48V bank charging 12V battery

[mackadoo]

Hi all,
I've got a 48V Lifepo4 bank being charged by two MPP Solar 3048 units in parallel. The whole thing is at a remote off grid location that I don't get to for a few weeks at a time and I'm setting up some remote monitoring stuff.

I'm going to be hooking up a Raspberry Pi running Solar Assistant and a sim-card enabled router to connect the RasPi to the internet. My issue comes in wanting the RasPi and internet to work even if for some reason the BMS or some cells in the battery fails. I've already got one of these 12V Lifepo4 batteries from aliexpress that I only use once or twice a year and only when I'm at said property, so I'd be happy to power my monitoring devices from there. So I have two questions:

1) Can I have this plugged in all the time at full charge without worrying about destroying the 12V battery?
2) Is there a better way to charge this directly from the 48V bank without being crazy expensive? Going through my inverter to charge it with the included 120VAC > 12VDC charger seems silly

I was considering getting a Victron Orion or possibly just a buck converter to save on costs if that would work.

Any thoughts?

 

[Samsonite801]

Hi all,
I've got a 48V Lifepo4 bank being charged by two MPP Solar 3048 units in parallel. The whole thing is at a remote off grid location that I don't get to for a few weeks at a time and I'm setting up some remote monitoring stuff.

I'm going to be hooking up a Raspberry Pi running Solar Assistant and a sim-card enabled router to connect the RasPi to the internet. My issue comes in wanting the RasPi and internet to work even if for some reason the BMS or some cells in the battery fails. I've already got one of these 12V Lifepo4 batteries from aliexpress that I only use once or twice a year and only when I'm at said property, so I'd be happy to power my monitoring devices from there. So I have two questions:

1) Can I have this plugged in all the time at full charge without worrying about destroying the 12V battery?
2) Is there a better way to charge this directly from the 48V bank without being crazy expensive? Going through my inverter to charge it with the included 120VAC > 12VDC charger seems silly

I was considering getting a Victron Orion or possibly just a buck converter to save on costs if that would work.

Any thoughts?

The problem with a buck converter (provided it had adjustable output voltage on it), is it could only float the battery up to full SoC (being CV charging), so it would charge at a slower amp rate.

I have pondered doing the same thing as you (in wanting to charge a 12v bank off of 48v bank), and I had been thinking of using the Victron Orion 48/12-30A, mainly because it is specifically designed to manage an LFP battery bank (has a LiFePO4 charge profile) on the output, so it could have both stages for charging, the CC and the CV and get the charging done faster when the demand is there for it (Orion can at least charge up to a 30a continuous at nominal output voltage, and 40a max).

 

[mackadoo]

The problem with a buck converter (provided it had adjustable output voltage on it), is it could only float the battery up to full SoC (being CV charging), so it would charge at a slower amp rate.

I'm ok with slow charging since my application is pretty low power. Even the 48/12-9A is overkill for what I need.

I have pondered doing the same thing as you (in wanting to charge a 12v bank off of 48v bank), and I had been thinking of using the Victron Orion 48/12-30A, mainly because it is specifically designed to manage an LFP battery bank (has a LiFePO4 charge profile) on the output, so it could have both cycles for charging, the CC and the CV and get the charging done faster when the demand is there for it (Orion can at least charge up to a 30a continuous at nominal output voltage, and 40a max).

Having the right profiles for this application as you've stated is what I'm more worried about. Don't really want to fry a $400 battery because I "saved" $100 on charging it with a buck converter vs. the Orion.

 

[Samsonite801]

Biggest key if you don't care about fast charging, is you can use a single-stage charging (with a buck converter) that has a regulated set screw for the output voltage (aka float only, constant voltage, CV), and say you set it to 13.4v CV it would hold voltage there (allowing amperage to drift down), until it comes down to zero amps and then would still sit at 13.4v (90% SoC on an LFP)...

Lithium Iron Phosphave Charge Range

Sheet5 Lithium Iron Phosphate Charge Voltage Range (LiFePO4) What to charge your cells/batteries/packs to. (SOC),Never More than,3.650,Shown is the "FLAT CURVE" of LiFePO4 cells/batteries/packs. What to not discharge past. (DOC),Nominal,3.200,The steap parts on the left and the right are the "Le...

docs.google.com

A proper 2-stage LFP charger can use a first-stage of CC (constant current), and let the voltage peak up to a higher number before it flips to CV on final-stage, and allow the amperage to taper off.

Yeah, I do still personally prefer to get a proper LFP DC-DC charger for my own stuff.

 

[740GLE]

If you’re worried about a 48v BMS failure or cell going bad, won’t the 12v lifepo battery have the same failure modes?

 

[mackadoo]

If you’re worried about a 48v BMS failure or cell going bad, won’t the 12v lifepo battery have the same failure modes?

Ultimately yes, but I'm looking at making the system as robust as possible so that I can work on some other projects in the future that would otherwise fail when the battery system fails. The install is at a cottage a 4 hour drive away in northern Ontario so winterizing is a real issue and if I can get the system reliable enough I can get something like a heat pump going. This is just a backup for communication so I can know if the system has failed before spending an 8 hour round trip to see what's up.

 

[mackadoo]

Biggest key if you don't care about fast charging, is you can use a single-stage charging (with a buck converter) that has a regulated set screw for the output voltage (aka float only, constant voltage, CV), and say you set it to 13.4v CV it would hold voltage there (allowing amperage to drift down), until it comes down to zero amps and then would still sit at 13.4v (90% SoC on an LFP)...

Considering how cheap these are for low current, I think I'm going to start with this and just monitor the battery stays at around 90% charge. Thanks.

 

[GrumpyOldRetiree]

Just spitballing here, but: Assuming that both the Pi and the router have a somewhat constant and predictable draw, couldn't you use the Pi to control a relay via a gpio pin. The relay would switch the DC-DC converter on/off based on a timer loop (or cron job) running on the Pi. This might maintain the battery SOC within an acceptable range. Additionally, with the addition of a temp sensor, the Pi could act as low temp cutoff if you needed that function.