battery cable selection

[Isetech]

This is an interesting topic, looking at some of the installations.

How do you decide what size cable and fuse protection to install.

The first thing you need to look at, the battery rating. A 100 amp hr (5 kwh) battery with a 0.5 C rating would mean that the fuse protection should be around 70 amp and the cable would need to be at least 16mm. I have noticed some 25 mm silicon multi cord flexible cable is rated at over 200 amps, why would you need a 200 amp cable for a 0.5 C rated battery.

Lets say you decided to install 2 batteries, then a 125 amp fuse would be a wise choice using 25 or 35 mm cable.

Its not always that simple, the lithium batteries have special terminals, which have a 25mm lugs.

I have an installation where we will be installing a 8 kva unit with 4 x 5 kwh batteries, that job is going to get interesting. Selecting the fuse and cable layout is going to be interesting. You have the choice of parallel or cascade parallel connections.

[Dylboy]

Hey uncle !

Happy new year sorry for my ignorance but was it casacding parallel ?

My guess is that parallel with the cables at the battery terminals opposed to all the batteries terminated to a bus bar ?

Sent from my CPH2197 using Tapatalk

[Isetech]

I am references using 5.3 kwh 48 VDC lithium batteries

The way I understand it, and I could be way off the mark (feel free to correct me)

The red wire from the fuse/disconnector to the master battery first red terminal, then from terminal 2 (red) of the master battery to terminal 1 of slave battery (battery 2)

The black wire from the fuse/disconnector to terminal 2 of the slave battery (battery 2), from terminal 1 of the slave (battery 2) to terminal 1 on the master battery (battery 1)

By doing this, you can use 25 mm cables and limit the charge/discharge to 100 amps, most lithium batteries recommend you only use a C0.5 rating, so it saves cable and the size of the fuse holder and fuses. I use a 160 amp fuse disconnector with 125 amp fuses


If you connect batteries in parallel, you just connect each battery directly from the battery to the fuse/disconnector using 25 mm wire and you can charge and discharge the batteries at 200 amp, not recommended for 5 kwh batteries, it will reduce the cycle life of the batteries. you would need to use 50 mm battery cables from the fuse holder to the inverter.

Hey uncle !

Happy new year sorry for my ignorance but was it casacding parallel ?

My guess is that parallel with the cables at the battery terminals opposed to all the batteries terminated to a bus bar ?

Sent from my CPH2197 using Tapatalk

[Justloadit]

I am references using 5.3 kwh 48 VDC lithium batteries

The way I understand it, and I could be way off the mark (feel free to correct me)

The red wire from the fuse/disconnector to the master battery first red terminal, then from terminal 2 (red) of the master battery to terminal 1 of slave battery (battery 2)

The black wire from the fuse/disconnector to terminal 2 of the slave battery (battery 2), from terminal 1 of the slave (battery 2) to terminal 1 on the master battery (battery 1)

By doing this, you can use 25 mm cables and limit the charge/discharge to 100 amps, most lithium batteries recommend you only use a C0.5 rating, so it saves cable and the size of the fuse holder and fuses. I use a 160 amp fuse disconnector with 125 amp fuses


If you connect batteries in parallel, you just connect each battery directly from the battery to the fuse/disconnector using 25 mm wire and you can charge and discharge the batteries at 200 amp, not recommended for 5 kwh batteries, it will reduce the cycle life of the batteries. you would need to use 50 mm battery cables from the fuse holder to the inverter.

Not knowing what battery system you have, I am not sure if the cascade connection is what you intend to do with this connection.

I understand Cascade as one system supplies the next system, and then the load.
If we imagine having say 2 hybrid inverters, mains would go into the first inverter, which then supplies the second inverter, which then supplies the load.
When the mains fails, the first inverter supplies the second inverter and the load at the same time. When the batteries in the first inverter discharges, the second inverter simply continues supplying the load until those batteries are exhausted. There is a seamless change of supply to the load. The only issue here is that the inverters must be able to supply the full power that the load requires.

[Isetech]

IF lets say you have a 5 kva unit connected to 2 x 5 kwh lithium batteries which has a max charge/discharge battery of 100 amps, some would say a perfect setup if the batteries have a C0.5 rating. The inverter is being used at its capacity.

Using an 8 kva inverter (has a 200 amp charge /discharge rating) with 2 x 5 kwh batteries adds a few challenges. the 2 x 5 kwh batteries are only rated at half the inverters full charge/discharge capacity, this is where the challenges come in. The batteries which have a C0.5 rating can charge/discharge at 200 amps, however it is not advisable (reduce the life cycles).

So do you fit a 160 amp fuse holder with a 125 amps fuses and restrict the charge/discharge. Something else you need to consider, the battery terminals on the 5 kwh batteries can only fit a 25 mm cable.

One of the projects we will be tackling will have 4 x 5 kwh batteries, in this case we will fit a 160 amp fuse holder with a 125 amp fuse at each battery, connect to a DC busbar(parallel) then fit a 250 amp fuse holder with 200 amp fuses.

Not knowing what battery system you have, I am not sure if the cascade connection is what you intend to do with this connection.

I understand Cascade as one system supplies the next system, and then the load.
If we imagine having say 2 hybrid inverters, mains would go into the first inverter, which then supplies the second inverter, which then supplies the load.
When the mains fails, the first inverter supplies the second inverter and the load at the same time. When the batteries in the first inverter discharges, the second inverter simply continues supplying the load until those batteries are exhausted. There is a seamless change of supply to the load. The only issue here is that the inverters must be able to supply the full power that the load requires.

[Justloadit]

What about if you have isolated one or 2 of the battery packs?
Then the one pack will absorb all the current available leading to battery heating up.

[Isetech]

If you isolate or remove a battery you would have to adjust the charge discharge rate. Even if you were using 50 mm cable, you still cant just disconnect and remove 1 battery, even if the cable can carry the load, you would damage the other batteries or shorten the life cycle of the battery.

This why it is so important that there is documentation with all the relevant data, isolation procedures, commissioning sheets etc. A backup system becomes a single complete unit made up of all the components. There is an isolation procedure required, components have to be spec'd to the system, so you cant just go in and take out or isolate individual parts. Like an aircon, you cant just remove the condenser unit and switch it on and expect it to work.

Technology is moving forward at such a fast pace and we as an industry are going backwards. We are so far behind with training and regulation.

Those days of threading and bending metal conduit, securing it to rafters in a neat orderly fashion, using house wire and just being a wire puller, using old COC documents, test reports, fax machines (a device for sending documents) and pagers ( its a device we use to carry for customer to contact us.), they are gone.

The solar PV industry is growing as such a fast pace, especially in the past 6 months I have seen it grow faster than in the past 5 years , I hear a lot of talk about standard and updates, I also hear about certain people targeting the industry and installers, yet we are still using standards that apply to lead acid batteries

Its not 1950 anymore, you cant sit around a table and discuss future regulations then take another 5 years to implement them, its time we figure out how move at the same pace as the industry and technology. By the time the SANS book is updated the lithium battery could be like a lead acid battery.

What about if you have isolated one or 2 of the battery packs?
Then the one pack will absorb all the current available leading to battery heating up.

[Isetech]

Something else I have noticed, people join groups and take anything said by certain individuals as fact.

It would be interesting if there ever was a court case:

Prosecutor : Who told you that you have to permanently bond the neutral/earth or install a relay to bond the neutral earth?

Electrician: I read it on a social media group or forum.

Prosecutor: Why did you install the lithium battery in the front of a garage where motor vehicles park, knowing that it would explode if bumped by the vehicle.

Electrician: I saw pictures of everyone else doing it on social media.

There seems to be more confusion than facts in this industry at the moment, good leadership and fast paced regulation updates are required.