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Old 26-07-2017, 09:31   #1
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Hybrid Battery Bank Idea

This is really just a Gedankenexperiment,

But what if you supplemented a lead-acid battery bank with a LiFePo battery of 10% or 20% of the capacity of the lead-acid bank?

You would need some kind of controller which would start charging the LiFePo battery when the charge current to the lead-acid batteries tapers off or whenever there is excess charging capacity in the system.

THEN, when charging stops, the LiFePo battery can be used to finish charge the lead acid bank, taking it from 90% to 100%.

Would greatly increase the efficiency of charging the lead acid bank when off shore power. Would use the high acceptance rate of LiFePo to efficiently soak up available charging capacity when the lead acid bank is not accepting much power.

It's not for me, because I gave up my mooring with no shore power, so now I have shore power whenever I'm not out cruising, and when I am out cruising, I move often enough that I get a full charge often enough from motoring or being in a marina. So for me, it's just an idle thought, but for someone?

It would add enough complexity to question why you would not just go "the whole hog" and change over entirely to LiFePo. But maybe for someone, something like this could be worthwhile.
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Old 26-07-2017, 10:06   #2
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Re: Hybrid Battery Bank Idea

I mentioned this in another thread but of the opposite design. Get a lithium battery that has 20-30% of the capacity of the lead-acid bank and charge that from alternative energy sources. Charge the LA batts from the Lithium.

It would allow you to capture the wind and sun much more efficiently, especially when the LA gets charged over 80% or so. When the sun went down it would continue to trickle charge the lead-acids over the next however many hours.

I think this may significantly extend the life of the LA batteries.
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Old 26-07-2017, 10:13   #3
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Re: Hybrid Battery Bank Idea

Lead acid has a higher charge voltage than the LFP batteries.
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Old 26-07-2017, 10:29   #4
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Re: Hybrid Battery Bank Idea

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Lead acid has a higher charge voltage than the LFP batteries.
Wouldn't that depend on the battery? The charge voltage for trojans is 14.8 volts.
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Old 26-07-2017, 11:15   #5
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Re: Hybrid Battery Bank Idea

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Originally Posted by DeepFrz View Post
Lead acid has a higher charge voltage than the LFP batteries.
Doesn't matter at all. You use a battery powered multistage charger. You don't even need to use the same nominal voltage.
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Old 26-07-2017, 11:51   #6
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Re: Hybrid Battery Bank Idea

Wouldn't you be better off purchasing a second lithium ion battery and getting rid of the lead acid batteries altogether? I went the route I did with lead acid and solar, because it was $10,000 cheaper even with the additional $4000 being spent on solar. Otherwise, I would have preferred 100% lithium.
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Old 26-07-2017, 12:28   #7
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Re: Hybrid Battery Bank Idea

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Wouldn't you be better off purchasing a second lithium ion battery and getting rid of the lead acid batteries altogether? I went the route I did with lead acid and solar, because it was $10,000 cheaper even with the additional $4000 being spent on solar. Otherwise, I would have preferred 100% lithium.
Yes, as I mentioned -- it's not for me. Lead acid is working well in the undemanding service on my boat.

Might have considered it when I still has my mooring.
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Old 26-07-2017, 13:22   #8
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Re: Hybrid Battery Bank Idea

YES, finally another out of the box maniac, I will be testing this idea IRL one day.

Note a big LFP bank used for the "long tail" on a small lead bank (reserve/starter) should have the same issues.

I was originally looking at DCDC chargers capable of being switched on manually. As long as the resting voltage of the LFP bank is high enough, should be able to get a decent Absorb cycle in to get the lead bank to 100% full.

But I believe a direct connection would also do the same, just there you need to consider really robust wiring, safety issues, monitoring ampacity, heat build-up etc.

The ​Sterling "Pro-Con" (Pro Connect) CVSR has current-limiting features that may help.
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Old 26-07-2017, 13:24   #9
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Re: Hybrid Battery Bank Idea

IF you really wanted to do this the advantage of lifepo is that is has a huge acceptance rate.

To make the best use of this it means keeping the FLA's as charged as possible and drawing primarily from the lifepo until they are dead. No matter what the usage is. So you would charge the FLA's from them, run the deck lights, etc... Then on the charging side first charge the FLA's then the lifepos with any remaining charge.

Ideally this would allow you to run a generator or engine the least amount possible to keep the battery banks as close to full as possible. In reality the amount of overhead would probably drive the cruiser nuts, and the cost would be crazy.
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Old 26-07-2017, 13:43   #10
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Re: Hybrid Battery Bank Idea

This idea would not be too hard to implement. But it complicates things if you want to use the LiFePo bank for anything other than charging the LA bank. The concept could be simple. You create a small LiFePo battery bank and charge it from multiple sources (solar, alternator, AC mains) and then you have a separate isolated DC-DC charger that uses the LiFePo battery as the source to charge the LA bank. Then you run the boat off the LA bank.

For example, if the Li bank were 25% of the AH of the LA bank then you could stop charging from mains AC when the LA bank reached 80%. The rest of the top up could come from Li and if there was renewable energy available the Li would not discharge as much so the 25% sizing might end up needing to be only 10-15%. But the LA would get 100% charge every time.

This would work well in regions with poor sunlight (high latitudes) as well as places with lots of solar. Poor sunlight could be supplemented with short duration generator runs that would quickly bring the Li bank to 100% once LA bank reaches 80%. The fuel powered charging would run at maximum power and thus efficiency without the long taper that LA forces on us now.
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Old 26-07-2017, 15:46   #11
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Re: Hybrid Battery Bank Idea

Actually, there is no inherent need to juggle using one bank type over the other, all depends on relative size and your design intentions.

Assume a short-term dino-juice (or pit-stop shore power) charge source, solar being insufficient at least some of the time, or there's no reason for the concept.

Ideally the lead bank is also high CAR, not as high as the LFP but...

So the genny or alts run, charging at max amps for a limited period of time.

The LFP bank sucks it all up fast as possible, and doesn't care whether it gets to "full" or not.

The lead bank gets at least up to absorbV, and ideally well into where amps accepted are really declining, when it's hardly worth burning more fuel, so then shut the charge source down.

Over the next 4-6 hours, continue to top up the lead bank from the LFP one, until it is 100% full.

Ideal IMO would be a nice Sterling B2B charger, and isolating loads from the lead bank so it can intelligently track the declining amps acceptance.

Yes there is some inefficiency here in charging batt to batt, but for me that's OK.

Note if solar **is** in the picture, the fuel run is done in the AM, solar is used to get the lead to as full as possible, and the LFP-sourced charging only takes place finally after we know the solar was insufficient.
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Old 26-07-2017, 15:55   #12
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Re: Hybrid Battery Bank Idea

Here is a thread from six months ago (feels like a lifetime!) where I noobishly proposed something similar, including (mostly negative) feedback from some much more expert than me.

http://www.sailnet.com/forums/showthread.php?t=274969

Which of course is to be expected so radically out of the box and departing from KISS principles, from those rightfully concerned about idiots playing with fire aboard boats at sea!
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Old 26-07-2017, 16:03   #13
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Re: Hybrid Battery Bank Idea

John,

Lead acid has many advantages. It's main disadvantage on a cruising boat is shortened life due to chronic undercharging. Weight and acceptance are secondary issues for a heavy cruising boat. Racing and light boats may see it different.

But it's cost prohibitive for most cruisers to build big Li banks. A small Li bank that fixes the problem of cycle life of the LA bank by getting it full without wasting fuel seems like a good idea. And it is the least disruptive to the whole boat. Not to mention if either bank dies the other can be a backup.
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Old 26-07-2017, 16:03   #14
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Re: Hybrid Battery Bank Idea

Dockhead,

Great idea. Creative use of hybrid systems can really improve efficiency at moderate cost. Solar-powered charging to get to 100% is great when available, but your idea seems like it could work well for the many situations in which solar isn't available. I like it.
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Old 26-07-2017, 17:24   #15
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Re: Hybrid Battery Bank Idea

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it's cost prohibitive for most cruisers to build big Li banks.
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.
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