Battery EQ and BMS discussion.

[offgridQLD]

Just  type in LyteFyba bms into google and you can find it.

Links are being moderated I think or something like that . I have removed the other posts with broken links.

Kurt

[Cniemand]

The idea of Bottom Balancing isn't simply for Electric cars. You all are correct that a house battery tends to remain in the upper voltages, however that is moot to the idea of a balanced system. One balances cells amongst themselves because of rare instances when you accidently fall into the extremes.

Bottom Balancing is a one time off thing. You bring them all down to the same level at the bottom and then charge them all up together until they hit a voltage that all of them can use without deviation. If they deviate slightly at the top it doesn't matter.

You wouldn't push your cells to use 100% of their top end anyway. No cell is ever going to be exact as the others regardless of batch. One will be 100aH, one will be 101aH. One could be 110aH. The only capacity you have available to use is the least capacity cell.

As you shouldn't be programming your Classics to have your batteries at such high voltage (There really isn't much useful capacity between 3.5vpc and 3.6vpc) the danger is if your system doesn't shut down in a low voltage state.

If you are waiting until your inverter kicks off for lithiums (and you havent changed the setpoints specific in the inverter for lithiums) you will find that one of your cells is going to be close to 2v while the others are near 3. That is the point of Bottom Balancing. When it matters, you do not have 15 cells at 3v trying to kill the smallest capacitance cell to below 2v. If they all remain at the same voltage at 0% then no harm - no foul. 

PC Boards fail all the time. So do alarms. I simply do no trust $30 boards running all over the place to give me insurance.

Side note: those little boards each draw mA. If I leave my system, I can hit the giant breaker and kill the entire pack. Come back months later to a bank that was just as full as I left it (Lithiums have no self-discharge mechanism) however, if I have a ton of BMS wires all over the place, they will kill that pack through small loads while I am gone.

- Cloud.

At the end of the day it is everyones choice. I choose no on bms in an off-grid setting. (3 yrs full time living)

[australsolarier]

Cniemand,

i follow your logic. and we all agree when all the cells have a very close capacity and work 100%, then there is no balancing to do.

as some of us do top balancing we are very aware how far apart the voltages of the individual cells at the top end are. could you do as all a favour and measure your individual cells just before your charging goes into float? (you might have to hold the voltage there and disconnect the inverter etc to have a stable total voltage)

this is absolutely not to criticize you at all. it is purely a matter of curiosity.

the balancing circuit boards i am using use 2mA. that means in my case it would take 11 years to discharge the lithium battery  to 50% SOC. ( taking  what you said that there is absolute no internal self discharge in lithium batteries.)
read this here:
http://forums.energymatters.com.au/off-topic/topic6629.html

Kurt went through the trouble to measure all the details of the balancing boards for the people that are interested about it.
(we generally believe those circuit boards do balancing at a somewhat too high of voltage)

if you could take the time to give us those measurements we would all appreciate it.
thanks in advance for your effort
urs

[mike90045]

I'm going to agree with Cniemand on the bottom balance.  Do it right and set you OVP and LVD properly, and except for an annual double check, there's nothing else to do.   do you trust the LVD in a $5K inverter, or a dozen PCB's  with questionable QA?

And the balance boards, in a 48V bank, you have a dozen chances or more, one will fail and take out a cell with it.
It's not like NASA where the contract is awarded to the cheapest bidder !! (but the DESIGN is fail-safe or redundant)

[offgridQLD]

I think by using a reasonably conservative primary inverter LVD and backing that up with cell level board LVD as a secondary measure. You have double redundancy.

I actually have a 3rd backup that is my Auto start generator(never gets used other than emergency backup) It will kick in if the bank SOC gets to 20% SOC and will recharge it to 80% SOC.

So not likely that my cells will be drained to 0% soc.

My issue is that bottom balancing isn't a once off thing (no balancing is)  and to get a accurate bottom balance you need to drain the cells down to a very low SOC. So every 3 - 6 months (or how often you are doing it) it would be very difficult to hold a bank at such a low SOC while the system was in use (actively powering your home) Then your stuck with a flat battery and a offgrid house ....(wife & kids) that wants the tv, lights and fridge to work and couldn't give a rats about  your 10% SOC bank and balancing.

You only get the opportunity to see how well balanced the pack is when you drain it down so low. As most people would spend the most part with there bank in the 40% SOC - 95% SOC range. Top balancing you get some indication of balance every time you hit absorb. Sure it's not as telling as when you take the voltage up higher to do a proper balance. Though you still get some clue that something isnt way out at absorb voltage level.

A bottom balanced bank you are going to have no clue whats going on until your willing to drain the bank almost completely.

What I do agree with and I think most of us do is balancing is only done to keep the cells aligned at the extreme ends of SOC both low and high to avoid issues of capacity scope limitation .

So it's not good saying . I want a bottom balanced bank as I don't have to rely on  $30 electronics that could fail to protect from low voltage" because the same can be said for a bottom balanced bank and high voltage..... whats protecting your bottom balanced bank from a run away charger as the cell balance is going to be out at the top end. If your charger....(electronics)  didn't stop charging or did some stray EQ or over charge situation the cells will be toast. You need the electronics to save the day.

At some point with lithium cells we need to really on electronics. Having multiple stages of protecting is the key .

Using the 5k inverter example isn't a good one as Im sure the components within the inverter that monitor and trigger LVD/HVD are most likely only a $5 portion of that 5k. At least with the cell boards they have just one function HVD/LVD and the full $15 - $30 is dedicated to that one function.

Kurt

[CDN-VT]

Wish we had a "like , Like " button  or something , It's good to read others thinking .

VT

[australsolarier]

like always,
well said Kurt

[TomW]

No offence, folks, but this thread has gone far afield from a blacked out  MNGP issue.

Should I go back and split it off to a new topic or do you want to  start another thread for it.

It makes for better search results later on as folks hunt answers.

Tom

[Resthome]

No offence, folks, but this thread has gone far afield from a blacked out  MNGP issue.

Should I go back and split it off to a new topic or do you want to  start another thread for it.

It makes for better search results later on as folks hunt answers.

Tom

Tom..  I vote to split it off, so the topics are better searched.

[Cniemand]

@ OFFGRIDQLD : What mechanism is causing the "bottom balanced" pack to drift that creates a need for quarterly rebalancing? The idea behind it is to balance at near 0% and then charge them all up to the level of the smallest capacity cell. So a range of 0-98aH (in a 100aH pack) so that you never see the voltage deviation/spikes at the top. In this thread I have read about some people adding slight charges at the top which of course would "ruin" a bottom balanced pack. That would be the only way to disrupt the balancing as you switched it to be a "top balanced" pack.

I, Personally do not see a Top Approach as a balanced pack. The variances are inherently there. You are relying on shunt current around to keep it all going.

@ Austrolsolarier : I could measure each cell specific. I may edit back and do so, but what occures is thus. In my 16 cell 100aH setup I have 15 cells that are above capacitance than one cell in particular. I've programmed the Classic to charge the pack at 3.5vpc and as the pack fills up each cell is in very tight tolerances (within a mv or two) of each other (having been balanced at 0% SOC years ago). When it gets close to FLOAT, cell # 2 begins to deviate several mV and then starts to shoot off. (due to having a lower aH capacity than others = full = higher voltages) at which the whizbang jr aids the Classic to switch over due to it reaching a ENDAMP of 5a.  The surface charge is slowly burned off from the pack and they all register the same voltages across the pack while in FLOAT mode. At night as they draw down, they remain within a couple mV of each other.  Rinse. Repeat. 3 years later.

EDIT : Also, If you all are suggesting that most people do not use a majority of their LIFE cell capacities,  (which is probably true. I rarely use more than 40-50% of my pack on a daily run. My Pack rarely is below 55%SOC in the morning) why are those that believe in "Top Balancing" trying to eek out every last drop of their packs and risking the problems of having voltages deviate and shoot while playing with that portions of the charge curve? You guys aren't really gaining any real useful capacity through it. Just more headaches having to deal with shunting amps around to keep it all together....

[BobWhite]

No offence, folks, but this thread has gone far afield from a blacked out  MNGP issue.

Should I go back and split it off to a new topic or do you want to  start another thread for it.

It makes for better search results later on as folks hunt answers.

Tom

Tom I'm with John, split for an easier search. I personally get frustrated.with difficult searches and so far it has been great here!!

Walt

[Cniemand]

You only get the opportunity to see how well balanced the pack is when you drain it down so low. As most people would spend the most part with there bank in the 40% SOC - 95% SOC range. Top balancing you get some indication of balance every time you hit absorb. Sure it's not as telling as when you take the voltage up higher to do a proper balance. Though you still get some clue that something isnt way out at absorb voltage level.

A bottom balanced bank you are going to have no clue whats going on until your willing to drain the bank almost completely.

I would counter that in Top Balancing... you never have an indication of a balance. One is still allowing the system to use every available aH in the pack. Cells only begin to deviate once they start to fill up based on their own specific capacity irregardless of the others in the pack with them. All Lithium batteries will reach a certain voltage indicative of a certain SOC. A 85%SOC 100aH cell will have the same voltage as an 85%SOC 400aH cell.

I'm very curious as to why everyone wants to use every last drop in their cells? Over-charging cells is just as detrimental to cycle-count as over-discharging....

[offgridQLD]

I think your misunderstanding how often we are top balancing. Its a bit like me saying "oh your running your bank down to 0% soc all the time as your bottom balancing every day"

I'm not using every last drop of my cells . I charge to the same conservative absorb voltages as you mentioned...  3.5v pr cell. Your making it sound like people top balance every day. Going over 3.5v pr cell once every few months is not a issue. Im not shunting current around all the time. In-fact I don't really use the shunt feature on my boards. I just give the low cells a zap with a single cell charger while doing a very slow low rate high absorb every few months.

I'm not to worried what the MV variation is in the middle of the SOC as it's meaningless and is always tight (within a MV or two) in this SOC range with lifepo4. Even if the cells are out of balance. Every body's bank holds a tight MV reading in this area no matter what balance condition its in (well within reason)

Te way I see it if you have a one cell that shoots high first then its just the same as a person who top balance's and has one cell that shoots low first.  Though every time that cells shoots a little bit high its getting slightly out of balance to the other cells. Its a tiny amount but over time it grows.

there is no perfect battery. expensive OEM cells like the ones in my Electric car are very good. All 88 cells hold tight and are always within 5mv of each other at full SOC. They are absolute abused in comparison to stationary storage batterys .Though there is a cost to this kind of uniformity.

As mentioned just out of interest run the cells down to the very low SOC what ever voltage you initially used to bottom balance and report back on how well balanced they all are now 3 years on.

Edit: at 3.5v pr cell average absorb voltage setting your using. What voltage do you see on (Cell 2, the low capacity cell that shoots high first) befor end amps are triggered?

Kurt

[australsolarier]

Cniemand,

nobody here is criticizing or saying anything you do or say is wrong.

all i asked is the voltages. we top balancers are aware of those top voltages, because every so often, once a month or every three months, we look at them. and take action or at the very least worry when some voltages go apart.

so just out of curiosity i asked for YOUR voltages. i can then compare them. get information.

the charging procedures of those lifepo4 batteries is still in the experimental stage. mostly because the chinese manufacturers just manufacture and sell them and don't seem to be interested to figure out proper charging algorithms, etc.  as a matter of fact i do not think they would really care if 40% of their cells  fail after 5 years.

so the top balancers generally absorb the battery 3.5V/cell, the same like you. we then go up to about 3.6V every so often to give us an idea if there is a big divergence of battery capacity. we go up there not to eek out the last .01% of capacity, but to get an indication. we then either manually balance the cells or let the bms circuit boards do the balancing.

seeing how well kurt's battery bank is balanced, it would probably take ten years to create problems with inbalance. how ever  there is another person we know of that seems to have a serious problem with one of his cells. to the degree where he thinks about replacing it. but it probably is still 95% of the rest of the cells.
so we check our investment, as you would check the oil levels in your car, or the water level in lead acid batteries.

greetings urs

[russ_drinkwater]

I think I will stick with the FLA trojans for now and let time sort out the pros and cons of lithium batteries.
Hopefully in another few years the designs and construction will be better and cheaper.
Anyone know what the age of the oldest fully functioning lithium cell is?