Change to LifePO4 - Emergency engine starting ?

[geem]

I'm not sure it is incorrect.

Heres a datasheet for a random EVE LF280K cell https://www.battery-germany.de/wp-c...80K-280Ah-Product-Specification-Version-B.pdf

It pretty clearly shows the maximum allowable discharge rate is 1C, or up to 2C for a maximum of 30 seconds:
View attachment 201478

Where are you seeing that it is capable of safely discharging at more than 2C?

It seems that the Internet is full of conflicting info. I have seen 3C for 30s and up to 10C for cooled cells for less than 1C. The issue is that you don't want to create a high temperature gradient across the individual cell. The +ve terminal get supper hot very quickly at high current. Short bursts are OK but sustained high C rates create a huge temperature gradient. This destroys the cell

 

[jakew009]

I didn't say that.

You said "So a 3584Wh 12.8V battery with a 1C rating can discharge at 280A."

I said "The battery is capable of discharging many more amps than that if it's not protected by a BMS."

The charge above proves that, as it can safely be discharged at 2C for very short periods. But, it can discharge at even higher currents, although that could/will damage the cells or shorten their like.

My point was, 1C isn't something that the cells cannot physically exceed. Some LFP cells even have higher C rating than 1C for continuous use.

I think we are agreeing then. My point was that in general terms, the bigger the battery bank, the bigger the instantaneous maximum current that it can deliver.

10C rating on a 100Ah battery = 1000A
10C rating on a 700Ah battery = 7kA

 

[jakew009]

It seems that the Internet is full of conflicting info. I have seen 3C for 30s and up to 10C for cooled cells for less than 1C. The issue is that you don't want to create a high temperature gradient across the individual cell. The +ve terminal get supper hot very quickly at high current. Short bursts are OK but sustained high C rates create a huge temperature gradient. This destroys the cell

The internet is also full of rubbish I don't think you will find any manufacturer datasheet for a large prismatic cell (>100Ah) intended for energy storage like we use in boats that allows much more than 2C.

 

[AntarcticPilot]

The internet is also full of rubbish I don't think you will find any manufacturer datasheet for a large prismatic cell (>100Ah) intended for energy storage like we use in boats that allows much more than 2C.

I note that the higher discharge rates are for COOLED cells. This is a non-trivial distinction; the cells in my EV have an elaborate water-cooling system to allow high discharge rates. Such a cooling system is unlikely on an after-market installation in a boat.

Without such a cooling system, damage to the cells will result from high discharge rates.

 

[geem]

I note that the higher discharge rates are for COOLED cells. This is a non-trivial distinction; the cells in my EV have an elaborate water-cooling system to allow high discharge rates. Such a cooling system is unlikely on an after-market installation in a boat.

Without such a cooling system, damage to the cells will result from high discharge rates.

In an emergency, high C rate to start an engine once, is unlikely to damage the cells. Repeatedly starting it, especially if the cells are hot would cause a problem. Most starter motors are unlikely to need 10C to start

 

[Sea Change]

I had an engine battery failure on my last boat, although it had always been on charge. The emergency parallel switch made it a no event.

That's a lot of faff if you're in the shit, drifting towards some rocks, a lee shore etc, etc and there is no wind. The time for emergency solutions is well in advance of needing them.

In my case, being a motor boat, not having a way to start the engines would be a serious issue, i can hardly sail home, i'm going to need a tow.

True, but we all have a different view of risk management. I don't consider the sudden failure of the engine battery to be a very high risk, so I chose not to build an emergency parallel system.
I'd be more inclined to put my time and effort in to an emergency fuel filter changeover system, because I've experienced clogged filters several times over the years. It's not an either/or and ideally we'd have all of these safety systems in place. But I don't consider it reckless not to fit every last precautionary measure available.

 

[geem]

True, but we all have a different view of risk management. I don't consider the sudden failure of the engine battery to be a very high risk, so I chose not to build an emergency parallel system.
I'd be more inclined to put my time and effort in to an emergency fuel filter changeover system, because I've experienced clogged filters several times over the years. It's not an either/or and ideally we'd have all of these safety systems in place. But I don't consider it reckless not to fit every last precautionary measure available.

I am used to building mechanical and electrical systems for data centres. The standard was N+1 for everything on the IT side and cooling.
I am not advocating n+1 for boats. It would be way over the top, but i do like to minimise risk and have alternatives. For my fuel tank, I cleaned it out a few years ago by climbing inside, and now I vacuum the bottom of the tank every 6 months. With the Donaldson 1/4 turn commercial filter system on my boat, I can change filters in about 30 seconds, so no need for a secondary filter system.
If you were building a second filter system, it should have an independent pick up pipe from the tank other wise the single point of failure is the pick up pipe itself. They can get blocked if you get a bad dose of diesel bug. The question is where do you stop. We could go on for ever with back up systems

 

[PaulRainbow]

True, but we all have a different view of risk management. I don't consider the sudden failure of the engine battery to be a very high risk, so I chose not to build an emergency parallel system.
I'd be more inclined to put my time and effort in to an emergency fuel filter changeover system, because I've experienced clogged filters several times over the years. It's not an either/or and ideally we'd have all of these safety systems in place. But I don't consider it reckless not to fit every last precautionary measure available.

I obviously come into contact with more electrical failures than most, because of my job, so where you say "i've never had a flat engine battery" i've seen many flat/dead/exploded engine batteries or domestic batteries.

"I chose not to build an emergency parallel system." makes it sound like a big, complicated affair. For a boat/setup like yours it's as simple as connecting an isolator switch between the load terminals of the engine battery and domestic bank.

1) Flat engine battery, turn the switch on, start the engine, turn it off.
2) Engine battery exploded or internally shorted, isolate it and turn the emergency switch on. (careful how the alternator charges)
3) LFP or BMS failure, isolate them, turn the switch on, run everything from the engine battery to get you "home".

 

[Refueler]

Jumper cables ?

 

[PaulRainbow]

Well, it’s been an interesting thread, thanks to everyone that contributed. I’d be looking for a permanent solution that’s as simple to implement in case of emergency. I also want a solution that works if the engine battery has shorted internally or even exploded and yes, those two things happen more often than many of you might expect.

Also, there have been some posts where people claim that it’s impossible to start an engine from an LFP battery, which causes some confusion and concern for some of those considering making the change, hopefully this thread clears that up.

So with respect to those who suggested jump leads, emergency start packs etc, whilst they may well do the job of starting the engine, in a pinch and for a simple flat/partly discharged battery, they’re don’t fulfil the criteria. There are scenarios when they wouldn’t work or could be downright dangerous. Fir instance, if the battery has shorted internally it will be circa 10V, connected to 14V or more, so will be getting pretty hot and gassing, going near that with jump leads or a jump pack will put you at a serious risk of being showered with bits of jagged battery case and hot acid, it’s also unlikely you’d start the engine anyway. If the battery has actually exploded you obviously can’t connect jump leads to it, you need to disconnect it first, not a nice job, especially in any sort of sea.

With lead acid engine and domestic systems the answer is simple, fit an isolator switch between the load terminals of the domestic and engine battery isolators. You can then close the emergency switch to jump start the engine and turn it back off again to maintain isolation of the two systems. In the event of one system suffering a catastrophic failure that system can be isolated by it’s own isolator switch, then turn the emergency switch on to run everything from the good system.

Fitting LFP domestic systems can pose some additional challenges/considerations, particularly for boats with bigger engines, such as my own. For smaller engine boats, like many/most here own the above still works fine, as long as the starter current doesn’t exceed the max current rating of the BMS, as several people have confirmed and I’ve found to be the case with installations on customers boats, where I always fit the emergency switch.

In my case it’s not possible to start the engines by using the emergency parallel switch. The 280Ah 24V battery has more than enough power to start the 2 x 7 litre diesels, but the BMS has a max current rating of 200A, the second the starter button is pressed the BMS shuts the load off. The starter draw 70A/80A while it’s cranking the engine, but the initial inrush will be several times that, although it only last milliseconds, it’s enough to “trip” the BMS, but within the burst rating of the batteries. A temporary battery cable between the LFP negative terminal and the negative busbar confirmed that, with the BMS bypass the LFP battery easily start the engine, with no noticeable voltage drop that you get with lead acid batteries.

So, I fitted a pair of heavy duty solenoids, one to replace the manual emergency parallel switch and one to bypass the BMS. These are energised by a pair of key switches in the main electrical panel, which is situated next to the lower helm. I used key switches so the keys can be removed to prevent crew accidentally turning them on. The main battery isolators are also key operated and in the same panel. So, open the panel door, turn the engine battery isolator off, parallel switch and BMS bypass switches on and press the starter buttons, job done.

 

[jakew009]

Well, it’s been an interesting thread, thanks to everyone that contributed. I’d be looking for a permanent solution that’s as simple to implement in case of emergency. I also want a solution that works if the engine battery has shorted internally or even exploded and yes, those two things happen more often than many of you might expect.

Also, there have been some posts where people claim that it’s impossible to start an engine from an LFP battery, which causes some confusion and concern for some of those considering making the change, hopefully this thread clears that up.

So with respect to those who suggested jump leads, emergency start packs etc, whilst they may well do the job of starting the engine, in a pinch and for a simple flat/partly discharged battery, they’re don’t fulfil the criteria. There are scenarios when they wouldn’t work or could be downright dangerous. Fir instance, if the battery has shorted internally it will be circa 10V, connected to 14V or more, so will be getting pretty hot and gassing, going near that with jump leads or a jump pack will put you at a serious risk of being showered with bits of jagged battery case and hot acid, it’s also unlikely you’d start the engine anyway. If the battery has actually exploded you obviously can’t connect jump leads to it, you need to disconnect it first, not a nice job, especially in any sort of sea.

With lead acid engine and domestic systems the answer is simple, fit an isolator switch between the load terminals of the domestic and engine battery isolators. You can then close the emergency switch to jump start the engine and turn it back off again to maintain isolation of the two systems. In the event of one system suffering a catastrophic failure that system can be isolated by it’s own isolator switch, then turn the emergency switch on to run everything from the good system.

Fitting LFP domestic systems can pose some additional challenges/considerations, particularly for boats with bigger engines, such as my own. For smaller engine boats, like many/most here own the above still works fine, as long as the starter current doesn’t exceed the max current rating of the BMS, as several people have confirmed and I’ve found to be the case with installations on customers boats, where I always fit the emergency switch.

In my case it’s not possible to start the engines by using the emergency parallel switch. The 280Ah 24V battery has more than enough power to start the 2 x 7 litre diesels, but the BMS has a max current rating of 200A, the second the starter button is pressed the BMS shuts the load off. The starter draw 70A/80A while it’s cranking the engine, but the initial inrush will be several times that, although it only last milliseconds, it’s enough to “trip” the BMS, but within the burst rating of the batteries. A temporary battery cable between the LFP negative terminal and the negative busbar confirmed that, with the BMS bypass the LFP battery easily start the engine, with no noticeable voltage drop that you get with lead acid batteries.

So, I fitted a pair of heavy duty solenoids, one to replace the manual emergency parallel switch and one to bypass the BMS. These are energised by a pair of key switches in the main electrical panel, which is situated next to the lower helm. I used key switches so the keys can be removed to prevent crew accidentally turning them on. The main battery isolators are also key operated and in the same panel. So, open the panel door, turn the engine battery isolator off, parallel switch and BMS bypass switches on and press the starter buttons, job done.

I guess the only potential downside is that if a future owner doesn’t fully understand the risk of bypassing the BMS, they could be forgiven for leaving that key switched on.

Perhaps it would be better with a push button that you have to hold down for the bypass solenoid?

 

[PaulRainbow]

I guess the only potential downside is that if a future owner doesn’t fully understand the risk of bypassing the BMS, they could be forgiven for leaving that key switched on.

Perhaps it would be better with a push button that you have to hold down for the bypass solenoid?

The keys should not be in the switches other than in an emergency and the key switch does cover you in the event of a BMS failure.

But i take your point, a button could be an alternative.

 

[tomaz_slo]

Well, it’s been an interesting thread, thanks to everyone that contributed. I’d be looking for a permanent solution that’s as simple to implement in case of emergency. I also want a solution that works if the engine battery has shorted internally or even exploded and yes, those two things happen more often than many of you might expect.

Also, there have been some posts where people claim that it’s impossible to start an engine from an LFP battery, which causes some confusion and concern for some of those considering making the change, hopefully this thread clears that up.

So with respect to those who suggested jump leads, emergency start packs etc, whilst they may well do the job of starting the engine, in a pinch and for a simple flat/partly discharged battery, they’re don’t fulfil the criteria. There are scenarios when they wouldn’t work or could be downright dangerous. Fir instance, if the battery has shorted internally it will be circa 10V, connected to 14V or more, so will be getting pretty hot and gassing, going near that with jump leads or a jump pack will put you at a serious risk of being showered with bits of jagged battery case and hot acid, it’s also unlikely you’d start the engine anyway. If the battery has actually exploded you obviously can’t connect jump leads to it, you need to disconnect it first, not a nice job, especially in any sort of sea.

With lead acid engine and domestic systems the answer is simple, fit an isolator switch between the load terminals of the domestic and engine battery isolators. You can then close the emergency switch to jump start the engine and turn it back off again to maintain isolation of the two systems. In the event of one system suffering a catastrophic failure that system can be isolated by it’s own isolator switch, then turn the emergency switch on to run everything from the good system.

Fitting LFP domestic systems can pose some additional challenges/considerations, particularly for boats with bigger engines, such as my own. For smaller engine boats, like many/most here own the above still works fine, as long as the starter current doesn’t exceed the max current rating of the BMS, as several people have confirmed and I’ve found to be the case with installations on customers boats, where I always fit the emergency switch.

In my case it’s not possible to start the engines by using the emergency parallel switch. The 280Ah 24V battery has more than enough power to start the 2 x 7 litre diesels, but the BMS has a max current rating of 200A, the second the starter button is pressed the BMS shuts the load off. The starter draw 70A/80A while it’s cranking the engine, but the initial inrush will be several times that, although it only last milliseconds, it’s enough to “trip” the BMS, but within the burst rating of the batteries. A temporary battery cable between the LFP negative terminal and the negative busbar confirmed that, with the BMS bypass the LFP battery easily start the engine, with no noticeable voltage drop that you get with lead acid batteries.

So, I fitted a pair of heavy duty solenoids, one to replace the manual emergency parallel switch and one to bypass the BMS. These are energised by a pair of key switches in the main electrical panel, which is situated next to the lower helm. I used key switches so the keys can be removed to prevent crew accidentally turning them on. The main battery isolators are also key operated and in the same panel. So, open the panel door, turn the engine battery isolator off, parallel switch and BMS bypass switches on and press the starter buttons, job done.

Paul, did you build your LFP by your self? I assume you have the cells and BMS dislocated. Must the cells be in some closed case and how all wires are then connected to BMS?

 

[PaulRainbow]

Paul, did you build your LFP by your self? I assume you have the cells and BMS dislocated. Must the cells be in some closed case and how all wires are then connected to BMS?

Yes, i build my own batteries, it's part of my business. The BMS can be a short distance from the cell pack. You can see my setup in post #39, the BMS is just to the left of the grey LA starter batteries, the battery box has a GRP lid. All batteries are secured from moving with wooden battens screwed to the bottom of the battery box. Other installation i have done vary, sometimes the batteries are in a location where they just need securing with battens and strapping down, other times they need some ply to protect them. The tops of the batteries are always covered. I use EVE cells and JK BMS, the BMS cell monitoring wires are bolted to the top of each cell with crimped on ring terminals and the main cables with lugs, hydraulically crimped on, as you can see from the pic in post #39.

 

[Zing]

My system works very well, see post #39.

Fuses are indeed long established methods for circuit protection, but they are not suitable for over current limit protection for LFP batteries. My BMS will protect by batteries from loads exceeding the C rating of the cells, yours will not, unless the highest rated fuse you have is less than the C rating of the cells.

How big is your battery bank ? Which bow thruster do you have ?

Not relevant. Also wrong. You size fuses to protect cables. You surely know that? A 25 Hp motor needs a big fuse and yes, you can protect the circuit with fuses.

 

[PaulRainbow]

Not relevant. Also wrong. You size fuses to protect cables. You surely know that? A 25 Hp motor needs a big fuse and yes, you can protect the circuit with fuses.

You clearly have no understanding of how a LFP system supposed to be installed and the purpose of fuses.

Of course you can protect a circuit with a fuse, it's generally their primary purpose, but they are not for protecting an LFP battery from exceeding the cells C rating, that's the job of the BMS. What happens with your system if something does exceed the C rating ?

You still did not answer "How big is your battery bank ? Which bow thruster do you have ?"

 

[shanemax]

I think we are agreeing then. My point was that in general terms, the bigger the battery bank, the bigger the instantaneous maximum current that it can deliver.

10C rating on a 100Ah battery = 1000A
10C rating on a 700Ah batte

 

[shanemax]

Subject to the gauge and loading ability of the wire.

 

[shanemax]

You clearly have no understanding of how a LFP system supposed to be installed and the purpose of fuses.

Of course you can protect a circuit with a fuse, it's generally their primary purpose, but they are not for protecting an LFP battery from exceeding the cells C rating, that's the job of the BMS. What happens with your system if something does exceed the C rating ?

You still did not answer "How big is your battery bank ? Which bow thruster do you have ?"

I understand what he is saying. if the battery is good for discharging (hypotheticly) 100amps but will get damaged if you try to pull 200amps out of it then a 110 amp fuse would blow saving the battery.

 

[PaulRainbow]

I understand what he is saying. if the battery is good for discharging (hypotheticly) 100amps but will get damaged if you try to pull 200amps out of it then a 110 amp fuse would blow saving the battery.

I understood what he's saying, but it's not they way to do it. Systems should be designed so that in normal use they will not exceed the batteries C rating, with the BMS protecting them from a accidental or unexpected over current event.