Equal length battery connections, double landed buss bars

[KRP]

I know equal length battery connections (between battery stack and inverters) is always recommended, but how important is it? I have 2 EX-6500 inverters (in split phase configuration) and a EG4 48V 600Ah stack. Due to layout, there is some difference between the cable lengths and SS is telling me that could be causing inverter problems (one fan running full all the time and one array showing impossibly high Watts and Amps). My battery cables are oversized (4/0). Seems to me that with oversized cables, the resistance difference between slightly mismatched lengths would be negligible, but tell me if I'm wrong.
Also, is there a problem with using a 2-stud buss bar for each battery cable, with the cable to the battery on one stud, and both inverters connected to the other? I've been told this "double landing" causes problems but I don't understand why. Thank you.

 

[12VoltInstalls]

The busbar has resistance. Stacking cables has basically only two scenarios that work correctly. Therin could be the issue. Battery A and Battery B? Several batteries? Just two?

Need to know exactly to avoid drawing three diagrams and describing 7 elaborate wiring schemes.

But if you are using a busbar stud to connect cables it has to be done in a very specific manner and most generally stacking ring terminals is bad.

Using 4/0 is something I’d be likely to do ? (Actually I would have probs done 2/0)

 

[KRP]

The "double landing" I'm referring to is one battery cable (from a 48V battery stack) connected to one stud on the buss bar. The other terminal of the buss bar has both cables that go to the inverters (one cable to each). The inverters each carry one leg in a split phase configuration. Since the terminals are stacked, there is no difference in path length from battery cable to both inverter cables. I don't see how that causes a problem, but am asking what I might be missing.

 

[DIYrich]

How "unequal" are the lenghts? 2 ft vs 30 ft: Upsize the wire for the longer length. 2ft vs 3ft: don't worry about it (or make both 3 ft). Calculate the voltage drop across the wire, and try to get something close to each other.

Stacking connections on a busbar: High current, not advised. the connector on the bottom has to pass all the power, including the power to the connector above it. It is designed to pass power only to itself.

 

[12VoltInstalls]

Since the terminals are stacked, there is no difference in path length from battery cable to both inverter cables. I don't see how that causes a problem, but am asking what I might be missing.

Not what’s missing- what’s added: ohms.
Stacked cables have higher resistance through the terminals.

IF you are going to stack on a busbar there is only one way/thing that is ok: battery cable terminal goes against the busbar, inverter on top of battery terminal. There’s a couple other “ok-ish” ways to stack but I’m not getting into that rabbit hole.

 

[Craig]

None of my stuff is the same length and I have ring stacked as well with zero issues in 3 years of continuous use. I have a 24V system with only a 2500 watt inverter though so I am not pulling many amps at once. I think with a 48v system you will be even better off. Personally I wouldn't get to caught up in a few inches difference.

 

[12VoltInstalls]

None of my stuff is the same length and I have ring stacked as well with zero issues in 3 years of continuous use. I have a 24V system with only a 2500 watt inverter though so I am not pulling many amps at once. I think with a 48v system you will be even better off. Personally I wouldn't get to caught up in a few inches difference.

Not trying to be argumentative nor “diss” a moderator, either, but having had issues in the past (when I was lead acid 12V, 8P) and having tested resistance and voltage with a meter on several configurations to duplicate prior helpful posts by members here… I’m not sure that it’s wise to categorically say length and stacking are irrelevant.

In practical terms, I don’t think “getting caught up in a few inches” is an area of great concern. With adequate, appropriate sized cables (I prefer to be oversized on battery-to-inverter cables or other high-amp connections) even a couple feet of variance there isn’t enough difference to matter- in practical terms. Especially with LiFePo.

However, the stacking of terminals on a busbar:
Numbers of posters with “melting” parts (some even experiencing small fires, a few bad fires) is most often related to a point of resistance that is an installation flaw. In your installation, the 2500W inverter is ~100A of draw. Still enough to pay attention to; I weld stuff with amperage less than that.
As I recall, there is a mention in ABYC regarding stacked terminals, and one of the purposes of N and G busbars in entrance and distribution panels is not merely convenience but also safety.

I am just concerned that putting a stamp of approval on stacking terminals (which imho is a terrible idea in vehicle and boat power systems from a reliability perspective) is going to lead to some poor practices and people can / could experience reliability issues, perhaps fires and loss, or god forbid loss of life for themselves or others.

While perhaps less of a concern for AC current, with DC current arcing damage and resistance are the primary failure points we design against and use more expensive crimping tools to ensure safe, reliable service.

I just think advising that stacking of terminals is acceptable isn’t a good idea.

 

[Craig]

Not trying to be argumentative nor “diss” a moderator, either, but having had issues in the past (when I was lead acid 12V, 8P) and having tested resistance and voltage with a meter on several configurations to duplicate prior helpful posts by members here… I’m not sure that it’s wise to categorically say length and stacking are irrelevant.

In practical terms, I don’t think “getting caught up in a few inches” is an area of great concern. With adequate, appropriate sized cables (I prefer to be oversized on battery-to-inverter cables or other high-amp connections) even a couple feet of variance there isn’t enough difference to matter- in practical terms. Especially with LiFePo.

However, the stacking of terminals on a busbar:
Numbers of posters with “melting” parts (some even experiencing small fires, a few bad fires) is most often related to a point of resistance that is an installation flaw. In your installation, the 2500W inverter is ~100A of draw. Still enough to pay attention to; I weld stuff with amperage less than that.
As I recall, there is a mention in ABYC regarding stacked terminals, and one of the purposes of N and G busbars in entrance and distribution panels is not merely convenience but also safety.

I am just concerned that putting a stamp of approval on stacking terminals (which imho is a terrible idea in vehicle and boat power systems from a reliability perspective) is going to lead to some poor practices and people can / could experience reliability issues, perhaps fires and loss, or god forbid loss of life for themselves or others.

While perhaps less of a concern for AC current, with DC current arcing damage and resistance are the primary failure points we design against and use more expensive crimping tools to ensure safe, reliable service.

I just think advising that stacking of terminals is acceptable isn’t a good idea.

We may need to address the definition of stacking terminals and what the consequences could be. I personally have a Inverter attached to bus bar followed by a water pump and then followed by my 24 v DC loads. The only thing of consequence is the Inverter I MIGHT have a whopping 5 amps in all the DC loads but I will test next time I am at my location. But however I am technically illiterate about the codes and what have you regarding stacking rings. I'm sure there is also a big difference in loads between a 12v Rv/marine system and a 24v home system designed to draw low amperage. Now just to instigate a conversation I don't see the difference between having properly clamped rings on one post vs having the same number of rings clamped to a bus bar. Please do not be afraid to DIS me I don't care if I am right or wrong I want everybody to learn. Anyways I have never had a problem but that does not mean its correct.

 

[12VoltInstalls]

conversation I don't see the difference between having properly clamped rings on one post vs having the same number of rings clamped to a bus bar. Please do not be afraid to DIS me I don't care if I am right or wrong I want everybody to learn. Anyways I have never had a problem but that does not mean its correct.

I am only ornery and mean to fools. It’s embarrassing to me when I do it but I’m quite good at it- that’s also to my shame and embarrassing. I wouldn’t diss you.

The critical thing is to not have the battery cable to load more than ‘one step away’ from a load. A busbar should be and is generally at nearly the same resistance as the battery cable terminal itself- when properly installed.
So a busbar is ‘zero resistance’ in practical terms (not scientific terms; everything from wiring to terminal ends has resistance) and is considered as the same as the battery cable.

Problems can and do occur when you two-steps-away a cable with a larger load. I had for quite some time a fla 4P2P 12V battery bank. The 2P was achieved at a single stud terminal block, with the 2/0 main cable to the inside busbar and fusebox etc sandwiched between the two 4P battery cables- not outside the 2P terminals; “one or zero steps away” in practical terms. Each terminal end sees no more reductions in amp capacity than it would measured alone in practical terms. (Again, there IS a difference scientifically)
However, if you did put the main cable outside there was a decimals voltage difference between the 4P banks measured at full sun (yes, I had to play)

So if (real situation on a ‘customer’ rig) you stacked multiple things on one battery stud - alternator, winch, floodlights, stereo, PDC feed- the battery stud gets very hot to the point of melting wire insulation and the PDC was even getting hot. One $35 busbar and a coupla hours later and he never melted wires again.

So stacking can make a difference.

In my own system I have 4-stud busbars. They are full. So when installing the second inverter I landed the inverter feed on top of the main battery feed. This is essentially the same or better than buying busbars with more studs- the load is not ‘removed’ by any “steps away” from the battery cable, and neither the battery cable nor the inverter know the difference.

Some will say that’s pickyuni and I’m not going to argue with that. But my stuff never gets hot, I’ve never blown a fuse or breaker, and it duplicates nothing I’ve seen on people’s stuff I’ve fixed after things got too hot.

A 5A DC load is probably fine in your situation, but my head would still ask the question, “is that terminal end robust enough to let me put 40ftlbs of torque on the nut?”

That’s all - thanks

 

[Craig]

Very good explanation. I have no doubt that what you say is true especially given your real world experience. I think the big difference that I am not taking in to account is that my loads are relatively small which has never created a problem for me. I will conduct a heat by running everything I can at once just for fun and see what happens. But with 12v and using all those devices you have I can at least see how things can heat up.

 

[Solar Guppy]

While it is ideal to have perfect paths for flow of energy so parallel batteries equally discharge, it is also somewhat self regulating with differences.

Take for example two batteries in parallel, but one has a higher resistance path.. When the pair provided energy one battery initially provides more than the other but as it discharges, its Voltage decreases to the point the second battery is carrying more of the total load. This can see-saw back in forth but in the end shouldn't be much of an issue to drain the capacity of both batteries, just won't be a constant on which is providing the most.

Here an external SOC monitor would be best, not relying on communications on one battery for the packs energy status.

 

[ampmike]

The busbar has resistance. Stacking cables has basically only two scenarios that work correctly. Therin could be the issue. Battery A and Battery B? Several batteries? Just two?

Need to know exactly to avoid drawing three diagrams and describing 7 elaborate wiring schemes.

But if you are using a busbar stud to connect cables it has to be done in a very specific manner and most generally stacking ring terminals is bad.

Using 4/0 is something I’d be likely to do ? (Actually I would have probs done 2/0)

If stacking ring terminals are bad how else would you connect batteries in parallel. like this drawing using 4/0 cable?

 

[MisterSandals]

If stacking ring terminals are bad how else would you connect batteries in parallel. like this drawing using 4/0 cable?

Sometimes it's unavoidable.

 

[ampmike]

Sometimes it's unavoidable.

Is there a correct way to stack terminals?

 

[glandpuck]

I would suggest that this concept of "equal length battery cables" would be a relegated to discussion on flooded lead acid batteries and AGM's.

Modern LFP packs from reputable companies accept the amperage and voltage input at their terminals. The microprocessor of the BMS uses the onboard programmed logic to handle the proper charging of the cells. It knows not the length of your cables.

So don't drive yourself into a panic attack trying to cut your cables to the exact equal lengths.

The key thing is to use cables that have specifications for your use. So for example, if you have 2 5.1kW lithium packs and each has a rated charge and discharge amperage of 100 amps DC, then your would need a wire gauge rated to handle 200 amps of current with an insulation rated for your temperatures and whether it is in conduit or not as well as fire and solvent risks.

 

[rhino]

Is there a correct way to stack terminals?

Do not put a washer between the terminals/lugs. They should be in direct contact of each other and they should be in direct contact with the battery terminal.

 

[MisterSandals]

The microprocessor of the BMS uses the onboard programmed logic to handle the proper charging of the cells.

I think you are over estimating what most BMSs do.

Other than trivial balancing current, a BMS is pretty much an On/Off switch.

 

[ampmike]

I would suggest that this concept of "equal length battery cables" would be a relegated to discussion on flooded lead acid batteries and AGM's.

Modern LFP packs from reputable companies accept the amperage and voltage input at their terminals. The microprocessor of the BMS uses the onboard programmed logic to handle the proper charging of the cells. It knows not the length of your cables.

So don't drive yourself into a panic attack trying to cut your cables to the exact equal lengths.

The key thing is to use cables that have specifications for your use. So for example, if you have 2 5.1kW lithium packs and each has a rated charge and discharge amperage of 100 amps DC, then your would need a wire gauge rated to handle 200 amps of current with an insulation rated for your temperatures and whether it is in conduit or not as well as fire and solvent risks.

I have 4 100 ah lifepo4 batteries and will use 4/0 cable through a 300 amp fuse on + and smart shut on -, to bus bars, to a 3000 watt inverter charger like this

 

[ampmike]

Do not put a washer between the terminals/lugs. They should be in direct contact of each other and they should be in direct contact with the battery terminal.

like so

 

[MisterSandals]

I have 4 100 ah lifepo4 batteries and will use 4/0 cable through a 300 amp fuse on + and smart shut on -, to bus bars, to a 3000 watt inverter charger like this

If it were me, i'd put the cutoff switch between 300A fuse and the bus bar to cut off battery from everything.
And, i'd put a fuse sized for your inverter and wiring between bus bar and inverter. Fuse between bus bar and 12V CPAP receptacle.