Lifepo4 BMS Recommendations

[feudalkaos]

I know that Lifepo4 has been discussed many times in other posts. I have gone through each of the posts that I could find and come up with some answers to my questions however I wanted to post a specific request for BMS recommendations.

I have 16 Sinopoly 100ah cells. I have worked out a design that would include a High Voltage Cutoff relay (HVC) and a Low Voltage Cutoff (LVC) relay.

I plan to modify my alternator (Hitachi 80A on Yanmar) to be externally regulated using a Balmar MC-614.

I also plan to add source relays to each of my charging sources (Outback VFX2812 Inverter, Outback Flexmax 60 Solar, cheap wind generator). These relays would serve to open the source side (shore power & generator power, PV array power, and wind turbine connection) when the batteries reach a high voltage. This could be triggered by the HVC output as a last resort protection device. I was thinking of going this way instead of just opening the battery side so I do not risk destroying the chargers.

I have researched many different BMS devices and have yet to decide on one. My current favorite is the EMUS BMS with individual cell boards for each cell. If I went this route I would configure the cells as 4S4P (4 in series then 4 in parallel) so I can monitor and balance each individual cell. Unfortunately, the EMUS system is very far from cheap. It looks very good and highly configurable which I like but I would love to hear about others to make sure I am not buying more BMS then I really need.

I am not opposed to going 4P4S (4 in parallel then 4 in series) which would mean reducing the number of cell boards required from 16 to 4. My only hesitation is not seeing each cell as its own component. Thoughts about this wouldn't hurt my feelings. There has been a lot of discussion on this and it seems most go parallel first to reduce BMS costs and I get it but is that the "safest" direction to go?

I have also looked into the 123 Smart BMS, the Orion BMS, the Rec Active BMS, Lithiumate and several much lesser systems.

I would like to know from people who have used any of the mentioned ones or anything else what their experience has been.

Thank you for the feedback!

[nebster]

I can't help you on the specific BMS advice request.

If you go series-first and then parallel, in addition to more cell monitors (which do give you the advantage of watching every cell), you need a way to isolate each string. Contactors or FET gates are more expensive. Four BMSes (or a fancy BMS that can manage four strings) is more expensive. The wiring to a central bus usually becomes a bit more expensive.

For this reason, many people do some degree of paralleling first. That's fine and justifiable. (Sometimes, they also use other technical justifications outside of cost that I am not sure anyone has ever actually substantiated. It's likely those folks will be around soon to espouse those ideas.)

The big benefit of series-first is redundancy and uptime. You can isolate part of your battery for maintenance or if an issue arises. Whether that is important to you is pretty situational.

"Safety" is a loaded term:

(a) From the point of view of a battery thermal event, I think either topology is about the same. LFP is very safe. Cell-level monitoring with HVC and LVC will work fine in either scenario.

(b) If system uptime is critical, series-first is "safer." Someone with maneuvering thrusters tied into a battery system might care a lot about that. Or, in our case, we leave a dog on board unattended in places where reliable air-conditioning is important.

(c) In theory, a series-first design handles a single-cell hard short failure better, but in practice as a community I think we have basically zero documented cases of this ever happening. (Cells do fail, but in gradual and less exciting ways.)

Last thought on series v parallel approach:

Most people buying drop-in (commercial) LFP batteries today are in effect building a series-first battery. Each drop-in unit has an internal BMS and isolation mechanism. However many of them they buy -- and it is almost always more than one -- are ganged up in parallel. It is more expensive to do this. It works just fine. There are way more people doing this now as compared to us hobbyists building up big DIY banks. I suspect LFP house banks are being assembled 100:1 or more as series-first designs now, because that is the only practical way for the commercial providers to deliver scalable storage.

But, horses for courses.

Good luck!

[feudalkaos]

Originally Posted by nebster

I can't help you on the specific BMS advice request.

If you go series-first and then parallel, in addition to more cell monitors (which do give you the advantage of watching every cell), you need a way to isolate each string. Contactors or FET gates are more expensive. Four BMSes (or a fancy BMS that can manage four strings) is more expensive. The wiring to a central bus usually becomes a bit more expensive.

For this reason, many people do some degree of paralleling first. That's fine and justifiable. (Sometimes, they also use other technical justifications outside of cost that I am not sure anyone has ever actually substantiated. It's likely those folks will be around soon to espouse those ideas.)

The big benefit of series-first is redundancy and uptime. You can isolate part of your battery for maintenance or if an issue arises. Whether that is important to you is pretty situational.

"Safety" is a loaded term:

(a) From the point of view of a battery thermal event, I think either topology is about the same. LFP is very safe. Cell-level monitoring with HVC and LVC will work fine in either scenario.

(b) If system uptime is critical, series-first is "safer." Someone with maneuvering thrusters tied into a battery system might care a lot about that. Or, in our case, we leave a dog on board unattended in places where reliable air-conditioning is important.

(c) In theory, a series-first design handles a single-cell hard short failure better, but in practice as a community I think we have basically zero documented cases of this ever happening. (Cells do fail, but in gradual and less exciting ways.)

Last thought on series v parallel approach:

Most people buying drop-in (commercial) LFP batteries today are in effect building a series-first battery. Each drop-in unit has an internal BMS and isolation mechanism. However many of them they buy -- and it is almost always more than one -- are ganged up in parallel. It is more expensive to do this. It works just fine. There are way more people doing this now as compared to us hobbyists building up big DIY banks. I suspect LFP house banks are being assembled 100:1 or more as series-first designs now, because that is the only practical way for the commercial providers to deliver scalable storage.

But, horses for courses.

Good luck!

Thanks for the reply. I hear what you are saying about serial first being more expensive. I had not considered independent HVC and LVC for a series first system but without them it would defeat the advantage.

I am now considering splitting it in half and doing 2 battery banks in parallel, each with 2P4S and their own independant BMS, HVC and LVC. This would give me the best of both worlds I think. I like the idea of being able to disconnect a failing cell in some way without having to take the entire system offline.

[nebster]

Originally Posted by feudalkaos

I am now considering splitting it in half and doing 2 battery banks in parallel, each with 2P4S and their own independant BMS, HVC and LVC. This would give me the best of both worlds I think. I like the idea of being able to disconnect a failing cell in some way without having to take the entire system offline.

I like it. Think carefully about how you want/need the system to behave if one of the strings faults and disconnects itself under load.

[Flod]

I have used the 123bms for a season and it works great. Lot’s of options and nice app with bluetooth for monitoring and config. Also very good support from their tech people. I have emailed them when having problems and the answer quick and are very knowledgeable. I can recommend the product even though it’s on the more expensive side.