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

[mcarling]

Without the cell-level BMS, I don't think you would have had the loose bolt because your battery bank would not have required re-balancing.

[diugo]

Originally Posted by mcarling

Without the cell-level BMS, I don't think you would have had the loose bolt because your battery bank would not have required re-balancing.

Umm, the "BMS" was likely not the sole reason for the cell imbalance. I fully expect to manually rebalance a year from now.

[OceanSeaSpray]

Originally Posted by mcarling

The above illustrates one of the reasons why I would never install a cell-level BMS on a LiFePO4 battery bank. In my opinion and experience, it is sufficient and safer to do a top balance and then monitor only overall bank voltage for LVE and HVE cutoffs, and ensure the bank is never overcharged. I see no reason to check cell-level voltages other than before a passage or otherwise once per year. Adding a cell-level BMS is just adding voyeurism and additional points of failure, in my opinion. Snake oil.

Considering that you cannot both top AND bottom balance, you are only going to be safe at one end if you don't monitor cell voltages, this is the unavoidable reality.

The next thing I would say is that there is a big difference between a Battery Protection System and a Battery Management System. My view is that if you have a good BPS, you don't need a BMS, but you may need manual intervention one day.

A BPS ensures that cells can't be damaged through lithium plating/electrolyte decomposition during charging, or plain chemical decomposition due to over-discharge. Without individual cell voltage monitoring, you can't ensure all cells remain in their safe operating range, you are only hoping they will. It forces you to adopt exceedingly conservative operating limits and as a result you end up accessing a smaller fraction of the installed capacity. It also reduces charging efficiency because you end up doing absorption at constant voltage when you could bulk-charge instead.

If you are going to have HV monitoring, LV monitoring, one (or two) disconnect device(s) and possibly early alarms, in my view you can just as well add another 3 voltage inputs to pick up the cell readings and act on that - at virtually no additional cost or complexity. Note that I said "act on them" not "display and watch them". The point is not watching the lithium channel on Battery TV. Then you are 100% protected and you don't need to worry about one cell doing its own thing at the high or low end.

I don't buy into the myth of the "ever lower charging voltage". It originated from trying to use inadequate SLA charging gear and plain automotive systems with lithium cells. If you bulk charge and terminate at a target voltage rather than continuously trickling lowish voltages into the bank hoping it won't mind too much, you get more usable capacity, faster charging and it is much healthier for the cells.

A decent BPS is a ridiculously cheap insurance policy, because then you can't damage the cells no matter what. Using some half-baked approximation costs you essentially the same, but gives you no assurance that things won't go wrong, because they still can, and you end up with reduced available capacity and reduced charging efficiency.
A well-designed BPS should draw virtually no power and voltage sensing currents should all be identical and well below the milliamp mark. The risk is not there. Issues came from playing with high-voltage shunting cell boards and designs that weren't fail-safe or adequate and there is no shortage of those.

[OceanSeaSpray]

Diugo,

Remember that if you are using resistive voltage dividers against ground, having equal sensing currents in the sensing wires still doesn't result in balanced drain on the cells!
This is because the first cell is still contributing current to make up the sensing voltage of the others. It takes more than 8 resistors to make a balanced sensing circuit.

Eric

[senormechanico]

Originally Posted by OceanSeaSpray

Diugo,

Remember that if you are using resistive voltage dividers against ground, having equal sensing currents in the sensing wires still doesn't result in balanced drain on the cells!
This is because the first cell is still contributing current to make up the sensing voltage of the others. It takes more than 8 resistors to make a balanced sensing circuit.

Eric

Great point.

[T1 Terry]

Wouldn't it really be a CPS rather than BPS? Look after the cells and the battery will look after itself..... well I sort of stole that one, it was pennies and pounds but.....:lol:
The bottom balance only puts a strain on the cells that really isn't required, not for a house system, an EV system is a different story but cell monitoring is still the key there as well. The only value I can see in pulling a battery down to a low state of charge deliberately is to check capacity, but stop at either the advertised capacity or as soon as a cell dips below 3v. If there is a big differential when the below 3v occurs, then have a good look why, but that's about it. Even top balancing needs to be put into perspective, as long as the cell holds better than 3.4v rested it is full, taking it to 3.6v and holding it there till all the cells reach 3.6v is torturing the cells for no value and bring the capacity of the high cell down, not the low cell up. Eventually the internal resistance of the cells that reached 3.6v first will result in those cells reaching 3.6v before they are 100% full, the tell tail of this is a cell that drops from 3.6v to under 3.4v as soon as the charge current is removed.... but that is just telling you the damage you have done really, you can't fix it, just give yourself an upper cut for damaging the cell in the first place


T1 Terry

[SV THIRD DAY]

Originally Posted by OceanSeaSpray

A decent BPS is a ridiculously cheap insurance policy, because then you can't damage the cells no matter what. .

OR.....
Set the Charge Voltage on your charging devices (Alternator, Battery Charger, Solar Controller, or hamster in a cage) to 13.4v and stop with all the fuss and worry about over charging your bank. So that takes care of the top end.

For the bottom end, if I can't keep an eye on my battery voltage and SOC while living aboard, without some fancy-dancy computer doing it for me...they I shouldn't be messing around with LiFePO4 batteries in the first place! Seriously. When I'm low....I turn on the generator and charge...how hard it that?

Don't trust yourself or your charge devices...easy...put a high and low voltage alarm on the bank. Put cell voltage monitoring in...all cheap and easy.

Life is easy when you stop trying to make it hard.

[T1 Terry]

Originally Posted by SV THIRD DAY

OR.....
Set the Charge Voltage on your charging devices (Alternator, Battery Charger, Solar Controller, or hamster in a cage) to 13.4v and stop with all the fuss and worry about over charging your bank. So that takes care of the top end.

For the bottom end, if I can't keep an eye on my battery voltage and SOC while living aboard, without some fancy-dancy computer doing it for me...they I shouldn't be messing around with LiFePO4 batteries in the first place! Seriously. When I'm low....I turn on the generator and charge...how hard it that?

Don't trust yourself or your charge devices...easy...put a high and low voltage alarm on the bank. Put cell voltage monitoring in...all cheap and easy.

Life is easy when you stop trying to make it hard.

I think Main Sail called it the human error factor...... given enough time it will catch you

T1 Terry

[senormechanico]

"Don't trust yourself or your charge devices...easy...put a high and low voltage alarm on the bank. Put cell voltage monitoring in...all cheap and easy."

Isn't that just shifting the trust to and alarm instead?
Maybe an alarm to let us know when the first alarm fails?

Oh, the irony...

[SV THIRD DAY]

Originally Posted by senormechanico

"Don't trust yourself or your charge devices...easy...put a high and low voltage alarm on the bank. Put cell voltage monitoring in...all cheap and easy."

Isn't that just shifting the trust to and alarm instead?
Maybe an alarm to let us know when the first alarm fails?

Oh, the irony...

I guess a PFD is a back-up for a Jackline and a Jackline is back-up for not tripping over **** on deck and falling overboard. A cruising boat is typically built upon layer and layer of systems to keep bad things from happening.

My mindset follows the KISS principal to make this battery technology more real life cruiser friendly and usable.

Charge at a rate that CAN NOT overcharge and hurt the batteries and then pay attention (just like every cruiser does now) to the low end level of your batteries and charge when they need it. Remember undercharging will NOT hurt these batteries like the old school LA, so why not use that property to your advantage? Ok...maybe my alternator regulator craps out and tried to overcharge my bank...well...that's why you can have an alarm siren to warn you. Worried about Solar and Wind...when you are away....just turn them off then....drama solved.

Can mistakes happen as T1 Terry points out...sure...I have a long list of them. But again...I say if you don't know what you are doing at this point then LiFePO4 batteries are NOT for you. But the KISS approach at least to me with these batteries solves so many problems and headaches for damn near FREE....someone will have to make a damn good agument of why my KISS approach is crazy before I will add in the extra complexity and cost of a full BMS auto-disconnect system that in my opinion will suck people into battery complacency. Running a Cruising boat isn't a Ronco table top rotisserie project of "Set It and Forget it". That type of mentality usually has an unhappy ending of shipping repair parts down to Paradise.

Lets also be real here...will I trash my $1700 400AH bank if I accidentally run them to flat dead once/yr because I left my back warming pad on and slept through the siren alarm? I don't think so. The low voltage alarm (Not cut off...that's more BS trouble than it's worth) isn't really for me, but for my wife and kids when I'm away on a business trip. The alarm goes off and they start the generator and my son gets in trouble for not doing his job while I'm away...which is making sure the power and water systems on the boat are working in good order so the wife and daughter don't have to!

If my teenage son can figure this out, I think we are making it MUCH more difficult than it has to be. That's all I'm saying. If you love the electronic control package go for it, but what I'm trying to get across is that with most things in life, there often is an easier/simpler way to get something done than what a bunch of electrical engineers come up with...no offense to EEs....but you know what I mean.

[OceanSeaSpray]

Originally Posted by SV THIRD DAY

OR.....
Set the Charge Voltage on your charging devices (Alternator, Battery Charger, Solar Controller, or hamster in a cage) to 13.4v and stop with all the fuss and worry about over charging your bank. So that takes care of the top end.

For the bottom end, if I can't keep an eye on my battery voltage and SOC while living aboard, without some fancy-dancy computer doing it for me...they I shouldn't be messing around with LiFePO4 batteries in the first place! Seriously. When I'm low....I turn on the generator and charge...how hard it that?

Don't trust yourself or your charge devices...easy...put a high and low voltage alarm on the bank. Put cell voltage monitoring in...all cheap and easy.

Life is easy when you stop trying to make it hard.

Life would be even easier if polarisation voltage didn't exist and colder days made no difference to cell voltage. By the time you take that off your 3.35V/cell, capacity has taken a good hit and charging has reverted to the old SLA absorption method... great. There is a good reason why the target charging voltage is above the OCV. All that now puts the bottom end a lot closer.

This is until something goes wrong unexpectedly just once and the cells end up parked in a trash can, easy too by the way, quite a few have done it even though it couldn't possibly happen to them either. In hindsight it was "Simple and Stupid" alright.
This really is a case of advocating anything except doing the job properly.

[SV THIRD DAY]

Originally Posted by OceanSeaSpray

This really is a case of advocating anything except doing the job properly.

How long have you been running your LiFePO4 bank?
And share with us your set-up for comparison....that would be great to see.

[transmitterdan]

Originally Posted by SV THIRD DAY

Lets also be real here...will I trash my $1700 400AH bank if I accidentally run them to flat dead once/yr because I left my back warming pad on and slept through the siren alarm?

I think the answer to this question is yes.

[SV THIRD DAY]

Originally Posted by transmitterdan

I think the answer to this question is yes.

Hmmm...well we did the testing where we did just that intentionally and it didn't trash the battery or capacity....what....sacrilege....heresy...blasphem er!

Was it good for the battery...probably not...but did it make me need to throw the bank in the trash for that single simulated "opps" event of draining the batter flat down? No. Despite the panic, these banks can take more abuse than most people want to admit and why would a battery company WANT to admit it? It would only hurt them by opening themselves up to more warranty claims. Easier for them is to say everything will screw the battery up and void your warranty...it covers their ass without really giving you any real data/info. But it's how companies work when dealing with what you can, should, and shouldn't do with their product.

The research scientist in me from my former life just had to find out what would really happen compared to just listening to what people say would happen. I'm not saying all the experts and great advice on this thread isn't valuable. But a bunch of what is "known" is just myth that gets repeated over and over until it becomes true. Not because people are doing it intentionally, but because sane people don't do a destructive test on a $1700 battery bank "just to see what happens", so they just repeat the "your bank will die" story until no sane person would ever try it to find out! But as my wife will say, I stopped being sane a long time ago...it's pretty liberating.

Was I lucky on my drain dead battery test?
I can't rule that out, I've always said I would rather be lucky than good, but a one time suck down and leave empty for 24hrs hasn't killed the 400AH bank. It did need a top balancing when I fully recharged it up the following day, but that's not rocket science either and I'm back in test cycling business like it never happened. Did I cut life out of the bank with that test? Well maybe, but the world didn't stop spinning and Repubs and Dems didn't suddenly start hugging each other and start working together so all still seems right with the Universe, plus I got the bonus of learning something without having to pay $1700 for the education...which in the words of Green Keeper Carl from Caddy Shack, "so I got that going for me, which is nice".

[transmitterdan]

Most of the banks on boats are being charged by devices capable of 100+ amperes. There is a greater risk of fire if a dead zero voltage LiFePo bank is charged with a charger delivering 100+ amperes. To restore a dead zero LiFePo battery requires a charge profile that most chargers simply are not programmed to do. That's why some (most?) LiFePo specific chargers simply refuse to charge a battery with a cell voltage < 2.5V.