LiFePO4 and absorption charging

[Putt-Putt]

Originally Posted by Chotu

the guy you’re responding to has probably one of the biggest trolling motors on the whole forum powered by a 150,000 watt battery bank ha ha

Thats a significant battery bank. What is the voltage and how many Ahs?

What model is the trolling motor?

[s/v Jedi]

Originally Posted by 00X

:P,

Mine as well if i charge at the manufacturer recommended voltage (14.6). The manufacturer of my batteries (epoch) have a very good reputation amoung the boating community and yet, even they recommend 14.6 volts charging and expect the BMS to trip at high voltage to discontinue charging. Luckily their batteries are really well made and can handle this without issue, but their main consumer base was/is golf cars, electric vehicles and for trolling motors. RV'rs and boaters have noticed and adopted their batteries because they are very good, but if i called them up for advice, they would tell me to charge at 14.6 volts and not understand why thats a problem for me. The enthusiast crowd and internet forums have been instrumental in refining HOW to define best practice for us.

It means that you are not talking with someone knowledgeable on the technical aspects of the battery, just read out the official specifications.

For good background information, read the wiki from Rod Collins, MarineHowto.

But even at novice level one can understand that when a battery is full and then you drain 100Ah from it, followed by putting 100Ah back in, it is full again apart for some inefficiencies. This means that all you care about is putting the Ah back in, not a specific voltage.

The finish voltage (terminating voltage) is valid for the specified charge rate, like 0.5C or 0.2C. If you charge slower then the battery will be full before reaching that voltage.

[Johan Leopard51]

Originally Posted by s/v Jedi

Manufacturers want you to fully charge each and every cycle.

That (full charge every cycle) is going to be difficult in many cases, whether marine or land-based. Manufacturers should realize that in many use cases, the battery might be 100% once a week and spend the rest of the week varying between 35% and 90% with an average of say 75%

In Lithium Ion, there are also internet claims that it is bd for battery life to charge full every day, to charge just because iphone is at 90% and to leave battery on charge for long periods. Apple and others nowadays manage recharge to do the last bit just before they think you’re likely to disconnect. Not sure how that works in lifepo4

[Johan Leopard51]

Looked at some stats on my 600kWh lifepo4 land-based this morning:

Overnight the overall system showed 100% SOC but every hour there is a 1% dip when the BMS is doing something.

For past 24h the individual cel max voltage and min voltage (remember this is thousands of cells) ran 3.33V and 3.32V respectively. During that time the discharge rate was mild at only 60kW and then after solar kicked in (the system first charges battery before exporting to grid) the max discharge rate was 90kW.

Overall battery voltage (42 modules arranged in 3 x 14 of 14.6kWh “52V” per module) ranged for the last seven days ranged 738Vdc to 758Vdc with I’d say 95% of the time stable at 745V which in our size systems of a module of “52V” translates to 53.2V.

If you take my narrow range on thousands of cells and 42 modules, it seems to me that good BMS tend to focus on Amps that will keep the Voltage in what they regard as their happy place.

[barcoMeCasa]

Originally Posted by s/v Jedi

Manufacturers want you to fully charge each and every cycle, .

Yet to see that on any datasheet, only the definition of what they think of as fully charged.

Question still stands,

"While this is correct, the main reason is simply to fully recharge the battery, which must happen periodically, let’s say once every 1-2 weeks."

Why when there is so much published work showing PSOC is fine?

[s/v Jedi]

Originally Posted by Johan Leopard51

That (full charge every cycle) is going to be difficult in many cases, whether marine or land-based. Manufacturers should realize that in many use cases, the battery might be 100% once a week and spend the rest of the week varying between 35% and 90% with an average of say 75%

In Lithium Ion, there are also internet claims that it is bd for battery life to charge full every day, to charge just because iphone is at 90% and to leave battery on charge for long periods. Apple and others nowadays manage recharge to do the last bit just before they think you’re likely to disconnect. Not sure how that works in lifepo4

Originally Posted by barcoMeCasa

Yet to see that on any datasheet, only the definition of what they think of as fully charged.

Question still stands,

"While this is correct, the main reason is simply to fully recharge the battery, which must happen periodically, let’s say once every 1-2 weeks."

Why when there is so much published work showing PSOC is fine?

I did include screenshots from LiTime showing the CC/CV charge termination voltage and charge rate for their 24V batteries. This is a full charge and an algorithm that is only the bulk charge phase of most marine chargers.

That this isn’t the reality for boats is true. This means you can’t use any of the manufacturer provided specs/settings because those are only valid for the complete set of parameters, you can’t just pick one.

This is why I recommend to buy a Victron Smart BMV and really study and understand how it works and how to correctly set it up. It will then offer guidance in the dynamic environment for which the manufacturer can’t give settings because there are simply too many possibilities.

I am against BMS’s controlling chargers, I want them to be limited to managing and protecting the battery cells. Even that off grid garage guy down under has now decided that this is the way if you want a flexible system independent of manufacturers.

[s/v Jedi]

Originally Posted by barcoMeCasa

Why when there is so much published work showing PSOC is fine?

Show it. I say PSOC is fine as long as you periodically do a full charge and I don’t think that any engineer will disagree with me on that.

[Johan Leopard51]

Originally Posted by s/v Jedi

I am against BMS’s controlling chargers, I want them to be limited to managing and protecting the battery cells.

I’m with you but also not sure what control really means. In my big land system there is an advanced inverter/PCS that “performs” the charge/discharge but it is closely related to what the BMS allows it to do. The way I have it:

My batteries and their BMS might offer a range of charge, discharge abilities - for example that it can do 1.5C charge and discharge. I plan to get 8000 cycles out of this investment so I set up in the inverter that it not charge or discharge at more than 1/2C or 300kW (which in my factory case is also approximately the peak house load, the generator ability and a bit less than the solar). So I am setting more conservative values for that just like I set rules for grid capping, grid export, at what point the gennie comes and helps the battery and at what point the gennie stops again.

I do at times have 400kW solar and if loads are low I still do not allow recharge at more than 300kW even if the BMS says it could take it (assuming this is while damned grid is off again so can’t export surplus and this means I have to limit my solar)

But imho in no way does the inverter determine charge / discharge other than within min-max limits that BMS says. It is the BMS that has per cel temperature and voltage and it has relays that will remove a cell from taking part. Inverter cannot give BMS more volts / amps than it reckons it wants, and since current is taken not given, that tells me BMS in in charge of the inverter.

At yacht scale, I think when I go lifepo4 I will go for one or more ‘52Vdc’ roughly 15kWh modules and then leave everything 12V on the boat as 12V, including likely a 12V house battery that can stay AGM until it dies. A DC to DC or rectifier can be the bridge between OEM yacht and the new power system, basically imagine I replace the 9kW Northern Lights genset with a 15kW inverter and battery bank - literally I will put the battery and Victron where the gennie currently stands. When/if we do that, likely will change to 48V alternator/generator on the two motors.

[Johan Leopard51]

Originally Posted by s/v Jedi

Show it. I say PSOC is fine as long as you periodically do a full charge and I don’t think that any engineer will disagree with me on that.

Over last day, when battery is at 100%, each and every hour something (I presume the BMS) dips the battery bank to 99% and this tells me it does NOT want the battery to just sit there very full.

Note that cel max and cel min voltages dropped a bit when started discharge, then climbed steeply during recharge, tapered down from when battery 100% and then it stayed constant.

Me, I don’t want to try and get involved in the math or managing settings. I’ll trust what BMS does and what reputable inverter does after chatting to somebody that has same BMS and same inverter and asking the old question : given a do-over, would you buy the same again?

[00X]

Originally Posted by s/v Jedi

It means that you are not talking with someone knowledgeable on the technical aspects of the battery, just read out the official specifications.

For good background information, read the wiki from Rod Collins, MarineHowto.

But even at novice level one can understand that when a battery is full and then you drain 100Ah from it, followed by putting 100Ah back in, it is full again apart for some inefficiencies. This means that all you care about is putting the Ah back in, not a specific voltage.

The finish voltage (terminating voltage) is valid for the specified charge rate, like 0.5C or 0.2C. If you charge slower then the battery will be full before reaching that voltage.

Thank you I have read his web site and understand your points. My reply was generally directed at SailorBoys suggestion to just call up the manufacturer, do what they say and all will be good. Many people are buying cheap Chinese batteries. You would be lucky to get a knowledgeable person on the phone from a decent Western company, one from China, not likely if you can get anyone on the phone at all. I think someone new to lithium still trying to navigate the technology and still learning likely will not know the difference, set the charger to 14.6 bulk/absorb and wonder why everything keeps cutting off. I realize the onus is on the users though so back to my earlier point about these questions, how ever many times they come up are still not trivial when asked.

[00X]

Originally Posted by s/v Jedi

I am against BMS’s controlling chargers, I want them to be limited to managing and protecting the battery cells. Even that off grid garage guy down under has now decided that this is the way if you want a flexible system independent of manufacturers.

I have been watching his videos for a couple of years. That was an interesting and revelatory conclusion after all the tinkering and testing he has done. I agree and do not let the BMS control my chargers and only want them to manage the battery.

[sailorboy1]

Originally Posted by 00X

Thank you I have read his web site and understand your points. My reply was generally directed at SailorBoys suggestion to just call up the manufacturer, do what they say and all will be good.

I didn't say "all will be good".

I said you should ask your manufacturer the question instead of unknown forum people. Forums are full of expert parrots and just because you read it on one doesn't make it true no matter how often a parrot repeats it. But hey, go ahead and ask a forum and then chose the "expert" answer you wish.

[s/v Jedi]

Originally Posted by sailorboy1

I didn't say "all will be good".

I said you should ask your manufacturer the question instead of unknown forum people. Forums are full of expert parrots and just because you read it on one doesn't make it true no matter how often a parrot repeats it. But hey, go ahead and ask a forum and then chose the "expert" answer you wish.

Even the more questionable “expert answers” on this forum are often better than what the help line from the manufacturer has to offer.

[Chotu]

Originally Posted by s/v Jedi

Even the more questionable “expert answers” on this forum are often better than what the help line from the manufacturer has to offer.

agree wholeheartedly.

for instance, i was waiting for you to find this thread. ha ha ha

With your professional background and all your experience and care putting together these systems in your own boat, I knew you knew the answer to this already.

there is a lot more knowledge here than you can get from someone in India reading a tech support script to you. not that Li-Time has that. They actually have a really good company. But as you said, they can’t account for every single use case

[sailorboy1]

Wow that sure sounds like delusion thinking. Pointless to discuss anything with people with belief of super knowledge.