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
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
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