-do the wire lengths going from inverter 1 to the lynx distributor need to be the same as the wire length from inverter 2 to the lynx distributor? I’m assuming yes but I would like to check.

-do I need to account for solar load when calculating the inverter loads? Not sure how to phrase that, but in my diagram where would the solar load go once it reaches the lynx distributor? To the batteries or to the inverters?

im not entirely clear how to calculate my solar loads, i will need to read more. I was assuming it was 340 watts per panel x 4 / 12 volts = 113.3 AMPs coming out of the MPPT...?
 
Victron inverters DO NOT have native low temp charge protection, but they might be able to through virtual switch or assistants. MPPT does, but it requires a BMV/smartshunt or smart battery sense to provide life temp data. A GX device MIGHT enable the inverter inverter to have low temp charging protection as well.

I have to correct myself here. The most current firmware (not sure what version it was enabled), low temp charge protection can be enabled in Victron Connect. At a minimum, you would need a temp sensor for the inverter, or possibly a shunt temp sensor passed to the inverter via GX device.
 
I have to correct myself here. The most current firmware (not sure what version it was enabled), low temp charge protection can be enabled in Victron Connect. At a minimum, you would need a temp sensor for the inverter, or possibly a shunt temp sensor passed to the inverter via GX device.
Thanks! I also have a Victron BMV 702 shunt and CCGX, so it sounds like it’s in the realm of possibilities. The batteries will be located inside the Motorhome in a closet, so the chances of them being used in colder temps without the heat on are pretty small to zero. I’ll still use temp sensors with the Victron equipment, but I’ll also turn the battery bank switch off during winter storage.
 
One more question, unrelated. Once I add my lithium batteries, my solar will only charge those, and in turn the lithium will only be used for the inverter.

I have several other batteries in my Motorhome. House batterie, engine start batteries, and generator start batteries. All 12v AGM. The only way to charge these batteries is with a AC charger, which is mounted in the engine compartment.

I don’t have a DC-DC charger at this time, hopefully next year. Would I be able to charge the Lithium batteries with solar, then use the inverter to run the AC powered battery charger to charge up the AGM batteries? It sounds horribly inefficient, but I doubt this will be used often. Will it work in some capacity?
 
Yes. I do exactly this in a manner.

My off-grid stuff is in a shipping container with a 50A RV power pedestal. I hook the RV up to it just like shore power. Thus all my 12V is via the AC-DC converter and the RV's 12V battery. It's about 75% efficient from the inverter battery, but my 12V loads are relatively small compared to the total, so I don't care at all. Convenience wins.

Once your RV batteries are floated the background draw is very low. It's just that your RV 12V loads will be less efficient.
 
I’m full of ignorance, so I hope another question won’t hurt.

Is there any way to convert my 12v Victron Multiplus inverter to 24v? I’ve been thinking about selling my inverters and buying 24v versions, but it would be nice if I could just convert the ones I have. Thanks!
 
Thank you sharing for your wealth of knowledge, it’s been very helpful. I will just run with what I got. I think it will work for what I need.
 
I installed a LiFePO4 battery to my inverters and solar charge controller using the Victron Lynx. Everything appears to be working good, but I still have concerns about the low temp cutoff. As a review, my system is set up is:

(1) Victron CCGX
(2) Multiplus 13/3000/120 - Connected to CCGX via VE.Bus (Ethernet cable, daisy chained)
(1) Solar Controller MPPT 150/100 - Connected to CCGX via VE.Can (Ethernet cable, VE.Can Plug at MPPT and CCGX)
(1) BMV-702 with Temp sensor on Battery + post - Connected to CCGX via VE.Direct (RJ12 cable from shunt to display, VE.Direct cable to CCGX).

I don't think the MPPT is reading the temp sensor data from the BMV-702. Do I need to set this up somehow?

Everything reads correctly on the CCGX, the BMV display, and the MPPT bluetooth app. However, I set the MPPT "Low Temp Cutoff" to 41 degrees F". This morning the battery was 35 degree F, and the MPPT was charging the battery. I have the battery switch "Off" at night so no charging occurs if it gets below freezing, but shouldn't the MPPT cut off below 41 degrees?

Multiplus and MPPT are updated to the lastest software level. Do I need to update the CCGX and BMV?
 
after some searching around, I think I figured this out.

On the CC GX, under settings, I enabled "DVCC". In the DVCC menu, there is an option to turn on STS (Shared Temperature Sense"). I did this. Now I can read the battery temp sensor (connected to the BMV-702) from the Victron Connect bluetooth app on my phone, under the SmartSolar device. It appears the low temp cutoff is working for the Smart Solar controller, but I will check it again.

Will this work as low temp cutoff for the Inverters? (I updated the software to the latest)
Should I use the other options under DVCC? (Shared Voltage Sense, Limit Charge Current)

Thanks for playing along!
 
You got it! DVCC + STS, SCS and SVS.

In order to get it to work for your inverters, you will need to update firmware to current. Low temp protection wasn't added until the version released around 10/2021... version 489 I think.
 
BTW, you can sign up for a free account here:


It will give you free access to training materials, firmware downloads as well as firmware changelogs.

Looks like I remembered correctly (rare):

xxxx489October 29, 2021
  • In order to account for external chargers, state of charge now synchronizes to the configured absorption percentage when the DC voltage reaches Absorption voltage ‑ 0.1V.
  • Added configuration option to stop charging at low temperature for Li-Ion systems that don’t feature that themselves.
  • Fix small PLL issue introduced in xxxx483 which in some cases can cause audible noise.
  • Compacts do now startup reliable on AC when DC is 0V.
  • Freconnect for Chilean gridcode changed from 50.1Hz to 50.2Hz.
  • Added support for VE.Bus BMS.
    With this firmware there is no need to use the VE.Bus BMS assistant anymore.
    Using this new firmware results in practically the same behaviour as previously when using the Assistant.

    As soon as the Multi (or Quattro) sees the VE.Bus BMS, and the (new) checkbox called “Configured for VE.Bus BMS” is not checked yet, it will automatically configure itself.
    The settings then auto-configured are:
  • The (new) ”Configured for VE.Bus BMS” setting is set, meaning that it will no longer charge in case it doesn’t see the VE.Bus BMS anymore. In more detail: it will go through passthrough when AC is present, and switches off if there is no AC BMS. This is a safety feature.
  • Battery type is set to lithium
  • Absorption voltage is set to 14.2V, Float to 13.5
  • Maximum absorption time to 60 minutes
  • Charge curve fixed (but reduced float is disabled, the settings “repeated absorption time” and “repeated absorption interval” are changed but ignored)
  • Storage mode is unchecked
  • State of charge when bulk is finished: 95%
  • Charge efficiency: 95%
  • Temperature compensation is disabled.

  • The recommended way to commission such system is to:
  • update the firmware
  • install and connect the VE.Bus BMS
  • unplug the VE.Bus BMS and wait for the Multi to switch to passthrough/switch off. This step ensures that the Multi has properly detected the VE.Bus BMS.
  • Reinsert the VE.Bus BMS.
  • Finished, or optionally connect with VictronConnect and make the rest of the configuration.

  • The firmware update can of course be done with VictronConnect, which will soon ship this new firmware.

    Related changes:
  • - The VE.Bus BMS Assistant, when installed on this new firmware, will issue a warning, that it needs no longer to be installed. (It will be harmless if it is).
    - The ESS Assistant as well as some others, with integrated VE.Bus BMS functionality are updated and will work with both old firmware & new firmware.
 
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