Adding capacity

[chilly2]

Hello!

As I write another blissfully sunny day has both covered the house load and filled my 5kWh of storage before 2pm. I'm fairly certain a second 5kWh, or a good percentage of it at least, could be filled before the end of the day, so I'm thinking about adding a server rack 5kWh battery.

Current configuration...

48v system.
5kWh battery (4S 12.8v LiFePO4s, AWG 2/0 connections), with active balancer. Referring to as 4S from here on in.
The charge controllers (2x), 4S and inverters share a single set of 4 bolt bus bars.
There's a 125A ANL fuse on the +ve and a Victron smart shunt on the -ve from the 4S.

The plan...

- Move the charge controllers and inverters to a second set of bus bars.
- Join the two sets of bus bars with 2/0.
- Connect a new server rack 5kWh battery to the free spaces on the original bus bars.

The questions...

- Compatibility - The 4S are about 8 months old now. Would there be any problem having them in parallel with a rackmount battery?

- Safety - as above there's a fuse on the +ve but no switch or anything to cut off the 4S. What sort of fuse / switch should I put in between the two sets of bus bars to protect the CCs & inverters?

- Monitoring / SOC across multiple batteries - The smart shunt shows the SOC on the 4S. What would be the best way to keep an eye on the 4S and the new battery, preferably in one place? Another smart shunt seems expensive. Can I put the existing shunt on the -ve between the sets of bus bars and see the SOC across the batteries as a whole (up to 10kWh)? I realise I lose visibility on the 4S if I do that. Of the new batteries I've looked at, one has RS485, and one has a screen and bluetooth app so I could get by that way.

As a picture tells a thousand words, here's the current rats nest. It looks worse than it is but it's working fine, nothing overheating, etc. The bus bars are full, so the new battery is the impetus to reorganize the setup.

Critique of the plan / suggesions of other schematics welcome!

 

[chrisski]

- Join the two sets of bus bars with 2/0.

How many amps will go through this? The ampacity of the wire drives this.

- Safety - as above there's a fuse on the +ve but no switch or anything to cut off the 4S. What sort of fuse / switch should I put in between the two sets of bus bars to protect the CCs & inverters?

Each of my two batteries have a Class 125 amp T fuse, with a 225 amp Class T fuse after combined. I would fuse all five battteries seperately.

Monitoring / SOC across multiple batteries - The smart shunt shows the SOC on the 4S. What would be the best way to keep an eye on the 4S and the new battery, preferably in one place? Another smart shunt seems expensive. Can I put the existing shunt on the -ve between the sets of bus bars and see the SOC across the batteries as a whole (up to 10kWh)? I realise I lose visibility on the 4S if I do that. Of the new batteries I've looked at, one has RS485, and one has a screen and bluetooth app so I could get by that way.

Unless you want to buy a cheaper AILI shunt to put in the less important place, or having a BMS that you can monitor each individual battery, can thinkof nothing other than adding a SMART SHUNT.

I monitor the two individual batteries with a BMS on bluetooth, and after combined with a shunt. The shunt tells me everything I need to know. I have only looked at the BMS a couple times in the last year.

 

[chilly2]

How many amps will go through this? The ampacity of the wire drives this.

I probably should have included the current setup in full in the initial post

It was originally a 12v setup with 2000W inverter. It's now 48V.

- 48v 3000w 120/240 inverter, so max sustained would be 62A at 48VDC (12.5A at 240VAC).
- powered by the 4S (4x12v, each has a 100A BMS, connected with 2/0).
- 125A fuse on the +ve.
- smart shunt on the -ve.
- set of 250A bus bars.

Both the rack mount batteries I'm looking at have a max 100A discharge, so I'm expecting although the load won't pull any more, the fusing between the bus bars should handle 1.25 on both, so am adding a 2P 250A 500V MCCB between the +ve bus bars.

I have attached three rather basic looking diagrams.

- battery1orig is the configuration right now.
- battery2orig is with the addition of the rackmount battery, mccb, and bus bars.
- battery2 is with the shunt moved to cover both batteries.

If I move the shunt I get the overall in one place but lose clarity on the 4S which has no other way to measure SOC, so I'm thinking I'll go with battery2orig for now. I could, if I wanted to burn money, add a second smart shunt between the two negative bus bars for overall.

 

[chilly2]

Hello again,

It's been a week. I'm in paralysis moving forward and need some reassurance on the plan.

To summarize the above...

- add a second set of bus bars.
- move the charge controllers and inverter off the first set onto the second set.
- join the two sets of bus bars with awg 2/0 (of which I already have some).
- connect the new 48v battery to the first set along with the original 4S.

Between the bus bars, on +ve side...

battery 1 -> 125a fuse -> bus bar 1
battery 2 -> 125a fuse -> bus bar 1
bus bar 1 -> 250a class t fuse -> 250a mccb -> bus bar 2

The math says 2 x 100a batteries in parallel could deliver 200a, so a 250a class t is necessary and 4/0 between the two sets of bus bars.

However...

The existing 4s battery has a 125a fuse on it.
Each of the 4 12v batteries comprising the 4S have a BMS with over current protection.
The rack mount battery I'm looking at also has a BMS with over current protection.
If the BMS over current protection doesn't work what is the point of saying it has it? Ergo, it works.
The inverter draw maxes out at 3000w, and 3000/48 = 62.5A, which is well under.

Is the plan overkill or inadequate? Do I really need 4/0 and the extra fusing? Could I safely just add the new bus bars and MCCB with 2/0. If not, why not?

Seeing that "house burned down" thread the other day suggests I should just go ahead and spend.

Please help!

 

[DougfromdaUP]

Wire and fuse sizes should be determined by the current your devices will draw, not the current a device is capable of delivering.

That kind of defeats the purpose of a fuse.