Zing
Well-Known Member
Correct. They work very well at starting because of this.
Change to LifePO4 - Emergency engine starting ?
- Thread starter PaulRainbow
- Start date
Zing
Well-Known Member
Yes, LTO are perfect for the job.
geem
Well-Known Member
But don't make much financial sense when you need enough cells for a 24v starter. A couple of lead batteries do the job well enough. Don't make the mistake of seeing an LTO battery on Alexi express and think you are getting a good deal. Lots of second hand or poor quality cells out there that still go for way too much money.
PaulRainbow
Well-Known Member
My system works perfectly well, thanks. If i did need any help it would be way above your pay grade
You said your BMS allows unlimited current and you rely on fuses ??Mine only blows a fuse for one consumer. That’s not poorly designed at all. It is well designed. However your system of having a BMS shut you down is worse. Everything gets shut down at once. Now that’s a poor design. Did you do it?Poorly designed indeed.
Frankklose
Well-Known Member
Even with a 100A JBD BMS 280ah Lithium the VP2003t will start.
Zing
Well-Known Member
I don’t think your system works well. Being unable to use house batteries as a back up for the engine battery qualifies it as not working well. I was offering to help, but after that insult I withdraw.My system works perfectly well, thanks. If i did need any help it would be way above your pay grade
You said your BMS allows unlimited current and you rely on fuses ??
Yes, my BMS allows unlimited current as I have not found any BMS capable of dealing with your starter problem and other similar problems, such as powering my high amp bowthruster and hydraulics. Fuses are a long established and excellent circuit protection system.
geem
Well-Known Member
I think i have had this conversation with you before. There is no such thing as unlimited current. You show yourself by arguing technically but then use a statement such as your 'BMS allows unlimited current'. What you have is a system where the current doesn't flow through the BMS. That isn't not the same thing as unlimited current. The conductors still need to be sized accordingly. You couldn't have a boat that could support 'unlimited current'. The contactor you use has a current carrying capacity and the wire size will also limit the current.
You style of BMS was popular when BMS development was in its infancy. It's a bit like VHS or Betamax. The world went down the Mosfet route not your way. Mosfet BMS are now the mainstream.
What Paul and I have is batteries built with cells and a JK BMS. It is very easy to bypass the mosfet based bms for engine starting in an emergency.
For those with a flat starter battery, not a battery with a failed cell, a switch to join the house battery to the starter battery will be sufficient to start the engine.
Several people have also posted on here that they can start their engines through the bms on their lithium domestic batteries.
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Sea Change
Well-Known Member
How often do people find themselves with a flat engine start battery? In nearly two decades of boat ownership I can't remember it ever happening to me.
Our boat has an engine start battery that is used for engine starting and nothing else. It's not possible for it to power anything else due to the way the boat is wired. There is no charging for this battery other than the alternator itself.
It's never failed to start the engine, even after we've sat at anchor for several weeks without running the engine.
I suppose if it did let me down, if I had some time in hand I'd just charge it up via the solar/lithium/inverter system using the 30A mains charger that I carry.
And if I didn't have time for that, I'd have to pull out the jump leads and bypass the BMS.
But I tend not to put myself in situations where I need the engine and have no alternatives.
Our boat has an engine start battery that is used for engine starting and nothing else. It's not possible for it to power anything else due to the way the boat is wired. There is no charging for this battery other than the alternator itself.
It's never failed to start the engine, even after we've sat at anchor for several weeks without running the engine.
I suppose if it did let me down, if I had some time in hand I'd just charge it up via the solar/lithium/inverter system using the 30A mains charger that I carry.
And if I didn't have time for that, I'd have to pull out the jump leads and bypass the BMS.
But I tend not to put myself in situations where I need the engine and have no alternatives.
geem
Well-Known Member
Mine won't go flat as it has a DC/DC charger keeping it on float from the lithium. Our windlass is connected to the engine battery and we do use that to lift the dinghy fairly regularly. It's nice to know the battery won't be flat
Plum
Well-Known Member
I have always had the traditional two LA battery system with a changeover to enable engine starting from the second battery. I have never had a flat engine starting battery in over 40 years so never used the changeover system, but I am not brave enough get rid of this facility.
Zing
Well-Known Member
All batteries die eventually including yours and it may happen when you are away and can’t replace in time. One day, sure as eggs are eggs you will press the start button and nothing will happen. Best is if people have those jumper cables, switch or whatever to keep the show on the road.
Zing
Well-Known Member
Obviously there is no such thing as unlimited current. It did not need a lengthy explanation to explain these semantics, but I’m glad you understood what I said. Actually it is now possible to have a BMS with effectively unlimited current for all practical] purposes and I will may upgrade to one Daly goes to 2,000A, not that I need it.
There is advantage to not having a system capable of a total shut down. When that happens due to a glitch it’s potentially an avoidable emergency. Exactly that has happened to me elsewhere.
jakew009
Well-Known Member
Ironically a battery of unlimited capacity would actually have unlimited current, if the current it can deliver is a function of it's capacity (ie. the C rating).
PaulRainbow
Well-Known Member
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 ?
geem
Well-Known Member
We have pair of 100A 12v lead acid batteries in series for starting. We have a 12v diesel generator start battery in the same box as the engine start batteries. Both sets are kept on float by dc/dc chargers independently. We have a combine switch for the lithium and engine batteries. We can swap the generator battery for one of the engine batteries. I can bypass the bms on the batteries as we have access to the cells so I can start from the lithium.
I can charge the engine batteries via solar or from the generator. Are there any more belt and braces you think we need?
PaulRainbow
Well-Known Member
Was it not the case with those early types of BMS that they should be connected to an external device to prevent excess load and charge currents ? Not relying on fuses.
jakew009
Well-Known Member
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:
Where are you seeing that it is capable of safely discharging at more than 2C?
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PaulRainbow
Well-Known Member
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.
geem
Well-Known Member
Nobody would understand what you said from the post. I only understood it because of your similar post a while ago when you used the same term 'unlimited current'. Anybody with an understanding of electrics know its a nonsense statement. You simply have a different type of bms. It has some advantages and some disadvantages. You can still have BMS failure and contactor failure.
I have a friend with the same system. His has already had contactor failure (and previous BMS failure)which I helped him to diagnose. The boat was new to him but we found a spare contactor onboard so fixed it fairly quickly if you don't count the time trying to work out what was wrong on an unfamiliar boat.
PaulRainbow
Well-Known Member
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.