Lithium battery abuse

[TheBiscuits]

 

Having just disturbed the moths in my wallet for lithium batteries, I have been idly wondering about @Dr Bob's "top up tank" theory that got shot down on the forum a while back.  It's probably a geek version of buyer's remorse!

 

The disparaged concept was simple enough: stick a medium sized LiFePO4 battery in parallel with your existing lead acids and let it keep the charge up at (not ideal) float voltage.

 

Lots of the experts said it was a bad idea at the time, but Sterling Power are selling a range of LiFePO4 12v nominal batteries with onboard BMS units that will handle high/low voltage/temperature/current disconnect/reconnect.

 

The smallest battery in the range - 20Ah - has the BMS but not the bluetooth link.  Charging profile as recommended by Sterling is 14.4v bulk/absorption, 13.8v float, no equalisation, no desulphation.

 

If you don't add a battery-to-battery charger between these and the alternator the warranty is void.  This is quite important on the 200Ah/£1000+ batteries, but the 20Ah version retails at £90 ....

 

Does this alter the thinking of the real Electrical Engineers?  What actually happens electrically if I take a tired but not broken bank of 4 x 110ah lead acid batteries - fairly much a standard narrowboat setup -  and add one or two of these small, cheap lithiums to it in parallel with no other hardware in the mix?

 

1. Will the lithium discharge and charge first? 
2. Will there be enough power in the small battery to "top up" via parallel equalisation that expensive and painful last 5% on a lead acid bank? 
3. Would this allow a boater on a tight budget to ignore the warranty and just abuse the battery, while keeping their Lead Acid bank at a much healthier voltage?

 

I don't know the answers, I'm asking for thoughts.  And possibly guinea pigs ...

 

[blackrose]

[Tony1]

 

I've heard of a few youtubers trying out this arrangement, and none so for reporting disaster. 

I didnt try this myself as far as I can recall, so I cant speak from experience/memory, but I would have a couple of things that I would check out, during the early experiments and tests with it. 

 

The first would be how to limit the charging current being output by the alternator.

The lithium battery would (I think) want to suck out current at whatever the alternator can provide, and I suspect the LA batteries wouldn't act as an impediment to this. 

The recent youtube adopter I watched used a length of 6mm sq wire to limit the amount of current and thus protect the alternator- her guideline was to ensure the alternator stayed below 120 degrees, which I think might be a tad too hot considering they run at about 55 degrees with lead acids. The alternator may not burn out, but it mighty shorten its lifespan.

 

(There was a thread about this on here recently too, I'm sure of it)

 

But anyway, the 'length of cable' system might provide a current limit, although it was suggested here that you'd want to use heat resistant cable..

 

The other thing that I would want to check out is a way to make sure that the battery voltage doesnt creep up to dangerous levels near to end of the charge cycle/when the lithium is full. 

In theory the lithiums should soak up almost all of the available charge initially, and when the lithiums are full the lead acids will start to soak up more of it.

But that said, the lead acids will provide some resistance to the charging right? And that will increase fairly quickly.

So some of the charging current will still be trying to get into the lithium even after it is full, which doesnt sound ideal. 

 

I'm not saying its a problem as such, just something to consider. The experts will I'm sure give you a proper steer shortly. 

 

 

[MtB]

 

26 minutes ago, Tony1 said:

But that said, the lead acids will provide some resistance to the charging right? And that will increase fairly quickly.

 

No. Being in parallel, the LFP battery will suck out as much current as the alternator is willing to deliver, LA batt present or not. Hence the need for a long bit of 6mm wire, or the B2B charger.

 

 

26 minutes ago, Tony1 said:

So some of the charging current will still be trying to get into the lithium even after it is full, which doesnt sound ideal. 

 

Its not ideal, but this is what the BMS is for. To disconnect the alternator when the first LFP cell gets to the max cell voltage set in the BMS. 

 

Something I've recently come to realise is modern BMS's are considered enough. Old skool thinking on here is that the BMS is the emergency backup disconnect for when your primary control goes wrong, but I got laughed at on (I think), the Will Prowse forum for thinking this, and got told "but controlling the charging is what the BMS is FOR!" It seems they are very reliable nowadays and can be used as the only method of protecting the cells. This certainly appears to be the case with most 'drop-in' replacements, including the Sterling offering highlighted by Biscuits. But we won't know for certain until someone buys one and takes it to bits for a look inside.

 

 

Edited February 15, 2022 by MtB
Dyslexia strikes again

[GUMPY]

Looking at this from the outside it strikes me that the whole failing of lithium battery powered systems is the fact that they are being forced to work at 12vdc connected to an outmoded power generation system.

I have 5.2kWh of lithium  here at home that charge from solar or 230vac. Six 365w panels in series will charge the battery (I have 2x6 panels) as does the mains. If a boat was built with or converted to 230vac with a 3.6kw 230v AC alternator or a genset rather than 12v DC there is rafts of commercial kit available to do the job. BMS is built in to the master battery and charge rates are also limited by the Hybrid inverter. 

If you must have 12vdc then a simple transformer/rectifier system can facilitate that.

Only downside I can see apart from cost is that you can be dealing with up to 230v DC which can be very unpleasant 😉

 

 

[Tony1]

9 minutes ago, MtB said:

 

 

No. Being in parallel, the LFP battery will suck out as much current as the alternator is willing to deliver, LA batt present or not. Hence the need for a long bit of 6mm wire, or the B2B charger.

 

 

I didnt write that very clearly- what I meant was that the lead acids will have internal resistance to charge, which might mean that the surplus charge they reject will be passed on to the lithiums- which might already be full. 

To be fair this doesnt seem to be a real issue, but I do wonder. 

 

[Tony1]

14 minutes ago, MtB said:

 

Something I've recently come to realise is modern BMS's are considered enough. Old skool thinking on here is that the BMS is the emergency backup disconnect for when your primary control goes wrong, but I got laughed at on (I think), the Will Prowse forum for thinking this, and got told "but controlling the charging is what the BMS is FOR!" It seems they are very reliable nowadays and can be used as the only method of protecting the cells. This certainly appears to be the case with most 'drop-in' replacements, including the Sterling offering highlighted by Biscuits. But we won't know for certain until someone buys one and takes it to bits for a look inside.

 

 

Maybe the BMS systems we see inside batteries should be called Battery Protection Systems, rather than BMSs?

 

If all that a BMS system does is to disconnect the battery in an extreme voltage, surely that's not controlling the charging?

That's just reacting to an overcharging situation isnt it? 

 

I would imagine that 'controlling the charging' is a job done largely by the alternator controller (for sensible people), and by a B2B in my case- or for solar users, an MPPT. 

Thats why they call them charge controllers isnt it?

 

 

[TheBiscuits]

5 minutes ago, Tony1 said:

 

Maybe the BMS systems we see inside batteries should be called Battery Protection Systems, rather than BMSs?

 

You listened to @peterboat and bought the Valence batteries though.  If you don't buy the complete system from Valence you don't get the magic box that does all the important work.

 

I ruled them out for myself a few years ago because of this. 

 

10 minutes ago, Tony1 said:

If all that a BMS system does is to disconnect the battery in an extreme voltage, surely that's not controlling the charging?

That's just reacting to an overcharging situation isnt it? 

 

I would imagine that 'controlling the charging' is a job done largely by the alternator controller (for sensible people), and by a B2B in my case- or for solar users, an MPPT. 

Thats why they call them charge controllers isnt it?

 

Pretty much any complete battery management system will control the charge - as @MtB says, it's what they are for.  What they won't do is protect your alternator - they'll just disconnect, which breaks the alternator.

 

The "correct" way to do it as we know is to have an intelligent alternator controller that talks to the BMS and shuts down gracefully at the correct point.  @nicknorman rolled his own, but that's beyond most people.

 

I chose the B2B route for simplicity and on a 100A alternator a 60A B2B that can be switched into half power mode is the correct size.  I don't get the benefit of very fast charging from the lithium batteries but I accept that as a tradeoff as it (hopefully!) won't cook my alternator either.

 

[BEngo]

The problem, to me, with relying on a built in BMS to control charging  is the way they switch high currents. They rely on a lot of smallish mosfets in parallel  to handle large currents, which is fine so long as they all work

  When one fails all the others start to be overloaded and don't last long.

You also may, or not, get any control over its operating settings.

 

The obvious solution is either a dedicated charge control layer or a BMS with plenty of spare current capacity. You can know what is in a charge control layer,  but you have no knowledge of which Daly BMS is inside the battery box unless you open it.

 

The Daly BMS reviews I have seen were not universally great either.

[MtB]

9 minutes ago, TheBiscuits said:

I chose the B2B route for simplicity and on a 100A alternator a 60A B2B that can be switched into half power mode is the correct size.  I don't get the benefit of very fast charging from the lithium batteries but I accept that as a tradeoff as it (hopefully!) won't cook my alternator either.

 

 

Yes this is the weak point I see in the whole Sterling offering. Well I see two weak points actually. 

 

The (say) 100A alternator needs a max continuous current of say 65% of rated output to stop it running too hot. This means you need a B2B that will draw no more than 65A. Given the B2B seems to be about 65% efficient too, this gives a charge current of 42A. And that wastes fuel and eats rather heavily into the primary reason for getting LFPs in the first place, for the rapid charging. 

 

 

Edited February 15, 2022 by MtB
Wrong number!

[TheBiscuits]

2 minutes ago, BEngo said:

The obvious solution is either a dedicated charge control layer or a BMS with plenty of spare current capacity. You can know what is in a charge control layer,  but you have no knowledge of which Daly BMS is inside the battery box unless you open it.

 

Absolutely.  I looked at quite a few different makes of all-in-one lithiums and most of the cheap ones have a weedy little BMS in the case.  300Ah batteries that have a rated continuous current draw of 50A or 100A are very common.

 

I have a 12v 3kW inverter, so I do need to be able to pull ~300A from the battery bank occasionally.

 

It's why I was impressed by these Sterling batteries - the 200Ah version can deliver 1.5C discharge at 300A, which probably means it's the same internal BMS as the 300Ah version but lower power cells in the box.

[Tony1]

33 minutes ago, TheBiscuits said:

 

You listened to @peterboat and bought the Valence batteries though.  If you don't buy the complete system from Valence you don't get the magic box that does all the important work.

 

I ruled them out for myself a few years ago because of this. 

 

 

Pretty much any complete battery management system will control the charge - as @MtB says, it's what they are for.  What they won't do is protect your alternator - they'll just disconnect, which breaks the alternator.

 

The "correct" way to do it as we know is to have an intelligent alternator controller that talks to the BMS and shuts down gracefully at the correct point.  @nicknorman rolled his own, but that's beyond most people.

 

I chose the B2B route for simplicity and on a 100A alternator a 60A B2B that can be switched into half power mode is the correct size.  I don't get the benefit of very fast charging from the lithium batteries but I accept that as a tradeoff as it (hopefully!) won't cook my alternator either.

 

 

The Valences are very well-made batteries to be fair, but buyers need to be clear that they dont have a BMS built in before making a decision. 

If I'd been a bit more persistent I could have probably found a used BMS box, but thats my fault. 

 

I suspect that you've got the Sterling BB1260 unit? I have two of those, one for each alternator, but one thing I found was that they sometimes go into some kind of reduced current output mode, which I think might be the internal temp sensor tripping, as they get quite warm. I spoke to the Sterling rep who thought it was related to the voltages involved in my setup, but I didnt have the know how to use his answer. You'll be able to resolve that issue I'm sure, if you come across it.

 

If I remember it right, the BB1260 doesnt put out 60amps- that's its input current.

What it puts out seems to vary a bit, but I've seen typical figures of 50-55 amps with mine, dipping to 30amps when it limits itself. I found in the summer months it was going into this limited power mode quite often- but it might be just my unit, of course- I dont want to accuse Sterling when it could very easily be something in my setup. 

 

I think my alternator must be a pretty poor unit, as it struggled to stay cool (ie under 110 degrees) when it was putting out anything over 40 amps at tickover.

If I rev it up to say 1300, it will stay cool enough to put out 50 amps, but on a cruise you cant keep the rpm at 1300rpm (what with waiting in locks, passing moored boats etc), so I felt I needed a solution that did not get too hot at tickover, and that limited me to about 40 amps per alternator.  

When I'm doing a static charge and not cruising, I can rev to 1300rpm the whole time, and get more current from the alternators.

 

I'm sure a better alternator will work well with the BB1260 and not overheat at 50 amps output, but in my case, when running at tickover (or cruising), I have to set my B2Bs to half power, which gives about 25amps each. 

 

 

 

Edited February 15, 2022 by Tony1

[nicknorman]

1 hour ago, MtB said:

 

Its not ideal, but this is what the BMS is for. To disconnect the alternator when the first LFP cell gets to the max cell voltage set in the BMS. 

 

Something I've recently come to realise is modern BMS's are considered enough. Old skool thinking on here is that the BMS is the emergency backup disconnect for when your primary control goes wrong, but I got laughed at on (I think), the Will Prowse forum for thinking this, and got told "but controlling the charging is what the BMS is FOR!" It seems they are very reliable nowadays and can be used as the only method of protecting the cells. This certainly appears to be the case with most 'drop-in' replacements, including the Sterling offering highlighted by Biscuits. But we won't know for certain until someone buys one and takes it to bits for a look inside.

 

Well I disagree for 2 reasons, one being that the BMS cutoffs are set to the limit of what is tolerable without rapid damage. I suggest that it is better to avoid routinely charging to the maximum voltage / SoC. And secondly a system where the battery is as a matter of course instantly disconnected when fully charged, is problematic unless you have a hybrid system with an LA battery as well (also known as a fudge system). The fact that little Willie thinks differently doesn’t impress me!

Edited February 15, 2022 by nicknorman

[TheBiscuits]

6 minutes ago, Tony1 said:

If I remember it right, the BB1260 doesnt put out 60amps- that's its input current.

What it puts out seems to vary a bit, but I've seen typical figures of 50-55 amps with mine, dipping to 30amps when it limits itself. I found in the summer months it was going into this limited power mode quite often- but it might be just my unit, of course- I dont want to accuse Sterling when it could very easily be something in my setup. 

 

Yes, that's correct.  I was also looking at the Renogy 20A/40A/60A B2B's and they rate them by output power rather than input power - their 60A version can pull up to 90A input.  They also have a very simple on / half power/ full power switching system that can easily be remotely switched.  A temperature sensor on the alternator and a position sensor on the throttle could trivially be wired into one of these.

 

It sounds like you need more cooling airflow to the alternator(s) and to the B2B's. 

 

What voltages are you using?  The Sterling batteries can take a charging voltage of 14.2V-14.6V, 14.4V recommended so I'm hoping the B2B won't be working very hard.

[Dr Bob]

9 hours ago, TheBiscuits said:

Having just disturbed the moths in my wallet for lithium batteries, I have been idly wondering about @Dr Bob's "top up tank" theory that got shot down on the forum a while back.  It's probably a geek version of buyer's remorse!

 

The disparaged concept was simple enough: stick a medium sized LiFePO4 battery in parallel with your existing lead acids and let it keep the charge up at (not ideal) float voltage.

 

Pete, your problem is that you believe too much written on here. The 'Dr Bob' concept works fine – but I dont think I would do what you are asking. Of course you can use Lithiums as a top up tank but you have to watch what you are doing. I've been doing it for 3 years now ….or is it 4?

I know what I am doing. It has worked extremely well since I installed the Li's back when only Simon and Tom had them on board. I had them on my old boat, Kharis and then switched them over to my new boat 18 months ago. On Kharis I had circa 250 Ahrs of workable Li capacity paralleled to 6 cheapo LA ( 3 sealed – now 5 years old and 3 wet – now 6 years old). They were in this config for 2 years with us using the system year round. We sold Kharis in mid 2020, unbolted the Li's and BMS leaving the original set up in place and those original 6 LA's are still going strong. No apparent degradation due to undercharging. The current owners havent replaced any batteries yet.

I transferred the Li's to my new Aqualine and bolted them up in a similar manner but with 2 Sterling B to B's in circuit, paralleled to 4 tubular gel LA batteries. The boat is out and about all summer but has spent the last 2 winters on shore power. Again I see zero deterioration in the LA's.

There is no doubt the way I use them works. I have not compromised any warranties. I swopped them from boat to boat ...and I will take them to the next one if we change again. After 4 years I see no degradation at all in the Li's.

I operate the system between 13.25V and 12.80V when on discharge. I get circa 200 Ahr out of the Li's from 80% to 30%. Our current bote is very power hungry and typically I am using 170 Ahrs in 24 hrs. When out and about, we charge every day for 1-2 hours. We have a 240A alternator which typically puts out 110A at the start of charge decaying to 50A as we get to 80% full. Originally I took the decision not to put solar on the roof as the alternator was so big, but last summer I bit the bullet and put 500W of solar on so we dont need to run the engine as much.

The key to running the paralleled Li's is not to discharge the LA's much. They are there in reserve if you need them but normally I will be charging as the Li's reach a lower limit of 12.75V so the LA's never see more than 5% discharge. The charging voltage on the Li's up to 80% is typically circa up to 13.8V on the Li's and that is 13.9V on the LA's (Li's have longer wire runs). 13.9V is fine for the last 5% of LA charge BUT ONLY IF THE DISCHARGE IS LIGHT. If you discharge the system down to say 12.3V then the LA's will be down to 50-60% and the final 10% of recharge HAS to be at 14.2V+ otherwise they will sulphate. On the few occasions we have stayed put for days without running the engine, I have charged the Li's in a few hours then when isolated, the alternator is then pushing 14.4V into the LA's to do the final charge. Yes it takes a few hours but it stops the LA's from damage.

The important bit here then is to size your additional Li bank to around your daily usage so you normally keep the LA's pretty full. This way you get the benefit of always being able to get charge into the LAs – ie you don't waste valuable solar. The problem with your question is that a 20 or 40 Ahr Li means that you will always be digging into the LA reserve so you need 14V+ to finish off the LA charge. My experience would suggest having at least 100Ahr of usable Li capacity in a paralleled system.

In the case of a 20/40 Ahr Li system, both the Li and LA will charge if depleted so loading the alternator (!) and likely the Li will get full first (as it is only 20-40 Ahr), then when isolated the alternator will be putting out the 14V+ the LA needs. It therefore seems pointless to have Li's as the benefit is that when you finish your day engine running, you still have space in your Li's as they dont need to be full – so you can take full benefit of your solar.

I would take with a pinch of salt much of what is said on here about how to control power into Li's when charging. Every boat is different and control can be done in many different ways. On Kharis, we had a Sterling AtoB as our alternator voltage was too low (13.9V max). The AtoB controlled the alternator output and so limited the temperature. I later realised the 5 metre run of 50mm battery cable was helping curb too much power. All I had was a high voltage isolation switch to isolate the Li bank when I got to 13.8V. On the new Aqualine I bought 2 * 60A Sterling BtoBs as I did not want to mess with the warranty on the new boat (alternator, existing LA's and the electrics in general). To be honest, those BtoB's were a waste of time. The Li's are again 5 metres from the main 12v system and I started only with 50mm cable. The boat is wired with 2*90mm cable as the alternator is a 240A with a 5kW inverter. I have recently upgraded the Li cables to 1*90mm but never see more than 120 A going into the Li's – although when we've been parked for a couple of days without running the engine and the LA's are down at 12.6V, I see 120A on the Li's and 50A on the LA's when turning on the engine. I now dont bother with the BtoB's and just let the alternator do all the charging – starting at 110A and dropping to 40A ish as it reaches 80-90% when the auto isolate operates. Most of the time I actually isolate the Li's myself as I know when they are getting up to full charge. My alternator never gets over 70°C even in high ambients. Bottom line is that choice of cabling helps control alternator temperature.

So, the 'Dr Bob' paralleling method does work but you need a reasonable size Li battery – and I would say that is about the amount of power you use in a day. Any smaller and you will discharge your LAs too deep and require 14V+ to get them back – which sort of destroys the point of having Li's.

You say you have taken the plunge with Li's. How have you configured them?

[MtB]

6 minutes ago, nicknorman said:

Well I disagree for 2 reasons, one being that the BMS cutoffs are set to the limit of what is tolerable without rapid damage. I suggest that it is better to avoid routinely charging to the maximum voltage / SoC. And secondly a system where the battery is as a matter of course instantly disconnected when fully charged, is problematic unless you have a hybrid system with an LA battery as well (also known as a fudge system). The fact that little Willie thinks differently doesn’t impress me!

 

Basically you are right on your first point, but the Overkill is user-configurable so you can set all your parameters yourself to deal with this.

 

Secondly point yes, if the fact that a hybrid fudge system works brilliantly troubles you, after all your effort building a more komplicated system! 

[Dr Bob]

1 hour ago, Tony1 said:

 

I

In theory the lithiums should soak up almost all of the available charge initially, and when the lithiums are full the lead acids will start to soak up more of it.

 

 

 

 

That's not what I see. Typically my LA's are 95% full and the Li's 30% full when I turn on the engine. The LA's fill up as fast with or without the Li''s connected. The tail current drops at the same rate. Both sets of batteis suck in power at their 'independent' rates.

[MtB]

There is another major advantage to having a hybrid fudge system. When a pure Li system gets fully discharged you either have to run the engine or sit in the dark.

 

With LAs in parallel, they take over and will continue to supply power and discharge as low as you like, until tomorrow when you can start the engine/genny/sun/whatever. You are in control of the LA DoD, not the BMS.  

[nicknorman]

12 minutes ago, MtB said:

 

Basically you are right on your first point, but the Overkill is user-configurable so you can set all your parameters yourself to deal with this.

 

Secondly point yes, if the fact that a hybrid fudge system works brilliantly troubles you, after all your effort building a more komplicated system! 

It doesn’t trouble me. It is a fudge, albeit a fudge that works adequately. It is ideal for people who can’t make their minds up. Rosé wine was invented for the same sort of people.

 

As a boiler person you will know that such systems (and plenty of others) are designed around the concept of having a normal control system, and an emergency cutoff system that would never normally be expected to operate. It is standard engineering practice. So I don’t see why a boat Li electrical system should be any difference, and certainly not just because Will says so.

 

Also I rather prefer to have a mechanical cutoff device (eg the bistable relay) rather than a paralleled mosfet system, since the former is much more tolerant of short term over-current.

1 minute ago, MtB said:

There is another major advantage to having a hybrid fudge system. When a pure Li system gets fully discharged you either have to run the engine or sit in the dark.

 

With LAs in parallel, they take over and will continue to supply power and discharge as low as you like, until tomorrow when you can start the engine/genny/sun/whatever. You are in control of the LA DoD, not the BMS.  

Or you could just have an adequately sized Li system with adequate control and monitoring.

[Tony1]

9 minutes ago, TheBiscuits said:

 

Yes, that's correct.  I was also looking at the Renogy 20A/40A/60A B2B's and they rate them by output power rather than input power - their 60A version can pull up to 90A input.  They also have a very simple on / half power/ full power switching system that can easily be remotely switched.  A temperature sensor on the alternator and a position sensor on the throttle could trivially be wired into one of these.

 

It sounds like you need more cooling airflow to the alternator(s) and to the B2B's. 

 

What voltages are you using?  The Sterling batteries can take a charging voltage of 14.2V-14.6V, 14.4V recommended so I'm hoping the B2B won't be working very hard.

 

I did try some very basic cooling tests on the alternator in early Jan of 2021, with an ambient temp of about 2 degrees, on a breezy day and with the engine cover off and a fan blowing onto the alternator. But it kept going above 110 degrees if it was giving out anything over 50 amps. I think it was around this time I decided to go with a B2B, which was ok but I got fed up having to run the engine for 2 or 3 hours each day to recharge.  

 

I did some more testing in late August after the second B2B was fitted, and (possibly with the higher ambient temp), it seemed like the most the alternators could give out whilst staying under 100 degrees was 40 amps, and maybe 45 if it went up to 110+ degrees.  

 

I did fit a low power extraction fan to the cupboard with the B2Bs in it, and tried leaving the door wide open, but they do produce a fair bit of heat.

In the end I set them to half power as a default for cruising, and I added an extra smaller B2B, to be switched on only when static charging (and thus revving higher). I've ended up with a right old mish mash, but it does work at least.

 

I think one of the alternators runs at 13.9v and the other one at 13.5v. 

 

 

[Dr Bob]

1 hour ago, Tony1 said:

 

I didnt write that very clearly- what I meant was that the lead acids will have internal resistance to charge, which might mean that the surplus charge they reject will be passed on to the lithiums- which might already be full. 

To be fair this doesnt seem to be a real issue, but I do wonder. 

 

I dont see that as an issue. When my Li's are full, they isolate (or are isolated) so only the LA's take charge. Both sets will take charge when they want it. On the LA's the tail current will drop to very low and the Li as they get over 90% need to be isolated (or a BMS recognises it and limits charge). For our application it is simpler to just isolate. Worked great for me for 4 years.

[nicknorman]

26 minutes ago, Dr Bob said:

 

Pete, your problem is that you believe too much written on here. The 'Dr Bob' concept works fine – but I dont think I would do what you are asking. Of course you can use Lithiums as a top up tank but you have to watch what you are doing. I've been doing it for 3 years now ….or is it 4?

I know what I am doing. It has worked extremely well since I installed the Li's back when only Simon and Tom had them on board. I had them on my old boat, Kharis and then switched them over to my new boat 18 months ago. On Kharis I had circa 250 Ahrs of workable Li capacity paralleled to 6 cheapo LA ( 3 sealed – now 5 years old and 3 wet – now 6 years old). They were in this config for 2 years with us using the system year round. We sold Kharis in mid 2020, unbolted the Li's and BMS leaving the original set up in place and those original 6 LA's are still going strong. No apparent degradation due to undercharging. The current owners havent replaced any batteries yet.

I transferred the Li's to my new Aqualine and bolted them up in a similar manner but with 2 Sterling B to B's in circuit, paralleled to 4 tubular gel LA batteries. The boat is out and about all summer but has spent the last 2 winters on shore power. Again I see zero deterioration in the LA's.

There is no doubt the way I use them works. I have not compromised any warranties. I swopped them from boat to boat ...and I will take them to the next one if we change again. After 4 years I see no degradation at all in the Li's.

I operate the system between 13.25V and 12.80V when on discharge. I get circa 200 Ahr out of the Li's from 80% to 30%. Our current bote is very power hungry and typically I am using 170 Ahrs in 24 hrs. When out and about, we charge every day for 1-2 hours. We have a 240A alternator which typically puts out 110A at the start of charge decaying to 50A as we get to 80% full. Originally I took the decision not to put solar on the roof as the alternator was so big, but last summer I bit the bullet and put 500W of solar on so we dont need to run the engine as much.

The key to running the paralleled Li's is not to discharge the LA's much. They are there in reserve if you need them but normally I will be charging as the Li's reach a lower limit of 12.75V so the LA's never see more than 5% discharge. The charging voltage on the Li's up to 80% is typically circa up to 13.8V on the Li's and that is 13.9V on the LA's (Li's have longer wire runs). 13.9V is fine for the last 5% of LA charge BUT ONLY IF THE DISCHARGE IS LIGHT. If you discharge the system down to say 12.3V then the LA's will be down to 50-60% and the final 10% of recharge HAS to be at 14.2V+ otherwise they will sulphate. On the few occasions we have stayed put for days without running the engine, I have charged the Li's in a few hours then when isolated, the alternator is then pushing 14.4V into the LA's to do the final charge. Yes it takes a few hours but it stops the LA's from damage.

The important bit here then is to size your additional Li bank to around your daily usage so you normally keep the LA's pretty full. This way you get the benefit of always being able to get charge into the LAs – ie you don't waste valuable solar. The problem with your question is that a 20 or 40 Ahr Li means that you will always be digging into the LA reserve so you need 14V+ to finish off the LA charge. My experience would suggest having at least 100Ahr of usable Li capacity in a paralleled system.

In the case of a 20/40 Ahr Li system, both the Li and LA will charge if depleted so loading the alternator (!) and likely the Li will get full first (as it is only 20-40 Ahr), then when isolated the alternator will be putting out the 14V+ the LA needs. It therefore seems pointless to have Li's as the benefit is that when you finish your day engine running, you still have space in your Li's as they dont need to be full – so you can take full benefit of your solar.

I would take with a pinch of salt much of what is said on here about how to control power into Li's when charging. Every boat is different and control can be done in many different ways. On Kharis, we had a Sterling AtoB as our alternator voltage was too low (13.9V max). The AtoB controlled the alternator output and so limited the temperature. I later realised the 5 metre run of 50mm battery cable was helping curb too much power. All I had was a high voltage isolation switch to isolate the Li bank when I got to 13.8V. On the new Aqualine I bought 2 * 60A Sterling BtoBs as I did not want to mess with the warranty on the new boat (alternator, existing LA's and the electrics in general). To be honest, those BtoB's were a waste of time. The Li's are again 5 metres from the main 12v system and I started only with 50mm cable. The boat is wired with 2*90mm cable as the alternator is a 240A with a 5kW inverter. I have recently upgraded the Li cables to 1*90mm but never see more than 120 A going into the Li's – although when we've been parked for a couple of days without running the engine and the LA's are down at 12.6V, I see 120A on the Li's and 50A on the LA's when turning on the engine. I now dont bother with the BtoB's and just let the alternator do all the charging – starting at 110A and dropping to 40A ish as it reaches 80-90% when the auto isolate operates. Most of the time I actually isolate the Li's myself as I know when they are getting up to full charge. My alternator never gets over 70°C even in high ambients. Bottom line is that choice of cabling helps control alternator temperature.

So, the 'Dr Bob' paralleling method does work but you need a reasonable size Li battery – and I would say that is about the amount of power you use in a day. Any smaller and you will discharge your LAs too deep and require 14V+ to get them back – which sort of destroys the point of having Li's.

You say you have taken the plunge with Li's. How have you configured them?

Where have you been? Does your mother know you were out all night!?

 

Anyway, obviously your system works for you, but it is quite complicated from the user’s perspective, which is indicated by the length of your post describing it.

[Dr Bob]

1 hour ago, MtB said:

 

Yes this is the weak point I see in the whole Sterling offering. Well I see two weak points actually. 

 

The (say) 100A alternator needs a max continuous current of say 65% of rated output to stop it running too hot. This means you need a B2B that will draw no more than 65A. Given the B2B seems to be about 65% efficient too, this gives a charge current of 42A. And that wastes fuel and eats rather heavily into the primary reason for getting LFPs in the first place, for the rapid charging. 

 

 

 

BtoB's are very inefficient and turn so much power into heat. My 2 units only ever put out a max of 90A (each rated at 60 A input). I've given up with mine.

[MtB]

1 minute ago, Dr Bob said:

I dont see that as an issue. When my Li's are full, they isolate (or are isolated) so only the LA's take charge.

 

May I ask, does this happen automatically or do you, like me, have a switch and do it manually? 

 

If the former, is it a relay operated by say a BMV712, or a BMS board, or something else? 

 

Welcome back by the way....

 

[Dr Bob]

3 minutes ago, nicknorman said:

Where have you been? Does your mother know you were out all night!?

 

Anyway, obviously your system works for you, but it is quite complicated from the user’s perspective, which is indicated by the length of your post describing it.

Hi Nick! I spent the last 12 month writing that reply.

It is actually quite simple really.