LiFePO4 and FLA: a match made in heaven?

[hzcruiser]

Let's just agree that it is early days for LFP, some people already have them on board for different reasons, others argue that LFP is still too expensive and more difficult to replace if they break in a far away place.

I believe LA and LFP can actually coexist peacefully even on the same boat!

Outrageous, some might say. Heresy, others will cry and start to pick up a rock, or at least pick up a keyboard to hurl in my direction


It might even be possible that the combination can give you the best of both worlds?
As an intro: What really happens if you connect LFP and LA together?

Not too long ago a gentleman on this forum had a stern warning for us to never, ever directly connect a fully charged batt to an empty batt as the current will be huge and the cables might melt or worse and the batts will get damaged beyond repair! Well I'm proud to say I just topped that and dared to connect an LFP sitting at 13.2V in my lab (not lap) to a dicharged LA batt measuring 12.1V open circuit. And I survived the stunt to tell the story to the world: nothing happened! Nada. Zilch. The Amp meter jumped to 1.5A and is now slowly dropping off. The "bus voltage" dropped to 12.9V. The battery controller is documenting this as we speak, so to speak. Tomorrow I can show the graph of this "disaster waiting to happen", if someone is interested.



Looking at the different voltage thresholds and recommended charge regimes for LA and LFP I can even see signs of a symbiosis here: the LFPs can provide the bulk of the power until they are down to 10 or 20% SOC, then we automatically take them off the bus and the LAs would provide the "reserve" power on those rainy days.



How can that ever work, you might ask? Say we set our charge controllers to simply bulk charge to 14.3V then drop to some float voltage around the LFPs resting voltage (say 13.3V?). Now once our charging source kicks in the LFP will likely absorb more current due to their lower impedance (internal resistance). Charging might continue until the threshold is reached, which means the LFPs are full, the LAs are maybe sitting at 80-90% SOC.

The charge source (if it's one we can control, e.g. alternator, generator, land power) now gets disconnected and the LFPs are topping off the LAs until they're full. LAs at 100% SOC is what we want anyway, right? Brilliant! The LFPs are getting slowly discharged immediately after reaching 100% which is also what's recommended to maximise their life span. Even better!

All the consumers are now driven by the LFPs higher voltage relative to the LAs. If, however, we can't fully charge the LFPs because we rely on wind and solar, nothing bad is going to happen either. The only thing we need to make sure is removing the LFPs from the bus when they're empty, nothing more (unless I missed something crucial here).


WDYT?

[transmitterdan]

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.

[LeaseOnLife]

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.

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. <== needs a bit of calc for sizing.

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..

[john61ct]

This idea has been discussed many times already.

Originally Posted by hzcruiser

Not too long ago a gentleman on this forum had a stern warning for us to never, ever directly connect a fully charged batt to an empty batt as the current will be huge and the cables might melt or worse and the batts will get damaged beyond repair!

It would be possible to set up a situation where the wires get too hot.

An "empty" batt is already damaged.

But yes, most of the time there is no problem doing this.

The relative sizes of the two banks changes things.

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.


> 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.

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

and of course your system should be designed to be doing that anyway

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

But IMO the concept is feasible.

But certainly not KISS.

[john61ct]

Originally Posted by LeaseOnLife

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. <== needs a bit of calc for sizing.

Not necessary for this scenario.

[john61ct]

Originally Posted by transmitterdan

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.

No. The current demanded by the lead bank will never approach even 10% of what would cause the slightest damage to the LFP.

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

[john61ct]

Recent discussion http://www.cruisersforum.com/forums/...ml#post2699270

An older one

http://www.cruisersforum.com/forums/...ge-180921.html

[john61ct]

For those with a very large lead bank and wanting to limit wire size over a long distance.

The current limiting function of Blue Sea's CL-Series BatteryLink[emoji769] ACR (PN 7600) can be used to limit current from a source battery to the target bank.

It limits the current to 60A in a 25°C ambient temperature environment.

Discontinued but available out there, usually $60-80, sometimes less on eBay.

[zboss]

You know... lead acid batteries never explode.

https://youtu.be/gQSsrgUxqIw

[transmitterdan]

Originally Posted by john61ct

No. The current demanded by the lead bank will never approach even 10% of what would cause the slightest damage to the LFP.

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

I did not say LFP would be damaged. Read before you type.

[Q Xopa]

Originally Posted by john61ct

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.

But yes, most of the time there is no problem doing this.

The relative sizes of the two banks changes things.

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.


> 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.

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

and of course your system should be designed to be doing that anyway

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

But IMO the concept is feasible.

But certainly not KISS.

Yes the Sterling B2B chargers are a good way to do it. Even better when he brings out the higher Amperage models he is talking about.

I've also looked at Midnite solar controllers. I haven't got too far into looking at this option. They seem reasonably configurable and have some greater capacity models available.

Further to this concept I also like the idea of running all of our DC charging sources, solar, alt, even an AC charger, through a single controller. That way there is the possibility to have some other less smart individual charge controllers/ regulators. Or perhaps no/ fewer controllers.

This is an extra device that seems to as you say further complicates things. But it may overall simplify the system if you can reduce some other controllers.

I haven't looked into this option too much but I think it should be doable.

I also like the concept being discussed recently on another thread of charging the LFP bank with all the Charging sources then using a DCDC charger to charge the FLA bank from the LFP bank.

Then having a manual batt switch to switch, in an emergency the FLA bank onto the system bus if the LFP bank gets dropped off line.

[john61ct]

Originally Posted by transmitterdan

I did not say LFP would be damaged. Read before you type.

Yes, typo meant LA.

You can put a 500A charge source on a 100AH FLA batt, no problem as long as V is regulated.

Even if you give Hold Absorb too much time, the actual amps flow rate is miniscule.

And all this is irrelevant to the OP topic, as I said will not come close to any situation where there is any danger.

As long as the infrastructure is designed and implemented properly.

And I also gave a reliable and inexpensive solution to limiting current if anyone is still worried about this non-problem.

____
Note the actual problem is that LFP and other high CAR chemistries, is that

they **demand** a very high current when depleted, so that they can destroy many charge sources designed for lead, with poor overcurrent protection,

Very common with alts still equipped with stock VR, need to be converted to external unit like Balmar MC-612

But again just clarifying background nothing to with the OP context.

[transmitterdan]

Originally Posted by john61ct

Yes, typo meant LA.

You can put a 500A charge source on a 100AH FLA batt, no problem as long as V is regulated.

Ok, I’m 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.

[a64pilot]

Originally Posted by transmitterdan

Ok, I’m 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.

I’m afraid your incorrect.
In fact Lifeline says their battery will take as high as 5C, that is of course over 3000 amps for my 660 AH bank, and just isn’t possible.
However as he stated if your voltage is held where it’s supposed to be, the battery will accept only what it can, you see this of course when a bank comes out of bulk and goes into absorption, that is the point where the battery won’t accept 100% of the chargers output, the battery backs down on what it will take, the charger does not.
So yes, you can for example connect a 500 amp charger to a Lifeline battery of 100 amps and it will accept all 500 amps for a short interval, then the battery continues to decline in charge acceptance until it gets to only .5 amp, then the battery is fully charged, however the charger is still able to output 500 amps, but since it is voltage regulated it won’t.
The battery is self regulating if you will

[a64pilot]

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 minimum