Small 12V Only System Wiring and Fuse Questions

[jl9322]

I am putting together a small portable system and have some questions about wires and fuses. This will be 12 volt only, no inverter. Max load will be well under 15 amps, but want to allow for a little more, maybe up to 25 amps total for all loads.

I want to connect two 100AH LiFePO4 batteries with 100A BMS in parallel.

Questions:

When connecting batteries to each other in parallel, do you size the wires for max load or BMS amps? If I size the wire to the 100A BMS with something like 2 or 4 AWG, then I assume no fuse needed between batteries. If I were to connect the batteries together with something like 10 AWG, I assume I would need to use something like a 30 amp fuse?

Considering small loads, is a 30A DC breaker and fuse box between batteries and loads sufficient using 10 AWG? Or would I still need a battery terminal fuse? (Will be using the DC breaker as a disconnect of system.)

I put together the attached drawing for reference. Feel free to point out anything else to consider. I will have a plug connector between charge controller and solar panel as it is going to be portable.

 

[chrisski]

I’d connect the to batteries to a 100 or 125 amp breaker with properly sized wire to a busbar and then have the 30 amp fuse after that.

I would see this quickly growing to be more than just that one device.

I would also not put in a circuit breaker as you picture. Seems to be a audio circuit breaker and not designed for solar loads.

When connecting batteries to each other in parallel, do you size the wires for max load or BMS amps?

Max load for the BMS. Hopefully this is bigger than the max load for each battery. You could have a battery go out in the future requireing the max load to each battery creep up to the BMS amperage.

If I size the wire to the 100A BMS with something like 2 or 4 AWG, then I assume no fuse needed between batteries.

I put a fuse on each battery and then a fuse after combined.

If I were to connect the batteries together with something like 10 AWG, I assume I would need to use something like a 30 amp fuse?

Yes. If the max load is 30 amps, you will have the fuse blow sometimes. If the max load is 25 amps, blowing a fuse is unlikely.

I also would not use 10 AWG as connection to a lithium battery with a 100 amp BMS.

 

[jl9322]

I’d connect the to batteries to a 100 or 125 amp breaker with properly sized wire to a busbar and then have the 30 amp fuse after that.

Are you saying that wiring two batteries in parallel should be done via bus bar? Is there some advantage to using a busbar for two batteries in parallel? I can see where you have three or more batteries, you will get better balancing of the batteries, but I am not sure what you would gain in a two battery set up.

I would see this quickly growing to be more than just that one device.

Setup will include USB and cigarette lighter style socket in addition to refrigerator. If everything in use at max load, I'm looking at around 11 amps. Still plenty room to add additional DC loads. No plans for large draws or inverter on this system.

I would also not put in a circuit breaker as you picture. Seems to be a audio circuit breaker and not designed for solar loads.

This particular breaker is rated for 30A 12 volts to 48 volts DC. What should I be looking for in a DC breaker?

Max load for the BMS. Hopefully this is bigger than the max load for each battery. You could have a battery go out in the future requireing the max load to each battery creep up to the BMS amperage.

I put a fuse on each battery and then a fuse after combined.

Yes. If the max load is 30 amps, you will have the fuse blow sometimes. If the max load is 25 amps, blowing a fuse is unlikely.

I also would not use 10 AWG as connection to a lithium battery with a 100 amp BMS.

I was referring to the battery BMS, not a separate BMS.

Assuming a max combined system load of 25 amps or less. So if I connect the batteries directly to each other in parallel, that could be done with 10 AWG and a 30 amp fuse?

 

[chrisski]

Are you saying that wiring two batteries in parallel should be done via bus bar? Is there some advantage to using a busbar for two batteries in parallel? I can see where you have three or more batteries, you will get better balancing of the batteries, but I am not sure what you would gain in a two battery set up.

I'm recommending you use a busbar after the batteries are combined to split the current. You can use something else like a fuse block.

This particular breaker is rated for 30A 12 volts to 48 volts DC. What should I be looking for in a DC breaker?

I'm reluctant to recommend a breaker. Has more to do with the high AIC of a lithium battery where if the OPCD trips or blows there's enough current to jump across the gap. I recommend a MRBF fuse ro Class T fuse. A MRBF fuse comes in as low as 30 amps. A Class T comes in 100 amps.

Assuming a max combined system load of 25 amps or less. So if I connect the batteries directly to each other in parallel, that could be done with 10 AWG and a 30 amp fuse?

If the load does not exceed 25 amps constant or 30 amps tops, then yes.

The problem I have with that is what I call the guest or family factor where they either don't understand the limits or think that those rules don't really need to be used. So if you build this with any way to get the full 100 amps off the battery, they will find a way to do this.

 

[jl9322]

I'm recommending you use a busbar after the batteries are combined to split the current. You can use something else like a fuse block.

Easy enough to wire both batteries directly to fuse box (+) and bus bar (-). In my planned layout, it just means using more wire than if I went battery to battery to fuse box, especially considering equal length wires.

I'm reluctant to recommend a breaker. Has more to do with the high AIC of a lithium battery where if the OPCD trips or blows there's enough current to jump across the gap. I recommend a MRBF fuse ro Class T fuse. A MRBF fuse comes in as low as 30 amps. A Class T comes in 100 amps.

I will plan to add MRBF fuse at each positive battery terminal. Would it be fine to still use a DC breaker as a disconnect if terminals fused with MRBF fuses? In that case, I would wire both batteries to the DC breaker and DC breaker to fuse box. Or should I look at some other type of disconnect option?

If the load does not exceed 25 amps constant or 30 amps tops, then yes.

The problem I have with that is what I call the guest or family factor where they either don't understand the limits or think that those rules don't really need to be used. So if you build this with any way to get the full 100 amps off the battery, they will find a way to do this.

No worries, just the wife and I will be using and she won't do more than plug into the USB sockets.

Thanks for you help!

 

[chrisski]

Would it be fine to still use a DC breaker as a disconnect if terminals fused with MRBF fuses? In that case, I would wire both batteries to the DC breaker and DC breaker to fuse box. Or should I look at some other type of disconnect option?

I'd not want to use anything except a quality breaker like you'd find on the Bluesea website. I have used this DC switch and fuses

 

[robbob2112]

The breaker style you picture has an AIC of 2ka if you can find a datasheet for it at all. If they ever trip under load it can damage the contacts. They already run hot and after a trip under load they run hotter.

Ideally you connect each battery to bus bars. This also gives you a spot to connect other wires without stacking lugs. If you stack lugs they are not perfectly flat and the connection is not as good as a single lug per stud. If you must stack lugs never more than 2. The small loops for shunt power and the like don't count. This sort of thing should never be the bottom, should be on top.

Do use no-ox-id special or equivalent to prevent corrosion. A thin coating on all contact surfaces.

Now... you can use a MRBF 2 stud mount with 2 fuses of appropriate size for multiple connections.

Were it me I would size the wire and fuses for 125% of BMS max current connected to the bus bars. From there fused wires to you loads. Fused wires to the mppt.