Lithium house bank. Affordable from China?

Bert, when cycling a lifepo4 house battery bank, the minimum voltage should not be below 2.8v per cell. You can go below this, but you are shortening the life of the battery. 2.8v per cell is generally considered the 20% SoC voltage. So an alternator should never see 2v per cell unless youre planning to damage your cells... in fact, one of the benfits of lifepo4 is that even at 20% SoC, when you should be cutting off the load to prevent damage, the 12v nominal battery is still delivering +13volts under load and all your devices are still running 100%!!!

Same thing at the top end occurs at 3.4V per cell for approx 98% SoC. Again you can go above this, but your shortening the life of the battery for only a few percent gain in capacity - its generally not considered worthwhile going after the last few percent for this reason. Some manufacturers even quote 3.6V to be 105% of capacity... The difference in thinking from old school LA chemistry, is one should NOT adopt this 100% charged voltage, as a float voltage as its slightly above the point where your reducing the life of the cells... Thus with a +50mv charge to accept current into the battery, 3.45v is about ideal for an almost full charge with excellent longevity. Anwhere between these volatages, and you can pretty much do your pleasing in a practical sense, charge/discharge/storage, with these batteries and expect them to last a very very long time... if only Lead acid were so simple!

The 2 considerations to prevent battery damage are avoiding draining the batteries too low, and avoid over charging them too high. By limiting the float charge voltage to 13.8V on a solar array in normal usage, and 13.6V when not in use, keeps the batteries in good condition whilst still close to fully charged. So just a tad lower settings on the regulator designed for LA batteries... Having a simple Low voltage cut off isolator, saving the battery from being drained for whatever reason or breakdown, is simple protection at the low end. Thats about as complicated as it needs to be. No BMS is even required. Manual checking and balancing from time to time would be a good idea tho, but according to the guys that are already using this simple system and checking manually, its proven that they dont even need to balance them after 12months, all cells are within 0.1V even after cycling 80% everyday during this time according to 2 different people. Neither are using a BMS. They both report Lifepo4 batteries to be much easier to deal with compared to Lead acid, provided simple measures are taken to keep the cells within the 2.8-3.4v range...

 

What I ment was, a LifePo4 battery start charging already at 2 Volt, although it is a 3.3 Volt battery, versus Lead Acid battery at approx (temperature dependent) 2.08 Volt per 2.35 Volt cell. I have now the 40 LifePo4 batteries for 3 years and I am quite happy with their performance. I charge sometimes 8 at the time in series on my variable charger and sometimes each cell separate on a single unit charger I have build. It depend how my mood is. I agree with you that by not deep charging and fully charging to the last bit, one gets away with a simple checking method.
Bert

 

Hi Groper,

I started some 2 years ago with a simple BMS, but never improved upon. I like to discuss with you, what you would do.
1) Assuming I have 6 batteries LiFePo4 in series. i.e. 6 x 3.35 = 20.1 Volt
2) I have each battery paralleled to 5 others i.e. 6 parallel. (6 x 6) (Suggested by Jeremy Harris)
3) I have 2 wires. One from the plus (20.1V) and one coming from the string of 4 batteries. (13.4V)
4) I take your suggestion and store the minimum value of 2.8 Volt per battery and 16.8 Volt in total somewhere in a register.
5) What algorithm would you create in the software program or would you agree to the following

6) Switch on.
7) Measure plus i.e. voltage of 36 batteries. Approx 20.1 Volt
8) Measure at 4 batteries and I will have approx 13.4 Volt
9) After a little while , the batteries are slightly discharged and I measure the 36 batteries and I measure 19.8 Volt . I calculate each battery = 19.8 /6 = 3.3 Volt
10) At the same time I measure the 4 and this is 13.2 Volt and I divide this by 4 and this is also 3.3 Volt
11) No problem. No battery out of norm
12) Now the Plus Voltage drops to 18 Volt and at the 4 batteries the voltage is still 13.2 Volt . i.e. I have a problem battery in the string. Because 18 – 13.2 = 4.8 Volt divided by 2 = 2.4 Volt which is lower than the minimum standard I set at 2.8 Volt. (or visa versa measured at the 4 batteries)
13) In the above case I have to shut the battery down and have to trace where my problem is.

Now my old scenario
a) I have 6 batteries in series and at each fourth battery I have a wire coming out to the microprocessor. i.e. 12 wires in total. (minus is common)
b) At each plus side I have a 40 Ampere Schottky diode going to a plus bar.
c) I loose 0.35 Volt x current of 50 Ampere = 17.5 watt loss at a usage of approx 1 Kilowatt.
d) Now I measure the plus voltage of each string of 6 batteries in series and also the 4 and I divide and compare with the other 5 strings. Any wild deviation I DO NOT SWITCH my motor off, but check that particular string and find the problem battery. The motor will run on the other 5 strings of 6 batteries. (Each battery is 5C , thus 25 Ampere x 5 = 125 Ampere maximum,. I use only 50 Ampere, good safety factor.
e) To charge the system with the solar panels (I bought 2 x 240 Watt = 480 watt absolute maximum) I consider to have the regulator feeding another set of 6 x 40 Ampere Schottky diodes. Diagram attached.
Groper, what would you improve or do differently ?

 

I dont follow everything youve posted just yet, but let me start for asking this;

Have you parralelled your cells first to get required capacity, then connected the parrallel strings in series to bring it upto required voltage?

Reason i ask, is if you do this the other way (which it sounds like in your post) and series the cells together first, then connect the series strings in parrallel, it creates a much bigger cell balance problem. By parralelling the cells first, each string of parralell cells self balance by themselves, then you only need to balance each parralell string with the other 5.

Not sure if thats what youve already done or not?

 

The thing with lithium, due to the very flat discharge voltage curve, as that you can only top balance or bottom balance the cells. Using voltage to acertain SoC only works at the top or bottom end, because the difference in the middle is not really great enough to be measured within manufacuring tolerances. ie. you cant tell the difference between a cell at 40% or 50% SoC as the voltage difference could be less than 1mv.

So using the sharp knee voltages, you can then begin to tell whether a cell is near fully charged or near fully discharged, ie top or bottom balancing.

Now, for a house bank, we dont want to bottom balance, because we want to keep our bank full... if we were to bottom balance, the BMS would never get the chance to do it unless you drained the pack leaving you with no power. So it makes sense to top balance.

One approach is to simply have individual cell loggers - this should only be required for each parralell string, as after initial balance, they will self balance within thier respective parrallel string. So we should only need to balance each string, with the other series strings. The method was very simple, using a charge voltage of 3.6V per series string, if one cell goes high, the cell logger alram trips a HVC when a string hits high @ 3.6V with a cutoff timer of say 10mins... The house loads pulls the high string back down quickly because the Lifepo4 chemistry doesnt hold any capacity above 3.4V. The low strings below 3.4V are barely affected in voltage due to the flat curve. The the charge kicks back in charging all the cells again, before tripping out again on the high string etc... the process repeats continously until all the low strings have caught upto the high string and thus are fully balanced at 100%. After that, reduce the float voltage to 3.45V per cell and all cells should be at the same SoC, which is what we are really trying to acheive...

 

There are 2 scenario's , a) 6 batteries parallel and then in series. b) 6 strings of 6 batteries in series and at the top via 6 Schottky diodes to a plusbar.

agrreed, but like all things in life one has advantages of the one system and one has advantages of the other system

No I haven't tried Jeremy's Harris proposal to parallel 6 batteries and than in serial.
Bear in mind I have Schottky diodes between the 6 strings of batteries and therefore I am able to disconnect one string without interruption of stopping the prop.

 

I got the feeling that I have been over-cautious. As you have said, the LiFePo4 batteries have a very flat voltage curve. What my supplier has in their instruction, max 2.0 Volt at the low side, max 4 Volt at the high side, but preferable 3.65 Volt maximum at 25 degrees C. What they haven't informed me is, at what point will the voltage sharply drop. at 5% of capacity or at 20% of capacity or at 1% of capacity. If I knew that, then it would be easier to make a simple BMS. I need a BMS to control the Solar panels in anyway. Thanks for the exchange of idea's. Bert

 

2.7-2.8v is considered the 20% SoC voltage, for long life i would avoid going below this level however a brief trip below this would not be catastrophic by any means - not till you go below 2.4V are you in serious trouble.

This is from the worlds largest lifepo4 manufacturer;



At the top end, the voltage rises extremely rapidly.

3.4v rested, is 100%. Above this voltage, there is no more capacity, only heat and damage from lithium plating inside the cell.

 

You know Groper, I may be a stupid old man, but why do we really have to balance the batteries?. For what? To get instead of 1500 cycles 1400 cycles? I will be than 140 years old instead of 150 years. One battery can have 5 C, i.e. 5 times the Ah rating. If one battery is feeding a little more, so what!

In my case , I take them out and charge them individual in any case until full or near 99% capacity. Also in my case I can carry on with running the prop and able to charge the string by the solar system or whatever, or at night when I come back.

For me it is extremely important to know, at what point the curve sharply bent. O.K. O.K. everybody will say take one battery and find it out. Put a globe (bulb) on it and monitor every 1 minute. and make a curve. Maybe I have been an idiot and should have done that a long time ago. But in my defense, I did not want 39 good batteries and 1 bad one, as I had a 8 x 5 configuration which I am in the process of changing to 6 x 6.
Bert

 

Thank you, Groper. It saves me from messing one battery up. We elder people are inclined to forget. I wouldn't be surprised that I forgot and had a battery discharged to below 2 Volt, if I got distracted.

 

Thank so much Groper. My supplier tells me max to 2 Volt with de-rating to 70% capacity at the end of 1000 cycles. In view that I am operating in the dark blue to green /yellow area, I take a chance and make my lower level between what you suggested 2.8 Volt and what the supplier stated 2.0 Volt. i.e. I will go for about 2.5 Volt. It will give me some more juice. Don't feel offended, I appreciate your input tremendously.

Now , I only have to make a decision on 36 single batteries, nicely monitored by 12 wires and can at any time replace or check a battery. VERSUS 6 batteries parallel.
Groper what would you do? Is this balancing really such a problem when one charge the batteries each single mode after a trip? ( 8 single charge stations 3.65 Volt) Thus, it takes me 2 nights and everything is ready for the next week-end.
What would you do.
Bert

 

The reason for balancing to get all cells to the same state of charge. Why? Several reasons... to get maximum capacity from the pack, And to prevent damage to the cells by over charging or too deeply discharging a cell in the pack.

If we have high cell, and cell voltage is not monitored, only battery voltage, that high cell could reach damaging voltages before the charger stopped charging. If were using a HV cutoff, then we wouldnt be charging to full capacity either as the low cells still have more to absorb.

Then in a similar fashion, if we have a low cell, and a BMS or alarm tells us to cut off the load to prevent battery damage, tripped by a low cell when the others still have plenty of charge left in them, were not using our full capacity either. So balancing is a good idea, i think keeping things simple is also a good idea tho

 

Groper, it is bedtime for you, thus I made a decision. I don't believe that if I charge my batteries all of them in a single mode i.e. charger with 8 or higher single charging stations. That during discharging !!! on one trip, the batteries will go out of balance so badly , that I need a balancing system. I made a decision to monitor 36 single (6 x 6) batteries with 12 wires. Thank you so much for clearing my thoughts.
If I have made a mistake, I will tell you in 5 years time and apologize. Because I cannot see that it will show up earlier.
Sleep well and thanks a lot.
bert

 

As far as "quality" I've noticed in hand power tools you 'get what you pay for' and rating numbers are meaningless.

Company bought some $24.99 Ryobi 12volt cordless drills and they literally could not drive two screws on a full charge. It would die 1/2 way through 2nd screw.

Then you got a 9.6volt Hilti and you can just about leave the charger at home.

 

Hi Squid,

Your company haven't bought Lithium powered drills. They normally are sold as 14 Volt or higher. Funny, I have good experience with that make. Are you sure, you did not buy one with a 220 Volt charger and you have in the San Fransisco area 110 Volt. You will never get the batteries fully charged.
Bert