Change to LifePO4 - Emergency engine starting ?

[AntarcticPilot]

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.

There's one factor not being addressed and that is that exceeding the battery's rating COULD result not merely in a dead battery but in a fire. Obviously, there's a large safety margin in manufacturers' figures, but the bottom line is that lithium batteries are prone to forming dendrites of lithium between the anode and cathode. That is the mechanism by which the capacity is reduced as the number of charge/discharge cycles increases; limiting dendrite formation is an active area of research in electrochemistry. In normal operation with a BMS and within the manufacturer's specification, that's taken into consideration and is not an issue. But if the discharge rate is greatly exceeded, the dendrites could short-circuit the cell, and a runaway reaction take place, with catastrophic results.

I'm not an expert in this, but I am aware of current research into this technology and because of that I would NOT operate lithium batteries outside the manufacturer's specifications.

 

[Refueler]

There's one factor not being addressed and that is that exceeding the battery's rating COULD result not merely in a dead battery but in a fire. Obviously, there's a large safety margin in manufacturers' figures, but the bottom line is that lithium batteries are prone to forming dendrites of lithium between the anode and cathode. That is the mechanism by which the capacity is reduced as the number of charge/discharge cycles increases; limiting dendrite formation is an active area of research in electrochemistry. In normal operation with a BMS and within the manufacturer's specification, that's taken into consideration and is not an issue. But if the discharge rate is greatly exceeded, the dendrites could short-circuit the cell, and a runaway reaction take place, with catastrophic results.

I'm not an expert in this, but I am aware of current research into this technology and because of that I would NOT operate lithium batteries outside the manufacturer's specifications.

While I agree with most you post .. the fact is though that the LiFePo4 cell is one of the safest Lithium formats. When designed for use such as here .. they usually have a BMS fitted - unlike near all the batterys I have for modelling / other use ... The BMS designed to cut off in extremes .. to safeguard battery .. to keep cells balanced.
Burst rates - can often be quoted by manufacturers from 2x rate upwards ... but as the figure increases - the time quoted reduces rapidly ... with 10 secs being a common figure for the not so extreme burst rates.
I have noticed that batterys such as we use here - often do not quote Burst rates on the labels and I think thats deliberate on the manufacturers part - to avoid stupid people thinking they can use that rate more widely. Funny though - that in Model world ... even with some of the 100C batts we use - manufacturers WILL state burst rates !! (Forget about what most people think abouy model batts ... we have models with large very powerful LiPo's that can start your car ...).

 

[AntarcticPilot]

While I agree with most you post .. the fact is though that the LiFePo4 cell is one of the safest Lithium formats. When designed for use such as here .. they usually have a BMS fitted - unlike near all the batterys I have for modelling / other use ... The BMS designed to cut off in extremes .. to safeguard battery .. to keep cells balanced.
Burst rates - can often be quoted by manufacturers from 2x rate upwards ... but as the figure increases - the time quoted reduces rapidly ... with 10 secs being a common figure for the not so extreme burst rates.
I have noticed that batterys such as we use here - often do not quote Burst rates on the labels and I think thats deliberate on the manufacturers part - to avoid stupid people thinking they can use that rate more widely. Funny though - that in Model world ... even with some of the 100C batts we use - manufacturers WILL state burst rates !! (Forget about what most people think abouy model batts ... we have models with large very powerful LiPo's that can start your car ...).

I don't usually disagree with you, and I'm not really disagreeing here. But the formation of dendrites is a factor in all lithium technologies. In operation as designed, LiFePO4 is very safe; we don't disagree on this. And, of course, BMS ensure that batteries are used within their design limits. But you, I am sure, use your batteries according to the manufacturers' guidance, and use batteries designed for the application. But here we are looking at batteries designed for relatively low "house" currents (tens of amps, not hundreds), and considering discharge rates substantially higher than the design limit. Also I did emphasize COULD! I highlighted a realistic risk, not something that would certainly happen. But I imagine that an insurance company would get very snotty about a fire resulting from discharging a battery (of any technology) at rates well beyond it's design limits.

 

[geem]

...

I don't usually disagree with you, and I'm not really disagreeing here. But the formation of dendrites is a factor in all lithium technologies. In operation as designed, LiFePO4 is very safe; we don't disagree on this. And, of course, BMS ensure that batteries are used within their design limits. But you, I am sure, use your batteries according to the manufacturers' guidance, and use batteries designed for the application. But here we are looking at batteries designed for relatively low "house" currents (tens of amps, not hundreds), and considering discharge rates substantially higher than the design limit. Also I did emphasize COULD! I highlighted a realistic risk, not something that would certainly happen. But I imagine that an insurance company would get very snotty about a fire resulting from discharging a battery (of any technology) at rates well beyond it's design limits.

It's the temperature gradient across individual cells that is the main problem. A short burst of 3 or 4C won't cause a problem. If you kept repeating the short burst. You start to create heat. You would have to do long bursts on the starter to raise the heat sufficiently to trip the high temperature protection. Mine is set at 70degC. Thermal runaway can't happen until in excess of 300degC.
Battery high temperature sensors should be connected to the +ve terminal of a cell. This is the hottest part of any cell in a high C situation

 

[Refueler]

I don't usually disagree with you, and I'm not really disagreeing here. But the formation of dendrites is a factor in all lithium technologies. In operation as designed, LiFePO4 is very safe; we don't disagree on this. And, of course, BMS ensure that batteries are used within their design limits. But you, I am sure, use your batteries according to the manufacturers' guidance, and use batteries designed for the application. But here we are looking at batteries designed for relatively low "house" currents (tens of amps, not hundreds), and considering discharge rates substantially higher than the design limit. Also I did emphasize COULD! I highlighted a realistic risk, not something that would certainly happen. But I imagine that an insurance company would get very snotty about a fire resulting from discharging a battery (of any technology) at rates well beyond it's design limits.

We are on same page ... and yes I not only value my extensive expensive model collection etc - but do not care to buy Li batterys too often !

I would be extremely wary and literally a no other course of action emergency to consider using a 'domestic rated' Li battery to start an engine.

 

[PaulRainbow]

I don't usually disagree with you, and I'm not really disagreeing here. But the formation of dendrites is a factor in all lithium technologies. In operation as designed, LiFePO4 is very safe; we don't disagree on this. And, of course, BMS ensure that batteries are used within their design limits. But you, I am sure, use your batteries according to the manufacturers' guidance, and use batteries designed for the application. But here we are looking at batteries designed for relatively low "house" currents (tens of amps, not hundreds), and considering discharge rates substantially higher than the design limit. Also I did emphasize COULD! I highlighted a realistic risk, not something that would certainly happen. But I imagine that an insurance company would get very snotty about a fire resulting from discharging a battery (of any technology) at rates well beyond it's design limits.

Sorry, but that's incorrect. The batteries that are the subject of this thread consist of 280Ah cells with a constant current rating of 1C, so that's 280A and in my case that's at 24V. All of my customer installs have used the same cells, or in some cases 314Ah cells, rated for 314A. There may be a reference in the thread from someone with a 100Ah battery, but even that's going to be rated at 100A.

I'm not considering "discharge rates substantially higher than the design limit". To start my engines requires a current higher than the BMS is rated for, for a few milliseconds of inrush and then 70A-80A to turn the engine over until it starts, usually about 2 seconds.

 

[PaulRainbow]

We are on same page ... and yes I not only value my extensive expensive model collection etc - but do not care to buy Li batterys too often !

I would be extremely wary and literally a no other course of action emergency to consider using a 'domestic rated' Li battery to start an engine.

These batteries are not "domestic rated" there's no such thing. They have a C rating and if your proposed current needs are within that rating they are fine to use. The batteries cannot tell, or care, what the current is powering.

 

[Refueler]

Sorry, but that's nonsense. These batteries are not "domestic rated" there's no such thing. They have a C rating and if your proposed current needs are within that rating they are fine to use. The batteries cannot tell, or care, what the current is powering.

I used the term loosely as I am sure you are well aware ... I know its not an official designation but it serves to highlight low C rated vs high C rated ...

And anyway .. C ratings are fine in the lower figures - but in higher figures - they are generally regarded as marketing hype.Some quoted figures - you wonder how they got them !!

 

[PaulRainbow]

I used the term loosely as I am sure you are well aware ...

I'm not.

I know its not an official designation but it serves to highlight low C rated vs high C rated ...

Sorry, you can't just invent designations, they make no sense and mislead people.

And anyway .. C ratings are fine in the lower figures - but in higher figures - they are generally regarded as marketing hype.Some quoted figures - you wonder how they got them !!

Not sure what "lower figures" you're referring to, but the batteries in question are rated at 1C, 280A. I only use EVE cells, but If we don't go by C ratings, what do we go by ?

 

[Refueler]

I'm not.

Sorry, you can't just invent designations, they make no sense and mislead people.

Not sure what "lower figures" you're referring to, but the batteries in question are rated at 1C, 280A. I only use EVE cells, but If we don't go by C ratings, what do we go by ?

You are now playing a game Paul ... you are well aware of what I have meant and I am sure you interpreted as I meant.

In terms of what do we use ... of course the C rating ... but as I said and I stand by it as do many people far more knowledgable than you or I - that the high quoted C ratings are often highly suspect. Using the lower C rated to the max is fine ... but if you have some of the high C rated - the general advice is to exercise caution. Quite often as you will be aware due to your work - that the limiting factors usually come down to wire gauge and ability to carry such loads ... and that often is before the cells (if no BMS) fail. NOTE I AM NOT QUOTING AMPS ... I am quoting C rate.

 

[PaulRainbow]

You are now playing a game Paul ... you are well aware of what I have meant and I am sure you interpreted as I meant.

No i am not and i have no way of "interpreting" what you meant, i only know what you typed.

For instance, you refer to "domestic rating, which, as you admit, doesn't exist.

In terms of what do we use ... of course the C rating ... but as I said and I stand by it as do many people far more knowledgable than you or I - that the high quoted C ratings are often highly suspect. Using the lower C rated to the max is fine ... but if you have some of the high C rated - the general advice is to exercise caution.

Do you have any sources relating to this ?

Quite often as you will be aware due to your work - that the limiting factors usually come down to wire gauge and ability to carry such loads ... and that often is before the cells (if no BMS) fail. NOTE I AM NOT QUOTING AMPS ... I am quoting C rate.

Wire gauge isn't a limiting factor with regard to C rating. C rating is what it is, you then install wiring appropriate to your current requirements, subject to not exceeding the C rating, and fuse accordingly.

Anyway, i've not advocated exceeding C rates, so i don't know where this all came from. A properly designed and installed system doesn't need to do so.

For example, my installation (as i've repeatedly posted) has a 280Ah 24V battery, rated at 1C continuous (280A). The BMS is 200A, that was the biggest JK BMS available at the time, i'm now fitting 300A ones. The cables are rated at 340A and the fuse is 250A. The 200A BMS shuts down when the starter button is pressed, due to the few milliseconds of current inrush, so it has to be bypassed to start the engine. Once the starter is spinning the current draw is a mere 70A-80A, i can (and do) draw double that with the inverter.

The BMS doesn't like the inrush, but it doesn't bother the fuse, so the C rating isn't even reached, let alone exceeded. Even if i left the BMS bypassed and tried to continuously exceed the C rating of the batteries the fuse would blow.

 

[shanemax]

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 curre having a BMS

 

[shanemax]

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.

Having a BMS and inline fuse ,in the system, will do no harm. Its a bit like having a seatbelt plus airbags when things go wrong.
Two lines of protection.

 

[PaulRainbow]

Having a BMS and inline fuse ,in the system, will do no harm. Its a bit like having a seatbelt plus airbags when things go wrong.
Two lines of protection.

Kind of, but slightly different. The fuse is there for the same reason you'd have a battery fuse in any system, it protects the battery cables. The BMS safeguards the battery against several things, over current from loads or charging, low or high temperatures, cell imbalances etc etc. The system should be designed so that in normal use you don't exceed the continuous current rating of the battery, if you unexpectedly did, the BMS will shut the loads down. If the fuse is rated accordingly it will, in addition to protecting the cables, act as an additional failsafe if the BMS didn't shut down.

 

[shanemax]

There's one factor not being addressed and that is that exceeding the battery's rating COULD result not merely in a dead battery but in a fire. Obviously, there's a large safety margin in manufacturers' figures, but the bottom line is that lithium batteries are prone to forming dendrites of lithium between the anode and cathode. That is the mechanism by which the capacity is reduced as the number of charge/discharge cycles increases; limiting dendrite formation is an active area of research in electrochemistry. In normal operation with a BMS and within the manufacturer's specification, that's taken into consideration and is not an issue. But if the discharge rate is greatly exceeded, the dendrites could short-circuit the cell, and a runaway reaction take place, with catastrophic results.

I'm not an expert in this, but I am aware of current research into this technology and because of that I would NOT operate lithium batteries outside the manufacturer's specifications.

I always thought if you find Dendrites you need to see the doctor fast

 

[geem]

Having a BMS and inline fuse ,in the system, will do no harm. Its a bit like having a seatbelt plus airbags when things go wrong.
Two lines of protection.

You don't seem to have a grasp on basic electrics. Do you not understand what Paul is saying?

 

[noelex]

To fully understand a battery’s performance, the better manufacturers publish multiple "C" ratings depending on the time scale.

For example, the EVE 280Ah cells publish three C ratings:

Standard discharge current : 0.5C
Maximum continuous discharge current : 1C
Pulse discharge (30s): 2C


We also need to consider the current rating of the BMS if it is in the circuit when starting the engine. This will also have multiple current ratings depending on the time scale.

The difficulty with engine starting is that it initially requires a very high current for a very short period of time (inrush current). For a typical marine diesel, this is likely to be 500-1000A for around 50 milliseconds. Some manufacturers will publish a C rating for shorter time scales, but often we are left guessing.

Fuses will not typically provide any protection for these high inrush currents, but we still need to ensure that the battery cells (and BMS if used) can deliver these currents safely and without unduly affecting the lifespan.

Below is a good discussion on starting currents:

https://www.bluesea.com/resources/114

The graph below shows a marine engine that requires a steady state starting current of around 200A, but the peak current is over 1000A.