270ah DIY LiFePO4 build

[Poey50]

... and to keep the thread going, just managed to built and test my new BMS, diyBMS an open source project by a brit called Stuart. Well developed, you get the files to built your own pcbs (one per cell, similar to 123bms) and one controller board. 304Ah EVE cells ordered, will be here late Jan (at best!) so plenty of time to test the setup. Got 4 18650cells, 2 new 2 used, hooked them up and testing:

View attachment 126786
works fine charges/passive balances/discharges as it should, quite impressed.
More impressive that it can talk to Victron kit via CANBUS, that's the task for w/e onwards!

cheers

V.

Great work! I like how the thread has become a place for these new builds - especially one so cutting edge (to my mind, anyway).

 

[vas]

Great work! I like how the thread has become a place for these new builds - especially one so cutting edge (to my mind, anyway).

well, that's the idea, we learn, we try, we share, we go forward!

now, if I can figure out a decent way to disconnect this lifepo4 bank and connect back my standby (half dead) trojans if/when lifepo4 is too empty to service needs I'd be a happy man.

Obvs difference between this diyBMS and many others is that it doesn't do disconnects (so no A rating, no mosfets, no contactor!), stopping charging (to avoid overcharge) and disconnecting (to avoid emptying the bank) have to be done in other ways.
Now, overcharging is easy as I'm Victron only so bms communicates with victronOS head and the head instructs inverter/mppt/b2b to cut off as per the bms levels. So happy with that (but also have 4relays on the bms that I could use as a belt and braces approach instructing inverter to stop inverting or stop charging-not easy on my mppt though!

BUT, when lifepo4 is empty I have to switch to some other source for my needs.
THAT, I've not got sorted yet, as contactors have to be rated for 150A (at least for extra safety-watermaker and a couple of other things that may need to run in parallel are circa 120-125A) and all current will be permanently passing through them. Various solutions suggested either too expensive or too much current consuming (from what I gather)

So I need a sort of 3way switch that disengages lifepo4 and engages something else (could be another LA bank, or could even be the engine starter bank) (or even parallel them so that LA can crudely keep the voltage acceptable in the circuit and in a B2B fashion "recharge" the lifepo4, not quite sure if that's possible, but would be ideal as I'd only need to "bring" an extra source into the system and not get lifepo4 offline.
Are there any solutions/tests on that, anyone knows?



V.

 

[gregcope]

Do victron not do a thing? Will be blue and probably expensive… but workable.

 

[Poey50]

well, that's the idea, we learn, we try, we share, we go forward!

now, if I can figure out a decent way to disconnect this lifepo4 bank and connect back my standby (half dead) trojans if/when lifepo4 is too empty to service needs I'd be a happy man.

Obvs difference between this diyBMS and many others is that it doesn't do disconnects (so no A rating, no mosfets, no contactor!), stopping charging (to avoid overcharge) and disconnecting (to avoid emptying the bank) have to be done in other ways.
Now, overcharging is easy as I'm Victron only so bms communicates with victronOS head and the head instructs inverter/mppt/b2b to cut off as per the bms levels. So happy with that (but also have 4relays on the bms that I could use as a belt and braces approach instructing inverter to stop inverting or stop charging-not easy on my mppt though!

BUT, when lifepo4 is empty I have to switch to some other source for my needs.
THAT, I've not got sorted yet, as contactors have to be rated for 150A (at least for extra safety-watermaker and a couple of other things that may need to run in parallel are circa 120-125A) and all current will be permanently passing through them. Various solutions suggested either too expensive or too much current consuming (from what I gather)

So I need a sort of 3way switch that disengages lifepo4 and engages something else (could be another LA bank, or could even be the engine starter bank) (or even parallel them so that LA can crudely keep the voltage acceptable in the circuit and in a B2B fashion "recharge" the lifepo4, not quite sure if that's possible, but would be ideal as I'd only need to "bring" an extra source into the system and not get lifepo4 offline.
Are there any solutions/tests on that, anyone knows?

Hmm ... the simplest method and one used increasingly by narrow-boaters from what I can gather (perhaps because they have space for both lead acid and LFP) is to have lead acid in permanent parallel with LFP in a mixed hybrid system. But this involves having the BMS isolate the LFP at low voltage cut-off in order that the lead acid takes over. The system works because low voltage for LFP is still high voltage and state of charge for lead acid. Not everyone like this as it means that the BMS is no longer used as a last ditch protection against catastrophe but, since lead acid is always in the system, the voltage spike problem of sudden removal of LFP disappears as an issue. Also there may be a problem in keeping the lead acid well-charged unless the LFP is taken to reasonably high charge fairly frequently.

However it would mean using contactors ...

 

[vas]

Do victron not do a thing? Will be blue and probably expensive… but workable.

not seen anything, but may be wrong, will have to check!

Hmm ... the simplest method and one used increasingly by narrow-boaters from what I can gather (perhaps because they have space for both lead acid and LFP) is to have lead acid in permanent parallel with LFP in a mixed hybrid system. But this involves having the BMS isolate the LFP at low voltage cut-off in order that the lead acid takes over. The system works because low voltage for LFP is still high voltage and state of charge for lead acid. Not everyone like this as it means that the BMS is no longer used as a last ditch protection against catastrophe but, since lead acid is always in the system, the voltage spike problem of sudden removal of LFP disappears as an issue. Also there may be a problem in keeping the lead acid well-charged unless the LFP is taken to reasonably high charge fairly frequently.

However it would mean using contactors ...

that's what I was thinking, if not for my dying trojans, but I also have 2 FLA 150Ah in series as engine starter batteries (doing bugger all most of the time!)
What does permanent parallel mean Poey? they are always one together with the LFP and charged in parallel/concurrent with the voltage going into the LFP? how does that work? have to think about it!

thought was that LFP is always online, if SOC drops to alarming levels, the FLA bank is paralleled giving some juice back to the LFP. This way I wont need a contactor for full current (as in these cases consumption will be I recon under 20A!) so can do with something smaller/cheaper.
However, on getting the numbers down, LFP "empty" is going to be more than FLA full, so nope back to the drawing board...

Looks like I wont be able to avoid a full blown 150A contactor (on all the time!), but can do with a smaller one for when getting the FLAs onboard on emergency. Not ideal...

 

[Travelling Westerly]

Does your BMS do balancing? One trick I saw from Solar Garage is to charge at really low amps when nearly fully charged. Logic is that when charging at reasonable amps (1A or above. - ie 20x above balance current) that one cell "runsaway" and hits high cell voltage cutt off (as you are seeing) way before others.

Balancing is often configured to happen when charging (ie above 3.5v). As balancer currents can be really low (50ma? even) then charging at say 0.5A or what every the packs see as a charging amperage (to enable balancing) but low enough to allow the balancer to keep up with a cell that is trying to "runaway" by bleeding power away from it, whilst the other cells get charged and things level out.

Do you have a BMS with bluetooth where you can validate charge Amps and balancer being on (or just confirm its seeing charge?)

I agree with what you are saying there, I'm starting to see results with reducing the CC down to 25 Ah from 50 Ah. This means each pack is getting a 'slower' 12.5 Ah per pack.
The BMS is Bluetooth and I've altered the cell balance start voltage to begin at 2.5v (iirc) which is giving the balance time to work (only 30ma I think).
Also set the Tail Current at 6% (for 240 Amp pack) which slows it down towards the end. It looks like the runners are getting slower the more charge cycles are completed.
Cheers

 

[Poey50]

What does permanent parallel mean Poey? they are always one together with the LFP and charged in parallel/concurrent with the voltage going into the LFP? how does that work? have to think about it!

Zwerfcat describe it here. I haven't been able to work out whether it is deceptively simple or brilliantly simple. Lithium-Hybrid

 

[starfire]

I would worry about a faulty lead acid, say a common failure of a cell with a short circuit.
What would the LFP do putting current into what is now a nominal 11.5V lead acid ?

 

[Poey50]

I would worry about a faulty lead acid, say a common failure of a cell with a short circuit.
What would the LFP do putting current into what is now a nominal 11.5V lead acid ?

Important point. Perhaps I'll add a third 'simple' - dangerously simple?

 

[Kelpie]

Does the switchover between LFP and LA need to be automatic?
I'm toying with a manual system but it would mean losing power during the change.

 

[vas]

Does the switchover between LFP and LA need to be automatic?
I'm toying with a manual system but it would mean losing power during the change.

I guess if you have two relays to play with (I've got them!) you could first turn on the LA and then take the LFP offline

thing is what contactors would you use?
I assume you need bi-stable relays 100-200A @12-24VDC with a control coil of either 12 or 24VDC.
On a first rough check on ebay, not that many around...

 

[vas]

er, sorry missed the point of your Q, you could similarly have two manual on/off switches, turn the LA on first, then turn off the LFP, job done.

 

[Kelpie]

er, sorry missed the point of your Q, you could similarly have two manual on/off switches, turn the LA on first, then turn off the LFP, job done.

That's basically what I'm thinking of at this point- two normal isolator switches but only one key.

 

[Pete7]

m ... the si

Not everyone like this as it means that the BMS is no longer used as a last ditch protection against catastrophe but, since lead acid is always in the system, the voltage spike problem of sudden removal of LFP disappears as an issue. Also there may be a problem in keeping the lead acid well-charged unless the LFP is taken to reasonably high charge fairly frequently.

I have separate switches so can isolate the LFP, crank up the charge voltage for FLA to 14.8v plus temperature compensation and let the solar panel do the work for a couple of days with long absorption and then switch it back down again to include the LFP. Victron's MPPTs enable you to save a setting rather than have to remember every single setting. This should give the FLA a good stir every couple of weeks.

 

[vas]

I have separate switches so can isolate the LFP, crank up the charge voltage for FLA to 14.8v plus temperature compensation and let the solar panel do the work for a couple of days with long absorption and then switch it back down again to include the LFP. Victron's MPPTs enable you to save a setting rather than have to remember every single setting. This should give the FLA a good stir every couple of weeks.

slightly confused, so Pete, you have the FLA always online (in parallel to the LFP) and occasionally get the LFP offline to fully charge the FLA or have I got that wrong?

cheers

 

[vas]

T-class fuse housing 200-400euro??? are they nuts??? ffs! Cannot order from Cactus, trying to find local distributor for BlueSea, not easy, going to order some stuff from Mouser, so thought to check there.
OK a 200A T-class is 40euro, a 400A is 70, I can stomach that, but the housing? What am I missing? Apparently looks like I do use T-class fuses at my house 3ph supply el. panel, everyday is a school day after all

 

[Poey50]

T-class fuse housing 200-400euro??? are they nuts??? ffs! Cannot order from Cactus, trying to find local distributor for BlueSea, not easy, going to order some stuff from Mouser, so thought to check there.
OK a 200A T-class is 40euro, a 400A is 70, I can stomach that, but the housing? What am I missing? Apparently looks like I do use T-class fuses at my house 3ph supply el. panel, everyday is a school day after all

My Class-T fuse holder was £59 from Cactus in June this year. There's a world-wide supply problem for Bluesea Class T. I've now been waiting 6 months for fuses after numerous delays. The latest due date is next week - I'm not holding my breath.

 

[sailaboutvic]

T-class fuse housing 200-400euro??? are they nuts??? ffs! Cannot order from Cactus, trying to find local distributor for BlueSea, not easy, going to order some stuff from Mouser, so thought to check there.
OK a 200A T-class is 40euro, a 400A is 70, I can stomach that, but the housing? What am I missing? Apparently looks like I do use T-class fuses at my house 3ph supply el. panel, everyday is a school day after all

We in the UK for a bit if you can find someone you can order from I can give you an address to have it sent to and I forward it on to you if that's any good .

 

[gregcope]

Two M8 lugs and some brass M8 nuts and bolts?

Yes makes changing it slightly harder.

 

[Kelpie]

We in the UK for a bit if you can find someone you can order from I can give you an address to have it sent to and I forward it on to you if that's any good .

I gave up trying to get one in the UK. Have fitted a Victron ANL instead, for now. I ordered back in June and was told delivery late July, which then slipped too end of August. My sail-away deadline was 1st August. Seems I did the right thing not waiting...