48 V Hybrid for Backup - Please Check my Work

Computerizer

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Apr 23, 2022
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I'm in the planning stages for a 48 V on/off-grid system for backup at home. I was given 2 kWh of brand new LiPo cells from my employer (they'd purchased them for a project that we never ended up doing). I know LiPo isn't the best for this, but they were free, so... I'll use them for now and maybe replace with something better later.

I'm currently leaning toward a 5000 W PowMr hybrid inverter/charger, however I'm open to suggestions if there's something else with similar capabilities that's worth looking at. Lower power is fine, actually; our emergency loads would be less than 1.5 kW under most circumstances, so a 2 kW inverter is sufficient (but hard to find 2 kW all-in-ones with 48 V).

Although it's not a requirement, I would like to have a 12 V tap off my batteries for various 12 V accessories I already own.

For now, I will charge the batteries from the grid. During grid outages, we'll use the inverter to run the fridge/freezer as needed.

If the outage lasts more than a day, we will then charge from one of our EVs. We have two EVs, one with a 66 kWh battery and one with an 82 kWh battery. I intend to use a 12 V - 48 V DC-DC charger for that, like this. I know both EV's own DC-DC chargers can put out at least 1000 W at 12 V, so that 560 W charger won't be a problem.

If an outage lasts long enough to use up a significant portion of our EV charge, then I would hook up solar panels. I'd have all the equipment ready, so I could just go out to my 12 kW grid-tied array, unhook a portion of the modules from their Enphase inverters, and connect them in series/parallel (as needed) for the charge controller. It's a ground-mounted array and very easy to access all of the connections.

Here is an electric diagram of my plan:

Battery Bank-4x 13S.png

The biggest question I have, is what's the right way to do my 12 V tap on parallel batteries? I drew in diodes before the 12 V bus, which seems like a good way to prevent energy from flowing between the parallel batteries, but would that really work? Or is there a better way? Or no way?

In addition to that, am I missing anything? Any suggestions to improve our plan? Questions?

TIA!
 

400bird

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You can't do 12 volts the way you have it drawn. That will cause imbalance in the cells as the 12 volt loads drain down those first 3 cells.
Also, 3 lithium ion (NMC or similar with a 3.6-3.7 nominal voltage) make a poor 12 volt substitute.
9.6 - 12.3 volts depending on how you cycle the pack.

I'd recommend a 48-12v DCDC if you need to support 12 volt loads
 

nbtesh

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The best way to to get 12v from a higher voltage is a DCtoDC converter.
48v to 12V
 

Computerizer

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Okay. In the searching I did I saw mixed opinions on tapping 12 V light that. I figured since it would be small loads it wouldn't be a big deal and not worth spending extra money on another DC-DC converter.

Does everything else look okay with my plan?
 

nbtesh

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" I figured since it would be small loads it wouldn't be a big deal and not worth spending extra money on another DC-DC converter."
Its actually the opposite. 48V - 12V is limited in the amount of amps it can carry. check Specs when buying.
Lipos are very unstable. Dont tap directly into them bypassing the BMS.

Diagram
Add a Breaker after each BMS
Add a Breaker after each Busbar
I dont get the 48-12v EV dcdc

Change PowMr All in one for MPPsolar or growatt
 

400bird

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I don't like pushing a MOSFET based past 50% of it's rated current. Plus if you take one or two packs out of service, you'd want enough head room in the remaining packs to power the inverter.
I'd choose a 100a BMS
Also, considering adding a per pack breaker, it makes a handy disconnect switch for maintenance needs.

I'd also double check that 120a breaker going to the inverter. It feels smaller than I'd expect. My 6.8kw inverter requires a 250a breaker.

I can't comment on your choice on inverters, I went with a diff tier manufacturer. I also didn't pull the spec sheet to ensure your PV is assembled correctly to match the inverters limits.


Are the packs pre-built? I ask because 14s is a more common way to get a 48 volt pack.

Depending on how you cycle the cells:
13s is 41.6 - 54.6 volts
14s is 44.8 - 58.8 volts
 

Computerizer

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Change PowMr All in one for MPPsolar or growatt
I've been having trouble locating all-in-one units that meet my requirements: 110-120 Vac (US), 48 V battery, 320 V(oc) solar; and don't cost a hell of a lot more than the PowMr (and PowMr also sells Growatt on their website -- not sure what their relationship is).
 

Computerizer

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I don't like pushing a MOSFET based past 50% of it's rated current. Plus if you take one or two packs out of service, you'd want enough head room in the remaining packs to power the inverter.
I'd choose a 100a BMS
I'd rather go with a smaller inverter -- I don't need 5 kW; I'd be fine with 2 kW. The challenge I had was finding a 48 V all-in-one with lower power that also accepted the higher solar voltage. (I could put solar in parallel instead, but it's a 100 ft run so I'm trying to keep the wire size down by using higher voltage.) I thought I would get the 5 kW and reduce its maximum current settings for the batteries, e.g. to 60 A or so.

Also, considering adding a per pack breaker, it makes a handy disconnect switch for maintenance needs.
Good point, I'll do that.

I'd also double check that 120a breaker going to the inverter. It feels smaller than I'd expect. My 6.8kw inverter requires a 250a breaker.
You're right, I misread the manual. It says max current is 120 A but recommends a 200 A breaker.

I can't comment on your choice on inverters, I went with a diff tier manufacturer. I also didn't pull the spec sheet to ensure your PV is assembled correctly to match the inverters limits.


Are the packs pre-built? I ask because 14s is a more common way to get a 48 volt pack.

Depending on how you cycle the cells:
13s is 41.6 - 54.6 volts
14s is 44.8 - 58.8 volts

They are not assembled. I was given 54 individual internal-resistance-matched 10000 mAh lipo cells. 13S x 4 is the easiest way to use the most of them; I'll only have 2 extra cells. 14S x 3 would give me 12 extra cells. So that's why I went with 13S. The voltage range is supported by all the equipment I've looked at, and 13S BMSs are available.
 

Computerizer

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Okay, here's an updated plan based on all your feedback.

  • Switched to Growatt SPF 3000TL LVM-ES.
  • Added breakers in useful places.
  • Adjusted braker sizes
  • Added minimum wire sizes (some are larger due to length).
How's it look now?
 

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bishnoi

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May 4, 2022
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Okay, here's an updated plan based on all your feedback.

  • Switched to Growatt SPF 3000TL LVM-ES.
  • Added breakers in useful places.
  • Adjusted braker sizes
  • Added minimum wire sizes (some are larger due to length).
How's it look now?
hi, what values do you give for the hybrid inverter for
bulk voltage program 26 ?
floating voltage program 27 ?
for use with leaf cells 14s
 
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