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dkenny64 · 26-12-2020, 16:04

setup..
2 house banks( DIY with BMS on each bank, separate charge ports).
boat has a 1-2-both-off switch.. for the house banks and this also ties
to the engine start..hold this thought..

now..
each house bank is 200ah of LiFePO.. each with is own BMS 150A output..
expected max load is under 100amp...loads under circuit breakers that are 120amp

question..
now we've run bank 1 down to the point the BMS disconnects.
flip the 1-2-both to change to bank2 to run the boat...
what happens when the banks are in parallel...
bank2 is full bank1 is almost dead.. hugh current from 2 into 1? or

little since a quick switch? what if its not a quick switch..they stay parallel for hours??

the LiFepo are in parallel the the alternator output so at full lifepo charge the bms disconnect won't be a problem..

do I need a breaker on the output of each bank?? to stop the charge dump from the charged bank to the dead bank?

-dkenny64

T1 Terry · 27-12-2020, 17:38

Ouch, what point does the BMS disconnect the drained battery? Not sure what lead you to believe this was a good idea to deeply discharge the lithium battery every time, just because they can handle being deeply discharged doesn't mean they need to be deeply discharged every cycle.
question..
now we've run bank 1 down to the point the BMS disconnects.
flip the 1-2-both to change to bank2 to run the boat...
what happens when the banks are in parallel...
bank2 is full bank1 is almost dead.. hugh current from 2 into 1?

Yes, as you cross the connection between the two batteries, you will momentarily create a path from one battery to discharge into the other battery, then you will break that path creating a very large arc that will in time destroy the 3 way switch or actually weld the contacts so you can't disconnect the link between both batteries.
The circuit breaker might be 120 amps, but is it a DC circuit breaker or an AC circuit breaker, double pole or single pole? Most circuit breakers have at least a 150% capacity rating short term and it is only after the whole unit heats up from the over current that it will trip. The greater the over load the faster the trip. That means at least 180 amps could pass through the circuit breaker without it tripping so that would be enough to make quite a decent arc across the 3 way switch contacts.

the LiFepo are in parallel the the alternator output so at full lifepo charge the bms disconnect won't be a problem..

do I need a breaker on the output of each bank?? to stop the charge dump from the charged bank to the dead bank?
What you need is a DC to DC charger that only turns on when you have the engine running, or a high capacity contactor (not a relay because they only have one contact and this could weld closed, a contactor has two contacts that actually link the input and output terminals so the load/current is broken twice.)
This DC to DC or contactor will charge both batteries from the alternator when the alternator is running without the need to link both batteries via the three way switch. The alternator will priority charge the lowest battery first, the BMS should stop the battery from over charging, but what happens if both battery BMS systems open because both batteries are fully charged, where does the alternator output go?

By connecting the system up this way you have 2 separate batteries and would only need to link them together if the output from one battery was not enough to supply the load.

T1 Terry

dkenny64 · 28-12-2020, 06:14

T1 Terry. thanks for replying. I don't plan to run the LFP banks dead..but it could happen. so this would be a worst case scenario. the alternator charges thru a diode isolator. so both banks and the engine start get sharged at the same time. the engine start will remain an FLA. so even if the BMSs disconnect the charging the alternator will be fine. the circuit break is DC rated. most likely something from BlueSea systems the BMS I'm looking has separate discharge/charing ports. thanks for asking the the 3 way switch.. I need to check its ratings. to make sure it can handle a switching surge. -dkenny64

smac999 · 28-12-2020, 11:12

Connect them both together permanently. The dual bms still gives you redundancy from a bad cell or other issues as only 1 bank will shunt down.

Use the 1-2-all switch from lithuim and start battery to dc panel. So in the event of total lithuim failure. You can power boat from start battery.

dkenny64 · 28-12-2020, 12:01

Quote:

Originally Posted by smac999

Connect them both together permanently. The dual bms still gives you redundancy from a bad cell or other issues as only 1 bank will shunt down.

Use the 1-2-all switch from lithuim and start battery to dc panel. So in the event of total lithuim failure. You can power boat from start battery.

the boat already has the switches in place.. allowing 2 banks that are separate. it also the means to tied all banks together. i would like to keep it this way.

all the boat systems run on the house banks. imagine the problem if you are trying to weigh anchor and the LFP banks are dead(for any reason). the engine start is on the other end of the boat..(long wires. lots of loss).

thanks for the thoughts
-dkenny64

GoneDiving · 28-12-2020, 12:32

What's the over current rating for the full bank's BMS? You will have essentially a dead short until the faster of the BMS or circuit breaker trips. This will be orders of magnitude higher than 120A. As an indication most LiFePo4 cells are rated for a 3C tempoary discharge so thats 600A on a 200AH cell. Actual delivery can be much higher. Lets hope the breaker and/or BMS are fast acting.

If the aim is a redundant bank then I'd agree that a dcdc charger would work well. It can be used to maintain the charge of both banks or to top up one if you should reach the low voltage disconnect. Of course, the best solution is to not run any bank until it is dead flat.

Why do you need two smaller banks? One larger bank, with BMS protection, would be much simpler and offer double the capacity until it is drawn dowm to low voltage disconnect.

dkenny64 · 28-12-2020, 14:57

Quote:

Originally Posted by GoneDiving

What's the over current rating for the full bank's BMS? You will have essentially a dead short until the faster of the BMS or circuit breaker trips. This will be orders of magnitude higher than 120A. As an indication most LiFePo4 cells are rated for a 3C tempoary discharge so thats 600A on a 200AH cell. Actual delivery can be much higher. Lets hope the breaker and/or BMS are fast acting.

If the aim is a redundant bank then I'd agree that a dcdc charger would work well. It can be used to maintain the charge of both banks or to top up one if you should reach the low voltage disconnect. Of course, the best solution is to not run any bank until it is dead flat.

Why do you need two smaller banks? One larger bank, with BMS protection, would be much simpler and offer double the capacity until it is drawn dowm to low voltage disconnect.

The BMS I looking to put on each bank is rated for 150a continuous with 200a surge.
redundant is just that...you wake up in the morning after running on bank 1 and now is 30% left.. is this enough to run a 120a load for 10-15 to raise the anchor..if not..then I would have to sit and wait for the engine/solar to put enough power back in the bank..with a 2nd bank(sitting offline) its then is just a simple switch to run on the full bank..
agree I don't want to run until dead...

best guess is I'll only need 150a for a surge..
what's the peak current on a 1000w windlass? I've tried to find out, but no luck.. its lofrans.

-thanks for replying
dkenny64

T1 Terry · 28-12-2020, 17:04

Maybe I missed it some where, but what voltage are the battery packs and the 1000w windlass? Need that to determine current draw in amps. The 10 to 15 mins to raise the anchor would not be all at 1000w. That 1000w could possibly handle an overload of 100% for a second or two, maybe a 50% overload for 30 secs, so a momentary peak of 2000w or 1500w for 30 secs (you can't have both) then the load would likely drop back to around 30% max for continuous running to avoid burn out.

If you want to do the math yourself, watts divided by volts = amps. Say the battery and windlass are 12v, peak 2000w for a few secs = 2000 / 12 = 166 amps ... this is an absolute max I'd imagine, the heat generated in the unit at a 100% overload would be huge. 166 amp divided by 60 gives amp mins, divided by 60 again gives amp secs, 166 / 60 = 2.77 amp mins, 2.77 amp mins / 60 = 0.046 amp secs x 2 seconds = .092 amp secs = the number of Ah used is so small I doubt it could be measured by most of the equipment used outside the lab.
So now you are looking at the 15 mins max run at 30% load. 1000 / 30% = 330w. 330 / 12 = 27.5 amps / 60 = 0.46 amp minutes x 15 mins = 6.9 amp mins / 60 to get Ah hrs and you haven't draw very much out of the battery have you?

I know it's hard to forget the old issues with lead acid batteries where anything much below 60% SOC meant any serious load was going to drop the battery voltage down to unusable levels, but a quality 200Ah lithium battery @ 30% SOC will still deliver the initial 166 amps for the max overload and still have heaps left to run the windlass for 15 mins without the voltage dropping below 12v at any stage ..... not even imaginable with lead acid batteries but all in a days work for lithium batteries.

To give an example, we have pulled our 200Ah 24v battery down to 5% SOC, this is where our BMS is set to isolate the battery, we can bypass the isolation and still start the diesel engine that requires 100 amps for 30 secs for the glow plugs and often a 30 sec crank @ 400 amps in the real cold (well for around here anyway) before it actually wants to fire up. Sometimes we have to repeat this process, but there is still ample in the battery @ 5% SOC to start the engine ... now that is something special in my books and has allowed me the luxury of getting rid of the start battery and running on the house battery alone. We still have an all electric house set up, hot water, air fryer, toaster, coffee machine and reverse cycle air conditioning, diesel heater, electric blankets for winter as well as the usual 24v and 12v stuff ... all from the 200Ah @ 24v lithium battery ... but we do have 1620w of solar on the roof ......

T1 Terry

dkenny64 · 28-12-2020, 17:22

sorry if I left out some details.
the windlass and the electric winches are 12v.

yes. you are right about using 1000w all the time..when its just pulling up the chain. I would guess its less..but when its loaded..is it more than 1000w. 1200w-1500w for a short time..what's a short time? 20sec. 5 sec? 45 sec?

I know when we've weighed the anchor with our current setup..I can draw enough power out of 200ah of FLA to cause the alternator belt to slip while the engine is running. 90a alternator..single belt(not figured out how to use 2 belts nor serpentine). so its seems to be a hugh load.

the more constant voltage of LFP is a big consideration..not to mention that if I put the batteries together from cells the cost is close to FLA..or less..

sounds like you have a nice 24v setup..I can only have 12v and only have 800w of solar on the roof.. I would like to overkill the problem than find out I don't have enough power to start the engine or the generator.

side note.. I almost had this problem in the hurricane in 2019.. got back to the boat.. both house banks were DEAD..got the engine running and generator only because I disconnected the start battery from everything.

thanks for replying

-dkenny64

T1 Terry · 28-12-2020, 21:46

Even if the windlass was trying to pull 1000w on the lead acid batteries, the voltage probably drops to around 10v under load, that means the current required is 100 amps and the 90 amp alternator trying to supply that would certainly make the belts squeal.
How long wold it be overloaded, that depends on how stuck the anchor is I guess, but a 100% overload (so drawing 2000w) could only occur for a few seconds, then either a breaker would trip or the smoke would come out of the Windlass. The initial torque available from a DC motor from zero RPM is the full torque available from the electric motor, so multiply that by the reduction ratio within the windlass and there is big torque applied to the chain right from the start. Generally the anchor would release within a few seconds or drag the boat toward the anchor until it released, so the maximum overload would be very brief, especially with an LFP battery because it would be supplied full voltage rather than the sagged voltage from the lead acid batteries.

T1 Terry

dkenny64 · 30-12-2020, 17:17

I've been doing lots of thinking about this..
seems that using a DC-DC charger to charge the LFP banks.might be the answer. another problem I have it belt slippage on the alternator..
but if the alternator outputs go the the start battery..it high load is short term..but running the engine and using a DC-DC to charge the LFP limits the current output of the alternator..the DC-DC would be only a 20amp model.
the Daly BMS is a separate model( charge and out are separated)..
seem I could tied the LFP charge sources together..setup to charge LFP..if one bank got fully charged before the other the BMS should stop that batteries charging..right? then all the charge goes to the other battery..

only other hickup..its how to keep the engine start fully charged( these are FLA). idea add a 2nd solar controller to the output of one of the panels(2 sets of 400w). solar is setup 400w on port and 400w on starboard..each side of the boom...so I'm expecting some shading and less than 800w max. ideally I hope to get around 600w.. I already have the other controller so I would only need to buy the DC-DC charger and the stuff for LFP..

sound like a plan or should I forget it?
-dkenny64

T1 Terry · 30-12-2020, 18:49

A mob over this side of the world called Enerdrive, sell 40 amp DC to DC chargers that are designed to handle the drop in lithium batteries hard disconnect without suffering any problems, many can't handle the target battery being disconnected while they are running at full output.
They have programmable setting for lithium battery use, a cut in/cut out designed to protect the start battery from over discharge and a separate switchable wire that will allow the start battery to be pulled to a low voltage, required for "smart"alternators that are computer controlled to drop the battery voltage down to 12.6v once it considers the start battery should be fully recharged .... a real pain but such is the wisdom of vehicle manufacturers these days ....

With one of these units, you could run the alternator and all the solar to the start battery set up to fully charge flooded cell lead acid batteries, then simply charge the lithium batteries from the DC to DC output.
Personally, I'd build the lithium battery as one high capacity battery rather than two separate batteries and have the start battery as the redundancy factor. You can then wire a switch to link the lithium battery to the start battery if the start battery ever fails, or in the extremely unlikely case the lithium battery fails.
If you build the lithium battery from multiple cells in parallel to build the desired capacity, then link these groups in series to build the desired voltage, you have redundancy within the lithium battery as well as the start battery as a redundancy fall back while you sort the lithium battery.

T1 Terry

dkenny64 · 30-12-2020, 19:12

running a single house bank is your choice, but it provides no back up...
I want a back up bank... the engine start is only for that use..
I want the house too be redundant..

think about 8days of cloudy weather. you've left you boat in hook...
you've left the house(LPF) in parallel( by a switch)..
you come back only to find the LPF are offline(discharge protected)
if the start batteries were tied to this...well call someone for a jump..
you cannot start the engine..
but think about if one house bank was online...
problem solved...start the engine on the unused LFP..and wait..
the alternator will charge the start....
if the start bank is charge, then I can start the generator. or start the generator instead of the engine...
3 banks one for starting only. the house using one bank normally but tied together in extreme cases. like a hurricane...

-dkenny64

GoneDiving · 30-12-2020, 19:34

Quote:

Originally Posted by dkenny64

think about 8days of cloudy weather. you've left you boat in hook...
you've left the house(LPF) in parallel( by a switch)..
you come back only to find the LPF are offline(discharge protected)
if the start batteries were tied to this...well call someone for a jump..
you cannot start the engine..

If your low voltage cut off is selectable, it doesn't have to be set to dead flat. It can be set to flat plus a margin to restart your charge sources (main and aux engines). After your 8 days of discharge (and I do agree with you that in many areas this is a real consideration) only the load disconnects, you return to a isolated battery, reset the cut off point, start your charge sources and away you go.

Note that if one battery lasts 8 days before requiring the redundant battery then a combined battery would last 16 before the cut off even becomes a consideration.

As you say, what you do with your boat is your choice. I just wanted to highlight that configuration setting often open up a range of settings beyond "full" and "flat".

Good luck.