8s 18p LiFePo4 setup
- Thread starter Goldbug
- Start date
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
8s 18p implies you have 18 parallel groups of 8 serial modules. That would imply you need 18 8cell BMS systems.
Is there any way you can flip this around and do 8 serial groups of 18 parallel modules (18p8s)? This would allow you to use a single 8 cell BMS (Each group of 18 is treated as a single module.
I am curious, is there a reason you are using small cells for your build?
Is there any way you can flip this around and do 8 serial groups of 18 parallel modules (18p8s)? This would allow you to use a single 8 cell BMS (Each group of 18 is treated as a single module.
I am curious, is there a reason you are using small cells for your build?
KBWaldron
Solar Enthusiast
We would need to know how you are going to use them. What maximum discharge current do you anticipate? Each battery is apparently capable of 200A. 18 in parallel would support 3600 amps, which I suspect is not likely, but as I said we would need to know. If you are looking at a significant discharge you'll likely want to use a relay to cutoff rather than the BMS directly.
@FilterGuy is correct that 18 in parallel makes more sense. If new, these cells should likely be fairly close in performance, and paralleling 18 will average out to just about the same capacity for each parallel cell set. If used you should try to match the 8 18 parallel sets to have more or less the same aggregate capacity.
If your discharge rate is high, then your BMS will need to work overtime if your parallel sets aren't matched to each other.
@FilterGuy is correct that 18 in parallel makes more sense. If new, these cells should likely be fairly close in performance, and paralleling 18 will average out to just about the same capacity for each parallel cell set. If used you should try to match the 8 18 parallel sets to have more or less the same aggregate capacity.
If your discharge rate is high, then your BMS will need to work overtime if your parallel sets aren't matched to each other.
Goldbug
New Member
- Joined
- Dec 31, 2019
- Messages
- 2
I have a 100amp fuse on each 24v string 18 fuses in all. My batteries are new and I bottom balanced them (it took a long time). My load is small unless the air conditioner is running(which I have done for 4 continuous hours with 100+ weather). By the end of the day my solar has recharger the batteries to 27.5v Do I really need a BMS?
KBWaldron
Solar Enthusiast
If you’re running between 10-90% SOC, probably not. I suspect I will be bottom balancing and check the state of balance once a month, and not use a BMS.
I will however have breakers and configuration to ensure I don’t overdraw or over discharge.
I will however have breakers and configuration to ensure I don’t overdraw or over discharge.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Good question and fodder for a huge debate. A good BMS offers a lot of different functions:
- Under/over temp protection
The under/over temp can be skipped if you are confident that your environment is temp controlled and you aren't pushing the battery hard.
If you think you need under/over temp, you can implement it separately and still not use a BMS. - Over current protection
Over current protection can be implemented without a BMS, but you really should have at least a fuse or breaker in places even if you think your load would never be too high. - Per Cell under/over voltage protection
You can approximate this by setting your charge and load to cut off at the proper voltages, but you need to set enough margin that any week cell does not get over/under charged. If you don't have per cell protection, you are kinda making a contract with yourself to keep an eye on the cell voltages in case they drift out of balance. - Cell balancing
There is debate on how important this is. Some people are perfectly willing to set their charge/discharge voltages and currents conservatively and don't use any type of balancing method. This seems to work fine for a lot of people. However, if you are driving the batteries hard and trying to get every last electron out of your bank.... you really should have some kind of balance method.
KBWaldron
Solar Enthusiast
I guess one of the main questions to ask is, how frequently can or do you want to check on your batteries? If your solution envisions long-time autonomous running then a BMS is almost a must. But if you‘re willing to check the state of your batteries regularly then you can probably do without.
