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I'm lost down the rabbit hole - RV Electrical Upgrade with Lifepo4 Inverter Charger

techguy22

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Dec 1, 2020
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Hello, I'm new to the forum and I recently purchased an older RV that has FLA house batteries and happens to have a bad power converter/charger in it. In my search for electrical upgrades for the house batteries I realized that Lifepo4 is the way to go so I went that direction. I also learned that if I got a combination inverter/charger I could replace my power converter with it and not have to buy a new one of those. So, I spent hours researching and landed on the Sungold 3000W Inverter Charger and a 4S bank of 280AH prismatic Lifepo4 cells. I purchased these items because I was pretty sure that as a base I wanted to have these as part of my system.

Now I'm trying to figure out the BMS and all the moving parts. I've spent hours researching options and honestly I feel like I'm chasing my tail right now. I keep on ending back up at a Chargery BMS8T with one of the Chargery DCC relays. I thought I had it pretty much nailed down until I read a post on this forum about how the DCC disconnecting and reconnecting the charger to my inverter might possibly fry the inverter. So then I go back to trying to figure out how to turn off the charger in my inverter before the DCC disconnects. I can tap into the remote power switch on the inverter easy enough, but it means I need to design ANOTHER system to stop charging the batteries on the control side before the Chargery disconnects the DCC from the batteries. I'm starting to feel like I'm trying to build a mission to the moon instead of powering my RV.

Here's what I have:
12v 280ah LifePo4 Battery Bank (4S)
Sungold 3000w Inverter Charger

Goals:
  • Replace old RV Power Converter with Charger on Inverter. The old RV Converter was a 40 amp model.
  • Power the RV 120V AC System with the inverter from batteries when not on shore power. RV has 30 Amp AC Power system.
  • Charge the LifePo4 Battery Bank when connected to shore power or onboard generator (installing in the future)
  • Charge the LifePo4 Battery Bank with the vehicle alternator while driving down the road
  • Need to be able to stop charging the batteries at a set point (90% SOC?) and still have the charger on the inverter provide 12v power to the coach when on shore power
  • A bonus would be able to monitor the battery capacity and information (I’m kind of a geeky IT/IoT guy so I like being able to interface and read data if I want to )

The BMS​

I’ve been back and forth between standard BMS Systems and Chargery BMS8T. From what I’m reading, the Chargery works but nobody seems to really love it. Because I’m powering a high wattage inverter, standard BMS systems get expensive. At 3000w on the inverter I’m pulling around 250A and they say to only count on 50% capacity from Chinese BMS systems so I would need at least a 500A BMS. Everyone also says that you shouldn’t use the BMS as your everyday disconnection for your loads. (The BMS should be last resort, but then you need to design ANOTHER cutoff system to cutoff so the BMS doesn’t cutoff :/)

I thought of using a standard BMS for my coach 12V loads and then using a separate relay to turn off the control wire of the inverter in case of low voltage or overcharging. However, if I turn off the control wire to the inverter it will stop functioning as my power converter for my 12v loads also.

The Inverter​

I went with the Sungold inverter because I liked the features and I can’t afford to go up to a Victron Multiplus. As I understand its functionality right now it has a remote control switch that plugs into the unit. Here is the Switch behavior:

  • Switch Off = Inverter Off and Charger Off (Even with Shore Power present)
  • Switch ON = Inverter ON or Charger On (With Shore Power)
  • Switch Power Save = Same as Switch ON except it searches for loads when on battery
  • I’m still unsure if I have AC Power passthrough by default to the coach with Shore Power connected and the inverter powered off. I can’t figure it out from the docs and I’ll have to hook up the inverter to test it.

I could tap into the wire for the remote control and power down the inverter that way if needed in case of low voltage in the battery. However I can’t use that wire to power off the charger in the case of the battery being full because it would stop the charger from supplying 12V dc to the coach (in that case it would pull from the batteries while on shore power which is not ideal)
Here’s a post on this forum discussing similar to what I would like to do.

Charging from Alternator​

I know I need something for a DC to DC Charger and I found the Renogy DCC50S that allows me to have my alternator charge and optionally add solar later on. It seems that all DC to DC chargers are kind of expensive. I’m open to suggestions here. I need to verify the Amperage of my alternator still also.

Where I’m At Currently​

I’m leaning towards the Chargery BMS8T system with the Chargery 300A DCC from them at this point for last resort overcharging and low voltage cutoff for the battery bank. I’ll have to set the overcharging cutoff on the Chargery to be HIGHER than the charging cutoff on the Sungold charger (and the alternator charger) so that it only cuts off the DCC in an emergency situation (the Sungold is malfunctioning) I’ve read on this forum that if the DCC cuts off the inverter while its charging and then the batteries settle to a lower voltage and then the DCC switches charging on again, it can confuse the inverter charger and possibly damage it.

Thinking this thru in my head my Chargery settings would be something like:

Charging:
  • Sungold float charges at 14.0 (it has alternate charging profiles… not sure if I could set it lithium?)
  • Turn Off Chargery DCC in case of voltage above 14.4
  • Turn Off Chargery DCC in case of freezing temps (But this would stop me from discharging batteries below freezing also :(
Discharging:
  • Sungold Internal cutoff kicks in at 10.5 Volts (too low for safety I’m told)
  • Possibly an additional low voltage sensing relay to power down the inverter via control switch at 11ish volts?
  • Chargery DCC does low voltage protect at 10.5ish volts??

I’m open to suggestions and ideas though on how I could protect my battery and charge it using other methods. What I keep coming back to is that I’m going to end up spending as much protecting my cells as I paid for them… and its not even a BMS that I like. I feel like I’ve gone down the rabbit hole with this project and I’m close to just building my own BMS to handle everything with all the external relays and things I’ll need. I'm hoping that I can start a discussion and help others in a similar situation as me.
 
Welcome to the forum.

A budget often forces compromises. You often can't get everything you want. A custom LVD and restricted charge peak may be two of those things.

10.5V = 2.625V/cell - fine for a cut off.

Recommend RTFM:


You absolutely, positively don't want to float LFP at 14.0. I don't see a program that has float that high. AGM 1 is 14.1 absorp (3.525) and 13.4V. "Classic LFP" is 13.6 and 13.4. Personally, I would choose the AGM 1 program.

The "Classic" setting will probably undercharge your battery notably. The T0 time will likely be pretty short because the battery will hit 3.4V/cell pretty quickly, so your absorption time will likely only be 1-2 hours max. You'd likely need 4+ hours to get near 90%.

Do not use any program with a float above 13.6V.

Remember that you're buying cells with 2000+ cycles - that's FULL RANGE CHARGE/DISCHARGE. If you charge to full and discharge to empty, you're down to 1999 cycles. AND after those 1999 cycles, you'll still have 224Ah of capacity (80% of rated). If your normal usage is well inside of the max limits, each of those cycles will increase cycle life count. Folks often get really wrapped up in the details and don't consider how the system will typically be used. If you've designed it that you don't need the full capacity of your bank, that's something of a safety in and of itself.

BMS limits should be set just outside the limits. 3.65 for charge, 2.5 for discharge. They are safeties. In many cases "canned" BMS have non-adjustable limits of 3.75±.05 and something like 2.3-2.4V. Those are also acceptable.

The addition of a good battery monitor will help you keep track of your actual SoC. If you go for the "Budget" version, make sure you select the higher current shunt.
 
Thanks for help guide me with some good info and experience. I've read the manual for the inverter several times and its one of those manuals where I don't feel like everything is clear so I needed some help.

Since you've made me more comfortable with it I'm OK with relying more on the inverter itself to cutoff the discharge or charge as needed. I think I'll try going with the AGM 1 program for the inverter charger and see how it works. Also, I'm thinking of maybe doing some sort of hybrid disconnect for the BMS. What do you think of getting a smaller BMS, 100A or so, to handle the draw from the 12v system on my coach (Only has a 45 amp power converter right now) and then hooking up a relay from the output of the BMS to control an override on the power inverter? I would just hookup the inverter direct to the batteries bypassing the BMS and rely on the small relay to operate the inverter controls. This would save me buying a giant SSR or BMS to handle the 300A load of the inverter.

Like I said, I'm not super excited about the Chargery. I would rather invest in Victron BMV-712 and get a cheaper BMS to mainly monitor cell voltages and cutoff in case of abnormality. Any suggestions on a smaller reasonably priced BMS?

I'll keep mulling this over and try to come up with a draft schematic to figure out the details on how to hook it up. I really appreciate your help :)
 
I keep coming back to the Chargery... I don't think I really have other options. I live in a freezing climate so I really would like to have low temp cutoff. I think that the Chargery with the Chargery 300A DCC is what I need to to do the job. Should cost about $250 from aliexpress...

Here is a schematic I have as to how I would setup the system right now. I would love any feedback or suggestions. I put in a manual battery cutoff switch in case I want to disconnect the battery and in the case that the battery is frozen and I want the inverter charger to provide power in that case.

My only unknown question is if my Sungold inverter will have issues if the Chargery disconnects the battery from it. I've read that some cheaper inverters have an issue with this and could even let out magic blue smoke. I have no idea of how to know this or test this with my inverter.

If the Chargery can't provide me a good state of charge indication I'll plan on upgrading to a Victron BMV-712 and swap out the chargery Shunt and use that for SOC purposes.
 

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