LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[ebaugh]

I've been away way to long...but my BMS (a management system) using cell boards did cause an imbalance due to a faulty cell board. Even when the charging system remained well below the balancing voltage. It can happen...anytime a load is imposed on an individual cell...or group of cells. It shouldn't but can.

[simat]

Originally Posted by Maine Sail

Would love to know how you tested this?

I, like you only charge to 3.45 volts/cell and let the current drop to 0.05C, actually I charge to 3.375 V/cell for six days and then to 3.45 V, then back to 3.375 V. The maximum charge current I get from my solar panels is ~0.1C. I was interested to see how much extra charge would go into the battery by pushing the voltage up to 3.6 V/cell. I started logging the results when the cell voltage reached 3.4 V/cell and kept logging till the average V/cell reached 3.6 and the current dropped to 5 amps (0.014C)
Attachment 82016
Attachment 82017
As you can see the battery is certainly not balanced and the voltage started to diverge rapidly towards the end. A salutatory lesson.

Our loads on boats, when using LFP for house banks, are nowhere near .25 or .5C.... My bank is now beyond 400 cycles, most to 80% DOD, and with 9 complete capacity tests, it is still putting up the same exact capacity it was 400 cycles ago.

I must confess that my system is not on a boat, the closest I get to a cruising yacht is a windsurfer. My system powers an off grid house, we rely on the solar panels for 100% of our power. We have a 4kW inverter, battery load is rarely above 0.5C.

This is the best thread I have found on the net for information on using LiFePO4 batteries for a low charge and discharge current application, where extending the calendar life of the batteries is a major focus. Thank you to everyone who has contributed.

Our LiFePO4 batteries have been in use for nearly fourteen months so we have clocked up about 400 cycles. I also have found no change in the cells over the 400 cycles.

I still have a BMS for a LVE or HVE but do not use it for balancing, just insurance..

For my system I have a Cellog 8 to do the LV and HV alarms and have been doing the balancing manually, still need to do more work on this. For a system I set up for a friend who is not very technically orientated the balancing is done automatically.

[mcarling]

Originally Posted by T1 Terry

But you quite emphatically tell us you do not do cell level monitoring, so how would you know? It's the bury the head in the sand theory that what I don't know can't hurt me?

If you read what I wrote above, you will read that I check the balance about once a year. That includes checking the voltage across each cell (with a voltmeter) while 42V is applied across the 12 cells. I leave them with the 42.0V charge applied for several days during which I check the cell voltages from time to time. I have never recorded an individual cell +/- more than a few thousandths of a volt from 3.500V. After disconnecting the charger, they are all still within a few thousandths of a volt of each other.

Originally Posted by T1 Terry

Think of the number of combinations 12 numbers added together could equal 42v, can't you see that cell level monitoring tells you what each number is, battery level monitoring only tells you the sum 12 of these voltages? Without an alarm warning you cell has reached 3.6v you simply would ot realise it had happened.

I do not believe that my cells are willful. I believe, based on both theory and empirical evidence, that all my cells behave similarly. Maybe there a god of batteries who decides the voltage of each battery all the time and usually decides for batteries to behave in accordance with what we believe are the laws of chemistry, but who every once in a while gets into a bad mood and causes a cell to behave in surprising ways. Maybe, but I'll continue to believe that batteries behave deterministically and that when the overall voltage reaches 42.0V that each cell is near enough to 3.5V.

Originally Posted by T1 Terry

Battery Monitoring Systems do not cause an imbalance if they fail, automated cell balancing systems that use a lod to discharge high cell can and often do, I agree with your loathing of such system, I feel they are far more likely to damage a battery than any other issue, besides over voltage charging of course. The way I see it, only by monitoring cell level voltages can this be achieved.

Battery Management Systems have more failure modes than Battery Monitoring Systems but Battery Monitoring Systems can fail in such a way as to short an individual cell. I'm not willing to take that risk for what I believe to be zero gain. The assertion that the only way to avoid overcharging is to monitor cell-level voltages is obviously not correct. HVE disconnect based on overall voltage surely has avoid overcharging in countless installations.

Originally Posted by T1 Terry

let me assure you, brand new cells charged to 4.6v I think it was, causes a one time fatallity, I have 2 cells to prove it, neither of them burst their cases but both of them slowly vented their electrolyte resulting in no ability to accept charge current of ability to hold a charge. As soon as current was applied, the voltage rose the point the current fell to zero, all te way back up to 4.6v, I stopped the test at this point as the heat build up within the cell was becoming obvious.

I don't think anyone here is recommending that LiFePO4 cells be charged to 4.6V or that it would take a very long time for 4.6V to damage a LiFePO4 cell.

[goboatingnow]

In my own experience , 18650 lifepo4 cells used in fractional C environments do not go out of balance. In fact I've seen them converge. Hence in that regard they can be viewed like we do LA cells, where we habitually ignore individual cells.

HOWEVER , at the present time, given the costs, I would always recommend cell monitoring for lifepo4., equally I also recommend it for expensive LA traction cells too.

Dave


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[donradcliffe]

There's sad rumors about Equus, a high dollar wooden boat, which just burned and sank due to a fire in their Li battery bank. Any of the experts have any more details??

[OceanPlanet]

Originally Posted by donradcliffe

There's sad rumors about Equus, a high dollar wooden boat, which just burned and sank due to a fire in their Li battery bank. Any of the experts have any more details??

All we know here is that it was not a GLi system. Have no idea whose it was. So sad, was a really pretty boat.

[deckofficer]

Originally Posted by donradcliffe

There's sad rumors about Equus, a high dollar wooden boat, which just burned and sank due to a fire in their Li battery bank. Any of the experts have any more details??

Your a better detective than me, as I can't find any mention of a lithium battery on Wendy Schmidt's boat.

[CharlieJ]

Here is a link: W-46 Equus burns and sinks

Very little information regarding the specifics.

[deckofficer]

Originally Posted by CharlieJ

Here is a link: W-46 Equus burns and sinks

Very little information regarding the specifics.

Thanks for the link. LiPO isn't the same animal as LiFePO4. If you need the energy density of LiPO, there is the slim chance of a fire, but this thread is about LiFePO4 and they are safer than lead acid.

[CharlieJ]

Bob-
Absolutely agree. Will be landing 2000Ahr of LFP on a total refit project in about three months.

Where did you glean the type of Li chemistry?

[deckofficer]

Originally Posted by CharlieJ

Bob-
Absolutely agree. Will be landing 2000Ahr of LFP on a total refit project in about three months.

Where did you glean the type of Li chemistry?

Post #6 on your link.

The refit, is it on your Gulfstar or a customer's boat?

[Maine Sail]

Originally Posted by CharlieJ

Here is a link: W-46 Equus burns and sinks

Very little information regarding the specifics.

All I know, that is fact, is that the boat was originally not sold or built with Lithium. If someone converted it, that certainly may be possible.

Would love more information on the system, design, who did the install or what yard did the work..?

[CharlieJ]

Bob-I thought that is where you found it…I think it is a casual comment…Li is Li to the uninitiated.

The LFP bank is going into a 60' Sun Reef sailing catamaran.

[deckofficer]

Originally Posted by CharlieJ

Bob-I thought that is where you found it…I think it is a casual comment…Li is Li to the uninitiated.

The LFP bank is going into a 60' Sun Reef sailing catamaran.

Sounds like they want to be able to run the A/C at night without the drone and vibration of the gen set. Even at 12 volts that is a big 26 kwhr bank.

[CharlieJ]

Sounds like they want to be able to run the A/C at night without the drone and vibration of the gen set.

24VDC and