Merged LiFeYPO4 1000Ah Winston prismatic cells and all electric galley...

[mbullock]

Originally Posted by john61ct

There is no accurate objective measure of SoC directly.

If you keep between

12.0V (11.9 under high current discharge) at the bottom

and 13.8 (or however you choose to define *your* 100% Full) at the top,

then your bank will last much longer than it would following the vendor's spec for min/max.

And your "sacrifice" of working capacity compared to rated AH will likely be well under 15% total.

Using a chart like the one attached, or doing my own test gives a fairly accurate soc with LFP surely.

I’m 24v but my v-max coincides with your figure but I am starting v-min at 12.6v equiv. obviously based upon experience I can amend the value in the BMS . Also with such a conservative vmin I’m only going to start an alarm, cut the inverter off and leave other dc Loads to run a while longer. All loads will cut before the manufacturers vmin is reached to keep the cells safe.

[CatNewBee]

I have experimented with different settings, 13.8V was too low for me. If you charge full throttle with 300A you hit the mark at 70%, If you charge low current (Solar in the evening), you may reach 90+%, so I decided to go higher. With my setting, I hit always above 90% and below 3.6V cell voltage, so safe enough for me.

[john61ct]

Originally Posted by mbullock

Attachment 175135Using a chart like the one attached, or doing my own test gives a fairly accurate soc with LFP surely.

I’m 24v but my v-max coincides with your figure but I am starting v-min at 12.6v equiv. obviously based upon experience I can amend the value in the BMS . Also with such a conservative vmin I’m only going to start an alarm, cut the inverter off and leave other dc Loads to run a while longer. All loads will cut before the manufacturers vmin is reached to keep the cells safe.

That chart shows way higher a voltage than I would ever allow my bank. My 100% would be at or below the top "shoulder" maybe the 90% mark.

For most quality brand prismatics that will still be higher than their 20-hour rate AH capacity.

To the degree you go up into that shoulder, you are sacrificing years off the bank lifetime.

These are arbitrary chosen setpoints, define your own 0% and 100%, only go outside for occasional testing / maintenance protocols.

[CatNewBee]

Originally Posted by john61ct

That chart shows way higher a voltage than I would ever allow my bank. My 100% would be at or below the top "shoulder" maybe the 90% mark.

For most quality brand prismatics that will still be higher than their 20-hour rate AH capacity.

To the degree you go up into that shoulder, you are sacrificing years off the bank lifetime.

These are arbitrary chosen setpoints, define your own 0% and 100%, only go outside for occasional testing / maintenance protocols.

I will report, when my cells are dying.
Expect the post in 20 years...

[robywire]

The REC BMS is a good find. The DC-DC converter seems a lot more elegant than a heater (resistor) on top the battery to balance the cells. Though, is it possible to disengage the REC balancing function and only do it occasionally? I wouldn't think it necessary to top balance an LFP pack on every cycle.

Have you considered a FLIR imaging of the cell connections and the REC box while you are charging at between 13.8V and 14.3V where the balancing is engaged. I'm curious about any heat generated.

To my way of thinking, the mean time between failure (MTBF) of this balancing circuit could accidentally define the MTBF of the pack. This is why I'd want to "monitor" the pack with the REC and then periodically "manage" the top balance when I can be hands on.

Great work!
Rob

[CatNewBee]

Originally Posted by robywire

The REC BMS is a good find. The DC-DC converter seems a lot more elegant than a heater (resistor) on top the battery to balance the cells. Though, is it possible to disengage the REC balancing function and only do it occasionally? I wouldn't think it necessary to top balance an LFP pack on every cycle.

Have you considered a FLIR imaging of the cell connections and the REC box while you are charging at between 13.8V and 14.3V where the balancing is engaged. I'm curious about any heat generated.

To my way of thinking, the mean time between failure (MTBF) of this balancing circuit could accidentally define the MTBF of the pack. This is why I'd want to "monitor" the pack with the REC and then periodically "manage" the top balance when I can be hands on.

Great work!
Rob

No too much heat inside the BMS, it is self- regulated. a 1000Ah cell ewen won't notice 3A current, nor increase temp.

[CatNewBee]

Originally Posted by CatNewBee

No too much heat inside the BMS, it is self- regulated. a 1000Ah cell ewen won't notice 3A current, nor increase temp.

Regarding balancing: no, keep it on all the time! If the cells are in balance, no balancing will occure anyway - if not, you should do it.

You can play around with the parameters of the BMS and the charge sources to not hit the point where balancing starts, but this is not adviseable at all. Balancing occurs mainly while charging, so some cells simply get less Amps than the others, it does not hurt at all.

A charge current of 100A then charges the high cell with 98A and the low cell with 102A. That's all, Don't worry about it.

It's a religious fight about balancing, there is no reason not to do it.

[hellosailor]

I suggest that actually running and testing BMSes to generate a real MTBF is beyond the budget of any firm making them. You'd have to actually deploy some battery packs (preferably a reasonable number like 1000) with BMSes, and then cycle all the packs through their lifetime (perhaps 2000 cycles) to generate some reliable numbers. But even if you tried to do that for one pack? 2000 cycles? If you did one cycle per day you'd need six years to get the data, by which time your product would be obsolete.

You may remember 10(?) years ago a transatlantic airliner made an emergency glide landing in the Azores or the Canaries, after a total engine failure. There was a lot fo question about why the bug jets now usually have only two engines instead of the old "more reliable" four. Turns out the engineers say that as you increase the number of parts (from 2 engines to 4) the odds of a failure go up geometrically. 2x more engines, 2^2, that's four times more likely to have a problem.

So if you start with a BMS that has discrete electronics on it, those have a typical life expectancy of 100 years. Put 25 of them on one board...eh, my math doesn't count that high. It becomes more important to plan on a failsoft, anticipating that there WILL be a failure of various kinds, and the question is not when will that happen, but does the design mitigate the consequences of every possible failure? (Like, ensuring the alternator won't burn out if the BMS takes the battery off circuit.)

Folks are going naked, without a BMS, and horror stories are not happening at the predicted rate, either. Decisions, decisions.

[john61ct]

Originally Posted by CatNewBee

Don't worry about it.

It's a religious fight about balancing, there is no reason not to do it.

Many owners that avoid the shoulders, only monitor at the bank- or per-4S pack- level, and even after years, report no significant imbalance issues.

Some rebalance manually as part of their routine / annual maintenance.

Many LFP banks have been damaged by the per-cell circuitry going bad or poorly designed in the first place.

So no, not just "religious" if by that you mean "irrational".

I recall reading MaineSail's account of installing a packaged-BMS system, and the first thing he did was remove the per-cell electronics.

If I am recalling that wrong someone please correct me, I can't find the post in my notes.

[CatNewBee]

I am not a fan of cell modules either. I prefer a integrated BMS, that does all on one sealed board and connects only by wires to the cells. That is a neat design, that prevents user mistakes, shortcuts and other unexpected things like broken communication chains across the cells etc....
But its not about the modules, but about best practise. I do opt for a system, that monitors and if necessary engages to fix issues when they occure, instead of relying on the user to check and maintain his cells once a year. Usually he is sailing and checking the weather, not the cell voltage difference.

[john61ct]

I did not distinguish between electronics physically mounted on cell posts vs centrally located gadgets with just wiring going to the cells.

I suppose there is a difference there thanks fr that.

But I imagine either type can be useful or dangerous, and for myself will likely continue to avoid both while in daily use.

[robywire]

Quoting RC's Marine How To article;
https://marinehowto.com/lifepo4-batteries-on-boats/

"AUTOMATIC CELL BALANCING:
Let me first say;
I DO NOT BELIEVE AUTOMATED CELL BALANCING IS NECESSARY FOR A WELL EXECUTED FRACTIONAL “C” SYSTEM !!!!
.......
.......
I DO NOT ALLOW MY BMS SYSTEM TO BALANCE AUTOMATICALLY !
I am a believer that shunt balancing should be a “monitored” event just like “equalizing” lead acid batteries."

That said, it seems that much of RC's work was with the shunting resistor (heater) type of BMS. Still, even with a more reliable shunting methodology like REC, his point that the LFP bank should not need to be balanced still remains.

If the pack is not out of balance, you don't need shunting. If the shunt circuitry is regularly balancing at the top, then you may have a problem that your balancing routine is hiding. Add to that, the LFP packs hate a high SOC and love a low SOC.

Jack Rickard of EVTV Motor Werks gives an interesting analogy of how a top balancing shunt creates the need for a top balancing shunt.
The Thrill of Victory, and the Agony of the Feet - EVTV Motor Verks

Fully automatic has its appeal and I think CatNewBee has provided a first class methodolgy that appears to solve the automatic top balancing catastrophes of the past, time tells all.

I think bottom balancing and staying safely between the knees is for me, but I like the REC BMS monitoring and management functionality. I just have to make sure I can turn off the balancing function.

Best regards,
Rob

[CatNewBee]

Originally Posted by robywire

Quoting RC's Marine How To article;
https://marinehowto.com/lifepo4-batteries-on-boats/


That said, it seems that much of RC's work was with the shunting resistor (heater) type of BMS. Still, even with a more reliable shunting methodology like REC, his point that the LFP bank should not need to be balanced still remains.

If the pack is not out of balance, you don't need shunting. If the shunt circuitry is regularly balancing at the top, then you may have a problem that your balancing routine is hiding. Add to that, the LFP packs hate a high SOC and love a low SOC.

Jack Rickard of EVTV Motor Werks gives an interesting analogy of how a top balancing shunt creates the need for a top balancing shunt.
The Thrill of Victory, and the Agony of the Feet - EVTV Motor Verks

Fully automatic has its appeal and I think CatNewBee has provided a first class methodolgy that appears to solve the automatic top balancing catastrophes of the past, time tells all.

I think bottom balancing and staying safely between the knees is for me, but I like the REC BMS monitoring and management functionality. I just have to make sure I can turn off the balancing function.

Best regards,
Rob

When it comes to Religion, reason is not appreciated..

I DO NOT BELIEVE AUTOMATED CELL BALANCING IS NECESSARY

I am not a believer in human maintenance of LFP, so I stick to a well engineered BMS for a good reason. A guru is a guru, and has followers, who would never question any of his wisdoms. A scientist always question everything until proven wrong.

It is your chice, I am not saying, use balancers, I am saying, make your own educated decission and be prepared to cope with the concequences.

[john61ct]

MS would be the first to say he's not infallible.

He definitely does not advocate bare cells + DIY protective gear for us newbies, and generally recommends & installs full-fledged BMS packaged integrated systems like Victron and Lithionics.

Just for his own system, he disabled the live per-cell balancing.

That would probably not be a good idea charging up to the high voltages that Catnewbee uses.

Human maintenance, at least routinely checking that all is well, is needed IMO even with NASA-level redundancies.

[robywire]

Originally Posted by john61ct

MS would be the first to say he's not infallible.

But his practical experience makes him damn near. I know that I am just one of many that consider his contributions in this space invaluable.

Originally Posted by john61ct

Human maintenance, at least routinely checking that all is well, is needed IMO even with NASA-level redundancies.