Crosspost: RV Travel Trailer System - LiFePO4, solar, B2B, safety and fuse check please

[shortyjacobs]

Hopefully I'm not breaking rules - I had this in the Safety Check forum but now realize that since I'm looking for a general review, that might be the wrong forum. After 8 days with no comments there, I'm asking here.

• 202 Ah LiFePO4 4s battery: 1C discharge rating
• Using DMT1250 DC-DC to modify converter voltage to lithium voltages (converter is FLA only)
• Running 1 gauge to front of travel trailer with anderson connector. Will have Anderson connector direct to alternator in tow vehicle. Provides 50A charging in a pinch
• To reduce wire runs, use a DPDT manual switch to go between TV charging mode and "normal" mode where jack and other front electrics are powered. Would ONLY be in TV Charging mode (open contacts on DPDT), when stationary, (anderson connector on Tow Vehicle is at FRONT...can't be connected while driving).
• Future Inverter and Auto Transfer Switch (with correct make/break of ground bonding). Inverter will have manual enable/disable relay switch powered by BMS, but will bypass BMS due to amp limits on BMS.
• Solar input to DMT1250 as well.

Do I have enough fuses in the right spots? Anything I'm missing?

 

[chrisski]

Why are you bypassing the BMS because of amp limits? I am getting 2 X 24 volt 280 ah batteries. My original BMS I wanted was 250 ah per battery, but it was pointed out. Had no need for 500 amps of power, so I went with 120 amps per battery. That is still way more than I need. If your inverter needs more amps by bypassing a BMS, may be better to step up n voltage, like 12 volts to 24 volts, or even 48 volts.

Perhaps instead of using a converter and a DC to DC converter, you get a RV Lithium DC to DC converter.

You mention 1 gauge. I could not find 1 gauge locally. The odd numbers were just not sold.

Also, with the block diagram, I’m expecting to see an inverter or a power requirement. That helps if fuse size and wire size.

 

[shortyjacobs]

I'm using an overkill solar (rebadged xiaoxiang) BMS, 120A limit. The batteries can do 200 amps, but I wanted to use the overkill BMS. I could do 24V, or parallel 12V with parallel BMSs, but this is what I've got right now. I've already purchased most/all of the hardware here, so that's the path. I'm only bypassing the BMS for the inverter, the rest of the system current will run through the BMS. For the inverter, I'll use the BMS controlled output to trigger a small relay to enable/disable the inverter, (I think Will has a video out there about this even), so my batteries will still be protected from LVD.

The DC DC converter is to give me options for solar, high amp tow vehicle charging, and AC charging with minimal components. It's still TBD to see if this will work....I plan on this weekend hooking up my battery with the DCDC converter to my RV's existing converter to see if it works!

1 gauge comes from here...cheapest option I found for high amp long wire runs with anderson connectors included: https://www.amazon.com/gp/product/B07GD8V653/ . Based on the diameter, looks like my 2 gauge lugs will work fine. Plus it's cheap cheap cheap!

Inverter is in the upper left corner of the block diagram. 2000W inverter, estimated 185 amps input at max draw. That's why I'm bypassing the BMS. I protect the inverter with the 200a T-fuse 2 awg wire and then step down to smaller 6 awg and a 60a fuse.

 

[chrisski]

Inverter is in the upper left corner of the block diagram. 2000W inverter, estimated 185 amps input at max draw. That's why I'm bypassing the BMS. I protect the inverter with the 200a T-fuse 2 awg wire and then step down to smaller 6 awg and a 60a fuse.

I have a 12 volt 2000 watt inverter which I use to run a microwave and for the 2 minutes its on, I see 2200 watts from the battery. THat is 14.6 charging volts and 155 amps draw from the battery. I use 4/0 wire. IMO 1 gauge, if you can find it, will be to small. That may be OK for a 24 volt system, but out of my comfort level.

I am putting together a 24 volt lithium battery bank. I have no idea how you would bypass the BMS system. I plan on Overkill. I think bypassing the BMS, if possible, is a terrible idea to run most stuff, like a microwave in a build. If this were something like a mobile wheel chair stuck in the middle of the road and perhaps result in the person being runover a car because of it, then perhaps bypassing the BMS would be fine.

 

[shortyjacobs]

The 1 gauge wire is for the 25-30 foot run from the tow vehicle to the DC-DC charger. Max amps 50 Max voltage drop according to a calculator online is 0.37 volts, or 2.55%, based on 14.6V in and 50 amps flowing. Is that not enough?

For the inverter, 2000 watts out at 87% is 165 amps in. At 13.6V that’s a volt drop of 0.15 or 1.14% over 2 AWG over 3 feet. Do I really need to go to 4/0 if 2 gauge gets me only a 1.14% loss?

The bypass of the BMS is bypassing the current to the inverter around the BMS. But I still use the BMS to control the inverter, so if I hit a LVD the BMS shuts off and the inverter shuts off. The battery is still BMS protected, I’m just not running the inverter current through the MOSFET circuitry.

 

[chrisski]

AFAIK, there is no code designed specifically for trailers. NEC code is buildings, not mobile vehicles and not available for download, but you can purchase for thousands of dollars. There is a several year old copy of the ABYCC that is available for download for free that you can get some info from. I get my system on the safe side.

If you go with 2 gauge wire, it has the potential to get very hot, as in it could burn you if you touch it or do whatever a wire that approaches boiling temps do to things it is next to. I wish I could tell you how hot it will become, but I could not find a formula, but got told it depends.

Also, with the amps, should look at not at a 13.6 volt float, but at a night time 12.4 volt capacity (Inverter wattage / voltage * 1.15 to cover inverter losses= amps). Also include an inverter low voltage cutoff which is probably 10.5 volts. The * 1.15 is to compensate the 15% losses in converting DC to AC power. So, a 2000 watt inverter will pull close to 2300 DC watts from the battery to make 2000 watts.

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So for a 2000 watt inverter putting out 2000 watts at the following volts, this is what it will pull for amps:

14.5 volts = 158 amps

12 volts = 191 amps

10.5 volts = 219 amps

I mention this is because I will never push my inverter to the limits when the sun is not shining, but I go to sleep and the other three in my family don't respect that. So, we turn on the microwave in the day and it will pull 158 amps. Then I ask them to not use the microwave before I go to sleep, but they will use it anyway. So my batteries may be stabilized at 12.6 volts, but then when they turn the microwave on, the voltage sags to 12 volts and the load which was 158 amps jumps to 191 amps. Now they may have decided to turn a 1000 watt hot plate on and deplete my batteries to near 50%, and now when its microwave popcorn time, my inverter at 2000 watts stresses the batteries more and the voltage drops to 10.5, and I have 219 amps through the wire.

My point to the story is its more than just voltage loss, because an undersized battery bank will cause voltage to sag, and as voltage goes down, that microwave will still pull 2000 watts, which causes the voltage to sag more, and with the loss built in, what I thought was a 158 amp load could be 219 amps.

To me, that's how a lot of the 300k+ electric fires a year in the states work. So, I'm erring on the conservative side.

I'm also the only one answering your post, and I know others would think 4/0 wire would be ridiculous. One bad thing about being on a forum that has so many different threads is a thread can get lost with postings every two minutes.

 

[jjkd]

Note that the 1 gauge wire you link to is CCA - copper clad aluminum. This is going to have higher resistance and hence higher voltage drop than copper wire, along with a different current limit. Make sure you have taken that into your consideration during calculations.

 

[shortyjacobs]

Note that the 1 gauge wire you link to is CCA - copper clad aluminum. This is going to have higher resistance and hence higher voltage drop than copper wire, along with a different current limit. Make sure you have taken that into your consideration during calculations.

Good catch, I missed that. Voltage drop for pure copper is 0.37 volts in my application. Pure aluminum is 0.59 volts. CCA is somewhere in between, (literature seems to vary on where exactly). Seems close enough for me, I think, although it is a bit more than I was hoping for. I'm only pushing 50 amps through it, so current limit won't be a big deal.