Incrementally adding DC batteries

svetz

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I had been hoping Craig would bring this thread across as I'm in the same boat. Hopefully one of you all know...
So I have always figured that I could incrementally add storage to my system.

What I'm reading is that you shouldn't parallel batteries that do not have same IR and capacity. Seems to me that say 2 years down the road the original pack would be somewhat worn out and have say only 75% capacity.

So question is: how can I add new batteries later without prematurely aging the new ones? What is the procedure?
As Craig said, adding a new parallel bank the following year means the two banks will have different internal resistances. Which means the new bank will supply more power and get more wear. But the old bank isn't working as hard, so it shouldn't be aging as fast. Similarly, the new bank isn't handling all of the power the first bank did the first year, so it won't age as fast as the first bank did. So, no net problem?

For charging, both banks would be controlled by thier own BMS, so there shouldn't be any stranded power as you might have with an unbalanced cell in series, right?

Despite one bank working harder, both banks would still always be the same voltage. So it doesn't seem like there would be any extra wear and tear in terms of one bank backfeeding the other?

So, what do you think?

Bonus Question:
Do the BMSes have to be exactly matched for parallel operation or are all BMSes the same? If they're not all the same, how do you get matched ones?
 

gnubie

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Hmm, forgot about that completely. I meant to do some practical testing on the weekend.
 

b.james

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What happens is the battery pack with the lower resistance passes more current.
People parralel LFP like Battleborn etc without care but they will have different resistances too.
Old lead acids are paralleled all the time and the same happens there.

I have found that new LFP from the one supplier are nearly all the same internal resistance.
If it worries you get a battery resistance measuring device and put another resistor in series with a low string .
There is a way of calculating internal resistance with a stock resistor and a multimeter but I forgot how to do that years ago.
 
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svetz

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Someone in the battery crowd came up with a tool where you input all the internal resistances and how many s and p you want; then it pumps out a battery build plan. But I didn't want to rebuild the packs every year.
 

rkw

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What happens is the battery pack with the lower resistance passes more current.
People parralel LFP like Battleborn etc without care but they will have different resistances too.
Old lead acids are paralleled all the time and the same happens there.

I have found that new LFP from the one supplier are nearly all the same internal resistance.
If it worries you get a battery resistance measuring device and put another resistor in series with a low string .
There is a way of calculating internal resistance with a stock resistor and a multimeter but I forgot how to do that years ago.

That makes sense. What brands currently viewed as lower end (just perception due to lack of familiarity) change cell suppliers? By the logic ya showed, is it possible there could be a significant difference in IR between the of cells used to build a battery? So maybe a batch from February might perform differently from a batch made in October?

in my mind my guess is a "maybe" which always helps ooooh so much... haha
 

b.james

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Most but not all manufacturers quote less than 3 milliohms . My guess though is they are mostly using the same methods techniques and chemical mixes to make the plates and separators so cell resistances should be pretty darn close when purchased.

This is useful to explain resistance

A quote from that is :-
Now, the internal resistance of the battery is making the charging voltage at the terminals look higher than it actually is in terms of actual state of charge of the battery. Here is a real-world illustration of this behaviour:

We were in the process of building a brand new lithium iron phosphate battery bank on a sailing catamaran, charging 400Ah of cells for the first time with both engines running. The charging current had been a solid 180A for almost an hour. The cell voltages, which had initially jumped up around 3.40V, were gradually rising. When they reached 3.60V, we shut one engine down in order not to exceed this value, reducing the current by half, down to 90A.

The cell voltages instantly dropped down to 3.45V.

We therefore lost 0.15V in cell voltage by reducing the current by 90A. We can use these figures to calculate the internal resistance of the cells using the relation presented earlier, ΔV = R x I:

In this case, we have ΔV = 0.15V and I = 90A. As a result, we can write R = ΔV / I = 0.15 / 90 = 1.66mΩ

1.66 milliohms is a very small resistance figure typical of lithium battery cells, but it is nevertheless enough to significantly skew the voltage reading at high amperage. At a current of 10A, its contribution becomes only ΔV = R x I = 0.00166 x 10 = 0.0166V = 16.6mV, but still enough to be measured.

If you think about that for a bit you start to wonder how they had the cells connected at that point . My guess would be all in parralel for that low resistance but then they had the motors and alternators connected which might indicate series connection? Don't understand that yet. Perhaps its just a made up story.
 
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svetz

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So, what's the best way to add new batteries to a battery bank every couple of years?

A number of us "had" plans like the one proposed in the hotch-botch-system; e.g., 25% for 4 years to make it affordable. The question of premature aging as described above has us scratching our heads as to if it's a real problem or not. Could be there just is no choice.
 

burtinator

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You have to be careful of internal currents between the parallelled batteries. I have an electric scooter with a 3kwh battery to which i have added a 2kwh battery in parallel, after i have been driving i can see that there's a current flowing from the one pack to the other. So while the voltage might be the same because they are parallelled, they are charging each other.
In my situation it's not really a problem because the batteries can handle the extra current, so as long as you arent using the maximum current the batteries can handle and the batteries get the time to balance each other you should be safe.
But as always on the internet, don't take my word for it, always measure it yourselves.
 

svetz

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A you probably guessed from what I cobbled together in the Battery FAQ, I've been reading up on this and trying to figure it out... I think I might have an answer, see what you all think....

TL;DR: The procedure to minimize aging effects is to add the additional battery banks as quickly as possible, sizing the first string such that it won't exceed 4C.

From this paper, the largest factor in LiFePO4 ageing is heat. That led to C-Rate is what causes the heat and this paper quantifies the relationship:

Capture.PNG

Unlike lead acid, lithium has two saving graces: a High C-Rate and a high number of cycles. As you can see from the chart, staying below 4C minimizes the effects of degradation and if cycling once per day 3 years has less than a 1% degradation effect. So while you can't eliminate the impacts,you can minimize them

So, using that table let’s compare two cases: Option 1) Buy 3 parallel banks up front. Option 2) buy one bank per year over 3 years.

Let’s assume:
  • There will be 365 cycles per year
  • Under normal use in option 1 each bank runs at 1C
  • Under normal use, bank 1 runs 3C in year 1, then 1.5C in year 2, than 1 C thereafter.
  • Starting internal resistance is 60 milliOhms/cycle/cell (for 48V that’s .96 ohms)
  • So from the formula at 1C the batteries degrade .1% /yr, at 2C the batteries degrade .12%/yr, and at 3C they degrade .14%/yr under 4 years.
Capture.PNG
 
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svetz

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One of the ideas in the hotch-botch-system proposal was to also incrementally add inverter/chargers. Using inverters that could be paralleled together would be an effective way to prevent excessive C-rates with a current limiting device, current would be naturally limited since the inverter could only pull so much power.
 
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Craig

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One of the ideas in the hotch-botch-system proposal was to also incrementally add inverters. Using inverters that could be paralleled together would be an effective way to prevent excessive C-rates with a current limiting device, current would be naturally limited since the inverter could only pull so much power.
Im missing something here if you add inverters wouldnt that possibly increase C rate?


Also, just to make you do more work, If my battery supply equals my average daily demand for 1 day and I add a bank equal to my first bank. my C rate would cut in half and my discharge amount would also cut in half. Reason for adding more batteries would be to have more storage for those times when we get no sun for a prolonged period.

Do 2 1/2 cycles = 1 cycle?
If it does you are still prolonging battery life by adding more storage because now you are cycling only half as many times per year.
 

svetz

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Great work SVETZ!!!!

Or utter hogwash. It's not like I'm an expert, I just think about stuff I find on the web, hope it's accurate, and that you guys point out the flaws.

Im missing something here if you add inverters wouldnt that possibly increase C rate?
Year one you have 1kW inverter and size X battery bank. The most it can draw is 1kW.
Year two you add a second 1KW inverter and add another sized X battery bank giving you 2X. Now you draw 2kW, but you have 2X batteries, so C-Rate is the same as year 1.

Do 2 1/2 cycles = 1 cycle?
Yes. Even a 50% depth of discharge is counted as 1 cycle on both banks. But it's a lower depth of discharge and a lower C-Rate; so overall has less degradation to the battery.

What would be really interesting to know is what happens if you didn't recharge at 50%; but waited until the next day. Did you just double the life of the battery by cutting the number of cycles in half? Or, would charging at the lower depth of discharge give you a longer lasting bank?
 
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svetz

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Updated a bit, also attached the circuit source for the program. At a draw of 5.5 kW in year 10 there's a 1 amp imbalance.

Capture.PNG
 

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HighTechLab

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I had made a post on a user asking about batteries that were used and how to set them up in a 5p4s configuration, and here is what I had wrote:

I would parallel your strings before putting them in series - this makes balancing so much easier. So you would make 4ea 5p1s strings then tie it together to be the 5p4s.

If you do it this way you only need a 4s balancer/monitor as it can't pull any single cell out of range, it would have to pull 5 cells out of range.

I would try to make each parallel string of 5 cells average out in capacity, e.g. if you have a 100ah cell and a 90ah cell, then put those together in parallel to get 95ah average capacity, then series with another group of equal average capacity.

you don't want to series up differing capacity for a variety of reasons - if one group of 5 gets near full and the other 3 groups are at 80%, then you will have to stop charging to prevent overcharging that one group - Meanwhile you have 3 other groups that are only 80% full - leading to wasted capacity.

If I were adding cells incrementally, I would add them in parallel with EACH CELL instead of as as a 12v string then parallel the 2 or 3 12v strings. Then the main 4s BMS can keep track of all the cells and as mentioned earlier in this thread, the IR from the worn cells being higher would over time cause these cells to even out.

My balancer (Not technically a battery managment system, just a power shuffler) can move 6 amps between series interconnected cells and makes my bank of used CALB cells stay nicely in balance and over time the "hotter" cells have come down and the "colder" cells have been preserved pretty well.

Thoughts??
 

svetz

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If I were adding cells incrementally, I would add them in parallel with EACH CELL ...

Less anyone get confused, the thread's question isn't about building batteries, it's about how to add a new string of batteries yearly to an existing battery bank to minimize the internal resistance (IR) problem.

That said, HighTechLab's approach is what I think battery builders do when adding cells to an existing pack. When they have 4 new cells they can extend an existing 4s4p to a 4s5p; and they'll live with the IR problem degrading their cells. But, if they had enough batteries for a whole new second pack they'd just build the second pack to completely get around the IR problem since usually it's for an eBike rather than a PowerWall they can easily swap out batteries.

I don't see how a BMS or balancer would help minimize the artificial aging problem of mixing new cells with old cells.

With HighTechLab's suggestion the first year battery-bank might be 18650s as 14s36p (i.e., 5kWh, 48V). In year 2 you extend that to 14s72p, and year 3 14s108p. It would be interesting to see how the currents would play out per cell in a 10 year scenario. But overall, that's more "p" in a single pack than I'd want.
 
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svetz

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Solutions to date:
1) Each year fully disassemble the battery banks into cells, individually re-measure every IR, use a program like repacker to redesign and rebuild into two or more new battery banks (might not be too bad with a design like Jehu's circuit boards with the spring clips).
2) Size the first bank to run at < 4C, then you can take advantage of the information of the table in this post to minimize the imbalance.
3) Add new cells equally onto "rows" of old cells (14s36p -> 14s72p , 14s108p)

1 & 3 won't work obviously if you're not working in cells (e.g., Battle Born)
 
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Rider

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1 & 3 won't work obviously if you're not working in cells (e.g., Battle Born)
Yeah.... building batteries is a nice challenge, you can tailor it to your specs and save a bunch of money, but I'd much rather just get a completed battery, no wiring of a BMS, no muss, no fuss, no maintenance, just hook it up and use it.

Which brings us back to the original post... how to insert whole batteries into an existing battery array, keeping in mind the degradation of the original batteries. If the batteries are added within 5 years, I don't see a problem at all. A typical LiFePO4 still has 80% usability after 25 years! So after 2-3, even 5 years, the change in the cells is minimal, especially if you haven't been deep cycling. Beyond that, consider creating a new battery array and sell off the old ones!! They still have tons of value to someone else. The sale of those batteries helps pay for the new ones.
 

svetz

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In one of Will's posts he recommends a max charge rate of .4C; not sure where the number comes from; but based on the chart it should help to reduce mismatch down the road. I know the posts above say less then 4C for discharge based on the curve, but after hearing 2C is more the upper limit for LiFePO4 cells suspect that 4C is too high. Obviously the lower you go the less heat, the less damage.

In a recent test I did, I found that my cells were getting hotter over time while discharging at the same rate; more than I believe was simple heat buildup. So, while I suspect it's okay to go to 90% DoD, I suspect the maximum C-Rate should be function of SoC. I'll see if I can find anything about internal resistance as a function of SoC.
 
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