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One large battery vs multiple smaller ones

Peer

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Dec 25, 2021
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6
Hey everyone!

I am planing to switch to an electric propulsion on my sailboat. And I am currently thinking about the battery.

My current plan is to build 3 separate 16S 48V Batteries from 105Ah EVE Cells (probably from Luyuan). Each battery will have its own BMS and circuit breaker. Currently I am tending towards a 200A JK BMS.

That would give me roughly 15kWh in total (3x 5kWh) or 300Ah on 48V.

The reasons for not just doing a single 16S Battery with 304Ah EVE cells are mainly weight and space. Its far easier to handle 3 40kg batteries on the boat than one 100kg one. Also it would be much easier for me to find space for three smaller boxes still close to each other but apart than one single huge battery box.

I did some research, both here in this forum and youtube. Especially, I found one paper from Orion BMS that is heavily against using multiple strings of batteries and prefer to just parallel multiple cells in one battery: https://www.orionbms.com/manuals/pdf/parallel_strings.pdf



For my comparison I will take the prices and specs from Luyuan. Just the base cell prices from the store excluding shipping.

EVE 105Ah (per cell):
- Price: €45.13 / $49.80
- Weight: 2.0kg / 4.41lb

EVE 304Ah (per cell):
- Price: €126.85 / $140.00
- Weight: 5.5kg / 12.13lb

So to get the desired 15kWh battery capacity there are two possibilities:
Option 1: 16S 105Ah Battery (per Battery)
- Price: €722.08 / $796.80
- Weight: 32kg / 70.56lb

for 3 Batteries this amounts to:
- Price: €2166.24 / $2390.4
- Weight: 96kg / 211.68lb

Option 2: 16S 304Ah Battery
- Price: 2029.60€ / $2240
- Weight: 88kg / 194lb


The actual cost will most likely be even higher for the 105Ah cells compared to the 304Ah ones as I will probably have to pay a lot more shipping. The 105Ah are shipped 4 batteries per package while the 304Ah are two per package. So roughly 50% more packages.


Requirements:
The electric motor can pull around 15kW (300A at 48V) continuous. My plan is to limit this to around 10kW (200A) continuous. But the usual draw is planned to be between 1kW (20A) and 5kW (100A) in normal conditions. Everything above is just for bad weather and emergencies. Which of course can happen, so the system should be able to handle 10kW (200A) continuous. If I later feel that this is not sufficient enought, I can increase this up to 15kW (300A).


Now I will go into each individual point and compare the two versions while referencing many of the major issues pointed out by the orion bms paper:

Weight:
For simplicity I will add around 10kg / 22lb to each 105Ah Battery and 20kg / 44lb to the 304Ah. So this comes up to around 42kg / 92lb for one of the three smaller batteries or 108kg / 238lb for the big one. The first I can take out of the boat with two people for maintenance or winter storage. The big one, well, I guess I will probably need a crane or some really strong helpers. I do not care that much about the total weight as I will be removing around 400kg (900lb) of diesel engine, fuel tanks, fuel and other stuff. For a car this would be a bigger issue.

Space:
Same goes for the space. Three batteries will need more space. Each having larger cells (per Ah), their own case, mounting, bms, breakers etc. I would assume that two 105Ah batteries will be roughly as big as one 304Ah battery. But as each one is beeing drastically smaller, I think its easier to put them into the available space on board.

BMS:
Using 3 smaller batteries allows me to use a "classical" (aka mosfet) BMS like a JK with around 200A max current each. So even if I pull the full 15kW (300A) this would "only" be 100A per battery. If I consider double the amount peak, that would max out the 200A BMS, but that should only happen for short times if at all. The 105Ah cells allow 3C (so 300A or 900A for all three) for 30 seconds while the 304 only allows for 2C (600A) for 30 seconds. And both have 1C continuous. So this would be fine in both cases. But I wouldn't like using a mosfet BMS for currents around 600A (even though they exist). In case of a single large battery I would tend towards a batrium BMS or something similar. But with a trip shunt and everything this will probably cost me a lot more than 1000€ ($1100) compared to roughly 600€ for three JK BMS.

Price:
I believe the three battery version will be more expensive. I will need everything three times. BMS, Case, Breakers etc. But, each component will be cheaper. But including the shipping, I assume the three battery version will be quite a bit more expensive in the end. How much? Until I requested some reliable quotes on all parts, I can only guess.

Cascading failure:
Imagine one BMS or circuit beaker tripps while being at full throttle. Then the remaining two batteries have to provide the full power by themselves. Without reserves, the remaining two bms will probably shut down quite quickly aswell due to overload. This is called a cascading failure. This can be addressed by having excess power available. I have planned that a single 100Ah battery can provide enough power to keep the max. cruise speed continuous (5kW / 100A). Two batteries can provide the (limited) maximum power of the motor of 10kW (200A) continuous. And if I would in crease the max power of the motor to 15kW, I would at least add one more 105Ah Battery to my system. So in case a single battery fails for whatever reason, the remaining ones should still be able to fully power the boat. On the flip side: If the BMS tripps (for whatever reason) on the single battery, you are immediately dead in the water. If this happens close to land or while docking, this can quickly lead to dangerous situations. For the three battery version, a single failure would not be a problem. At least if it does not go unnoticed.

Total capacity:
According to the Orion BMS paper linked above the total capacity of 3 battery version will be lower than the single battery. They explain this mainly due to the "Eddy currents". A term, I learned, some (many?) here do not like. Basically, if one battery is discharged to a lower level (e.g. due to a lower capacity or bad connection) that battery will be charged back up by the other batteries. So you have currents flowing inside your battery system. This effect should mainly show if you really use the full capacity of your battery bank. And even then it is disputed by some. Their reasoning is that due to the lower voltage of the depleted battery, it will just provide less power than the fuller ones until the voltages equalize again. In one test I saw on youtube a 5Ah battery was combined with a 300Ah battery and even in these conditions it worked out fine (within certain boundaries). But I don't know if these effects might still appear for equal capacity banks at very low SOC for example. So to minimise this effect I would buy high quality matched cells (thus Luyuan). Also, I never intend to use more than 50% SOC from the batteries except for emergencies. So I can live with a slightly lower max capacity I think.

Impact of bad cells:
Another point is that a single bad cell will pull down the entire string. In my case this holds true for both batteries. But a 30% lower 105A cell would reduce one of the three strings by 30% (so about 10% total capacity lost) compared to the direct 30% total capacity lost of a single 304Ah cell having 30% less capacity. And even if I parallel the 3 105Ah cells in one big battery I would suffer about the same capacity loss (around 10%). So unless I missed something, even in an 3s16p or 16p3s comparison this should have no different effect. Except for being harder to detect. And in my case, it would be even worse to have a single battery compared to 3 parallel ones (by a factor of three). But also a reason to go for good and matched cells.

Inrush currents when parallel connecting:
Well, basically don't connect Li* batteries in parallel if they sit at different voltages...

Lower maximum power:
I honestly don't see that point. Unless a single cell is seriously depleted and you try to pull the maximum current. For capacity, sure. But for power? Am I missing something?

Possibility of interrupted charge power:
In my eyes this makes no difference for 1 vs 3 batteries. If they shut down, its bad either way. But with three the chance is far less if you have reserves (see cascading failure). The only advantage I can see is if the BMS can talk with the charger. But as I will have multiple methods of charging, mainly shore power, generator, regenerative breaking while sailing and solar, I don't think I will find a system that can communicate with three different kinds of chargers.

Extensibility:
If I want more capacity later, I can increment in 5kW steps (instead of directly doubling it) without having to mix different batteries.


So to sum it up, the advantages and disadvantages of using three 105Ah batteries compared to one 304Ah battery:

Pros:
- Less weight per battery (more manageable)
- Less space required per battery
- Redundancy
- Easier to maintain
- Less currents per battery
- Easier to extend later

Cons:
- More total weight
- More total space required
- More expensive
- Cascading failure when used without reserves
- Less total capacity?
- Less power output???


What do you think? Am I missing something major?

Also, many "professional" electric boat propulsion solutions use 12V LiFePo4 Batteries in series and parallel. E.g. 4 strings of 4x 100Ah 12V Battleborns for 48V 400Ah. So this does not seem to be as bad as described in the orion bms paper? And those don't even have balancing inside the strings.

Thank you and sorry for the long post!

Best regards,
Peer
 
Last edited:
Using 3 smaller batteries allows me to use a "classical" (aka mosfet) BMS like a JK with around 200A max current each. So even if I pull the full 15kW (300A) this would "only" be 100A per battery. If I consider double the amount peak, that would max out the 200A BMS, but that should only happen for short times if at all.
Your analysis is pretty complete and sound. I think this piece (quoted above) is one of the critical bits that you NEED to make sure works for your system/loads.

Most other things are more of a matter of "few vs many" parts and connections.

See related:

 
I noticed I had a too low price for the JK BMS. I corrected it in the original post.

Does anyone here has experience with multiple banks in parallel? I would be especially interested if there is indeed a capacity loss or even a power loss?
 
Here is an excerpt from an article you may want to read: "
A sales manager at Sinopoly I was talking to was adamant about using 100Ah or 200Ah cells only for assembling marine battery banks, with 100Ah being preferred and 200Ah acceptable. Large cells simply don’t have the structural strength-to-weight ratio required to be taken to sea on board small crafts and would exhibit shortened life due to internal mechanical damage arising from on-going vessel motion. It is common sense: as a cell becomes larger, its internal weight increases much faster than the rigidity and surface area of the casing and the casing is all what holds the plates together in a prismatic cell.


Failures have been reported on vessels equipped with 700Ah cells following ocean passages: some cells were suddenly found to be losing charge inexplicably, rendering the battery bank completely unmanageable and the matter ended in a complete write-off. All big-brand commercial marine lithium battery packs on the market today are built from cells no larger than 200Ah." https://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/
 
I love this post, so well thought out and well considered ! Gotta say it, I really wish more folks thought things out so well. You wouldn't happen to be a Virgo would ya ? LMAO (I am so I can deride Virgo's, lol)

If you are using 105AH cells, 8-cells together is 13-3/8" long as an 8S and with a double row (Block format) for 16 cells the block would be 10-1/4" wide X 13-3/8" long (Cells only) so casing & space for BMS etc is extra space.
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I mostly deal with Land Based offgrid stuff BUT I do other stuff such as E-MudTrucks etc... (I don't get into any of that here). BUT there is a LESSON LEARNED which applies to your setup and many other Mobile applications. The Large Cells are NOT good for mobile ! There are too many complications and issues. Cells from 50AH to 100AH(max) are preferable as long as they are Properly Matched & Batched. The smaller cells allow for faster charge to saturate them for a good deep charge.

Cutting through it, my suggestion would be:
Eve 105AH Cells, Matched, setup as 16S Block Pack, JK-BMS-[JK-B2A24S15P] (With Bluetooth, 2A Active Balancing, 150A handling)
* Cells can output 105A and take 52A to charge, no need to go beyond. Set Balancing to start no lower than 3.300Vpc.
Parallel the batteries into the Bank with EQUAL length wires bound together (emi/rfi noise control) and fuse each with 150A fuse, I personally really like the MRBF Fuses.

NB: You really do not want to exceed 1C Discharge Rate, which is like an Emergency Door, only to be touched when absolutely necessary!. IF you want to get into Above 1C Draws, then you are looking at much costlier LFP from other sources, and they are quite a bit more $. Also keep in mind that battery packs in parallel DIVIDE Load & Charge and you do need to consider the Last Man Standing in your battery bank. If 3 of 4 packs cutoff for any reason, can the Last one take full charge & discharge as demanded ? This is where things get tricky.

TIP, Time & Money saver!
If you order the cells from Amy Wan @ Luyuan, and you give her the Make/Model of BMS and any accessories you require, she can source that & ship it at once (saves on shipping instantly) and some other costs as well. Plus the bonus that everything arrives at once, so minimized Thumb Twiddling and you can get going.

Hope it helps, Good Luck
 
A sales manager at Sinopoly I was talking to was adamant about using 100Ah or 200Ah cells only for assembling marine battery banks
Thats actually a great point that I did not think about yet. Definitely one more big advantage point for the multiple batteries version!

You wouldn't happen to be a Virgo would ya ?
I hate to disapoint, but no ?

If you are using 105AH cells, 8-cells together is 13-3/8" long as an 8S and with a double row (Block format) for 16 cells the block would be 10-1/4" wide X 13-3/8" long (Cells only) so casing & space for BMS etc is extra space.
I have thought about this a little aswell. Basically I see three different possibilities: 1x16, 2x8 and 4x4.

One cell is 130.3mm x 36.7mm or 5.13'' x 1.44 inches so:
mmcminch
1x16130,3 x 587,213,03 x 58,725,13 x 23,12
2x8260,6 x 293,626,06 x 29,3610,26 x 11,56
4x4521,2 x 146,852,12 x 14,6820,52 x 5,78

I will probably choose the design depending on actual space I have available. So whichever design I can fit into the existing spots the best.

1x16 and 4x4 are quite similar in size (if you rotate one of them). But the 1x16 would require one long return wire and the 4x4 more mounting (if compressed, a topic I have not yet taken a deeper look into). Of couse plus the space for bms, brakers and case.


You really do not want to exceed 1C Discharge Rate, which is like an Emergency Door,
That is actually a good point aswell. That would also cut the cost on the BMS again as 150A should then be enough. And in case I notice I need more extend the battery even those cells would not be matched to the others then. So maybe even start with 4 packs from the start would be the safer way. But oviously more expencive...

Also keep in mind that battery packs in parallel DIVIDE Load & Charge and you do need to consider the Last Man Standing in your battery bank. If 3 of 4 packs cutoff for any reason, can the Last one take full charge & discharge as demanded ? This is where things get tricky.
That would be similar to the cascading failure. The bank has a size that under maximum cruise speed one of the 3 banks would be enough to run the boat on itself with a max of 1C. So this would be fine. Obviously this should not happen, though. But if it does, it would work. Although, not for long of course. For the max cont. power from the engine, 2 banks would be needed. I have planned with at least 3 banks, so if one fails, the other two should be able to provide full power when needed continuously. Even more banks would make this even safer obviously.

As for charging, I currently plan to have one 3500W / 70A charger. So that would stay below the 1C of a single pack. Recuperation on my boat length will probably only yield a few hundred watts. So this should be absolutely save. And if I have 1kW solar peak, so around 700W in a clear summer noon in my latitudes, I would be in heaven. Currently it would probably be more like 500W peak until I find the time and money to build a proper solar arch in the back. So again, no worries.

TIP, Time & Money saver!
If you order the cells from Amy Wan @ Luyuan, and you give her the Make/Model of BMS and any accessories you require, she can source that & ship it at once (saves on shipping instantly) and some other costs as well. Plus the bonus that everything arrives at once, so minimised Thumb Twiddling and you can get going.
That is actually a great tip! Thank you!


Bus bars:
The only thing I currently don't like with the EVE 105Ah from Amy is that I think the busbars look a bit flimsy. Also they don't have that bend in the middle to relieve stress from the terminals on movement. Sure, they should be at least strapped in very tight or even compressed. But I have an ocean going boat so it can get quite rocky there. I read that flexible busbars (the stranded ones) are heavily discouraged on a boat due to them being very prone to corrosion. Cables with ring terminals could be a solution, though with more resistance than busbars.

Or, when using the 4x4 setup, I would mainly go via the thin side of the batteries. Then only 3 bus bars would have to be between the expanding/contracting side of the batteries.

So does anyone here has some experience with the busbars delivered with the 105A EVE cells? Can they sustain the 100A / 1C without a problem?



And thank you all so much! I should have posted here much earlier. So much great feedback and input!
 
Multiple batteries is a good idea for a mobile system. As you (and others) have stated, smaller batteries are easier to package to prevent vibration damage.

I use the Sinopoly 100ah cells (sometimes parallel paired) to make smaller batteries.

One thing i would recommend is moving to completely seperate systems for each significant load (ie two separate systems capable of running the motor, and one additional system)

For this system i would use 5 inverters and 3 battery packs.

If you use the EVE style cells be careful to package them properly to prevent chafing.
 
I also have some of the EVE 105's at hand, Amy supplies them with 2mm Thick, 12mm Wide, 76mm long (for the Block Format Long Bridge-Bar ) and 50mm Long ones to go between cells. They ARE Copper & Tinned not brass & tinned ! Tell her you are from DIYSolar and would like a double set of bars. (not really needed). These are fine in a Marine Environment.

The JKBMS @ 150A is just fine for these 105's and this will not stress out the BMS either, I would not suggest it otherwise.

I understand your apprehension and why I mention fail over requirements. No idea if you go deep out to sea but as we all know Shit Happen when Least Wanted and so contingencies on contingencies. Last thing you want is to be at sea, in a storm and no power ! Which is of course When shit happens.... Never on a nice & perfect day, right.

Expansion & Contraction is MINIMAL and will only occur of being over charged or at a too high a rate. Seriously people get cookoo about this no matter.

The Compression Wabbit Warren ! DEAD HORSE keeps being revived. 10-12PSI is more than enough. Bind them when cells are at 3.200Vpc and at 25C/77F Temperature ! (I will not entertain discussions on this point again, BTDT far too many times)
Simply put, compress them to "hand squeeze tight) and bind blocks of 4 cells with Fibratape (2" wide, wrapped twice) and install the assembly into your box. (TIP: Putting a pull-string around each 4 cell block, makes it much easier to pull block from case for service if needed).

You likely do not see my Signature (that's off by default when account created I think now) so here is a link to help you
Luyuan Tech Basic Lifepo4 Assembly Guide There is a lot of handy in this.

Suggestion as you are furtling with which format to layout the cells. Make life simpler, make 16 cardboard cutouts that match the cells sizes and play "tetris". Thing is, keep in mind that you will want a Casing or Box preferably one that can be watertight and still serviceable if needed.

BTW: Amy has sources for Casings both Metal & ABS Plastic so once you know which Cells you want you can ask her about casings/boxes.

As I said, I am an offgridder residential, I have moose & deer and no powerlines. I am also quite rural/remote and can be snowed in for days, the main road is some distance away too. I KNOW Murphy & His Laws all too well, being prepared is a good solution to Murphy, hence I have triple redundancy for all essentials (heat/water/power). NEVER underestimate Murphy, his potential for havoc and of course happening at the worst possible times. With boats being prepared is never a luxury, IMO.

PS, this is page 2 of "About My System" which shows how I squeezed & bound cells to show you what I mentioned above.

Hope it helps, good luck.
 
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