Drop in Lithium Batteries - the basics?

[kwb78]

Not in any way my area of expertise, but wouldn't it be better to fit a regulator to protect the alternator, rather than a DC-DC charger? My little 18A Orion gets incredibly hot, surely that means a loss of be efficiency?

There are pros and cons of both methods. You will get faster charging of the lithium battery by having it charged directly by the alternator, however the alternator needs to be properly regulated both to ensure it does not overheat with the sustained current a LFP battery can pull, and also to protect against the possibility of a sudden BMS disconnection which can cause the alternator voltage to spike which kills the rectifier.

A DC-DC charger has the advantage that it does not require any modification of the alternator or starting battery. The alternator charges the start battery and feeds the charger while the engine is running, and selecting an appropriate sized charger also limits the current taken from the alternator. It's the simplest way to do it, but not necessarily the most efficient. For example, a 30A output charger will probably be pulling something like 40A from the alternator.

It probably depends on your use case as to which method is better. If you are replacing a leisure battery used for things like instruments, lighting, freshwater pumps etc, then a moderate sized DC-DC charger is probably adequate. If you want to make more extensive use of the power LFP batteries can provide with inverters and things like electric cooking or other more demanding applications then the faster charging would be beneficial and probably worth it. An external alternator regulator is likely to be quite a bit more expensive than a DC-DC charger, and may require changes to the alternator and its drive as well as the electronics.

 

[noelex]

_

A lot of confusion comes from 'internal resistance' often being quoted as dV/dI not V/I.
The slope of the volts/amps graph at the point of interest.
The internal resistance is valid for small changes of volts and amps.
It's wrong to extrapolate it outside of the working range.
For some devices, dV/dI can be negative. But only over a certain range.

The cold-cranking tests are not short circuit, they are the current to bring the volts down to something like 1.2V per cell. If you actually short circuit a battery fully, you may have bits of it in the ceiling before 30 seconds is up. Luckily it's pretty difficult to make a ten thousand amp short circuit.

I was looking at the graph below. The short circuit current of the 160Ahr lithium cell was limited to 1050A in testing. This is lower than I would have predicted.



Interestingly, the table in the same paper suggests discharge rates of 30C should be expected from LiFePO4. This would be a much higher 4800A for a 160Ahr cell.

Part of this discrepancy relates to the form factor of the cells. Prismatic cells will have a lower current delivery, but Winston themselves specify an "impulse current" of 10C or 1600A for prismatic cells. In a short circuit situation, the cells should be delivering much more than 1050A. I suspect the measurements in this paper are wrong or at least misleading.

 

[Kelpie]

There are pros and cons of both methods. You will get faster charging of the lithium battery by having it charged directly by the alternator, however the alternator needs to be properly regulated both to ensure it does not overheat with the sustained current a LFP battery can pull, and also to protect against the possibility of a sudden BMS disconnection which can cause the alternator voltage to spike which kills the rectifier.

A DC-DC charger has the advantage that it does not require any modification of the alternator or starting battery. The alternator charges the start battery and feeds the charger while the engine is running, and selecting an appropriate sized charger also limits the current taken from the alternator. It's the simplest way to do it, but not necessarily the most efficient. For example, a 30A output charger will probably be pulling something like 40A from the alternator.

It probably depends on your use case as to which method is better. If you are replacing a leisure battery used for things like instruments, lighting, freshwater pumps etc, then a moderate sized DC-DC charger is probably adequate. If you want to make more extensive use of the power LFP batteries can provide with inverters and things like electric cooking or other more demanding applications then the faster charging would be beneficial and probably worth it. An external alternator regulator is likely to be quite a bit more expensive than a DC-DC charger, and may require changes to the alternator and its drive as well as the electronics.

Thanks for the explanation. I knew there was a reason why I just went with a DC-DC!
In my mind a 140A alternator like the OP's *is* a big alternator.
I'm aware that a comprehensive alternator upgrade can cost a £1k+, which buys an awfully lot of solar panels. Hence why I didn't go that route and relied on solar only for over two years. I recently fitted a little Orion too but I have used it only twice so far.

 

[Kelpie]

_

I was looking at the graph below. The short circuit current of the 160Ahr lithium cell was limited to 1050A in testing. This is lower than I would have predicted.

View attachment 166175

Interestingly, the table in the same paper suggests discharge rates of 30C should be expected from LiFePO4. This would be a much higher 4800A for a 160Ahr cell.

Part of this discrepancy relates to the form factor of the cells. Prismatic cells will have a lower current delivery, but Winston themselves specify an "impulse current" of 10C or 1600A for prismatic cells. In a short circuit situation, the cells should be delivering much more than 1050A. I suspect the measurements in this paper are wrong or at least misleading.

These numbers have a huge variation. Thanks for digging in to it.
My class T fuse upgrade cost over £100. The previously fitted ANL was a fifth of that, and replacement fuses are much more readily available.
I'm not going to endanger my boat for the sake of £80 but I do wonder if class T is a bit overkill in reality? I know the ABYC insist on it but they have no interest in promoting cost effective measures.

 

[B27]

... I suspect the measurements in this paper are wrong or at least misleading.

I think the paper comes from a bloke with a PhD from a proper university.
It's not just some chancer on youtube with a multimeter and a lot of opinions.
To be fair the object of that test seems to be shorting a battery over a time frame of 15 minutes, it is possible the initial upward slope goes to a much higher peak of very short duration, like milliseconds. But then a fuse won't blow in milliseconds.

It may be a moot point, because it's actually quite hard to short anything in such a way that you'll get thousands of amps out of a few volts. Even a very thick piece of wire making very good contact at both ends will have limits.

 

[B27]

Thanks for the explanation. I knew there was a reason why I just went with a DC-DC!
In my mind a 140A alternator like the OP's *is* a big alternator.
I'm aware that a comprehensive alternator upgrade can cost a £1k+, which buys an awfully lot of solar panels. Hence why I didn't go that route and relied on solar only for over two years. I recently fitted a little Orion too but I have used it only twice so far.

In the third decade of the 20th century, a 140A alternator is medium sized for a private car.
A modern camper van might well have a 'smart' alternator.
A lot of boat engines are really industrial plant from last century.

The components for a current regulated Lifepo4 compatible alternator are in a breaker's yard near you!

 

[Pete7]

My class T fuse upgrade cost over £100. The previously fitted ANL was a fifth of that, and replacement fuses are much more readily available.
I'm not going to endanger my boat for the sake of £80 but I do wonder if class T is a bit overkill in reality? I know the ABYC insist on it but they have no interest in promoting cost effective measures.

T Class fuses were in short supply in the UK following Covid. However, there is an alternative a heavy duty NH Fuse:

DC Fused Disconnect 1-pole NH00 or NH000 size, M8 fixings, 160A DC max

We opted for the 160A version which matches the Blue Seas circuit breaker and the likely maximum load on our 2kW inverter.

 

[B27]

For those of you who prefer youtube:
Blog - Tests and diagnosis | GWL Group

You might manage to shorten the short (IYSWIM) if you really tried.

 

[lustyd]

For those of you who prefer youtube:
Blog - Tests and diagnosis | GWL Group

You might manage to shorten the short (IYSWIM) if you really tried.

Blimey. So instead of a 200A fuse I could have fitted a big switch and had 13 minutes to switch it off in a short circuit Really puts into perspective though, 1000A would need some chunky cables so that T class fuse seems to be money well spent

 

[B27]

Blimey. So instead of a 200A fuse I could have fitted a big switch and had 13 minutes to switch it off in a short circuit Really puts into perspective though, 1000A would need some chunky cables so that T class fuse seems to be money well spent

Something like that.
But if you try and switch off that switch, it might well weld shut!
A megafuse with a 2000A breaking capacity might be less money better spent?

There really ought to be youtube video of someone 'proving' that a cheaper 200A fuse can't break the short circuit current of a 200Ah battery, but I can't find it.
I think what happens when you exceed the breaking capacity of a fuse is that an arc and molten metal keeps the current flowing. It's very hard to sustain an arc with volts as low as 12 or so. 24V systems would be much better at sustaining arcs.

 

[Kelpie]

Something like that.
But if you try and switch off that switch, it might well weld shut!
A megafuse with a 2000A breaking capacity might be less money better spent?

There really ought to be youtube video of someone 'proving' that a cheaper 200A fuse can't break the short circuit current of a 200Ah battery, but I can't find it.
I think what happens when you exceed the breaking capacity of a fuse is that an arc and molten metal keeps the current flowing. It's very hard to sustain an arc with volts as low as 12 or so. 24V systems would be much better at sustaining arcs.

I think it's ionised air creating a pathway tr electricity, rather than molten metal.

 

[lustyd]

There really ought to be youtube video of someone 'proving' that a cheaper 200A fuse can't break the short circuit current of a 200Ah battery, but I can't find it.

Will Prowse might have a go if you ask. There’s another chap that blows big fuses for fun, and I mean very very large fuses!

 

[B27]

I think it's ionised air creating a pathway tr electricity, rather than molten metal.

In my experience if something arcs convincingly, there's often metal flying about. A good bit of arcing will usually leave traces of metal on a circuit board or fuse holder or the innards of a switch so that insulation is compromised.
It's a kilowatt or so in a very small space, it will often get hot enough to not just melt metal, but vaporise it.

Ionising air takes a lot of volts. You can get that by breaking an inductive circuit, but only briefly.

 

[B27]

Will Prowse might have a go if you ask. There’s another chap that blows big fuses for fun, and I mean very very large fuses!

With those kinds of currents, you should bolt your experiment to something very heavy.
The magnetic effect of lots of amps around a big loop like that can be significant.
I've seen cables leap around and break things!
Also seen mobile phones refuse to work after being near very high power stuff.

 

[dgadee]

Sterling suggest I put a BB12120 battery to battery charger into my proposed system. What does the panel think?

 

[B27]

Sterling suggest I put a BB12120 battery to battery charger into my proposed system. What does the panel think?

That sounds 'reassuringly expensive'.

 

[PaulRainbow]

Sterling suggest I put a BB12120 battery to battery charger into my proposed system. What does the panel think?

I think I'd avoid Sterling

 

[shanemax]

Please Google Lithium Batteries and airlines. You may only carry anything with Lithium batteries as hand luggage, you are not allowed under any circumstances to place them in your suitcases. I wonder why??? I would not dream of having two fastened down 200 amp batteries on my boat. I have nothing on my boat with Lithium batteries that I could not throw over the side in an emergency. Until they sort out the safety problems with this type of battery they are not going on my boat, and while you are at it try Youtube Lithium batteries catching fire. If your batteries (like mine) are between the companionway and your bunk you don't stand a chance.

 

[Kelpie]

How much engine charging do you actually need?
The bigger the DC-DC charger, the bigger the wiring, the more heat to get rid of.

 

[Kelpie]

Please Google Lithium Batteries and airlines. You may only carry anything with Lithium batteries as hand luggage, you are not allowed under any circumstances to place them in your suitcases. I wonder why??? I would not dream of having two fastened down 200 amp batteries on my boat. I have nothing on my boat with Lithium batteries that I could not throw over the side in an emergency. Until they sort out the safety problems with this type of battery they are not going on my boat, and while you are at it try Youtube Lithium batteries catching fire. If your batteries (like mine) are between the companionway and your bunk you don't stand a chance.

I'm glad that it now takes until the third page of a thread for this kind of comment to turn up. People are becoming better educated.

In brief- not all lithium batteries are the same. Boats use LiFePO4 which is not known for thermal runaway, and other than the high currents involved is no more dangerous than lead acid. In a properly designed system with correct fuses etc it is simply not a risk.

Laptops, power tools, phones, e-bikes, scooters, use a different and more volatile battery chemistry, and cheap products often lack safety features.

If you think lithium batteries are a safety risk, you haven't done enough research to safely install them, so it's probably best you stay away from the technology.