Portable power station and 12V LifePO4s externals

[nosolarjpn]

Hello,

I want to have two 12V Renogy 100Ah lifepo4 batteries as "backup" external batteries to supply additional power to my portable power station, a Bluetti 1500Wh (EB150?).

The idea is for the 12V lifepo4s to "charge" the Bluetti when it runs out.

I can't have solar panels, and a "DIY battery bank (i.e: imported Chinese cells)" is off limits. This config would only be used for emergencies (eg: blackout), so I'm opening to charging the 12V batteries and Bluetti using AC grid power.

My question: since the Renogy batteries have a built-in BMS, how do I efficiently "wire" everything for charging the Bluetti, and for charging the batteries?

Here's my plan so far:
1) connect the 12V batteries together (in parallel?)
2) add a battery monitor to check the volts and amps for each battery
3) use a power supply (eg: AC in -> 12V/30A DC out) to convert AC grid power to ~12V (should it be higher than 12V? is 13.5V ok?)
4) make a wire to connect the battery bank to MC4 port of Bluetti

I'm maybe missing some important details here but I can't figure it out. Any help would be appreciated.

EDIT: I am open to more batteries, ex: 4x12V to make a 48V battery bank, and not married to Renogy, I just thought the built-in BMS was convenient/easier.

 

[Bob142]

Welcome to the forum.

Here's my plan so far:
1) connect the 12V batteries together (in parallel?)

Yes. I believe with the Renogy LFPs you must wire them in parallel as they don't support series. You'll have a 12V 200Ah battery bank.

2) add a battery monitor to check the volts and amps for each battery

Good idea. Make sure to get a bi-directional Coulomb counting shunt-based battery monitor. That way it will measure current into and out of the battery to provide you with an accurate state of charge. It's difficult to impossible to do it based on voltage with LFP batteries.
Search for AiLi on this forum for a budget model. I also like the TF03K.

3) use a power supply (eg: AC in -> 12V/30A DC out) to convert AC grid power to ~12V (should it be higher than 12V? is 13.5V ok?)

For simplicity I recommend getting a dedicated LFP battery charger unless you have other reasons for a power supply.
I'm interpreting power supply to be a benchtop power supply like this. vs. a battery charger like this.

4) make a wire to connect the battery bank to MC4 port of Bluetti

If you really want to charge the EB150 from the LFPs:
I don't think the EB150 has a 12V car charger adapter option that would allow you to directly DC charge the unit. Maybe others can weigh in here but I recommend buying a small inverter, connecting that to your battery bank, and plugging your EB150 wall charger into it. Example below, shop around:

https://www.amazon.com/BESTEK-Inverter-Adapter-Converter-Charging/dp/B082MDRC64

There are losses associated with converting from DC to AC back to DC but the inverter is a useful for other things and I don't know of another general purpose way for you to charge the EB150 from your LFPs. Maybe @MBR has some thoughts on this.

Another option worth considering:
My preference, if your budget allows it, would be to just buy a larger 12V inverter and make the LFP system independent of the Bluetti. Then you could split the loads between the two, or just use the DIY system after the Bluetti runs out or vice versa. And you'd avoid the extra conversion loss.

Safety considerations:
Make sure you size your wires appropriately for the current they will carry and fuse everything. There are a lot of resources available on the forum to help you with that. One I find useful is below:

Circuit Wizard - Blue Sea Systems

If you're new to all this, check out the beginner video playlist:

Beginner Video Playlist

New to Solar? Watch these videos! They will help you understand the basics :) If you are a complete beginner and want to understand the basics of electricity, watch this video: If the video above was too easy and you want to understand how to calculate and size various components in your...

diysolarforum.com

 

[nosolarjpn]

Hi Bob142, thank you for the detailed response.

I was referring to a benchtop power supply, but I realize it would not auto-off once the batteries are full. I think a 12V inverter would be good, but now i'll need a lot of other things as you mention: fuses, wires, etc...

In the end I think i'll just get a second Bluetti and swap between the two, and make sure they're both fully charged via grid-power before the next blackout.

Thanks again.