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Old 25-08-2018, 13:24   #16
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Re: LiFePO4 and FLA: a match made in heaven?

Think I'll just stick with my golf cart batteries as they do everything I need and last 6-8yrs then I replace them.
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Old 25-08-2018, 15:05   #17
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by a64pilot View Post
This is a screenshot of lifelines Manual, note where it says that charging current should be set as high as is practical, that they can accept up to 5C and that .2 c is a minimumAttachment 176087
Ok, I see your point.
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Old 25-08-2018, 15:22   #18
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by transmitterdan View Post
Ok, Iím calling BS on this one. No LA battery vendor will condone putting 13.2V on a dead battery and letting the current go as high as it wants. They specifically limit charging current to some fraction of the 20 hour AH rating. High acceptance batteries like AGM typically specify current no higher than 0.25C.
What planet are you from? Every top quality AGM vendor specs a **minimum** charge rate of .2C or even .4C.

See Maine Sail's accounts of using .8C and more no problems.

And the topic here is FLA anyway, where acceptance is even lower.

The max rates battery vendors spec are just CYA wrt liability.

Any decent charge source has temperature sensing at the battery, and will derate if temps rise.

No one said anything about a dead battery anyway, the OP topic is trickling from LFP to cover "the long tail" last 4-6 hours,

which by definition is low amps no matter how much current is **made available** by a charge source, while the actual flow is determined by the resistance of the target bank. Since that is lead, resistance climbs with SoC, trailing amps fall.

This is basic 101 stuff here.

I stand by what I wrote, very easy to verify IRL.
Quote:
Originally Posted by john61ct View Post
You can put a 500A charge source on a 100AH FLA batt, no problem as long as V is regulated.
Ö
As long as the infrastructure is designed and implemented properly.
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Old 25-08-2018, 18:02   #19
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by transmitterdan View Post
Just because you didn't explode your LA battery doesn't mean it can't happen. Lithium batteries don't have any current limit so big ones can deliver more current than a LA battery is rated to take. This can destroy the LA battery and cause an explosion. It won't happen every time as it depends on how discharged the LA battery is as well as the internal resistance of both batteries.

Your theory about Lithium topping off LA is missing the point that to top off an LA you need a voltage higher than is safe for Lithium. And you cannot safely bulk charge Lithium at 14.3V. Leaving Lithium in parallel with LA will ensure the early death of the LA due to chronic under charging. Charging the LA to 100% will destroy the Lithium if they remain in parallel.

Thanks for your reply, Dan. This started as a thought experiment a few days ago and since there are knowledgable people here I thought to run it past you and even do a simple test. On my boat I actually have an AGM batt running in parallel with LFPs for about 6 or 8 weeks now but the AGM batt is pretty shot and is not accepting much current no matter how high the charge voltage. So that's not a good test as only 100-200mA are going into the AGM (and might never come out again).



Can we agree that every battery has a current limit, even LFPs? As mentioned though, their internal resistance is lower than LA.

You're saying "big lithiums can deliver more current than a LA is rated to take". In theory, yes. In reality, the voltage difference between them is not that high: If the charger puts out a more or less constant current and stops at some threshold voltage, the LFPs will continue to charge the LAs. We "just" need to make sure that no batt ever gets too hot, hence good chargers have temp sensors.


Quote:
Your theory about Lithium topping off LA is missing the point that to top off an LA you need a voltage higher than is safe for Lithium.
Maybe I'm wrong: my assumption here is that even at 14.3V or 14.0V or 13.6V you get a current from the LFP to the LA, which means you are charging the LA, correct?

The question then is, do we ever get the LAs to 100% SOC if we limit the charger voltage to 14.3 - 14.5V (and then drop off almost immediately since we don't want to overcharge the LFPs)?
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Old 25-08-2018, 18:19   #20
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Re: LiFePO4 and FLA: a match made in heaven?

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Originally Posted by hzcruiser View Post
Maybe I'm wrong: my assumption here is that even at 14.3V or 14.0V or 13.6V you get a current from the LFP to the LA, which means you are charging the LA, correct?
The only time you get over 13.2V is when a primary charge source is active.

The resting LFP voltage is never high enough to get the lead to 100% Full.

Without a Sterling BB charger to boost voltage.


> The question then is, do we ever get the LAs to 100% SOC if we limit the charger voltage to 14.3 - 14.5V (and then drop off almost immediately since we don't want to overcharge the LFPs

Anything over 13.8V (3.45Vpc) is overcharging. Very easy to just isolate the LFP from the charge buss when the circuit hits that.

The target Absorb voltage can be set to 14.6V if that is what the lead mfg specs, NP.

Then continue to pump high amps into the lead bank as long as possible, that is still many hours from Full.
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Old 25-08-2018, 18:22   #21
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Re: LiFePO4 and FLA: a match made in heaven?

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Originally Posted by LeaseOnLife View Post
I think this could work with a few add-ons:

Add a relay between the batteries and disconnect LiFePo4 when full, keep charging the LA with the rising voltage, to 14.4 or whatever needed for the LA type.

Then when discharging, LA will quickly drop below resting Voltage for LiFePo4, connect the batteries via the relay. Most discharge will come from LiFePo4. Disconnect the LifePo4 when below ~10 or 20%, rest of discharge to be done by LA.

When charging comes on, LA Voltage will come up to LiFePo4 voltage, connect batteries again via relay.

That's pretty much what I had in mind.



Quote:
To control the in-rush and max current from one battery to the other, I would use a length of wire coiled up. The coil will reduce the in-rush spike, the length of wire will provide some resistance and limit the max current.
From my experience (so far) that won't be necessary as you have enough resistance in the batts, wiring, breakers or fuses and connectors to limit the current.



Quote:
But for what purpose? Most / all of the work will be done by the LiFePo4, if it is small, it will go between, lets say, 10-90% every night, which does cost cycle-lifetime..
Now that is the real question! Why would someone in is right mind add latching relays to every batt on the boat? I'm glad you asked!

The point I have in mind is to lower or dampen the costs of migrating from LA to LFP. A lot of people are afraid these evil "Lithium batts" either go up in flames at the most inopportune time (e.g. in mid flight on a Boeing Dreamliner) or they get damaged beyond repair by once letting them go outside their safe specs. So they understandably don't want to risk a few boat units by going all LFP and maybe end up with a non-working chunk of dead batts.
However, if we could identify which of the existing LA batts are the weakest ones today, we can replace just those with a smaller number of LFPs and "get a feel for it" during these early days of the LFP technology.

That's why it's handy to have a latching relay on every batt so we can disconnect it from the bus and let it sit for a few hours or days to check the self-discharge rate.

And yes, the LFPs would do the bulk of the work with the LAs providing that reserve for extended periods of no charge source rather than "designing" an LFP bank that cannot sustain a few days of no charge.
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Old 25-08-2018, 18:28   #22
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Re: LiFePO4 and FLA: a match made in heaven?

If the goal is to delay scrapping parts of a FLA bank, that is not worth it.

The whole bank should be replaced at once.

Perhaps with a smaller one, if that becomes your Reserve / Starter, and the LFP bank your Main / House.
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Old 25-08-2018, 18:57   #23
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Re: LiFePO4 and FLA: a match made in heaven?

Quote:
Originally Posted by john61ct View Post
...


The whole bank should be replaced at once.

...

That is what they say. I never did. I have three banks of Lifeline AGMs, the newest 8D I had to buy a few months ago in St Thomas for $1000. The oldest (4D) in service is from 9 years ago. Basics capacity tests show that old battery at >=80%. All battery failures on our boat were cell shortages, every few years we have to take one battery out of commission and get a new one.


Similar to what Heinz says about mixing LiFePo4 with LA, the newest battery will do a little bit more work. But I only use ~10A continuously, maybe 15A when when the autopilot is working, plus radar etc. With this low load, the increased higher internal resistance of older batteries compared to a new battery will have little effect on which battery is loaded more. So I call BS on the industry wisdom of changing the whole bank.


Therefor I do like Heinz's approach of being able to gradually transition to LiFePo4.
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Old 25-08-2018, 20:33   #24
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Re: LiFePO4 and FLA: a match made in heaven?

IMHO, the combination of LA and Lithium is absolutely perfect. I have been running that way for some time. I have 700AH Lithium and 120 AH LA set up in two separate banks. I have a battery switch to switch between the banks for discharging. Both banks are set up to charge from the available charge sources (200A alternator and AC battery charger).

The lithium bank has high voltage and low voltage cutouts (adjustable).
When the boat is at the dock, I generally turn the lithium battery system off so that it doesn't keep cycling or charging to maximum. At that point the LA is connected to a normal AC charger as it was when the boat was new. If I go on a trip that requires more capacity, I enable the lithium bank and generally disable the LA bank which then becomes my emergency backup.
The nice thing here is that the LA is ALWAYS connected to the alternator so I do not need to worry about the alternator frying when the lithium bank reaches full charge and disconnects from the system.

Both banks have their own battery monitors.
In theory, I could set the alternator output to a normal LA charge profile if I wanted to. Since the lithium bank has a separate cut off (driven by the battery monitor which is in addition to the BMS cutoff safety system). It will shut down the charge path as soon as the maximum voltage is reached. At that point the alternator is still connected to the LA battery bank and will charge that as per normal LA charge profile.
Any sort of battery exploding incident would have to be caused by a shorted cell or similar in the LA bank which would cause a mess but nothing more and just the 200A alternator can cause this without the lithium enabled.
I suppose that the various safety systems on the lithium bank could all fail but I could also be hit by a meteor as I am minding my own business - it's a calculated (and dismissed) risk.
If both banks are enabled and thus connected in parallel, the LA bank will not ask for excessive current from the lithium bank unless it is damaged/dead to begin with. There simply is not enough voltage difference, even with the LA discharged, to do damage anywhere (assuming suitable cabling).
Both banks are fused to limit the maximum current to what the cabling can handle.
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Old 25-08-2018, 22:43   #25
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Re: LiFePO4 and FLA: a match made in heaven?

Hello John,


Quote:
Originally Posted by john61ct View Post
This idea has been discussed many times already.
It would be possible to set up a situation where the wires get too hot.

An "empty" batt is already damaged.
I didn't mean a damaged battery, but simply a discharged batt. That happens in the best of families.

Quote:
The relative sizes of the two banks changes things.
In which way? I'm not suggesting to add 40Ah of LFP to a 1000 Ah LA bank. But reducing the LA bank by the weakest 20% and adding a similar amount of LFP instead. Much smaller initial outlay and if those bloody LFPs really go belly up after a month or two not much is lost.



Quote:
The key problem is when the owner very wisely decides their LFP should never in daily cycling see voltages above 13.8V (3.45Vpc).

Also, the LFP full resting voltage is ~13.2V, which is not enough to get lead to 100% Full.

DCDC chargers like Sterling's BB series are IMO the best solution to both those problems.
Good point there! So it seems that we need to add a DC booster to the LAs. It doesn't have to be a big one, as it only needs to cover the last 10-20% of LA charging and we don't need to finish the charge cycle within a few hours. In the example above you might still have 3 * 200 Ah LA at maybe 80% SOC hence you need to put the remaining 40 Ah (plus losses) in to get to 100%. A small 2A DC booster per batt can do that in a good 20 h (from the LFP batts or from the charge source, whatever puts the higher voltage on the bus).


Quote:
> The only thing we need to make sure is removing the LFPs from the bus when they're empty

Well truly empty turns LFP into scrap.

I define my 0% SoC as 11.9V (2.975Vpc) in a low-amp discharge context.
Of course I didn't mean empty to the point of damaging them. Please give me a little bit of credit here. Empty as in "we have drawn as much energy out of them as we deem safe". My apologies for curtailing that mouthful simply as "empty". Whether that's 10% SOC or 20% or more is up to the user. My default would be 10% SOC.

Quote:
Note that there is a lot of inefficiency in transferring energy from one bank to another, so where it is not cheap and plentiful, this idea should only be used when actually needed.

Getting lead to 100% Full 3-4 times per week is enough
Fully agreed. There are conversion losses everywhere.

Quote:
So ideally this idea is only needed occasionally, in worst-case conditions.

But IMO the concept is feasible.

But certainly not KISS.
Thanks and yes, it's not KISS, it's an option for the cruiser who wants to maximise the efficiency or bang for his buck. We won't make our batts last longer without having additional or better tools available. As many people here are lamenting: throwing out and replacing the whole bank is "standard" nowadays. And if batts were cheap enough we would continue doing so.

But at least here in Oz all types of batts are surprisingly expensive and I don't want to treat mine as pure consumables, notwithstanding environmental aspects.
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Old 25-08-2018, 22:47   #26
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Re: LiFePO4 and FLA: a match made in heaven?

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Originally Posted by john61ct View Post

And there is zero risk of "explosion", pure uninformed fear-mongering that.

Exactly right, John! And this constant fear-mongering is what really annoys me the most. I still remember my first acid burns from not handling LA batts properly many moons ago. I was lucky that it only ruined all the clothes I was wearing at the time, no scars on my hands nor arms.
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Old 25-08-2018, 22:59   #27
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Re: LiFePO4 and FLA: a match made in heaven?

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Originally Posted by boatpoker View Post
Think I'll just stick with my golf cart batteries as they do everything I need and last 6-8yrs then I replace them.

Yes, if you're happy with the batts you have and the price you paid for them and the time they last then everything is hunky dory and reading this post or thread is just a waste of your time.

Fully agreed!
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Old 13-10-2018, 12:03   #28
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Re: LiFePO4 and FLA: a match made in heaven?

You describe well the parallel compatibility of a lithium battery on a lead/acid battery bank.


I've been using a 125 A/H LiFePO4 battery in parallel with a 165 A/H AGM bank in exactly the regime you describe here for two years and about 100 charge/recharge cycles. No problem.


Details are posted here: http://www.cruisersforum.com/forums/...ml#post2740583
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Old 13-10-2018, 14:19   #29
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Re: LiFePO4 and FLA: a match made in heaven?

JohnCt:
In one context for LiFePo4 you state:
"I define my 0% SoC as 11.9V (2.975Vpc) in a low-amp discharge context."
...And summarizing
  • LFP not above 13.8V (3.45Vpc) (in daily cycling).
  • LFP full resting voltage is ~13.2V (not enough to get lead to 100% Full)
  • LFP not below 11.9V (2.975Vpc) my 0% SoC (n a low amp discharge context).
And in an earlier context with LFP
  • Max knee wall voltage 13.8v (3.45vpc) "Stop Charging" 100% SoC
  • Min knee wall voltage 12.0 (3.0vpc) "Start Charging" 0% SoC
Has your thinking changed a little and what are your current "not above" and "not below"?

Also if MaineSail has previously identified:
  • Max charge allowed on his graph as 13.88v (3.47vpc)
  • Low kneewall voltage 11.6v (2.9vpc)
  • Discharge cutoff voltage 10.0v (2.5vpc)
What voltages are generally accepted as the Manufacturer's range for full capacity 100%SoC?

Also what percentage of the manufacturer's full capacity in ah is your current "not above" and "not below" voltages generally?

Thanks
And Thanks Cpt Pat for your description, very helpful.
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Old 13-10-2018, 15:28   #30
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Re: LiFePO4 and FLA: a match made in heaven?

My numbers are slightly different for my 125 A/H battery:


LFP not above 13.8 volts when charging at 20 amps - AND - not above 80% SOC when measured by A/H in/out. An LFP can be overcharged at 13.65 volts when the charge current is very low.

LFP 80% SOC resting voltage 13.30 volts.

LFP not below 12.7 volts while discharging at 5 to 10 amps - AND - not below 20% SOC when measured by A/H in/out. When discharging at 5 to 10 amps, the resting terminal voltage will rise to 12.95 volts after the load is removed. A 12.7 volt LFP discharge cutoff is coincidentally very convenient because this is the 100% SOC resting terminal voltage of lead/acid batteries. And that's where they take over. Yes, I agree, a match made in heaven.

Those numbers are for a SOC range regime of 20% to 80% SOC. Different LFP batteries or different charge and discharge rates will yield slightly different voltages that correspond to the 20% to 80% SOC parameters.
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