BMS capacity vs fuse size

[Sailorman Ed]

Question is where is all the stuff located?

Ok, here is my battery temporarily in position, my mounting board in its location ( now is painted and epoxied in place) and anticipated bus bars, shunt and Bussmann breakers. This is the standard battery location for a Gemini cat and all existing wires come here. I still need to reinforce the base and create a way to secure the new battery.

You said you are going to use AWG 2 between the battery and the bus bar....

My error, 2/0

The issue is your “200 mccb”. I am assuming that this is a 200A single pole circuit breaker (SPCB). What breaker do you plan on using in this application? Since it is the first overcurrent device (OCPD) off of the battery terminal, you have to be mindful of the battery’s available short circuit current (ASCC) so that the installed OCPD has sufficient ampere interrupt capacity to handle the ASCC of the LFP battery.

If you review my post #48 up thread you will see that the estimation (barring battery manufacturer’s data) for ASCC is 5kA/100Ahr. So for your 280Ahr battery the ASCC is
(280/100) x 5kA = 14kA. There are no readily available SPCB with this level of AIC. The standard “A” series circuit breaker (Blue Sea 50A single pole, white toggle #7230) has an AIC of 7500A at 65VDC which is insufficient.

CharlieJ - Does this apply only to Lithium phosphate or to LA as well. Doesn't a 200A breaker just trip? What would actually happen? I understand there "should" be a fuse within 7 inches but of all the boats I have been on, I have never actually seen this in practice. Not arguing, just trying to understand. So a MRBF fuse mounted on the battery does not work?

Ed

[CaptainRivet]

Originally Posted by CharlieJ

@CarlF #56


This is not an ABYC requirement.

The requirements are that if there is no fuse/circuit breaker (OCPD) equal to the ampacity of the conductor, than the next standard size OCPD may be use if it does not exceed 150% of the conductor’s ampacity.



1. There is no price differential for either the large or small format Class T fuses based on their rating.
2. 4/0 boat cable has an ampacity of 312A outside a machinery space and 264A inside a machinery space. Although, I do protect 4/0 boat cable with 400A Class T fuses for inverter/charger service and am comfortable because of the fast acting nature of a Class T fuse.
3. If NH fuses were more readily available in the USA, I would use them for high A, high AIC service.


ABYC has an exception for connections directly connected to a battery terminal: The conductor may be a maximum of 72” long if it is protected for its entire length. The easiest way to protect it is to put the conductor in self extinguishing split loom.

Be aware the bigger the fuse rating is the more voltage drop you have on that fuse and because its your main fuse on the whole system. So you don't want size bigger rating as necessary, even ABYC allows that.

Your advise to put class T as main fuse is wrong with multiple parallel batteries.
If you have 2 or more batteries in parallel, especially if the BMS is mosfet based which basically all dropins are, you want to put a class T on each battery with 2/3rd or better 50% continious current rating of the BMS. Means if your dropin has a 200A BMS use a 150A class T, for 100A a 63A class T as this protects the BMS from being roasted when current is taken in a peak from mainly one battery which can happen. For total current then as described i would use a breaker with the needed total current as overcurrent protection that also works as bank disconnect switch.

[CaptainRivet]

Originally Posted by CharlieJ

@CarlF #56


This is not an ABYC requirement.

The requirements are that if there is no fuse/circuit breaker (OCPD) equal to the ampacity of the conductor, than the next standard size OCPD may be use if it does not exceed 150% of the conductor’s ampacity.



1. There is no price differential for either the large or small format Class T fuses based on their rating.
2. 4/0 boat cable has an ampacity of 312A outside a machinery space and 264A inside a machinery space. Although, I do protect 4/0 boat cable with 400A Class T fuses for inverter/charger service and am comfortable because of the fast acting nature of a Class T fuse.
3. If NH fuses were more readily available in the USA, I would use them for high A, high AIC service.


ABYC has an exception for connections directly connected to a battery terminal: The conductor may be a maximum of 72” long if it is protected for its entire length. The easiest way to protect it is to put the conductor in self extinguishing split loom.

Be aware the bigger the fuse rating is the more voltage drop you have on that fuse and because its your main fuse on the whole system.

Your advise to put class T as main fuse is wrong with multiple parallel batteries.
If you have 2 or more batteries in parallel, especially if the BMS is mosfet based which basically all dropins are, you want to put a class T on each battery with 2/3rd or better 50% continious current rating of the BMS. Means if your dropin has a 200A BMS use a 150A class T, for 100A a 63A class T as this protects the BMS from being roasted when current is taken in a peak from mainly one battery which can happen. For total current then as described i would use a breaker with the needed total current that also works as bank disconnect switch.

[CaptainRivet]

Originally Posted by Sailorman Ed

Ok, here is my battery temporarily in position, my mounting board in its location ( now is painted and epoxied in place) and anticipated bus bars, shunt and Bussmann breakers. This is the standard battery location for a Gemini cat and all existing wires come here. I still need to reinforce the base and create a way to secure the new battery.


My error, 2/0


CharlieJ - Does this apply only to Lithium phosphate or to LA as well. Doesn't a 200A breaker just trip? What would actually happen? I understand there "should" be a fuse within 7 inches but of all the boats I have been on, I have never actually seen this in practice. Not arguing, just trying to understand. So a MRBF fuse mounted on the battery does not work?

Ed

It only applies to lithium batteries to use class T or NH due to their low internal resistance=basically a short which is not limiting the current and then these huge current can be achieved.
A lead battery has such an high internal resistance which therefor limits the current by itself that every type of fuse on the market is able to handle the resulting low short curcuit current.
An MRBF on a lead starter works, on a Lifepo4 not appropriate to use.

[CharlieJ]

@CaptainRivet #63#64:

If you have 2 or more batteries in parallel, especially if the BMS is mosfet based which basically all dropins are, you want to put a class T on each battery with 2/3rd or better 50% continious current rating of the BMS.

That is all well and good but we are discussing a one battery bank.

It only applies to lithium batteries to use class T or NH due to their low internal resistance=basically a short which is not limiting the current and then these huge current can be achieved.

Not so. Quality AGM and TPPL batteries all have internal resistance close to a LFP batteries. They can dump a tremendous current in a bolted fault.

[CharlieJ]

@SailorMan_Ed #64

Does this apply only to Lithium phosphate or to LA as well.

The lead acid family includes: flooded lead acid (FLA), absorbed glass mat (AGM), gelled electrolyte (GEL) and thin plate pure lead (TPPL). High quality AGMs (Lifeline) and TPPLs (Northstar) have very low internal resistance that is nearly the same as the internal resistance of a LFP battery. As an example: a Lifeline GPL-8DL has an internal resistance of 1.7mOhms and an available short circuit current (ASCC) of 7300A. So a modest house bank of three of these yields 765Ahrs and an ASCC of 21.9kA which marginally exceeds the AIC of a Class T fuse.

Doesn't a 200A breaker just trip? What would actually happen?

When a breaker/fuse is challenged with a short circuit, it will open (trip) in accordance with its current vs. time to open curve. When it opens, an arc is formed between the contacts that will be ionized into a plasma which is conductive which will cause the contacts and their operating mechanism to heat. If the breaker/fuse does not have an ample AIC rating for the ASSC of the source it may well experience an unplanned disassembly.

I understand there "should" be a fuse within 7 inches but of all the boats I have been on, I have never actually seen this in practice. Not arguing, just trying to understand

You probably weren’t sensitive to the requirement. I rarely find a boat with no OCPD in, at least, the main feed to the panel.

As I stated up thread, the 7” rule is the most stringent but there are exceptions one of which I explained earlier: if the wire is connected to the battery terminal, not a bus bar or a switch or a piece of equipment, and its is protected, then the 7” is extended to 72”. I install flame retardant split loom to this wire and I am compliant if I install the OCPD within 72” (measured along the conductor) of the attachment to the battery terminal.

The MRBF has an AIC of 10kA at 14VDC so, no it would not be compliant.

[Sailorman Ed]

CharlieJ thank you for the explanation. I understand now. I appreciate your assistance.
Ed