Reddit reviews Victron BMV-712 Battery Monitor

Also make sure the fuse holder is rated for the fuse. Some are only 100amps or 150a etc.

Check the max discharge rate on the batteries too. You don't want to go more than they can handle either.

I also find this battery monitor very handy and it's Bluetooth. It's not cheap, but if you're going with victron (is the charge controller Bluetooth? If not, get the smart version! It's way easier to monitor and program and only a few $ more usually) it may be a worthwhile addition and doing it while you cut the cables is more efficient than later in terms of materials.

specifically current - or amps. using the amps you can figure out the watts.

this will let you know how many amps you've taken out of the battery, how many you've put back in and roughly the state of charge (in percentage).

the victron will let you know how much longer you can keep going with your current load as well. (eg, you have 24 hours left before you hit 50% state of charge)

https://www.amazon.com/Victron-Energy-MONITOR-VTBMV-S-BMV-712-Battery/dp/B075RTSTKS/ref=sr_1_11?crid=3G11Z5715Z3NE&keywords=victron+battery+monitor&qid=1571575505&sprefix=victron+battery%2Caps%2C156&sr=8-11

I believe you are wasting money. The 3 lithium batteries at 100AH each are great. Is there a reason you need 300AH worth of batteries? Have you done an energy audit (easier than it sounds). I would look at the Life Blue batteries. The battery management is built in, it is bluetooth capable for monitoring. Essentially they replace your existing batteries and just work. The VictronBMV-712 is available on Amazon for $200. A Samlex PST-3000-12 runs ~$1,000.

Whether you buy from Larry at Starlight Solar or not, it is worth a call to him to get his opinion on your proposed system as well as his thoughts on a Life Blue system. There is a good chance that a small Honda or Yamaha generator hooked up to your trailer will allow the converter built in to your trailer will charge the Lithium Life Blue batteries without needing to buy a separate charger made for Lithium.

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With a 3000 watt inverter you will be able to run microwave, hair dryer, and air conditioner. The amount of batteries will be what limits the length of time you can run those items. I would be very surprised if you need more than 200AH worth of Lithium. Remember that you get to use almost all of a Lithium battery, not 50% like lead acid batteries. Lithium batteries also charge up a lot faster than lead acid.

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And you should really think about solar being integrated. You don't have to buy fuel for panels.

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If you have any questions, DM me and I can go into further detail.

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Where you need to be careful is in choosing how to charge the batteries.

I just ordered the Victron BMV-712 Battery Monitor, it seems to be almost perfect for my use case. I can then configure the DoD (Depth of Discharge) to whatever level I'd like and it'll flip a relay. From there, I can use a switch (and some other jazz) to 3D print an enclosure to contain a simple two position switch:

Low Battery (reached configured DoD). Either start generator, or turn off Inverter.

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It may be worth using a three position switch instead, which will turn off and keep the inverter off. I have a somewhat small 400Ah bank and I don't know if the fridge (the other large energy consumer) will make it through the night without switching over to propane. It may just be a nice feature to have anyway.

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Other considerations:

• The generator has a manual push button electric start. It's unclear until it arrives if the button must be held (manual starter actuation), or a single press starts the generator. I'll also need to control pushing the generator off button. That's two momentary switches
  
• The inverter doesn't seem to have an easy external method of controlling the power switch. I'm hoping that it's a low current switch so I can just control it with a relay. The docs state that with the remote panel installed, whichever is used to switch it off and on takes priority. Meaning that, if my system turns off the inverter, I can use the control panel to control it manually. (need to map out the logic there so it behaves as expected)
  
  So in the end, I will need to use some sort of logic such as an Arduino, or a custom circuit to accomplish my goals.
  
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  The Victron BMV-712 Battery Monitor just allows me to monitor the DoD (which the inverter doesn't have, it only has a hard 10V low voltage cutoff) and flip a relay. I'll need to do something custom to get the automation I'd like! (I think)
  
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  I'll have parts here this week, and the tinkering will begin! I'll have a more solid footing soon.

If you've got a cheap charge controller that's already working, or one that will fulfill your needs, perhaps you'd be better off, as Syonyk said, building your own or using a stand-alone monitor that can communicate with your computer.

An Arduino can do most of that with some extra stuff and a little effort, or if you want a pre-built charge monitor solution, check out Victron Energy's battery monitor:

https://www.amazon.com/Victron-BMV-712-Battery-Monitor-Bluetooth/dp/B075RTSTKS/

This has built in bluetooth and I believe you can talk to it with a computer or smartphone fairly easily. It's not a very DIY solution for this part of the system, but if you're not comfortable with computer-electronics, it might work well for you!

If you don't already have one, I recommend picking up a cheap battery meter with a shunt. The one I bought was only $20, and it measures the actual current coming out of the battery over time as Watt-hours. Similar to how a home electrical meter works. It's the only accurate way to know how much battery power you've used. It can be reset each day in order to track usage per day.

You can even buy two of them and wire the second one so that it measures current going into the battery. Then you know exactly how much power your panels provided during the day, and therefore how much you can use over night.

Or there are intelligent meters that do both and have Bluetooth and a phone app, if you want to spend $200. I'm probably going to buy one eventually. Or design my own.

Here's the model I have for now:

https://www.amazon.com/dp/B07KXCF8F8

That's not the exact brand I bought, but it looks identical, and I think they're all the same and just sold by different resellers. The exact one I bought is no longer in stock.

There are plenty of others to choose from. Some have "fuel gauge" style readings, too. Such gauges generally aren't very accurate unless they are programmed with the discharge curve of your specific battery type. They are probably tuned for 12V lead acid, and might be reasonably accurate for those, but then it wouldn't work for lithium ion, which is what I have. Then again, if they are smart enough, they might "learn" the behaviors of your battery, but at these prices, I doubt it.

Here's an example:

https://www.amazon.com/dp/B07CTKYFTG

Whatever you get, if you get one, make sure it has a shunt. The shunt is just a chunk of metal with a small, precise resistance value, and the meter continuously measures the voltage drop across that known resistance to determine how much current is flowing. The shunts are rated by maximum amperage, with 100A being quite common. The smaller the maximum amperage, the more accurate they are, but you need one big enough to cover your usage. 100A is a good bet.

And just for the sake of completion, here's the expensive one I mentioned:

Victron BMV-712 Battery Monitor https://www.amazon.com/dp/B075RTSTKS