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Old 15-04-2019, 08:11   #1
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Which reserve battery do you use with LiFePo4?

OK, I tried to search the LFP mother thread and could not seem to extract any ah-ha information in the 6K+ responses. But that could just be me (sigh)

Today, our usage is not the typical cruiser. We live 2.5 hours away and have our boat in a great marina local to all the of restaurants so it sits on shore power most of time. For us, it’s mostly about the activity of sailing and not the destination. We typically take day trips and so far have done a just a 3 day trip with a bit of anchoring. Then back to a marina… so in that environment I don’t worry about battery PSOC issues.

That said, I’m a geek. This boat is a great sailing boat, but just the ‘test boat’ for a future larger boat in an area with more consistent wind. The summer in Puget Sound is usually a bob-n-bake with marginal sailing, still enjoyable, but not exciting.

Anyways, I’m building a 200AH LiFePo4 bank from raw cells and I’ll stitch a BMS into it for a full system with LVC and HVC, etc, etc. Yes, my usage doesn’t need it, but it will be fun.

With the charge voltages differences between LiFePo4 and a reserve battery of FLA, Gel or AGM, I could design a charge system with a voltage differential to handle the different types.

However, before I design a wheel… what do the folks with boats using LFP banks use for a reserve battery? More LFP? AGM? Nothing?
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Old 15-04-2019, 08:19   #2
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Re: Which reserve battery do you use with LiFePo4?

Firefly Oasis.

Yes overkill if chronic PSOC is not unavoidable.

But then you're an overkill kind of guy. 8-)
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Old 15-04-2019, 08:58   #3
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Re: Which reserve battery do you use with LiFePo4?

Why do you need a reserve battery? Is the Li pack you are creating likely to fail. Will you be in some remote area where a dinghy or handheld radio won't solve tbe probkem?
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Old 15-04-2019, 09:02   #4
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Re: Which reserve battery do you use with LiFePo4?

Thanks! The charge voltages recommended for Firefly are 14.4V and 13.4V for float (but not needed). Optima has 13.8V min and 13.2V min for float, to that puts it closer to LFP voltages.

Also, just found an interesting article the folks at NREL did on an alternative charging algorithm that doubled the cycle life an Optima yellow top.

https://www.nrel.gov/transportation/.../evs17pres.pdf
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Old 15-04-2019, 09:06   #5
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Re: Which reserve battery do you use with LiFePo4?

Paul--

Yes, I was wondering if I really needed a reserve battery, heresy I know. Thanks for saying it first. ;^)

Also, this boat has an Atomic 4, which I like... and it comes with a hand crank. In my tests the fuel pump and ignition seem ok at 8V, but at 9V it's no problem at all. Just turn the crank for 5-10 seconds and Vroom.

*Much* easier than hand cranking a series Land Rover 4 cylinder. lol
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Old 15-04-2019, 09:36   #6
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Re: Which reserve battery do you use with LiFePo4?

LiFePo4 use battery management systems which will shut the battery system down if certain parameters relating to temperature, charge rate, voltage level, etc. are met. Not likely to happen on a well designed system yet still possible. As a full time cruiser I wanted a completely separate backup that could be used for navigation, autopilot, nav lights, etc. on my 12v system. With this in mind I installed a 50Ahr trolling battery with it's own solar panels and charge controller. This battery doubles as a power source for the electric hooka. I am planning on attaching a small inverter to this circuit and using it as a charging station for power tools, electric bicycles, spot lights, etc. In theory, if the main LiFePo bank gets shut down I can trim electrical demand to necessary only items and switch to this independent power source to get through until the main bank comes back on line.
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Old 15-04-2019, 09:53   #7
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Re: Which reserve battery do you use with LiFePo4?

I don't per se have a reserve battery to backup my DIY LFP house bank.

I otherwise have a diesel genny, alternator on the main, and (am now installing) solar as energy sources. The start battery is charged off the house bus (echo charger). I also have a genny that minds it's own (separate from the main engine) start battery. Smart monitoring/control all around.

If the house LFP bank should fail I will limp until I can replace it with whatever battery type is most convenient in that scenario. My inverter-charger, MPPT, and alternator regular each can charge whatever battery profile, such that replacing the LFP with any other battery type is do-able. I have effectively 3 separately wired battery banks that could, in a pinch, be tapped to run a radio, lights, pumps.
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Old 15-04-2019, 10:03   #8
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Re: Which reserve battery do you use with LiFePo4?

It depends on how much you want to spend. As you said, you are a hobbyist and not doing long distance cruising. I would (in order if expense).
A. Have no start/backup battery.
B. Use an agm or flooded with an echo charger.
C. Install battery of your choice with parallel charging system.

Regardless of choice, I would ensure your alternator is externally regulated with a bms controlled field shutoff.
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Old 15-04-2019, 11:11   #9
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Re: Which reserve battery do you use with LiFePo4?

Single 200 aH LiFePo4 bank, no reserve. 560 watts solar.

No problems in 8 years. Lots of equipment on boat.
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Old 15-04-2019, 12:01   #10
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Re: Which reserve battery do you use with LiFePo4?

Quote:
Originally Posted by ronstory View Post
The charge voltages recommended for Firefly are 14.4V and 13.4V for float (but not needed). Optima has 13.8V min and 13.2V min for float, to that puts it closer to LFP voltages.

Also, just found an interesting article the folks at NREL did on an alternative charging algorithm that doubled the cycle life an Optima yellow top.
IMO Optima no longer has anything to do with true deep cycling.

Firefly Float V is best kept to 13.2 but as you say can just stop.

You should front your LFP bank with a Sterling DCDC with user-custom settings, unless **all** your charge settings have that feature.

No lead bank will line up with LFP's needs precisely.

Maybe check out East Penn or Sonnenschein (Prevailers) datasheets?
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Old 15-04-2019, 13:14   #11
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Re: Which reserve battery do you use with LiFePo4?

I wouldn't care if the batteries had been hand crafted by the ghost of Thomas Edison. I'd still want a "Mayday" battery for the VHF (a handheld VHF will do that) and a reserve SLI battery for the engine, if I were going anyplace out of convenient sail/tow range. All the eggs in one basket...that's pushing things.

ron-
Bear in mind that lithium (of any type) is basically different from lead. No absorb-bulk-float nonsense, just "charge" and "don't charge". Instead of going to a float charge, you STOP all charging. The entire profile is totally different, regardless of how simple or complex you make the BMS. And of course, something must be done to prevent insulting the alternator (and blowing it out) if the lithium controller decides to cut off the lithium bank.
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Old 15-04-2019, 18:17   #12
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Re: Which reserve battery do you use with LiFePo4?

Quote:
Originally Posted by ronstory View Post

Today, our usage is not the typical cruiser. We live 2.5 hours away and have our boat in a great marina local to all the of restaurants so it sits on shore power most of time. [...]

Hello Ron,


that sounds like you want a system where you can access your boat from home at any time. Nowadays that's not that difficult, you might have WIFI in the marina or just need a 3G/4G/LTE router on the boat and some electronics on the boat that monitor what's going on.



E.g. I can see every voltage and current of every single battery on board, I can check the temperature, humidity and air pressure and also got a full history of graphs as what happened yesterday or during the last year, all from home.

I can individually disconnect every battery from the bus in order to test its resting voltage or to do a 20h-discharge test on the other batteries that are still online. That's using latching relays rated at 60A per set of four LFP cells.

With four LFPs I can draw 240A if needed, although the inverter maxes out at 2000W or less than 200A. Every LFP gets disconnected at HVC or LVC independantly of the others and I'd get a warning buzzer long before that happens.

Only if there is a solid short on the bus would all relays open, unless the breakers that are in series with them trip first. Why both? It's the regulation...





Quote:
Then back to a marina… so in that environment I don’t worry about battery PSOC issues.

In this scenario please keep in mind that most people don't want their LFPs sitting at 100% all the time as that supposedly reduces their life span. Best to stay between 10-90% for frequent use, in your scenario I would even keep the LFPs at 50% SOC, which sometimes gets mentioned by manufacturers as the long term storage level.
This can be achieved if you monitor the current in and out of your batts. If a 60Ah batt is "full" (at say 14.0V) you need a system that disconnects it after 30Ah have been taken out.





Quote:
Anyways, I’m building a 200AH LiFePo4 bank from raw cells and I’ll stitch a BMS into it for a full system with LVC and HVC, etc, etc. Yes, my usage doesn’t need it, but it will be fun.

Yes, it'll be a very good exercise to get to know the LFP characteristics.



Quote:
With the charge voltages differences between LiFePo4 and a reserve battery of FLA, Gel or AGM, I could design a charge system with a voltage differential to handle the different types.

However, before I design a wheel… what do the folks with boats using LFP banks use for a reserve battery? More LFP? AGM? Nothing?

As mentioned by others above, you don't really need a reserve batt. If you're monitoring the Ah you'll always know how much juice is left. By "you" I don't mean you personally sitting in front of your batt monitor for days, but an automated system doing that for you.
If you've heard of Peukert's law, it doesn't come into play with LFPs so that makes your life easier, too.


However, if you want to tinker and keep your expenses at a minimum, one could imagine replacing some of the oldest or weakest LA's with a small LFP, say 80 Ah or two 40 Ah LFPs, and keep the other LA's as your "reserve".



Now some people will immediately fly off the handle reading this, because they don't look at the whole picture: you can keep whatever shore or solar charge controller you have at whatever profile it's set to that keeps the LAs happy.

Then you only need to switch from LAs to LFPs until the LFPs have been charged to the level or voltage you want, then switch on the LAs and turn off the LFPs and Bob's your uncle.


The charge controller would then keep the LAs at their absorption level and then goes into float. The self discharge of LFP is negligible, hence if you plan a longer sailing trip, just turn on the LFPs again from home the day before you go to the boat.


Thinking of sailing trips, I can even turn on my boat fridge from home the day before I want to go out.





Full disclaimer: I'm one of the developers of a WMMS, a wireless monitoring and management system.
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Old 15-04-2019, 20:46   #13
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Re: Which reserve battery do you use with LiFePo4?

jdaltonpe--

I appreciate the advice. I'm not sure what I'm going to do yet... but it helps.
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Old 15-04-2019, 20:51   #14
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Re: Which reserve battery do you use with LiFePo4?

Singularity--

Good data point. After reading Mainesail's starting a diesel motor with a LFP back at the zero percent (9.5V) and having no problem turning over the motor a dozen times, I'm sure I could get it started with the hand crank. After that, it all alternator to help recover.
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Old 15-04-2019, 20:54   #15
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Re: Which reserve battery do you use with LiFePo4?

BradfordH--

Yes, I have a Balmar MC-614 ready to install, and new shiny 6-series alternator. So covered on that front.
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