LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[diugo]

Originally Posted by CharlieJ

OK. Your opinion; my opinion.

It's not an opinion---8 watts vs zero is a fact. By mentioning the shunt trip breaker, I am pointing out the existence of different, possibly superior disconnect technology. Let's leave it at that.

[goboatingnow]

#777
Dave-In fact the overcurrent protection device (OCPD), fuse or circuit breaker, has an ampere interrupt capacity (AIC) rating that is based on the short circuit current of the source.

The AIC is the maximum current that can be:
> interrupted by a circuit breaker that will leave the circuit breaker functional or
> interrupted by a fuse with the fuse and fuse holder remaining physically intact; i.e., not blown to bits!

ogh I understand all you are saying,

But what I am saying , is that if there is a fuse & breaker combination , then there is no need to rate the breaker at the interrupt capacity. The primary purpose of a BMS driven breaker is to disconnect normal loads from the battery in the event of a LVE. A suitablely rated fuse ( with a very high interrupt capability) should always be installed in any Li battery system given the potential short circuit currents. The breaker in this case is not there to perform an emergency short circuit disconnect. ( Ie its not acting as a "overcurrent protection device " device) This removes the need for a very heavy duty breaker.

Dave

[diugo]

All circuit breakers have a high interrupt capability already built in, to handle a direct short circuit situation. To be specific, the 100A shunt trip breaker I'm looking at is rated at something like 10,000 amps. Now, that may seem adequate---except that 10kA is only 14C to a 700Ah bank, and LFPs have been known to produce 20C or even more, when shorted. Therefore a "catastrophe fuse"---one with an interrupt rating much higher than 10kA---would also be prudent. The nominal fuse rating should be based on the ampacity of the interconnects and main lines (300A in my case).

[goboatingnow]

Again, I think there is a fundemental question about the purpose of LVE monitoring. IN my technical opinion ( and I have designed a couple of special purpose BMSs in the past), is that the disconnect device is purely to disconnect normal loads. The breaker ( relay) should be rated conservatively for this option.

That is very different from short circuit protection. for Li a fuse is the only practical method and one with as high an interrupt capacity as possible.

to put in in reverse, I don't believe LVE monitoring was EVER intended to act as short circuit protection.

Dave

[CharlieJ]

to put in in reverse, I don't believe LVE monitoring was EVER intended to act as short circuit protection.

My thought, exactly.

[CharlieJ]

But what I am saying , is that if there is a fuse & breaker combination , then there is no need to rate the breaker at the interrupt capacity. The primary purpose of a BMS driven breaker is to disconnect normal loads from the battery in the event of a LVE. A suitablely rated fuse ( with a very high interrupt capability) should always be installed in any Li battery system given the potential short circuit currents. The breaker in this case is not there to perform an emergency short circuit disconnect. ( Ie its not acting as a "overcurrent protection device " device) This removes the need for a very heavy duty breaker.

Got it. In my designs, I have chosen a high continuous current contactor, as mentioned above, as the control element. It is easy to interface with the output from the BMS. Short circuit protection is provided by a Class T fuse with a 20,000A AIC.

Unfortunately, ABYC Standard E-11 has this requirement:

11.10.1.2.3 For batteries or battery banks with a CCA rating greater than 2200 CCA, or 500 amp hours, battery overcurrent protection shall have a minimum ampere interrupting capacity (AIC) rating at least as great as the battery
manufacturer’s short circuit rating or 100 times the battery’s nominal amp hour (Ah) rating.

It is unfortunate because many battery manufacturers do not provide the short circuit information so I am forced to the "100 x" requirement and the Class T fuse would only be "legal" for a 200Ahr bank!!

[mcarling]

Originally Posted by CharlieJ

It is unfortunate because many battery manufacturers do not provide the short circuit information so I am forced to the "100 x" requirement and the Class T fuse would only be "legal" for a 200Ahr bank!!

Has anyone found the short circuit rating for Winston LFP batteries?

Does the 11.10.1.2.3 requirement even make sense for batteries wired in series? What is the probability that four or more batteries wired in series could short circuit? I guess a lot lower than the probability of the boat being hit by a meteor.

[diugo]

Littlefuse LDC fuses are interrupt rated for 50,000 amps DC. This should be adequate for banks up to 500Ah. For larger banks, one way to comply with the reg might be to limit the maximum cell size to 500Ah---and then use parallel strings of cells in series, each string with its own LDC fuse.

[deckofficer]

All this high amp stuff has me confused. On my next pack I was going to fuse for short circuit protection a series string. I was going to pick a rating around 50% higher than peak combined loads. The 48 volt bank will be used for propulsion and inverter. Max propulsion loads around 25KW, inverter 9KW, so I had planned to use a 1,000 amp fuse. Am I wrong?

[diugo]

Bob, for simple overload protection, the nominal size of the fuse (1000A in your case) should really be based on the ampacity (currently handling capability) of the conductors you plan to use---which in turn should be chosen to supply your maximum expected loads without significant voltage drop.

For true short circuit protection, the interrupt rating of the fuse (much higher than its nominal size) comes into play. It is a function only of the battery voltage divided by the resistance of all of the conductors and connections completing the short circuit.

[deckofficer]

Doug,

I will size all wire and connections for the loads. The main bank will be (16) 1000 a-hr cells in series. The most the inverter could draw at its surge rating is 8800 watts or 173 amps. (2) propulsion 12 Kw motors, 24000 watts or 470 amps and a DC-DC converter for 12 volts at 600 watts or 12 amps, for a total max amp draw (would probably never happen) of 655 amps.

What should I fuse the bank for? This isn't BMS, just safety fuse for short circuit protection.

[diugo]

Bob, your 48V 1000Ah bank is a major powerhouse that has the potential to liquify wiring if the wiring isn't sized appropriately. The good news is, your 655 amps of load requires thick wires simply to operate at all. The bad news is, those thick wires have very little resistance---probably well under a millohm (0.001 ohm).

Divide 48 volts by 0.001 and you get 48,000---which is already very close to the DC interrupt rating of the newest Class LDC fuses (50,000A).

In other words, even this fuse may explode or melt before simply "blowing", because your bank is so powerful. From an underwriter's standpoint, that's not acceptable. From a practical standpoint, you'll need to take extra measures that ensure that your vessel and passengers do not suffer if the fuse explodes or melts.

As far as the fuse's nominal rating goes, if you use a 1000A fuse, then the wiring needs to be properly sized to carry 1000A. So it may be more practical to size the wiring for 700A and then use a 700A Class LDC fuse.

[deckofficer]

Thanks Doug. Only the series buss bars will ever see that load. Each system will have its own dedicated cabling off of the bank, all very short runs, as both motor controllers and inverter will share the same space.

[diugo]

Meanwhile, back at the ranch, I noticed that Balqon's $99 160Ah clearance specials have been "out of stock" for a couple of days now. I didn't want them shipped during the pre-Christmas crunch, so I held off on ordering them!

What do you suppose the likelihood is of them finding some more?

[mcarling]

Originally Posted by diugo

What do you suppose the likelihood is of them finding some more?

Somewhere between slim and none.

My wildly speculative guess is that this clearance sale is consequence of deckofficer having noticed that they had shipped him old product.