Good thesis on preserving the life of LiFePo4 lithium batteries

[rgleason]

Cpt. Pat

I store my battery at 20% SOC. That is the SOC where my lithium battery is automatically taken off the bus and my AGMs take over. .

Would you mind giving us your voltage for stop discharging at 20% SOC?
Is that 12.7v?


What is your 0% SoC voltage? Your 100%SoC voltage?
What is the usable range and how many ah? Manuf battery bank capacity?



Thanks.

[rgleason]

Originally Posted by Maine Sail

Bingo! The best part is the 7713 fails open/safe when or if the BMS goes teats up... It also has a manual override so you can open the LFP battery switch, thus manually isolating it, and manually turn the 7713 back on. Now your reserve battery, I use a Firefly,

Top of the line, but just Firefly might be an answer for me.


Wouldn't Blue Seas L Solenoid 9012 SPST N.O. work for Lifepo up to 450ah?
https://www.bluesea.com/products/901...12_24V_DC_250A
available for $150-175


PS: It looks like Victron BVM 702 and BVM 712 have every relay alarm and setting needed for Lifepo.

The list is extensive

• Relay Mode DFLT Default mode. The relay thresholds Nos. 16 up to 31 can be used to control the relay.
• CHRG Charger mode. The relay will close when the state-of-charge falls below setting 16 (discharge floor) or when the battery voltage falls below setting 18 (low voltage relay). The relay will be open when the state-of-charge is higher than setting 17 (clear state-ofcharge relay) and the battery voltage is higher than setting 19 (clear low voltage relay).
• Relay Invert,
• Relay State, Relay closed min. time, Relay off delay,
• SoC Relay close for relay discharge floor, Clear SoC relay open relay,
• Low voltage relay when voltage falls for more than 10 seconds, clear low voltage when voltage rises for a user set time period.
• High voltage relay and clear HV relay

And the list goes on with about 40 settings for that one relay to the LiFePo4. It could probably provide the LVD and HVD functions as well.


The other approach to disconnect the alternator field wire to stop charging using VSR Alternator Regulator VSR Alternator Regulator
Might be a way of having the alternator directly connected to the Lifepo4 bank and use an ACR to charge the small reserve battery when current goes up. That one should probably be a Gel battery which would have a longer life. I have one that is 12 years old and at 80% it still starts the engine.

[travellerw]

Originally Posted by rgleason

Top of the line, but just Firefly might be an answer for me.


Wouldn't Blue Seas L Solenoid 9012 SPST N.O. work for Lifepo up to 450ah?
https://www.bluesea.com/products/901...12_24V_DC_250A
available for $150-175

Yes that will work.. However its 130 milliampere to hold closed. That is 10X the draw of the 7713 we listed earlier.

IMHO, you are better off spending the extra $25 (shop carefully, ebay and amazon) on the 7713. Not only does the draw go down by 10X but you also get over twice the amp rating. You also get the ability to use it as a manual switch, with service lockout. All those extra features are easily worth an extra $25, or even $50..

[hzcruiser]

Originally Posted by john61ct

Check out Blue Sea for ML switches that can be remote.

Their 9012 is great, NO, 0.13A draw while closed.

Thanks John! Yes, I've looked at Blue Sea before. They seem to be good quality devices with a fitting (read: fairly high) price tag. But robustness has its price, of course.
The 7713 and 9012 might be a bit overkill, as they can handle 250A or even 400A.

Originally Posted by travellerw

Of course the ML-RBS 7700 has the same specs but is a true "latching relay" and draws 0 current when sitting in a state. However, its not as safe as it will not switch to disconnected in the event of another failure (like the BMS dying).

That's a very good point and there will always be the ones that want this kind of "fail safe open" whereas others argue they don't want to be left in the dark at the most inopportune moment.

It's hard to please everyone.

[hzcruiser]

Originally Posted by travellerw

Alright.. I have to say that I think you guys are crazy to be nitpicking this kind of draw. Seriously, lets do the math.

First I will assume the 13 Milliampre in the spec sheet to be safe.

That would mean the following.
0.312ah/day
[...]

Yes, it would be nitpicking if there is only one relay in place. And the price wouldn't matter much. Only my approach is to have a relay at every single battery (a batt being a series combination of cells to make up the specific bus voltage).

Only then can the vessel or battery mgmt system disconnect every batt individually and test it for resting voltage, self-discharge rate etc. .

That way you can combine any mix of battery chemistries, any size batts, any age of batts (within reason, of course). You can also run 20 hour capacity checks without wasting energy simply by turning on the consumers you need anyway and let them run off the batts you want to test.


Hence with the 6 house batts I have at the moment, I need 6 relays to control them. But they can be smaller (as in 100A), since most of the time at least half of them are turned on, spreading the current load.

[john61ct]

Originally Posted by rgleason

Would you mind giving us your voltage for stop discharging at 20% SOC?

Pretty sure not voltage but AH counted down by the BMV

[john61ct]

Originally Posted by hzcruiser

my approach is to have a relay at every single battery (a batt being a series combination of cells to make up the specific bus voltage).

Perfect is enemy of the good
Paralysis by analysis
KISS

we teach that which we most need to learn

[Cpt Pat]

Originally Posted by john61ct

Breakers that robust are crazy pricey, good ones anyway,

not designed to be used as switches and much more complex to select compared to fuses for CP.

Check out Blue Sea for ML switches that can be remote.

Their 9012 is great, NO, 0.13A draw while closed.

The Blue Sea model 9012 is a good solenoid, and I use one elsewhere on my boat. But it draws 3.6 amps momentarily when changing state and the switch contacts on the Victron battery monitor I'm using are only rated at 2 amps. See https://www.bluesea.com/products/901...12_24V_DC_250A.

All of the low current latching type relays/solenoids/contactors I reviewed drew too large a state-change current. Some also specified current flow in only one direction: the magnetic field created by load current flowing the opposite direction will oppose keeping the contacts closed with the feeble holding current in the coil. Current must flow both ways with a battery.

I acknowledge that I am being super conservative, and perhaps too much so for other tastes. But I design circuits for things that fly far, far away that can never be repaired (albeit space is less harsh than seawater).

There are no high state-change transient currents with a conventional relay. Relays do produce a momentary reverse polarity pulse when they are de-energized -- very high voltages if there's nothing to absorb the stored energy (the "Tesla Effect" is how an ignition coil works). But that energy can be safely absorbed with a reversed polarity diode across the coil terminals (a "snubber" diode). Plus: my relay only cost $50.

[hzcruiser]

Originally Posted by Cpt Pat

Yes, your posting in the link above describes very well what I've been doing for two years.

Good on you, Pat! You beat me to it. I started combining AGM and LFPs less than a year ago. Unfortunately, there is still a lot of fearmongering around mixing batts of different types, size and age.

Of course I'm not saying you can dump them all together onto the bus, but with a bit of extra hardware and a proper control system, one can maximise the utilisation of the batts rather than replacing the lot every x years as preventative maintenance.

[hzcruiser]

Originally Posted by john61ct

Perfect is enemy of the good
Paralysis by analysis
KISS

we teach that which we most need to learn

I hear you, John! And I understand where you're coming from.

No system would ever be perfect or even close to it, as the requirements are too different for different boats, cruising areas and even seasons.

Nevertheless, times have changed and few people still drive manual cars or don't have variable interval windscreen wipers or rain sensors to make life easier and more comfortable.

The same goes for electronics on boats. A lot of new "gadgets" open boating up to a "larger audience" nowadays, compared to, say 50 years ago when only the "real men" sailed off shore. Whether that's good or bad is open for discussion, but it is reality. And for this reason a less KISS-like electrical system has its merits, too.

[john61ct]

Originally Posted by Cpt Pat

my relay only cost $50.

link?

[senormechanico]

Originally Posted by Delfin

I'd hold out for a picture of Big Foot wearing a pink tutu. More likely to get that.

You asked....

[Cpt Pat]

Originally Posted by john61ct

link?

Actually, it was much cheaper than I remembered: https://www.alliedelec.com/te-connec...91-1/70198896/

My operating currents are 30 amps max in and out. You'll want something more robust if your currents are higher.

[rgleason]

All good information. Thanks. The reason for Normally Open is to fail with the LifePo4 disconnected.

Travelerw wrote:
"IMHO, you are better off spending the extra $25 (shop carefully, ebay and amazon) on the 7713. Not only does the draw go down by 10X "

I guess they aren't the same draw! Thanks.

[rgleason]

Highway engineers have taught me that there is always another way to skin the cat.

John wrote:
"Plus there's always a Reserve or Starter bank in lead, could be the power source for the contactor.

So in that context maybe both the NO and zero-discharge features " less critical"

So does it make sense to power the relay and BMS off the starter battery?

John thanks for answering for Cpt Pat about the trigger for 20% SoC probably being ah. It would be nice to have the other answers too, so I continue lurking here.