Trying to match solar array and controller to 12v LifePO4 battery bank

[TurbineTester]

Hello, this is my first real post, so please be gentle

I'm upgrading my RV motorhome battery bank from 200Ah (2x100Ah)of BattleBorn Batteries to 1,000 Ah (5x200Ah) Ampere Time Batteries. Because i already have a 3000W Magnum 12v Inverter installed, i want to keep my house system at 12V. I'm trying to figure out how to get the max amount of permanently installed rooftop panels combined with an appropriately sized MPPT controller. I was thinking i was just going to use one of the solar kits Will recommends here. However, i'm not sure if i wanted to limit myself to only 800 watts or the 4x200W kit at Rich Solar, because i think i have enough rooftop space for at least 1200 w of panels on our 37' motorhome. I'm about to go out and measure and try some sample layouts of the 60"x30" panels at Rich Solar to see how many will actually fit. So i was looking at upgrading up to as much as the 1600W kit, but according to the diagram and manual it's only 1600w if it's connected to a 24v battery bank, it's limited to 800W at 12v output. So i'm trying to figure out what my best option for an MPPT controller might be that will work with a 12v battery bank. I know Victron makes 150v+/100A+ MPPT controllers, but i'm not sure i want to drop $800 (minimum) just on the charge controller if there is another option available. Going just on the amperage math alone, if i could fit 1600w of panels on the roof, in order to charge at 12-14.4v give or take i need to be able to output between 133A-111A (respectectively). If i can only fit 1200W of panels, that drops my demand down to 100A-83.3A needed output to not waste the PV capacity. In either case i'm already either at or really close to the max output of the $800 Victron 150v+/100A+ MPPT controller. Additionally, the panels themselves are 20V each, so in order to have 8 on the roof, i'll need to use Y branch connectors to get them in parrallel and up the amps and reduce the voltage, and i'm guessing this is ok? Like the MPPT algorithms work with whatever the input voltage/current is as long as you haven't exceeded either one, right? If i'm reading manuals correctly if you exceed the current it will limit the input current, but if you exceed the input voltage it will basically shut off the controller without damaging it. So i just want to be sure the voltage is always below the max input, right? Thanks for any help you can provide. I'm trying to get all this stuff ordered in the next 24-48 hours so it can arrive and i can install before we leave for our next trip. I already have all the batteries in our garage, just need to get the panels, controller and wires. I guess the questions i'm asking look like this:

1 - with a 3000w inverter can i run pretty much everything i need to include a roof top AC unit, or microwave provided i have appropriately sized wire (150-250A) wire and fuses/breaker from the battery bank to the inverter?

2 - What manufacturer, model and price point MPPT charge controller should i be looking at to take advantage of 1200-1600w of PN into a 12v bank?

3 - What other questions or concerns should i bee looking at?

Thanks in advance for any and all help!

 

[rmaddy]

Your math is correct. 1600W of solar on a 12V system means about 111A charge current depending on the charge voltage. Given that the panels will be mounted flat on the RV you will rarely get the full 1600W. So a 100A SCC should be plenty. The slight over paneling will help overall.

Those 200W Rich Solar panels have a Voc of 24.3V. If you put 8 of them (1600W) in a 4S2P arrangement then the Voc will be 97.2V. Since Voc goes up as the temperature goes down you can't use a 100V SCC so you need a 150V. So again, you are correct that you need something like the Victron 150/100 to support 1600W of panels on 12V system.

Another option would be to use 2 SCCs instead of 1. 800W of panels on each SCC means about 55A of charge current. If you put them in 2S2P then you can easily use a 100V SCC. So a pair of Victron 100/50 each with 4 panels in 2S2P would give you 1600W.

So $796 for a single 150/100 or $648 for a pair of 100/50.

with a 3000w inverter can i run pretty much everything i need to include a roof top AC unit, or microwave provided i have appropriately sized wire (150-250A) wire and fuses/breaker from the battery bank to the inverter?

A 3000W 12V inverter means 300A of current from the batteries. That means 4/0AWG wire and a 400A main battery fuse.

I can't speak to running the A/C but you'll have no trouble with the microwave unless it's really big.

 

[TurbineTester]

Ok, i checked the roof and have PLENTY of room for panels. 1600w is an easy fit with 1800-2000 well within reach. I'll stick with the 1600w plan for now. For the wire for a 3000w inverter wouldn't that be 3000/12=250A? or is that 3000 output, less efficiency loss equals higher amps draw from the battery than the output power would suggest? I'm going to go and check the wire gauge and fuse to be sure, but think all the wiring was upgraded when the prior owner installed the upgraded 3000w inverter over the stock electrical configuration.

 

[rmaddy]

An inverter isn't 100% efficient so a conservative calculation is 3000W / 12V / 85% efficiency = 300A. Some people even use a lower voltage. If you want to drain your LiFePO₄ batteries to 0% then you need to use 10V in the calculation and then you get an even higher amperage.

 

[TurbineTester]

Let me just say thank you so much for all your input. It's been very great seeing someone as knowledgeable as you chime in quickly. I really appreciate your help! I went out and did a little digging around in the battery bay and found a few things. I'm going to post some pictures in the next few posts with what i found. Biggest shocker was that i found out that i only have a 2000w inverter (Magna MS2000) and not a 3000w inverter

 

[sunshine]

Biggest shocker was that i found out that i only have a 2000w inverter (Magna MS2000) and not a 3000w inverter

19.1 KG...

2000W 12VDC Pure Sine Inverter Charger MS Series | Magnum Dimensions

www.magnum-dimensions.com

2000w.....that would be at least 3000ebay watts of any light weight HF inverter which by its weight I assume this one isn't!

The:

Input Battery Voltage Range	9-16.8

is also exceptionally good!.....no doubt you already realise how good this inverter is!

 

[Bobert]

Let me just say thank you so much for all your input. It's been very great seeing someone as knowledgeable as you chime in quickly. I really appreciate your help! I went out and did a little digging around in the battery bay and found a few things. I'm going to post some pictures in the next few posts with what i found. Biggest shocker was that i found out that i only have a 2000w inverter (Magna MS2000) and not a 3000w inverter

I would consider going with a higher voltage system at least do the math on the costs for equipment and wires. I have a 48v system on my travel trailer and the costs for setup of 2100 watts of solar and a 3500 watt inverter were way less at 48 than it would have been at 12 volt. 48 volt systems are becoming quite common on motor homes. It’s very possible that the saving on wires and controllers might buy you a top of the line inverter.

 

[TurbineTester]

 

[TurbineTester]

So i'm planning on using 4/0 cable from the batteries to the main existing power connections/switch/fuses/shunts. I was thinking of wiring the inverter connections reversing the negative and postive outputs in order to balance the cycles and current out of the batteries, kind of like what Will did in one of his 48v battery bank videos at about the 4' mark here. Like i have 12v batteries number 1 through 5, and i'll connect the main negative (4/0) lead to the inverter to the negative post on battery 1, and the main positive (4/0) lead to the inverter to the positive lug on battery 5, with all the batteries wired in parrallel with maybe 1/0 10-12" power-flex welder wire between each of the batteries. I have two questions:
1 - Am i over complicating trying to reverse the connection points to balance the current draw on the batteries?
2 - Either way, what gauge wire should i use to go between the batteries?

 

[rmaddy]

1 - Am i over complicating trying to reverse the connection points to balance the current draw on the batteries?

Not over complicating. You must do this or you will have terribly unbalanced batteries.

Read chapter 3 of the "Wiring Unlimited" book from Victron. It covers the issues with parallel batteries.

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

2 - Either way, what gauge wire should i use to go between the batteries?

Use the same 4/0AWG wire if using the "diagonal" battery wiring method.

If you use the "posts" or "bus bar" wiring method then you could use smaller wire for each battery parallel connection along with fusing each individual battery. The size of the wire and fuse depends on the max continuous discharge current supported by each battery.

 

[TurbineTester]

Not over complicating. You must do this or you will have terribly unbalanced batteries.

Read chapter 3 of the "Wiring Unlimited" book from Victron. It covers the issues with parallel batteries.

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

Use the same 4/0AWG wire if using the "diagonal" battery wiring method.

If you use the "posts" or "bus bar" wiring method then you could use smaller wire for each battery parallel connection along with fusing each individual battery. The size of the wire and fuse depends on the max continuous discharge current supported by each battery.

Awesome answers, can you suggest a resource to read up on the bus bar vs post methods and what components i should buy?

 

[TurbineTester]

This is the diagram from that Victron ebook

 

[rmaddy]

can you suggest a resource to read up on the bus bar vs post methods and what components i should buy?

There's not really any more to know beyond what is in that book. The post is really like a 1 stud bus bar. I'd use a regular bus bar with enough studs for each battery plus one for the main connection. Keep all of the negative parallel battery wires the same length. Keep all of the positive parallel battery wires the same length. Put a fuse between each battery and the positive battery bus bar.

 

[TurbineTester]

Is there any reason to "top balance" my set of 5 batteries or should i just heck the voltage and if they are close not worry about charging them all to the same level before installing them in the RV?

 

[rmaddy]

I would fully charge each battery separately to 100% SOC before connecting them all in parallel. Voltage is a terrible judge of SOC with LiFePO₄ due to the flat curve. Putting LiFePO₄ in parallel with sufficient difference in SOC can cause quite a surge of current between the unbalanced batteries.

Having said that, plenty of people have put such batteries in parallel without fully charging them beforehand. With 4/0AWG wire between the batteries you should have little issue if there is a large current due to imbalance. But personally, I would charge them all first to eliminate possible issues.

 

[TurbineTester]

Ok, currently charging the first one using myiCharger, at 40A it plugging along but may take some time for all 5.

 

[TurbineTester]

I would fully charge each battery separately to 100% SOC before connecting them all in parallel. Voltage is a terrible judge of SOC with LiFePO₄ due to the flat curve. Putting LiFePO₄ in parallel with sufficient difference in SOC can cause quite a surge of current between the unbalanced batteries.

Having said that, plenty of people have put such batteries in parallel without fully charging them beforehand. With 4/0AWG wire between the batteries you should have little issue if there is a large current due to imbalance. But personally, I would charge them all first to eliminate possible issues.

Ok I got them all charged up using the manufacturer recommended 14.6V CV charging spec. They all kept taking current until the BMS cut out. IN about 1-2 mins the BMS kicked back on and all 5 batteries eventually settled back down to 13.35V. I've got them all wired up in parrallel and hooked up to the MAGNUM Inverter. It runs the microwave and doesn't even deplete 1Ah for running it for one minute. All in all it seems to be working very well so far with one exception. When the magnum inverter gets close to the absorb charging stage, the BMV-702 actually drops all data. Even with the BMV connected to a phone that does not go into screen saver mode, the Victron Connect app looses the charge/voltage usage history, even though it stay connected to the bluetooth and still reports voltage, and it reports no Amps in/out even though the Magnum shows it's still sending current (20-80 Amps) to the batteries. It will reconnect shortly, and then reconnect again. If it weren't for the Magnum Inverter Panel saying it's sending a NOT insignificant amount of current into the batteries, i woudl say the BMV is behaving like one or more of the AmpereTime BMS may be cutting out. Should i adjust some parameters in the Magnum set up to change the settings from charging the prior 2x100Ah battleborn batteries to the 5x200Ah Ampere Time batteries?

 

[Bobert]

Ok I got them all charged up using the manufacturer recommended 14.6V CV charging spec. They all kept taking current until the BMS cut out. IN about 1-2 mins the BMS kicked back on and all 5 batteries eventually settled back down to 13.35V. I've got them all wired up in parrallel and hooked up to the MAGNUM Inverter. It runs the microwave and doesn't even deplete 1Ah for running it for one minute. All in all it seems to be working very well so far with one exception. When the magnum inverter gets close to the absorb charging stage, the BMV-702 actually drops all data. Even with the BMV connected to a phone that does not go into screen saver mode, the Victron Connect app looses the charge/voltage usage history, even though it stay connected to the bluetooth and still reports voltage, and it reports no Amps in/out even though the Magnum shows it's still sending current (20-80 Amps) to the batteries. It will reconnect shortly, and then reconnect again. If it weren't for the Magnum Inverter Panel saying it's sending a NOT insignificant amount of current into the batteries, i woudl say the BMV is behaving like one or more of the AmpereTime BMS may be cutting out. Should i adjust some parameters in the Magnum set up to change the settings from charging the prior 2x100Ah battleborn batteries to the 5x200Ah Ampere Time batteries?

I would give it a try set the top voltage just below when this event occurs. Could be your new batteries are slightly out of balance internally and if that’s the case they may correct themselves after cycling for a while.

 

[TurbineTester]

I would consider going with a higher voltage system at least do the math on the costs for equipment and wires. I have a 48v system on my travel trailer and the costs for setup of 2100 watts of solar and a 3500 watt inverter were way less at 48 than it would have been at 12 volt. 48 volt systems are becoming quite common on motor homes. It’s very possible that the saving on wires and controllers might buy you a top of the line inverter.

Thank you for this suggestion. Reviving this thread because i've decided to upgrade the system to a 4s2p 48v Battery bank and 48v inverter. This is mainly because the magnum inverter i have CAN run the center AC unit on our rig, but it is stretching it to the limit to do so. So have a few questions now. I got the 150/100 solar charge controller so i can charge 12v or 48v banks either way. So my questions are

1 - What is a good 48v 300-4000 watt inverter that would be the same level of quality and charge/invert/apss-through function as the Magnum i have now.

2 - What do i do to get the 48v battery level to the 12v house system? Assuming a buck converter?

3 - How do i charge the 48V batteries form the RV alternator when driving? add a 48v alternator?

Thanks again for all your help!

 

[Bobert]

1 - What is a good 48v 300-4000 watt inverter that would be the same level of quality and charge/invert/apss-through function as the Magnum i have now.

2 - What do i do to get the 48v battery level to the 12v house system? Assuming a buck converter?

3 - How do i charge the 48V batteries form the RV alternator when driving? add a 48v alternator?

Thanks again for all your help!

Victron makes the most popular inverter/ charger for rvs and has a 48v version they released for the us recently. They have a lot of features that are handy for campers like power sharing. Pricey but nice. Others on this forum are probably knowledgeable about other quality inverters that are available. In my case I had neither the budget nor the space for a modular system so I went with an AIO. The AIO has a bunch of peculiarities that reduce it’s convenience In an rv application. It can work really well but it is certainly not specifically designed for this application. Definitely not a set it and forget it system like the victron can be.

You can use a buck converter as long as it is large enough to power your high draw loads ( generator, levelers, slides ect. Another option is to ignore conversion losses and leave your converter charger and leave house battery system in tact. I already had a lithium compatible converter charger in my travel trailer so I simply left my original house system in place and swapped out the old fla battery with a 100 ah lifepo4 battery. a buck converter is cheaper and more efficient but more work and you loose the redundancy of a independent 12v power source. For me the extreme power draw of my old style hydraulic slides was the cementing factor in my decision to go this route. I have found the redundancy to be helpful on several occasions.
Before I upgraded from 2200 to 3200 watts of solar I had extra space and dedicated a separate 240 watt panel to charge the 12 volt battery and in this reduced the converter losses by quite a bit.

Adding a 48 volt alternator is the best but I have heard of good success using a dc to dc charger ( like the victron Orion) you just have to be sure not to overtax your original alternator. Upgraded alternators in conjunction with the dc to dc charger are also a possibility.