Lithium batteries

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Neeves

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I'm interested in Lithium, in my pursuit of weight reduction. But I'm a neophyte.

I note that lithium batteries, or at least some, have an automatic cut off (the ones I recall, a 200 amp hour unit the cut off was defined as - 10 volts)

Is the cut off a mechanism to save the battery or simply a 'restriction' imposed by the technology. Really - if your ability to maintain the charge in the battery fails does this 10 volt limit 'save' the battery?

Jonathan
 
LiFePO4 imposes no limitations on itself. It will accept charge until it dies and will also provide power until death. The sole purpose of the BMS is to look after the battery and protect the cells from their profligacy.
 
Neeves said:
I'm interested in Lithium, in my pursuit of weight reduction. But I'm a neophyte.

I note that lithium batteries, or at least some, have an automatic cut off (the ones I recall, a 200 amp hour unit the cut off was defined as - 10 volts)

Is the cut off a mechanism to save the battery or simply a 'restriction' imposed by the technology. Really - if your ability to maintain the charge in the battery fails does this 10 volt limit 'save' the battery?

Jonathan
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Basic lithium knowledge.
The BMS has settings that stop you over charging and under charging the battery. These will cut the battery off at predefined voltages to save the battery. They can be overridden in an emergency on most BMS.
Typical lower threshold would be 2.5v per cell and upper threshold would be 3.65v per cell. On a 12v battery with 4 cells the battery would operate between 10v and 14.6v. These are voltages that you would not ordinarily want to see on your battery. Lithium is best kept in the 10 to 90% charged range for long life.
The idea is to use a good MPPT solar controller with basic charge setting something like bulk set at 14v. 1/2 hour absorption time, then float at 13.4v. The float setting effectively turns off the solar as you don't want to float lithium. The absorption time is to hold the voltage up whilst the BMS does some cell balancing. You would typically turn on the cell balancing at 3.45v per cell, so 13.8v on a 12v battery. A good BMS will have active balancing. The JK BMS has 2A active balance. This takes amps from the highest voltage cell and gives it to the lowest voltage cell. There are other BMS such as Daly that don't have active balance. They use passive balance that is far less effective. This uses resistors to burn off the highest cell voltage. It only does this at millivolt levels so is not great at balancing.
If you are building your own battery, rather than buying a drop in battery, you need to top balance the cells. Lots of info on Utube on how to do this. It's an important phase in the assembly of a battery pack.
In addition, you need to compress the pack. Lots of info on utube on how to do this as well. None of it is hard and the beauty of building your own is you know what you are getting as you choose the components. You know its been properly top balanced. It's actually quite goof fun building your own battery.
The last thing is you need a class T fuse immediately after the battery on the positive side as close to the positive battery terminal as you can get.
 
Thanks to you both. I have a suspicion that both of you cover this question, or something similar, in your threads - but it seemed a Herculean task to sift through the threads - and I don't have that level of patience. :(

My question arose from looking at the spec of a drop in battery on a store shelf. It is, or was, very attractive ..... a 200amp/hr unit that is so much lighter than the 200amp/hr lead batteries we have currently.

We will not be changing the lead batteries 'immediately' - I'm just accumulating knowledge as the questions arise.

I'm grateful to you both for your threads on the topic (and you willingness to answer questions).

Jonathan
 
Jonathon, I'd certainly suggest you consider a DIY system. There is not only a considerable saving in money but you can design a better system than is easily possible with drop-ins - one that you will come to know well; a knowledge that will help you in the remote locations that you visit.
 
Poey50 said:
Jonathon, I'd certainly suggest you consider a DIY system. There is not only a considerable saving in money but you can design a better system than is easily possible with drop-ins - one that you will come to know well; a knowledge that will help you in the remote locations that you visit.
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Agreed, one of the great benefits of lithium is the the ability to use an induction hob. Most induction hobs are 2000w. A suitably sized inverter will run this from lithium.
One thing to consider is sizing of the BMS. 2000w is 167A at 12v. Two 12v batteries in parallel with their own BMS will reduce the amps to each battery to 133A. A 200A BMS on each battery will leave lots in reserve and should you have a BMS failure, you still have an active battery. Running a single BMS at high amps will create lots of heat in the MOSFETs. This increases the risk of BMS failure but also heats up the BMS and if you have drop in batteries (the BMS is inside the battery case) then this heat may cause heat to some of the cells. Not great for long battery life if you are already operating the battery in high ambient conditions. A DIY battery with a dedicated battery box will allow you to provide ventilation to the BMS and cells with a simple computer fan running 24/7. DIY can be a far more elegant solution if you have the time and Inclination to build your own
 
geem said:
Agreed, one of the great benefits of lithium is the the ability to use an induction hob. Most induction hobs are 2000w. A suitably sized inverter will run this from lithium.
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In areas of reasonable solar insolation we do all our cooking and water heating electrically on a lead acid system. So if you have a decent lead acid system don’t rule out using an induction hob. A lithium battery bank is not essential.

The main limitation is not the battery technology, but replacing the energy used.
 
I'll take your 'like' of my suggestion as a green light for more suggestions. If I were in your shoes I would be basing the system on a set of Winston Thundersky cells purchased from the factory by way of Julia Yu based in Hong Kong. She is utterly reliable and of course there are good cost savings. Winston are the premium product - you will never buy another cell. Rod Collins' 400 ah pack is not only still going strong after 13 years, the pack is still showing more than 400ah on his careful testing regime.
 
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noelex said:
In areas of reasonable solar insolation we do all our cooking and water heating electrically on a lead acid system. So if you have a decent lead acid system don’t rule out using an induction hob. A lithium battery bank is not essential.

The main limitation is not the battery technology, but replacing the energy used.
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I don't disagree, but you need a large lead acid battery bank of considerable weight to and energy density if you want to pull high loads for long period of time or lead acid battery life tends to suffer.
Lithium batteries are smaller and lighter and handle high loads very well. They also accept charge more efficiently than lead and have no need to achieve 100% charge levels on a daily basis.
 
I’m sold on the idea of lithium, it’s a perfect system for lightweight multis. When I do it, I will be investing in a decent MPPT controller. One further question, do I then run a small lead acid engine battery, or can I run the starter motor for a 15hp outboard, and use the alternator output to charge? Is another charge controller needed for that?
 
Johnathan, for once I disagree with the others and indeed we bought a drop in LFP battery with a 5 year warranty, as a proof of concept experiment. The alternative which I originally planned was 6v lead acids, but they wouldn't fit, so plan B with LFP. Works really well. If we get 5 or 6 years out of it, that's fine. If it lasts longer that's a bonus, though we might do something bigger next time around. Our average loads are 50% of the BMS which seems fine and maximum discharge to date is 33% SOC. Nor do we fuss about trying to fully charge it, as the last fraction is only a tiny amount.

Pete
 
Chiara’s slave said:
I’m sold on the idea of lithium, it’s a perfect system for lightweight multis. When I do it, I will be investing in a decent MPPT controller. One further question, do I then run a small lead acid engine battery, or can I run the starter motor for a 15hp outboard, and use the alternator output to charge? Is another charge controller needed for that?
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How big is the alternator on the outboard? and what other charging systems do you have? Can't see you wanting a stern arch with kWs of solar some how :eek: but you may have some.
 
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I currently(groan) have 180w of solar, and 148w of alternator, we use the engine very little. I could easily add another 200w of solar on the outrigger floats if I needed to. Nav lights, cabin lights, instuments, fridge, diesel heater on the load side.
 
Hmm, I would be tempted to keep the outboard alternator on a decent quality lead acid battery and leave it at that. Perhaps an emergency switch with a thin wire to the domestics with the plan of not using it very often. The thin wire will limit the LFP draw and if you can get it down to say 6A that should be okay, but it will be trial and error to work out the length and thickness of the wire and then test measuring the alternator temperature so it doesn't get too hot or drain your engine start battery.

The problem for alternators is LifePo4 (LFP) will take all the charging a yacht can throw at it. For us shorepower, 600w of solar and the engine together isn't going to worry it. For an alternator it could over heat trying to supply 100% continuously. Lower revs are worse because the cooling fans are turning slower.

That leaves solar to charge your new LFP domestic batteries. With good MPPTs, think Victron, they will do this very nicely. Last year 300w of solar generated 1.3kWh each day in good sunny conditions including using an electric kettle and induction hob, so your proposed 380w will do very nicely from March to October. On a drab January afternoon, forget it. We saw 19wh from 600w this year at 2pm in the afternoon.
 
Pete7 said:
Hmm, I would be tempted to keep the outboard alternator on a decent quality lead acid battery and leave it at that. Perhaps an emergency switch with a thin wire to the domestics with the plan of not using it very often. The thin wire will limit the LFP draw and if you can get it down to say 6A that should be okay, but it will be trial and error to work out the length and thickness of the wire and then test measuring the alternator temperature so it doesn't get too hot or drain your engine start battery.

The problem for alternators is LifePo4 (LFP) will take all the charging a yacht can throw at it. For us shorepower, 600w of solar and the engine together isn't going to worry it. For an alternator it could over heat trying to supply 100% continuously. Lower revs are worse because the cooling fans are turning slower.

That leaves solar to charge your new LFP domestic batteries. With good MPPTs, think Victron they will do this very nicely. 300w of solar generated 1.3kWh in good sunny conditions including using an electric kettle and induction hob, so your proposed 380w will do very nicely from March to October. On a drab January afternoon, forget it. We saw 19w from 600 this year at 2pm in the afternoon.
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We only spend the odd day on the boat in January, usually when the sun is shining. I think that massively alters our winter needs. We could run the outboard on a motorcycle battery, and keep it 100% separate. It has a recoil starter too, not a catastrophe if it gets a bit low. We usually start the engine every time we visit, so it's unlikely.
 
Chiara’s slave said:
We only spend the odd day on the boat in January, usually when the sun is shining. I think that massively alters our winter needs. We could run the outboard on a motorcycle battery, and keep it 100% separate. It has a recoil starter too, not a catastrophe if it gets a bit low. We usually start the engine every time we visit, so it's unlikely.
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Perfect. Since LFP wants to be stored at 50>60% state of charge (SOC) I find myself boiling the kettle sometimes if the battery is full before isolating it and leaving the boat. We have a LFP and lead-acid hybrid bank which means the bilge pump is active powered by lead acid and topped up with solar, though the bilges are always dry.

The other way is to leave the solar attached to the LFP and have a second set of MPPT custom settings with much lower absorption and float voltages which you switch to just before leaving the boat. You can do that with Victron.
 
You might consider a battery switch to completely isolate the LFP from the domestics. Then an emergency switch from a small but good quality engine start lead acid to feed the domestics in an emergency. For example the BMS has a hissy fit and shuts down just as your coming in through the Needles at night with no lights. Turn two switches and you are back in action for some hours on the engine battery.
 
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