diy solar

diy solar

Class T vs ANL fuse

I saw a short circuit test of a CALB 180ah cell on YouTube. He pulled 2500 amps on a dead short with a wrench. That's about 13c. Round up to 20c for a safety margin. That's also an older model CALB cell, so newer cells could potentially push out way more than that, especially ones like headway and other lithium chemistries.

A 280 amp hour cell at 20c is 5600 amps. A 2p version of that (which a lot of people have, and more) is almost 12k amps, which is higher than most MRBF or ANL fuses can handle. If you have a large bank, you could potentially short with tens of thousands of amps.

It's a class T for me! Unless I had a cell bank that was under 100ah, then I would consider another, still high interrupt rated fuse type. Even so, I have a class T on a 25ah LiFePO4 power bank.

Fuses are the absolute last line of defense against catastrophic failure, don't skimp on them.


Nice enough demo, but I wouldn't use the raw numbers reported to select a fuse.

3.3V cell
2.2V under load
2220A

Not quite maximum power transfer, with 4880W delivered; max would occur at 1.65V (and same power dissipated inside cell)

He dropped a large steel wrench across a single cell (effectively).

Steel ranges from 13 to 70 micro-ohm centimeters http://www.rfcafe.com/references/electrical/resistivity.htm
I'll use 30
Consider a wrench 20 cm long, 1.5 cm wide, 0.5 cm thick.

30 micro-ohm cm x 20 cm / 1.5 cm / 0.5 cm = 800 micro ohms = 0.0008 ohms.
Compare to a cell with spec 0.00025 ohms, typical measurement 0.00017 ohms.
Wrench plus typical cell is 5.7x higher resistance than cell alone. (hmm, doesn't agree with my estimate of close to max power transfer)

Consider same wrench shorting out a 16s pack. Then, resistance would be just 1.3x the cells alone.

I don't have cell resistance figures for his cell, but I would base fuse AIC rating on cell resistance figures. Using What I have seen quoted:
3.4V/0.00017 ohms = 20,000A. So class T would be good for a single string of LiFePO4 cells.
 
Solar for Dummies
I want to use a 24v 200Ah LiFePo4 battery wired to a 3500w inverter. What size class T fuse should I use?

I don't know if it matters but the inverter is "oversized" so it doesn't get pushed to it's limits. I doubt I'll ever use more than 2000w at most.
 
Solar for Dummies
I want to use a 24v 200Ah LiFePo4 battery wired to a 3500w inverter. What size class T fuse should I use?

I don't know if it matters but the inverter is "oversized" so it doesn't get pushed to it's limits. I doubt I'll ever use more than 2000w at most.
As a matter of policy I size the rest of the system components based on the max continuous rating of the inverter.

For a high frequency inverter
3500 ac watts / .85 conversion factor / 20 volts low cutoff = 205.882352941 service amps.
205.882352941 service amps / .8 fuse headroom = 257.352941176 fault amps.

That means a class t fuse of minimum 250 amps.
The wire that you use needs to be rated at 250 amps minimum.
The fuse is intended to stop the insulation melting/burning off the wire and protect the battery from venting toxic stuff.
 
The wire that you use needs to be rated at 250 amps minimum.
So, if I read an ampacity chart right I should use 4/0 wire
add a 250 amp fuse and I should sleep better at night ?

So, if I understand correctly the inverter is the deciding factor.
Although I don't ever plan on using the inverter to it's full potential I'll feel better using the "recommended" sizes. One never knows and should never say never.
 
Although I don't ever plan on using the inverter to it's full potential I'll feel better using the "recommended" sizes. One never knows and should never say never.

Inverters sometimes release the magic smoke in the mosfets and short out the input, so that's when you need the fuse to kick in. Remember: fuses protect wiring, not equipment.
 
So, if I read an ampacity chart right I should use 4/0 wire
add a 250 amp fuse and I should sleep better at night ?

So, if I understand correctly the inverter is the deciding factor.
Although I don't ever plan on using the inverter to it's full potential I'll feel better using the "recommended" sizes. One never knows and should never say never.

4/0 required maybe if in a multi-conductor cable or conduit.
The following chart says 1/0 OK to 260A, 2/0 OK to 300A, for single conductors in free air with 90 degree C insulation.
But do consider IR drop if more than a couple feet long.



As for fuse size, the goal is to never have it blow unless something goes wrong. So it is convenient if inverter fully loaded can't blow the fuse.
 
I just don't get it. My brain isn't suited for this stuff. Cows, plants and landscape design are my forte.

Inverters sometimes release the magic smoke in the mosfets and short out the input, so that's when you need the fuse to kick in. Remember: fuses protect wiring, not equipment.
Everything electrical is a Magic Smoke Machine in my hands. I could short out a brick
The following chart says 1/0 OK to 260A, 2/0 OK to 300A, for single conductors in free air with 90 degree C insulation.
So, my current 2/0 wire is good (about a 2 ft length) - Windy Nation Power Flex Welding and Battery Cable 600v -50c/+105c
and a 250 amp fuse would be OK, I'd rather replace fuses than wire or equipment

I appreciate your help and apologize for high jacking the thread
 
I have a class T on a 25ah LiFePO4 power bank.
This is the very thing I am debating now on a 25 ah battery pack I’m building with 3.2 volt Topband Cells.

On a side note, what a pain that little battery has been building.
 
This is the very thing I am debating now on a 25 ah battery pack I’m building with 3.2 volt Topband Cells.

Do you have an internal resistance spec or measurement? That would put an upper limit on short circuit current.

I found some specs.

This one doesn't mention internal resistance.


This says < 4 mOhm


3.2V / 0.004 ohm = 800A

Max continuous discharge 3C or 75A, peak 5C 10 seconds or 125A

There are probably some suitable ceramic fuses (to avoid arc blast) with AIC in the low thousands of amps, less money and volume than class T.
 
This one doesn't mention internal resistance.


Those are the ones. Bought them from Battery Hookup.
 
This is the very thing I am debating now on a 25 ah battery pack I’m building with 3.2 volt Topband Cells.

On a side note, what a pain that little battery has been building.
I use those with a 60 amp BMS, and 70a class T's I got used on ebay for about $5 each. eBay is a GREAT place to get really high quality, used, fuses for super cheap.
 
Do you have an internal resistance spec or measurement? That would put an upper limit on short circuit current.

I found some specs.

This one doesn't mention internal resistance.


This says < 4 mOhm


3.2V / 0.004 ohm = 800A

Max continuous discharge 3C or 75A, peak 5C 10 seconds or 125A

There are probably some suitable ceramic fuses (to avoid arc blast) with AIC in the low thousands of amps, less money and volume than class T.
I've used a 4s pack of those to jump a very large, very cold, V8. Winter CCA can be 600a+ on my truck, and it turned it over, immediately, with absolutely no hiccups.

Those little cells can put out some serious power.
 
I've used a 4s pack of those to jump a very large, very cold, V8. Winter CCA can be 600a+ on my truck, and it turned it over, immediately, with absolutely no hiccups.

Those little cells can put out some serious power.
the one 25 Ah topband cell i checked yesterday measured 4.2 milliOhms, 0.0042 Ohms. with a TR1035+ fossi battery internal resistance meter.
3.29 V / 0.0042 Ohm = 783 A implied ability at that state of charge.

battery internal resistance changes with state of charge so it might be different at other state of charge.

even for a small moment, 780 A from such a small cell is mindblowing to me. from a 25 Ah cell
 
I am measuring with a Battery tester, like Will uses to measure amp hours, but a different model a DL-24.

I am getting between 7 milli ohm fully charged to 14 and 17 mili ohm when doing a capacity test.







B484DCD8-54C0-4BBE-8058-C706BF4742FC.jpeg
 
I am measuring with a Battery tester, like Will uses to measure amp hours, but a different model a DL-24.

I am getting between 7 milli ohm fully charged to 14 and 17 mili ohm when doing a capacity test.







View attachment 61440
I ordered the Bluetooth version of that a few days ago (DL24P, I think). I'm excited to actually test some cells with something other than a simple shunt.

I tested some of the 25ah cells (with the cheap, but surprisingly accurate shunt) in a 4s configuration, that were top balanced, let rest for 24 hours, and at a 1c (give or take) rate from an inverter load. I got at about 320wh (as per spec). At a .3ish C discharge I got about 335wh. I didn't try and squeeze the last bit of juice out on either test, my LVD was set at 10.65v and they already met manufacturer A-grade specifications at that low voltage.

Those were from the Battery Hookup batch last year, I still have 48 of the new cell batch to test.

I freeking love these cells, so far all are more or less matched resistance and balanced (maybe my batch came from the same pack), all test full or more capacity, and the size makes a high voltage, small footprint, very safe pack possible.

Get 'em before I buy them all!
 
I ordered the Bluetooth version of that a few days ago (DL24P, I think). I'm excited to actually test some cells with something other than a simple shunt.
I also have the Bluetooth, but am trying to figure out how to download the app for apple. I bought this second hand and have a QR for for Android, and the android app is on the Google store, but through the iApp store, the E-Test is not to be found.

Looks like when I figure that out, I’ll get a lot of good data.
 
The highest rated class T fuse I’ve been able to find is 400 amp. I’ve decided that my 1120ah 12v system should have a 500a fuse. The only one I’ve found at that rating is ANL. What are all you class T absolutists doing about fusing for over 500a?
 
I have one battery feeding four inverters. I have two class T fuses, one for each two inverters.

How are your batteries wired? Are they lithium or lead-acid?
Maybe 6, 2V 1120 Ah cells in series? What is your peak continuous current draw, 400A? In that case you do want 500A fuse.
If you have two strings of cells in parallel, each can have one smaller fuse.

Would have suggested higher voltage, lower current.
 
Last edited:
Back
Top