Battery wiring

[Ewan123]

Evening all. Am I right in thinking that this cable arrangement wouldn't be good on my new hybrid bank (the second LiFePo4 will come later, these are the two at the bottom), i.e. one cable longer than the others (about 60cm vs. 20cm)? They would be 50mm² cables, so resistances are 0.000234 Ω and 0.000077 Ω at a nominal 12v. Is that 0.000156 Ω difference significant in this context?

 

Because of the space available, this is the only layout of the batteries available to me, so would I be better off making all the cables match the longest length?

 

[Tony Brooks]

If I understand you, the grey battery is the LA start battery and the one being charged. so my guess is that the long red cable will be far longer to limit the current the alternator supplies so it does not burn out. The equal length thing relates to the cables paralleling the domestic (Li) bank and they look they will be more or less equal.

 

Don't take this as gospel so wait for input from those who have done this.

[nicknorman]

11 hours ago, Ewan123 said:

Evening all. Am I right in thinking that this cable arrangement wouldn't be good on my new hybrid bank (the second LiFePo4 will come later, these are the two at the bottom), i.e. one cable longer than the others (about 60cm vs. 20cm)? They would be 50mm² cables, so resistances are 0.000234 Ω and 0.000077 Ω at a nominal 12v. Is that 0.000156 Ω difference significant in this context?

 

Because of the space available, this is the only layout of the batteries available to me, so would I be better off making all the cables match the longest length?

 

 

 

No it doesn’t matter. What you have drawn is fine. As Tony says you may need to consider a means of limiting the max current from the alternator to avoid it having a short life.

[Tony Brooks]

Just now, nicknorman said:

 

No it doesn’t matter. What you have drawn is fine. As Tony says you may need to consider a means of limiting the max current from the alternator to avoid it having a short life.

 

Thanks, Nick, good to know I at least partially understand the issues with Li systems.

[Ewan123]

Thanks both, I wasn't entirely clear which didn't help. These will just be the 'leisure' bank, with alternator feed being an extra cable (from one alternator to starter, not pictured, and the other to this bank). Yes, I'll be monitoring the setup closely for a while to be sure there's no overheating etc. and changing cables etc. as needed. I'll have my old lead acid bank ready to swap back in, in case I need to spend time making changes.

 

My main concern was the different resistances between the batteries, but hoped it would be negligible over that short distance. I'd just rather not have big loops of cable between the short distances if I can avoid it (maybe that's just me wanting it to look tidy??).

 

Thanks for your responses.

[Tony Brooks]

16 minutes ago, Ewan123 said:

Thanks both, I wasn't entirely clear which didn't help. These will just be the 'leisure' bank, with alternator feed being an extra cable (from one alternator to starter, not pictured, and the other to this bank). Yes, I'll be monitoring the setup closely for a while to be sure there's no overheating etc. and changing cables etc. as needed. I'll have my old lead acid bank ready to swap back in, in case I need to spend time making changes.

 

My main concern was the different resistances between the batteries, but hoped it would be negligible over that short distance. I'd just rather not have big loops of cable between the short distances if I can avoid it (maybe that's just me wanting it to look tidy??).

 

Thanks for your responses.

 

If the grey battery is a Li one then that setup is not ideal, but may well not produce any discernable ill effects. As the link cable between the LA batter will be limiting the current to perhaps about half the alternator's maximum output then it will also limit the volt drop across the cables.

 

I don't think Li batteries sulphate so as long as each battery's BMS is keeping that batteries cells balanced having a single battery more or less well charged than the rest probably won't present any ill effects.

Edited November 30, 2023 by Tony Brooks

[Ewan123]

9 minutes ago, Tony Brooks said:

 

If the grey battery is a Li one then that setup is not ideal, but may well not produce any discernable ill effects. As the link cable between the LA batter will be limiting the current to perhaps about half the alternator's maximum output then it will also limit the volt drop across the cables.

 

I don't think Li batteries sulphate so as long as each battery's BMS is keeping that batteries cells balanced having a single battery more or less well charged than the rest probably won't present any ill effects.

One day I'll learn to get all of the detail in the first post 🤦‍♂️

In the image, the grey battery is LA and the two long black ones are LiFePo4. So the LA to LiFePo4 connection is the one with differing cable lengths.

 

[nicknorman]

7 minutes ago, Tony Brooks said:

 

If the grey battery is a Li one then that setup is not ideal, but may well not produce any discernable ill effects. As the link cable between the LA batter will be limiting the current to perhaps about half the alternator's maximum output then it will also limit the volt drop across the cables.

 

I don't think Li batteries sulphate so as long as each battery's BMS is keeping that batteries cells balanced having a single battery more or less well charged than the rest probably won't present any ill effects.

Li batteries definitely don't sulphate! They like to be at a mid SoC. Because the voltage vs SoC profile is so flat in the mid range, with differing cable lengths in a bank there might be some slight SoC difference mid-charge but it is of absolutely zero consequence. As full charge is approached and the voltage vs SoC profile becomes very steep, it will all even out.

 

In my experience once a battery's cells are balanced, they don't go out of balance unless something is wrong. Which perhaps a bit surprising because in terms of % of capacity, in balance vs out of balance is small fractions of a %. I did have a connection issue on one cell a couple of years ago which caused incorrect balancing (actually putting the cells out of balance), but since that was remedied and the cells brought back into proper balance, the balancing function has never needed to kick in for at least the past year.

 

On your point about the link cable carrying half the alternator current, if I understand your logic (half is going into the LA) this is mostly not correct because the voltage of a Li battery remains above 13v until it is nearly flat, say below 10% SoC, and so the LA battery does not discharge significantly unless the Li is taken down to nearly flat or beyond. Only then does current flow out of the LA and thus only then will significant current flow back into the LA during recharge. Most people try to avoid this deep discharge state where the LA starts to discharge.

[Tony Brooks]

11 minutes ago, nicknorman said:

On your point about the link cable carrying half the alternator current, if I understand your logic (half is going into the LA) this is mostly not correct because the voltage of a Li battery remains above 13v until it is nearly flat, say below 10% SoC, and so the LA battery does not discharge significantly unless the Li is taken down to nearly flat or beyond. Only then does current flow out of the LA and thus only then will significant current flow back into the LA during recharge. Most people try to avoid this deep discharge state where the LA starts to discharge.

 

My thinking is that it is acknowledged that the maximum alternator output is best limited to stop it burning out before long and it is the LA to Li link cable that does that then the current in the link cable must also be limited. The actual alternator output would be the sum of what the LA battery wants plus what the Ls & link cable wants. From the date I have seen people seem to be limiting the alternator output to between half and 2/3 of the alternator's rated output, be that by a long LA to Li link cable, B to B, or alternator controller/regulator. Within a short time from start up the LA battery will be starting to limit the current, but until that time the alternator will probably be at maximum output, but that is within the design parameters of the alternator. It is the current demand from the Li that needs limiting.

[nicknorman]

14 minutes ago, Tony Brooks said:

 

My thinking is that it is acknowledged that the maximum alternator output is best limited to stop it burning out before long and it is the LA to Li link cable that does that then the current in the link cable must also be limited. The actual alternator output would be the sum of what the LA battery wants plus what the Ls & link cable wants. From the date I have seen people seem to be limiting the alternator output to between half and 2/3 of the alternator's rated output, be that by a long LA to Li link cable, B to B, or alternator controller/regulator. Within a short time from start up the LA battery will be starting to limit the current, but until that time the alternator will probably be at maximum output, but that is within the design parameters of the alternator. It is the current demand from the Li that needs limiting.

 

Ah yes I see. But if I were doing it I would keep the LA and Li close coupled and have the "long wire" between the alternator and both batteries. In part simply because it will be easier space-wise. Since the LA is rarely or never going to be discharged significantly its not going to affect LA charging and even if the LA is discharged, it will just slow its charging down a bit.

With the resistance between LA and Li, if the LA is well discharged, the Li is nearly flat and the alternator will be subjected to heavy load from the LA connected directly, plus from the Li connected via resistive path. So for a good while until the LA gets reasonably well charged, the total load on the alternator is likely to be max.

[Tony Brooks]

7 minutes ago, nicknorman said:

 

Ah yes I see. But if I were doing it I would keep the LA and Li close coupled and have the "long wire" between the alternator and both batteries. In part simply because it will be easier space-wise. Since the LA is rarely or never going to be discharged significantly its not going to affect LA charging and even if the LA is discharged, it will just slow its charging down a bit.

With the resistance between LA and Li, if the LA is well discharged, the Li is nearly flat and the alternator will be subjected to heavy load from the LA connected directly, plus from the Li connected via resistive path. So for a good while until the LA gets reasonably well charged, the total load on the alternator is likely to be max.

 

That makes even more sense, and it means @Ewan123's long red wire is redundant.

[nicknorman]

16 minutes ago, Tony Brooks said:

 

That makes even more sense, and it means @Ewan123's long red wire is redundant.

Well I think the length of that wire in 50mm^2 is not going to have any significant effect on current reduction. It would have to be a lot longer and a bit thinner.

[Sailbadthesinner]

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

[IanD]

8 minutes ago, Sailbadthesinner said:

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

Agreed -- but these cost a lot more and involve more design and fitting effort than long cables. Cheap is always best for some, others would say this is a false economy... 😉

Edited November 30, 2023 by IanD

[nicknorman]

7 minutes ago, Sailbadthesinner said:

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

It’s not a tactic I use, but I don’t think you can reasonably say it’s “absolutely crazy” without providing some justification. It does seem to be a popular tactic. In an ideal world you are right but we don’t live in an ideal world especially when it comes to narrowboat electrics!

[Tony Brooks]

4 minutes ago, Sailbadthesinner said:

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

 

The trouble with that is that on a single alternator boat the alternator settings to suit Li s will be far from suitable for LA's and it takes no account of what happens to the alternator with the Li BMS disconnects the alternator. A suitable B to B would solve the issues as long as it can be set for Li charging. A long piece of wire is simple, cheaper, and I suspect easier to fault find than the alternatives. I am not defending the practice, but just trying to  explain why it is done and maybe not such a horrible idea.

[ditchcrawler]

10 minutes ago, Sailbadthesinner said:

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

But why is it craze, up to about 15 years ago it was the norm and why people were fitting things like Ardvercs to overcome that limitation.

[Eeyore]

2 hours ago, Sailbadthesinner said:

The idea of using long cables to limit alternator current is absolutely crazy.

All battery cables should be as short as possible. If the alternator can't charge Lithium without overheating then you need a better alternator or an external controllable alternator regulator.

 

Looking for some technical info here.

Are the volt drop and resistance values used for the "long wire" method significantly different to the values found in a typical shunt installation?

 

[nicknorman]

2 minutes ago, Eeyore said:

Looking for some technical info here.

Are the volt drop and resistance values used for the "long wire" method significantly different to the values found in a typical shunt installation?

 

Yes much greater. A shunt only drops 50mV or 75mV at its maximum current (maybe 200A) whereas the long wire needs to drop around 0.75v at the proposed (reduced) charge current.

Edited November 30, 2023 by nicknorman

[Eeyore]

5 minutes ago, nicknorman said:

Yes much greater.

Thank you. So using correctly rated minimum length cables with a suitably installed resistor would be almost as cheap and a lot safer?

[nicknorman]

3 minutes ago, Eeyore said:

Thank you. So using correctly rated minimum length cables with a suitably installed resistor would be almost as cheap and a lot safer?

In theory, perhaps. But consider the power dissipation of the resistor (V x I) so say 70A x 0.75 = 53 watts. A very (physically) bit resistor that will get very hot. Hence it is better to distribute the power dissipation over a long cable that will only get warm. If the long cable is fused at the battery end, I don’t see any significant safety issue.

[IanD]

8 minutes ago, Eeyore said:

Thank you. So using correctly rated minimum length cables with a suitably installed resistor would be almost as cheap and a lot safer?

Yes, but be careful about resistor power dissipation and cooling -- if it drops 0.75V at 75A then you need a 10mohm resistor rated at >50W -- probably 100W since they get *very* hot at rated power. Like this...

 

https://www.ebay.co.uk/itm/264829994145

 

Note that these need mounting on a heatsink to keep them cool, you can't just dangle them in free air. Bolting them to the hull works very well, but make sure you use some heatsink compound to get good thermal contact. And don't drill through the hull... 😉

 

All of which is why people use long cables... 🙂

Edited November 30, 2023 by IanD

[ditchcrawler]

23 minutes ago, Eeyore said:

Thank you. So using correctly rated minimum length cables with a suitably installed resistor would be almost as cheap and a lot safer?

what Nick said. In the past lots of boats were wired with cable from the alternator that severally limited the charging of the old lead acid batteries. Its only in recent years that boat electricians have got away from the old autoelectical way of thinking and only using cable that is thick enough to carry the current with little thought to volt drop

[Ewan123]

4 hours ago, Tony Brooks said:

 

That makes even more sense, and it means @Ewan123's long red wire is redundant.

My long red wire is really only that long to physically reach between the battery terminals because of the space where I can fit the batteries (the LiFePo4 is significantly longer, so two of those lengthwise fills the space that my current 3x LA fill crosswise). I might have inadvertently made it all seem more interesting with the hybrid mention!

Edited November 30, 2023 by Ewan123

[Tony Brooks]

The Lucas LX Supreme is a sealed wet lead acid battery and you say it is a hybrid system. That looks conformed by there being one, later two, Li batters and one LA battery. You show no charging cables, so we are left to guess, but as you show the long red cable linking LA and Li positives it is reasonable to conclude the alternator is connected to the LA and then the Li takes what charge it wants via the long red positive cable.

 

What concerns me is that you seem to think that there may be a volt drop problem down the red cable because it is longer than the rest, when the real problem is that the Li will draw the maximum current the alternator can produce for many hours. That causes typical alternators to burn out through overheating.

 

So in your diagram the red cable would be made longer and thinner so it deliberately caused enough votlt drop to limit the current the LI would try to take. Nick suggests a better way is to do away with your red wire and twin the alternator connections, one to the LA and the other one (the longer, thinner one) to the Li.

 

It seems to me that there is a chance that you don't fully understand what you are trying to achieve, and ensuing equal lengths of interconnects as per LA practice is not, in this case, one of them