Replacing house batteries project

[KompetentKrew]

Question, as I am also buying batteries right now... Do the lithium batteries not have twice the usable capacity as the lead acid, and therefore the price being 3x is more like 1.5x? Buying 500 AH batteries that you can only use 250 AH or less seems to me like a bit of a fib by the dealers.

This is shown in the table of the post you replied to as "max useable discharge %" and therefore "adjusted ah".

I think there's maybe room for debate about how much more useable capacity lithium have - some people will try to maintain the condition and longevity of their AGM / lead-acid batteries by only discharging them 20% or 30% (i.e. keeping them above 70% or 80%). Whereas I think you can discharge lithium 80% and it will have no effect at all on its lifetime, in terms of number of useful cycles.

 

[PaulRainbow]

Spending too much time in here again, log off & delete password

Ahhh.... you're just teasing us .....

 

[ckris]

Alternator output needs to go direct to engine battery then get a b to b charger to charge lithium . The charge rates on lithium are very high .

It does not need to - this is one possible solution IF you have a problem with the alternator being overloaded.

In my case the lithium puts a 50-60a load on a 115a alternator. I decided to try direct charging from alternator and monitor it closely, happily realised a B2B or other solution was not needed.

 

[Poey50]

It does not need to - this is one possible solution IF you have a problem with the alternator being overloaded.

In my case the lithium puts a 50-60a load on a 115a alternator. I decided to try direct charging from alternator and monitor it closely, happily realised a B2B or other solution was not needed.

How are you protecting the alternator from BMS disconnect?

 

[ckris]

How are you protecting the alternator from BMS disconnect?

The lead acid batteries that are in parallel.

 

[Poey50]

The lead acid batteries that are in parallel.

That should do it. I'd be interested in how they work together in terms of charging etc. Do you keep them in parallel all the time for charge and discharge?

 

[ckris]

Just a couple of points as I'm in a rush.

The lifetime cost of LFP compared to AGM (and probably other lead acid depending on how both are used) is in favour of LFP. I think that argument is over. Of course, not everyone wants to, or can, make the upfront investment. For example, anyone planning to only keep the boat for a couple of years will not get their money back in the resale price.

For me the main questions are whether anyone considering LFP can make a safe and seaworthy system - and one that they actually need for their type of sailing.

ABYC explicitly rule out hybrid systems and it is likely that ISO have or will go the same way. This matters since eventually insurers - via surveyors - will demand compliance with emerging regulations. Their argument, as I understand it, is that two such different chemistries should not be connected in parallel. I know there are counter-arguments but that is what the emerging standards are likely to specify.

Buying drop-in batteries on cost per watt-hour is a bad idea. Paying for reliability is much more important and because the cells, welding, wiring, and BMS are hidden away in the case then lower costs can be gained by sub-standard construction and components. See the various break-down videos on Will Prowse YouTube channel, for example.

The point that I was trying to make, not very well, is that the UP FRONT cost of the LiFeP04 I bought last year were less than the equivalent premium AGMs that I was going to buy otherwise.

I think that is astonishing....and definitely not the case a few years ago.

And I believe this IS for well engineered units with good quality components..... from UK suppliers who you can pick up a phone to and offer 5 year and 10 year warranties (that are likely worth more than the typical FLA battery warranty) ........a world away from the DIY build with cells from Ali that was the only cost effective way to go LifeP04 5 years ago.

Will Prowse videos are great, and definitely worth pointing out there is some rubbish out there, but there are also some good quality reasonably priced batteries if you spend some time finding them.

 

[Poey50]

The point that I was trying to make, not very well, is that the UP FRONT cost of the LiFeP04 I bought last year were less than the equivalent premium AGMs that I was going to buy otherwise.

Yes, agreed. My wider point is that a safe and seaworthy LFP system involves much more than buying the battery. It's not personal to you or this thread. I just find that especially when it come to LFP people tend to be overly battery-focussed rather than systems-focussed.

 

[ckris]

That should do it. I'd be interested in how they work together in terms of charging etc. Do you keep them in parallel all the time for charge and discharge?

Yes at the moment it is 100AH AGM in parallel to 100AH LifePO4, as mentioned in previous posts I have been looking at this for a looong time so was being cautious and started with a small LiFeP04 and added a battery switch between them so I could isolate if needed and have been watching very carefully.

It has worked perfectly with LifePO4 doing the cycling and the AGM on float most of the time. Even the extra battery switch is unnecessary (I can turn off the LiFePO4 using bluetooth app anyway if I ever needed to) . Only when the LiFePO4 is discharged quite deeply (80%?) does the voltage drop enough for AGM to start contributing....which is quite rare.

It is also quite rare that I fully charge the LiFePO4 (it is a lovely change of mindset to realise it is not necessary). On a long motor the LiFePO4 just quietly switches itself off when full and I am back to having a slightly smaller LA bank.

The only problem so far is I got greedy and have ordered a second 280AH LifePO4 !!

 

[ckris]

Yes, agreed. My wider point is that a safe and seaworthy LFP system involves much more than buying the battery. It's not personal to you or this thread. I just find that especially when it come to LFP people tend to be overly battery-focussed rather than systems-focussed.

The work you and others have done in pioneering LiFePO4 systems in small boats and educating us about the pitfalls has been awesome.

The flip side is that over complicated systems are also a barrier to the average small boat owner going Lithium. Hybrid appears to be a simple, robust architecture which combined with decent quality LiFePO4 drop in gives an effective, safe, retro fit option at low cost.

Sometimes simple is the most elegant and robust solution. Pretty much the only thing to worry about with hybrid is over stressing the alternator, but in many cases this might not actually a problem!

 

[Poey50]

It has worked perfectly with LifePO4 doing the cycling and the AGM on float most of the time. Even the extra battery switch is unnecessary (I can turn off the LiFePO4 using bluetooth app anyway if I ever needed to) . Only when the LiFePO4 is discharged quite deeply (80%?) does the voltage drop enough for AGM to start contributing....which is quite rare.

As I said in another post, the lead acid will spend most of its time being float charged by the LFP. That's good for the lead acid but it isn't contributing anything until the LFP is nearly empty. At that point you might decide to keep discharging from your combined bank in which case you force a low voltage disconnect of the LFP. It would be interesting to know what cell voltages are at disconnect. With deeper discharge of the lead acid you will find over time some difficulty in getting it back to full charge since LFP generally uses a lower bulk voltage and requires very little or no absorption period. All lead acids need a long absorption period to avoid sulfation, so the AGM - not a cheap item -may not last long.

It is also quite rare that I fully charge the LiFePO4 (it is a lovely change of mindset to realise it is not necessary). On a long motor the LiFePO4 just quietly switches itself off when full and I am back to having a slightly smaller LA bank.

If I have understood correctly you charge the LFP to full on a long motor and force a high voltage disconnect - the lead acid battery avoiding the load dump / voltage spike that would otherwise occur. Is that right? Do you know the high cell voltages at which the BMS disconnects?

My point about low and high voltage disconnect is that, if I have understood correctly, you are using the last ditch protection of the battery as a routine form of battery management. Also depending on these figures (which are often placed too high and too low for manufacturers to claim bigger capacity) you could be shortening the service life of the LFP.

It may be that in fact you do a lot of monitoring and manual switching. If that is the case then you need to make sure every single time that you get the switching right. Automatic systems are safer systems. LFP gives you no credit for getting it right the first 2000 times if you are distracted on go number 2001.

 

[Buck Turgidson]

Don't know the full chemistry but the lithium batteries on the Ineos Americas Cup practice boat cough fire the other day after it capsized according to Sir Ben.

 

[Poey50]

The work you and others have done in pioneering LiFePO4 systems in small boats and educating us about the pitfalls has been awesome.

The flip side is that over complicated systems are also a barrier to the average small boat owner going Lithium. Hybrid appears to be a simple, robust architecture which combined with decent quality LiFePO4 drop in gives an effective, safe, retro fit option at low cost.

Sometimes simple is the most elegant and robust solution. Pretty much the only thing to worry about with hybrid is over stressing the alternator, but in many cases this might not actually a problem!

I wonder why you think that people don't just use this simple system?

 

[ckris]

As I said in another post, the lead acid will spend most of its time being float charged by the LFP. That's good for the lead acid but it isn't contributing anything until the LFP is nearly empty. At that point you might decide to keep discharging from your combined bank in which case you force a low voltage disconnect of the LFP. It would be interesting to know what cell voltages are at disconnect. With deeper discharge of the lead acid you will find over time some difficulty in getting it back to full charge since LFP generally uses a lower bulk voltage and requires very little or no absorption period. All lead acids need a long absorption period to avoid sulfation, so the AGM - not a cheap item -may not last long.

Yes, the AGM is acting as a safety buffer to allow easy retrofit in an existing system, it spends most of the time on float. Pack low voltage disconnect is set to 12v. The AGM only starts to contribute when LiFePO4 is over 80% discharged. There is no reason to discharge the AGM any more than you would otherwise, in practice you keep an eye on it in exactly the same way as you would without LiFePO4 as the time you would need to charge the AGM anyway is around the same point as the low voltage disconnect. Re-charging the AGM is no problem as your charge sources continue as before (the only change is to lower the float voltage). The AGM should live a very long and happy life spending most of it's time on float from the LiFePO4.

If I have understood correctly you charge the LFP to full on a long motor and force a high voltage disconnect - the lead acid battery avoiding the load dump / voltage spike that would otherwise occur. Is that right? Do you know the high cell voltages at which the BMS disconnects?

I would not describe as "force a high voltage disconnect", but yes - when the LiFePO4 pack voltage reaches 14.1v it quietly switches itself off and the alternator (or any other charge source) continues to charge just the AGM.

My point about low and high voltage disconnect is that, if I have understood correctly, you are using the last ditch protection of the battery as a routine form of battery management. Also depending on these figures (which are often placed too high and too low for manufacturers to claim bigger capacity) you could be shortening the service life of the LFP.

Not convinced by the idea that the BMS must only be a back up for last ditch protection - why? The risk of BMS failure is very low and we do not routinely double up on all our electric systems.

Anyway, hybrid is inherently more robust as it has the the AGM fallback if the BMS switches off the LiFePO4.

It may be that in fact you do a lot of monitoring and manual switching. If that is the case then you need to make sure every single time that you get the switching right. Automatic systems are safer systems. LFP gives you no credit for getting it right the first 2000 times if you are distracted on go number 2001.

Only extra monitoring was at the beginning because I was keen to see how it worked. Otherwise, just keep an eye on depth of discharge/voltage in exactly the same way as for a conventional LA bank so you know when it needs to charge. No manual switching, no extra monitoring, all automatic.....just like having a LA bank that is cheaper, larger capacity, faster charging and no stress if you cannot bring the bank up to full charge.

 

[Poey50]

Yes, the AGM is acting as a safety buffer to allow easy retrofit in an existing system, it spends most of the time on float. Pack low voltage disconnect is set to 12v. The AGM only starts to contribute when LiFePO4 is over 80% discharged. There is no reason to discharge the AGM any more than you would otherwise, in practice you keep an eye on it in exactly the same way as you would without LiFePO4 as the time you would need to charge the AGM anyway is around the same point as the low voltage disconnect. Re-charging the AGM is no problem as your charge sources continue as before (the only change is to lower the float voltage). The AGM should live a very long and happy life spending most of it's time on float from the LiFePO4.

I would not describe as "force a high voltage disconnect", but yes - when the LiFePO4 pack voltage reaches 14.1v it quietly switches itself off and the alternator (or any other charge source) continues to charge just the AGM.

Not convinced by the idea that the BMS must only be a back up for last ditch protection - why? The risk of BMS failure is very low and we do not routinely double up on all our electric systems.

Anyway, hybrid is inherently more robust as it has the the AGM fallback if the BMS switches off the LiFePO4.

Only extra monitoring was at the beginning because I was keen to see how it worked. Otherwise, just keep an eye on depth of discharge/voltage in exactly the same way as for a conventional LA bank so you know when it needs to charge. No manual switching, no extra monitoring, all automatic.....just like having a LA bank that is cheaper, larger capacity, faster charging and no stress if you cannot bring the bank up to full charge.

Thank you for the detailed explanation - very interesting. First up I would say that the continued presence of the lead acid battery regardless of what the LFP is doing, does offer a solution to many of the questions I raised on the other thread that I started recently. That is a very strong plus. My reservation with regard to the AGM is whether you get any other value from it other than as an excellent buffer. It should lead a long and happy life as long as you don't discharge deeply into its capacity since then it could struggle to get back to full charge since the LFP will always take charging priority and because lead acid has a long charging tail. If however you keep it purely to buffer and recharge when the LFP is at 20 % or above then it will stay at float as you say. In fact, any cheap small capacity lead acid would perform the same function.

My reservation about the LFP is whether you will get a long life from it. Are the high and low voltage disconnect figures within user control? I ask because the high voltage figure of 14.1 volts for disconnect sounds very low. Are you confident this is the voltage at the battery? Usually they are set much higher than this. Battleborn can high voltage disconnect 14.6- 15 volts for example. One of the issues Rod Collins raises is that high voltage disconnect is often set too high for long battery life on drop-ins. Manufacturers don't want their customers to have annoying cut-offs too soon and also as said before, an overly-high cut-off looks good on the capacity figures. It isn't generally anticipated that people will get to high and low disconnect on a regular basis in a lithium system but this form of hybrid system relies on that. As you properly say, it doesn't present danger to the vessel as your lead acid can then do it's job but longevity issues might be a problem.

As a general issue it is thought to be a better design feature to have BMS cut-off as a last resort not as a part of battery routine operation. My system with external relays has never had a high or low voltage disconnect in three years and that is not unusual. But, as agreed, you are covered for catastrophic failure by the lead acid. I hope you will report in the long term. It would be interesting to get a capacity tester to check every year. I just so happen to have one for sale on eBay! Battery Capacity Tester - 150 watts | eBay

 

[Poey50]

For interest - this is what Eric Bretscher has to say (author of Nordky Design series of 6 articles on LFP on boats)

Alternative 1 – Lead-Lithium Hybrid Bank
The simplest way of resolving all the challenges mentioned at the beginning of this article is running the lithium bank in parallel with some standard lead-acid capacity. If any issue arises with cell voltages or temperatures, the lithium bank can be disconnected and the installation will revert to a simple lead-acid system. In some instances, this lead-acid capacity could get damaged or destroyed if the event that resulted in the disconnection of the lithium cells was severe, like an alternator regulation failure.

The simplest safe lithium installation: leaving a sealed lead-acid battery in parallel with the lithium bank at all times allows disconnecting the lithium capacity in case of problem without any issues. The additional SLA doesn’t contribute to any meaningful capacity; its function is ensuring charging sources always see a battery in circuit.
The practical result of such an arrangement is that the lithium battery ends up doing virtually all the work, because it is first to discharge due to its higher operating voltage. The charging voltages are no longer high enough to provide effective charging for the lead-acid cells, but as those are being trickle-charged above 13V all the time, they can be expected to remain essentially full and it hardly matters.

The lead-acid battery needs to be able to absorb whatever “unwanted” current may come its way if the lithium bank gets disconnected due to a high voltage event for example. In some instances, a single sealed lead-acid (SLA) battery can be sufficient. SLAs are the best choice for this application as they don’t consume water and are very inexpensive; gel cells should be avoided as they are costly and a lot more intolerant to overcharging and AGMs would be a complete waste of money in this role.

The drawbacks are:

• Some charge gets lost trickling continuously into the SLA, more so in a lead-acid battery in poor condition.
• It doesn’t fully eliminate the lead and associated weight.
• Removal of the SLA from the system, at some point in the future, would create an unexpected liability.

Some advantages are to be found as well:

• Disconnection of the lithium bank can be managed with a single contactor; there is no need to implement a split bus. This can allow using some small BMS solutions incapable of managing a dual DC bus.
• The lithium bank is literally added to the installation in place, normally without cabling alterations required, but not without voltage and regulation adjustments.

With this in mind, it certainly is the simplest fully functional design one can build, as long as protection and automatic disconnection are still very properly implemented for the lithium bank.

Should the lithium bank ever become heavily discharged, the additional lead-acid capacity can start contributing, but this would also leave it at a reduced state of charge for a time afterwards and cause it to start sulphating. This is not automatically much of a concern, because it may not happen (this depends on the BMS low-voltage disconnect threshold) and it doesn’t actually result in much harm if it does. The SLA needs to remain in a reasonable condition however, in order to be able to absorb any transients if the lithium bank gets dropped off due to excessive voltage and not continuously discharge the lithium cells at an excessive rate.

Voltage Sensing
NEVER, EVER, SENSE THE CHARGING VOLTAGE DIRECTLY AT THE LITHIUM BANK TERMINALS IN THIS CONFIGURATION

The sensing voltage required for charge control must be sourced upstream of the lithium battery disconnector, or in other words from the SLA battery, so it remains valid even after a disconnection of the lithium capacity. This is very important, otherwise uncontrolled, unlimited charging of the lead-acid battery will occur after the lithium capacity gets isolated.

 

[Poey50]

I just saw a comment from Rod Collins which shed a little light on why ISO and ABYC don't support LFP-lead acid hybrid. As well as general concerns about different battery chemistries not playing well together - if a lead acid cell should short and effectively become a 10 volt battery the LFP would then pour power into it with Big Bang consequences.

 

[ckris]

In fact, any cheap small capacity lead acid would perform the same function.

Yes - I just used an old AGM that had lost ~25% capacity which would otherwise have been end of life. Curiously I was expecting this to be too much of a parasitic load and thought would need to replace with a new battery - but it is a negligible current draw from the LiFePO4.

My reservation about the LFP is whether you will get a long life from it. Are the high and low voltage disconnect figures within user control?

For the 2 suppliers I have used, the BMS can be easily configured using a bluetooth app, though their approach to the warranty varies depending on what you change so worth talking to them first. One of them (Life Batteries) will pre-configure the BMS for hybrid if asked using 12v low and 14v high disconnect.

Are you confident this is the voltage at the battery?

Yes

Usually they are set much higher than this. Battleborn can high voltage disconnect 14.6- 15 volts for example. One of the issues Rod Collins raises is that high voltage disconnect is often set too high for long battery life on drop-ins. Manufacturers don't want their customers to have annoying cut-offs too soon and also as said before, an overly-high cut-off looks good on the capacity figures. It isn't generally anticipated that people will get to high and low disconnect on a regular basis in a lithium system but this form of hybrid system relies on that.

The problem with a higher pack disconnect is if you go on a long motor with a conventional alternator like mine that regulates at 14.2v it will never disconnect and you would effectively end up with the LiFePO4 on 14.2v float charge.

TBH I am not absolutely sure how bad that would be but have read lots of opinions that leaving LiFePO4 on high float charge will shorten the life. I have also read this is damages Lithium Ion not LiFePO4. I would love to understand this better but for now having pack disconnect at 14v (or 14.1v for me) simply avoids the problem and still charges to 90%+.

One of the issues Rod Collins raises is that high voltage disconnect is often set too high for long battery life on drop-ins. Manufacturers don't want their customers to have annoying cut-offs too soon and also as said before, an overly-high cut-off looks good on the capacity figures.

Agree - it is important to find a good supplier. Batteries with user configurable BMS are readily available and some suppliers will be OK and stand by the warranty as long as you tell them and are making the settings more conservative than before.

 

[Poey50]

The problem with a higher pack disconnect is if you go on a long motor with a conventional alternator like mine that regulates at 14.2v it will never disconnect and you would effectively end up with the LiFePO4 on 14.2v float charge.

TBH I am not absolutely sure how bad that would be but have read lots of opinions that leaving LiFePO4 on high float charge will shorten the life. I have also read this is damages Lithium Ion not LiFePO4. I would love to understand this better but for now having pack disconnect at 14v (or 14.1v for me) simply avoids the problem and still charges to 90%+.

A good point that you need a user-configurable voltage disconnect settings (high and low) to avoid going into the upper and lower knees of the LFP charging curve. Cheaper drop-ins with high voltage disconnect and non-configurable settings would be a problem as said.

One of things I wasn't expecting with LFP was the need to be able to switch off charging sources including (safely) the alternator. Charging to full or nearly full is OK as long as it doesn't sit full when deterioration is said to rapidly increase (up to five times as fast according to one account). So the final days of a trip are spent trying to end up with about 60% state of charge before leaving the boat.

 

[goeasy123]

RISK is a function of PROBABILITY OF EVENT and SEVERITY OF OUTCOME.

SEVERITY OF OUTCOME is conditional on the SITUATION.

The failure modes of LFP all tend to binary. The failure modes of LA tend to incremental.

If the SITUATION is a long passage or presence in a remote location then the RISK associated with LFP only is very high compared to LA.

How do you configure a system that mitigates the risk?.... while maintaining the benefits for LFP and keep it simple. In particular, better harvesting of charging sources and more time-flexibility due to better partial discharge performance.

...and you don't need to be physics genius to install and maintain it.