Fusing for camper van solar

[Impatient]

I went down the wrong rabbit hole in a previous thread thinking my LiFePO battery had tanked in the cold, but then saw that a melted fuse/fuse holder might have caused the symptoms (It melted without completely blowing…allowing some current to pass….why I do not know, but it was a cheap Amazon/eBay inline fuse, so current thinking is it was just substandard, and I was “lucky” it lasted this long…and even more lucky nothing caught on fire).

here is a preliminary sketch of PROPOSED / REVISED fusing and wiring. Thoughts? WHAT SIZE / TYPE OF FUSES DO I ORDER FOR THE MAIN BATTERY CIRCUITS SO I CAN GET THIS PROJECT STARTED.

summary of proposed:
* an MRBF fuse on each battery string…wild guess: 80a…could go higher I guess
* 6 ga wire up to a panel, which is easily accessed. The batteries themselves (and therefore the MRBF’s) are NOT conveniently accessible.
* a MIDI fuse at the panel…easy to see and replace, For each string. (say 30a, though if there was a 25a version, I might prefer it). From the MIDI’s, wiresize drops to 10 awg (for <6”) to a “somewhat beefy” 30a switch (NKK S-823D) which controls which battery string is connected to the controller.
* another short (10-12”) 10 awg wire stretches from that switch to the controller. Max wiresize for Victron 100/20 controller terminal is 10 awg. So: battery > 80a MRBF > 6 ga > 30a MIDI > 10 ga > switch > 10 ga > controller
* battery negatives have 6awg wire to bonding busbar (BlueSea 2306 bonded to chassis) 1 ft long from the LiFePO, and about 5-6 ft from the pair of AGM’s). Additional wires to the negative bonding are 2x 10awg (the negative terminals in the Victron are internally bonded, so 1 10awg wire to each (load and battery) in the Victron effectively provides a 2x 10awg path (???), likewise, there are 2x 10awg negative wires to each of the 2 load fuse blocks. I didn’t draw all the load circuits because I was in a hurry, and assume it doesn’t affect the main question: WHAT SIZE / TYPE OF FUSES DO I ORDER FOR THE MAIN BATTERY CIRCUITS SO I CAN GET THIS PROJECT STARTED. Most of the wiring described is already in place, but will change to allow for the new fusing…but an extra pair of eyes or 2 or 3 would help convince me I might be doing this more correctly this time.

* solar is 45-50v 250w (so single digit amperage)
* loads never exceed 20a in practice: 12v fridge, fan, LED lights, water pump used sporadically, occasional 1.5a parking heater, and a couple 12v power outlets (fused at 15a, but rarely used beyond 3-4a phone & tablet charging, rare tire inflator (5a after ~10a inrush) use.
* no inverter
* shouldn’t matter, but there is a Victron Smart Shunt in the AGM negative cable.

in general, I use the LiFePO overnight while I sleep, until fully recharged mid- late- each morning. Thereafter, I switch the solar charging and loads to the AGM, until dark, and after until bedtime (back to LiFePO). So the LiFePO powers the fridge, heater while I sleep…a fairly light load; the AGM powers things afternoon and evening. Neither gets stressed, which is good for lifespan, and probably why the AGM’s haven’t failed yet.

lets not litigate why I have 2 battery strings. I have them, have had this arrangement for ~ 5 yrs as my install has evolved, I have always bought batteries at different times for each string, and didn’t want to mix old and new, and certainly not intermix LiFePO and AGM in the same string, or in such a way they could electrically interact (I don’t think current ever passes from one string to the other, or shared to a load, or from a charger, at the same time. I intend to continue to have this arrangement, though assuming the AGM’s will be the next to die in a year or 2, I would probably replace them with LiFePO When the time comes (They are under the floor board of the van and very hard to access…not trivial install…and near exhaust/muffler, plus more vulnerable to freezing temps.

 

[time2roll]

Looking at the picture feeding #6 wire I would reduce that MRBF to 60 amps. Not sure the other accessories to be powered but if that is enough that is where it would fall for me. For 20 amp loads it would seem to fit fine.

 

[Impatient]

Thanks for reply.

Um OK, sounds like you are OK with the general concept of MRBF’s at the batteries, and MIDI’s up near the controller (where I can easily check the fuse for “minor“ problems…leaving the MRBF’s for “catastrophic“ protection???) I was guessing at 80a, since it seemed that would protect the 6 ga (I thought), but I don’t really have a problem with 60a “catastrophic“ fuses since all MRBF’s have same AIC rating, I think. the inline fuse that melted was a mere 30a, and the whole set up was “cheap / minimal“ quality…but lasted 3-4 months... actually I think it lasted 5 years with the preceding AGM battery…I just swapped in the LIFePO, with a change from a Morningstar PWM controller to the Victron, wired the same way, and a somewhat more powerful Solar panel. 60a should never blow under my normal conditions (that’s a good thing).

I wasn’t sure anyone would “bless” the MIDI’s, and I have no choice but reduced wire size toward the controller, since 10:ga is the terminal limit on this Victron.

 

[Impatient]

@time2roll thanks for looking at previous diagram. Attached is a more “holistic” view, including load circuits and the “other“ charge controller…a CTEK d250s…older, no Lithium profile, but apparently the Battleborn BMS is able to handle it. apparently, once the BB is full, the BMS prevents further charge…so no floating. In practice, I use the solar (Victron SCC) more for the LiFePO4, and the CTEK more for the AGM’s.

all negatives share a single common bonding point (to chassis).

thoughts, anyone? This 2-string, 2-controller concept has been in place for ~ 5 yrs, though until a year ago, it was all AGM, with Morningstar SCC (as well as the CTEK B2B), and smaller solar panel.



some “controversy“ over whether I should use the “load out” terminals of the Victron, not sure, but it seems it works OK. Victron 100/20 was installed a year ago, replacing a Morningstar Prostar 15 PWM, on which I also used the load outs…so Victron was a “drop in“ replacement. Note, on diagram, the dashed lines represent current wiring, dotted lines are a potential change. Two things to mention on this: I could wire the Victron similar to the current CTEK arrangement, or I could change both, to connect the 2 load fuse blocks “more to the batteries” vs “more to the controllers.”

 

[HRTKD]

Cheap fuses/fuse holders are known to fail. There are numerous posts on the forum of audio quality fuse holders that became a puddle of plastic. Don't go cheap. You could use a breaker between the solar charge controller and the rest of the system. 30 amp is a good rating for that position. I use the Blue Seas 285 Series circuit breakers downstream of my solar charge controller.

I'm not a fan of an MRBF fuse for the battery. The general recommendation here on the forum is to use a Class T fuse with a LiFePO4 battery. Yes, they're expensive and often on back order. But for that position, they are the best over current protection.

I do have a dual pole circuit breaker between the PV and the solar charge controller. It functions mostly as a switch so I can maintain the system without getting on the roof to disconnect the MC4 connectors.

 

[Impatient]

well, I’ll delay things further researching T-class vs MRBF...both of which are far more robust than what I had before. Which brought me to this (attached). The question is, for 6ga wire, I’ve seen it is OK for ~100a, but @time2roll talked me down to 60a, OK, since I still never plan to get near that. but these T-class graphs make me worry about “rating” vs actual. Do I have this right: a 60 amp fuse will fester along for nearly 200 seconds at 100a? What am I missing?

 

[HRTKD]

These are not quick trip fuses. You can get quick trip fuses if you want but most of us use the slower fuses.

 

[Impatient]

confused. I thought I was looking at quick trip, when I found these graphs. Wouldn’t slow-blo be even worse?

 

[Impatient]

Maybe point me to a particular t-class brand/model/sku recommended for 6ga. I’ll probably get flamed for this too, most my wire is Home Depot THNN cable (Including the 6ga). I recently saw some Ancor marine, which appeared thicker/better.

for fusing, we’re worried about ampacity, right? I could probably get by with 10ga for 20a loads or charges. I opted for 6ga for less voltage drop (albeit Home Depot wire)…though most of my runs are pretty short...except the wires to the fridge, water pump, and galley lights…which are max 20ft round trip.

I’m not picking on T-class, but confused there are different shapes and dimensions. If I recall, there were similar blow curves for MRBF showing that a nominal 60a blows at higher (or longer) exposure than 60 amp.

 

[HRTKD]

I use the Class T fuse from Blue Sea. In their webpage they list it as "Extremely fast short-circuit response". Compare their trip time to the one you're looking at.

https://www.bluesea.com/products/5117/Class_T_Fuse_-_225_Amp

There are two physical sizes of Class T fuses from Blue Sea and two sizes of fuse holder.

As long as the wire has a rating higher than the current, you're good. I went with welding wire because it's so much more flexible and easier to deal with.

Fusing depends on max nominal load. Figure that out and then multiply by 1.25 which we use as a fudge factor to avoid nuisance trips.

 

[Powderday]

Do I have this right: a 60 amp fuse will fester along for nearly 200 seconds at 100a? What am I missing?

Yes that interpretation is correct based on the TCC (Time-Current-Curve) your provided. What you are missing is the damage curve for the cable you are using. A single conductor (I'm presuming you mean THHN cable from Home Depot, so I'll use 75degC for temperature) in open air is rated at 95A continuous (I'm using Canadian Electrical Code here... Table 1) welding cable is rating slightly higher. So the fuse would typically be ok in that situation.

However, de-rating the same cable (when you bundle it or it doesn't get enough ventilation), the rating is closer to ~65A in which case the cable would eventually melt at 100A and short out. The thing to consider is how you would end up with a 100A fault and not a full current fault--which is typically in the 1000s of Amps and would trip the fuse almost instantaneously.

In summary, this fuse would work well in a catastrophic short circuit but not necessarily in a partial short situation that you pointed out.