DIY Wiring Victron Smart Battery Protect and Lithium LIFEPO4 with $60 BMS and 2000W inverter

[FilterGuy]

Back to the Pre-charge with lead-acid and then switch to LiFePO4: I guess you could make something like that work, but I fail to see why one would do it that way.

 

[DW SD]

Back to the Pre-charge with lead-acid and then switch to LiFePO4: I guess you could make something like that work, but I fail to see why one would do it that way.

I didn’t read all the preexisting threads - not surprises that this isn’t new ground.

But in my case (and OP from his schematic video) I will have both types of battery on the system. So it is a matter of flipping switches in the right order, which will be very simple.
Then again it seems you will have to flip the same switch in the right order.
Thank you for providing other options and the clever design!

 

[FilterGuy]

I don't know exactly your situation, but you want to be careful not to connect the two banks inadvertently. We had a similar discussion on another thread and I recommended the following:

You should consider using these switches. (They don't have '1+2' positions)
https://shop.pkys.com/Blue-Sea-11001-Battery-Selector-Switch-1-2-OFF-No-Both-position_p_3640.html


With this arrangement you can't accidentally connect the two banks.

I don't know how applicable this is to your situation, but thought it might help.

 

[DW SD]

Hi Filter Guy,
(note, if this long post dilutes the thread, I can remove and post elsewhere, including starting my own thread about my particular mixed chemistry architecture).

Thanks for the thoughts. That certainly seems to work.
Candidly, I had spent some time thinking a few months ago about how I could transition to an LFP battery bank for my RV. My goals were adding substantial capacity to the 660Ah Lifeline bank (3 x 4D) and charging efficiency for my 900W solar setup and some learning without a lot of cost (<$1000 budget for meaningful capacity increase. And long term freeing up storage and weight on the 40k lb bus / RV if I eliminate some AGM batts.)

Knowing the voltage and charging characteristics were different, integration seemed tough. But then I came up with the idea of paralleling both. And considered that the AGMs would just float full in the operating range of the LFP batteries.

Then tapping my friend Google, found some references to hybrid banks as well as this GREAT FORUM. I know Nordkyn Design site seems to be the gospel around here. Check out his Alternative 1 design:

Electrical Design For a Marine Lithium Battery Bank | | Nordkyn Design

Electrical design considerations for the integration of lithium battery systems on board marine installations.

nordkyndesign.com

(yes there are pros and cons, which I can accept)

Here's folks using a mixed chemistry battery in practice, which was further validation.

Another Lithium battery thread

Yet another thread on LiFePo's. I'm getting a bit nearer thinking about putting some LiFePo's on board – purely to reduce the amount of hours I'm running the engine to charge the domestic bank. There were a few great threads at the end of last year and particularly https://www.canalworld.net/foru...

www.canalworld.net

Some of the issues the mixed chemistry solves:

1. alternator charging. In my case, the alternator is setup as Constant Voltage. 13.2V. So this happens to be LFP friendly. won't overcharge. But if the BMS disconnects, the alternator still sees AGMs. this is a 400horse cummins with commercial grade alternator. So I expect it could handle full output for a decent duration to get the LFP up to 13.2V if it were @ zero SOC. I still need to think about this.
2. occasional high or low ambient temp use. I live in southern California. Usually we camp locally in beach campgrounds. So temps aren't an issue. But if we travel cross country, I could envision having high road temps or possibly caught in cold weather. Then I could just disconnect the LFP (or let the BMS). My bays are heated.
3. if I stay long term at a campsite with a shore power, I could just turn off the LFP and float @ whatever voltage.
4. if I run the LFP low, the AGM is already connected and operates as normal.
5. Now, when I come back from the trip, often the battery isn't 100% charged, so I have to bring it home before putting it in storage to charge overnight. (to maximize AGM life). With mixed, the AGMs should be @ full all the time. I just have to condition the LFP to ~ 50% - wish before parking.

I think my big considerations now are process around flipping the switch. Making sure there isn't massive current flow from the LFP at the time of flipping the switch (including between battery banks). But I can ensure the voltages are similar and the inverter already charged.
PHEW... again, if this dilutes the thread, let me know and I'll rip this out and post elsewhere.

 

[grizzzman]

It amazes me all the fear around mixing LFP and FLA banks. Oh well, live-in fear I guess.

 

[Danny]

Thanks again for all your help. I just got this design working. It uses an on off inverter switch with a low voltage disconnect so I can have my inverter shutoff higher than my DC loads panel. The parts are about $15.00 . I'm guessing you guys will find a way to improve this. I think Will's method is simple but I don't see why a 12v relay module can't be used. I have not messed around with solid stat relays and I'm not sure how they power act without power or if they have any other issues that a relay module may solve with all the additional electronics.

 

[Danny]

Wow.... we have wandered back into the manual pre-charg switch discussion that has peppered several other threads. Here is what I will be doing in my next install:

View attachment 8057

Theory of operation.
Turn On

1. Make sure the inverter is off
2. Turn switch from Off to position one. The resister is now in the circuit and will quickly pre-charge the inverter capacitors.
   Once the capacitors are charged, the current will stop flowing.
   (Ignore the PTC thermistor for a moment. I will come back to that)
3. After a second or so move the switch to position 2. At this point the inverter is fully connected. The resistor circuit is parallel to the closed switch so it is effectively out of the circuit.
4. Turn on the inverter.
   
   Notes
   * The user can turn the switch two position 2, but does not need to.
   * Even if the user does not pause at position 1, the capacitors will probably be sufficiently charged to avoid any problems.

Turn off.

1. Turn off inverter.
2. Turn switch through position 1 to the off position. No pause is needed

User errors
It is possible for the user to leave the inverter on when they go to connect the inverter through the switch. In this case the resistors will be handling a large amount of current for potentially an extended time, creating excessive amount of heat and letting all the magic smoke out of the resistors. However, in this case the PTC thermistor will heat up and it's resistance will go way up. This in turn will drop the current to a manageable amount.

The above circuit is for 12 volts. The following circuit is for 24 volts:
View attachment 8058

Really good way to present this topic. It is a bit confusing. I will look around a bit for the discussion on pre-charge. Thanks.