Hybrid Battery Bank Idea

[Dockhead]

Originally Posted by john61ct

I did not mean to imply I was recommending one way over the other.

Just trying to isolate the critical concept here rather than making it dependant on one particular implementation of the other variables.

The smaller the LFP bank is relative to the lead, the more important it is to get the lead up to a relatively high SoC. If the LFP gets discharged to the point its Voltage drops then things get more complicated, especially for those running bare cells + OTS protection gear.

If LFP is ever allowed to truly flatten, I believe it is irrevocably damaged.

It's gratifying that others have seen what I saw in this idea!

Yes -- the idea is that when you've discharged your batteries to 50% or so at anchor or on a long passage, and you've got your generator running or you had to start up your main engine and big alternator, you let a small auxiliary LiFePo battery start to take charge when the charge acceptance of the lead acid batteries tapers off. Shut down the engine or generator when you get to 80% or 90% and then discharge the LiFePo battery into the lead acid bank through the battery to battery charger to give the lead acid bank its finish charge which it needs so much, but which you can't provide without putting a lot of extra hours on the generator, and maybe even harming it if you don't have other loads.

It violates KISS and I don't need it on my boat as I have other ways to get the periodic finishing charge, but could be useful for someone for whom this is a chronic problem. And the complexity is not all THAT huge.

[john61ct]

Of course the devil's in the details.

B2B idea - I think many require a higher voltage on the source circuit than LFP resting.

Direct connect - no control, possibly dangerous ampacity?

The Sterling current-limiting VSR may be my first choice.

[Rapanui]

My first thought was that it was a complex solution, but like all things, when you think further you can find real advantages.
One aspect often aired on here, is that people (including me) have no further adjacent space for extra house Ah, this approach would allow extra Ah remote from house bank with relatively small wire sizes. Going one step further, maybe you could replace the windlass battery with LiFePo and combine the functions that way?
If you used a 24V LiFePo, would a MPPT controller to charge the lead acids work? I only hesitate, as I have no idea how the max power point tracking would work or not, but if it works, it's an off the shelf solution.

[barnakiel]

I pondered a similar idea of a 20%80% hybrid battery.

I rejected it as it seems to bring little if any benefit at quite some extra cost in our specific case.

I think it is perfectly doable just not very reasonable (except some very specific use / case). Extra cost of different chargers / wiring / switches and monitoring is quite handsome and system complexity goes up too.

So I think it is 100% doable just not a good solution for our own boat.

Cheers,
b.

[john61ct]

Originally Posted by Rapanui

If you used a 24V LiFePo, would a MPPT controller to charge the lead acids work?

See Maine Sail's comment end of the six-month old thread I linked to.

Remember the biggest issue is that a bank may be pulling large amperages. A panel can only supply so much throughput, but a bank as source can easily overwhelm most regulating circuits.

As in releasing the magic smoke.

[Rapanui]

Yes, that is why I hesitated about using a MPPT controller. However, our controller limits output current irrespective of solar power available. My main concern would be if the max power tracking algorithm would function correctly with what is an almost infinite power source.
The hybrid concept is a neat idea which could be worth while in niche circumstances, eg where you have a separate bow thruster windlass battery. However, you could do this other ways with less installation work, e.g use a fuel cell to charge overnight or when solar is low thus effectively increasing battery capacity. Expensive, but not as much as a generator which is the other option when you cannot fit enough solar.

[john61ct]

To me genny or alt is never suitable for the long tail.

This solution IMO is only for when not enough solar.

Are you saying a fuel cell can be cheaper than a little genny??

[Dockhead]

Originally Posted by john61ct

To me genny or alt is never suitable for the long tail.

This solution IMO is only for when not enough solar.
. . .

Yes. Solar and lead-acid go together like cookies and cream

We had solar on the last boat, and we never ever thought about the health of the batteries. I think we got 10 years out of the last set . . .

But that was in Florida. Up here above 60N, we have a lot of wind, and every square inch of windage matters. So no permanent solar panels for us -- like most boats up here.

Alternator for the "long tail" is fine on a long motor -- when you're using the main engine for other purposes anyway.

Between that and shore power, I'm well fixed at the moment.

But when my boat was on a mooring most of the year, I really pulled my hair out over the "long tail", and my batteries didn't last very long (3 or 4 years max). I actually bought a 1kW Honda generator specially for the "long tail", because I didn't want to kill the heavy duty diesel generator with long light loads, but that was also too much complexity and a pain to store and keep the carburetor ungummed up, so I sold it.

[hzcruiser]

Originally Posted by DeepFrz

Lead acid has a higher charge voltage than the LFP batteries.

Even though the recommended charging voltage for LAs is about 13.6V (according to Charging Information For Lead Acid Batteries – Battery University) you will still charge your LA as long as the LFP voltage is higher than the resting voltage of your LAs, albeit slower.

The charging speed doesn't matter here as you only want to top up the LAs anyway and the current will be fairly low.

Hence you don't need any fancy DC-DC charger at all.

The LFP charging voltage is about 14V (according to https://www.powerstream.com/LLLF.htm) whereas their "resting voltage" is about 13.2-13.6V and will easily "flow" towards the LAs sitting at 12.7-13.0 V depending on their SOC. It worked a treat on my boat. About 3-4A current from the 180 Ah LFP to the 300Ah LA banks. ( I have four physical house banks on the boat plus a starter bank ).

The important bit is to disconnect one or the other type of battery (based on your discharge preferences) once the LAs are topped up. Otherwise you'll turn the extra voltage into heat at the LA batts and slowly drain your LFPs.

That's why I'm suggesting a small relay box at every battery or group of 4 LFPs for a nominal 12V system, centrally controlled wirelessly. No extra cables to run, just plug the box in series with your battery, the rest is done automatically or via a web browser.

[john61ct]

Originally Posted by hzcruiser

you don't need any fancy DC-DC charger at all.

The LFP charging voltage is about 14V (according to https://www.powerstream.com/LLLF.htm) whereas their "resting voltage" is about 13.2-13.6V and will easily "flow" towards the LAs sitting at 12.7-13.0 V depending on their SOC. It worked a treat on my boat. About 3-4A current from the 180 Ah LFP to the 300Ah LA banks.

Did you ever measure the current when the LFP was full and LA well depleted?

[seandepagnier]

This is similar to an idea of mine. You can mix flooded with agm as well.

Basically you build a charge controller that can handle many banks, maybe even 20 or more. Each bank can be any type of battery, or even just a single battery cell. Each bank has mosfets to be able to switch it into series/parallel with other banks.

The controller measures current in and out of each cell as well as voltage. It can know the efficiency curve of each cell and how it degrades over time. Lead acid is typically inefficient to charge when it is nearly full.

You could configure it in various modes, but normally it would try to cycle the lithium cells much more, and keep the lead acid at a higher state of charge.

If a bank fails or becomes inefficient it stops using that cell and you can replace just that cell.

This allows you to add and remove various batteries, without compromising the efficiency or life of the system.


Also, ultracapacitors are even more efficient than lithium, and can last longer than most of us will live. They are non-toxic and can handle very high currents. Disadvantage is cost for the size/capacity. They could be easilly dropped into the system to improve efficiency, especially from wind generators in gusting winds.

[john61ct]

Peukert means the lowest number of banks will be most efficient, greatest total capacity.

Also longest longevity.

Ideal IMO is one big Main bank plus a smaller Reserve for mission-critical backup.

Cranking starters easily switched between the two.

Which is LFP vs lead for purpose of this topic is as per owner's budget and preference.

[valhalla360]

A lot depends on specific boat configuration and use pattern.

If you motor a lot, are hooked up to shore power or have a big solar/wind array, it's really not gaining you anything. It would only be if your system struggles to keep the bank charged and you use your motor purely for charging, that there might be a benefit.

Assuming, you still get them fully charged periodically (so they have a decent lifespan) and you have sufficient space, a few extra lead acid batteries give you the same effective boost without the complications. Let's say you want 500ah for daily consumption:
- 1000ah bank (used down to 50%) you get maybe 400ah quickly going from 50% to 90% and then it slows down.
- Bump up to a 1200ah bank and you get 480ah quickly and only spend 20% as much time filling the last 20ah you need which makes a small solar/wind system much more viable. Yeah you may only get to 92-98% most days depending on solar output but if you hit fully charged once every week or so, shouldn't be a big issue on battery life.

How does the cost of an extra 200ah of batteries compare to the lithium batteries plus charging system that handles both types and routes the bulk charging as needed automatically. I suspect the extra LA batteries will be both cheaper and less prone to problems.

If lithium prices come down like some claim (I'm a bit skeptical but hey...) this is at best an interim solution. Once lithium are cost competitive, there is pretty much nothing to gain from lead acid and a change to full lithium will make more sense.

[hzcruiser]

Originally Posted by john61ct

Did you ever measure the current when the LFP was full and LA well depleted?

The 3-4 A I measured when the LAs were really low. However, the suggestion was to use the LFPs just for the last 10-20% of the LAs SOC. What I did was just a quick test a few weeks ago. The wireless module is not ready for beta testing just yet.

The beta test was envisioned last year and I had some positive feedback since then. Hopefully I can send out some "starter packs" to the testers within the next three months.

[hellosailor]

And by the time you've engineered in a way to prevent loads from eating at the Li battery while it is connected to charge the Pb bank...you'd have totally screwed the KISS concept to hell and back again.

Really, why not just save old pennies and citrus fruit, and use your garbage to make a battery capable of recharging the Pb bank?