diy solar

diy solar

What fuse to use

neoflyer

New Member
Joined
Oct 23, 2020
Messages
46
I replaced my 4 6v AGM batteries with two SOK LiFePO4 12v 206ah batteries with no other chamge to the system. There is 4/0 cable from the inverter/charger to the batteries through a cutoff switch with no fuse in the line. There is a 2/0 cable from the cutoff switch to the house items, as best I can tell. That line has a 175 amp Bussmann circuit breaker. Again I changed nothing but the batteries.

Attached is a picture of part of my system. The red curved canle from the right connecting to the switch is from the inverter and the red cable from the switch down is to the batteries. The smaller red cable from the switch goes through the Bussmann cb to the house 12v items.

Mike Sokol of RVElectricity.com says there should be a Bussmann type cb in the cable between the inverter and batteries, positive cable I assume, rated at 120% max battery current. Nate Yarborough of Explorist.life did a video on 8 types of fuses common to RVs and he suggest an MRBF fuse for this purpose.

Can anyone give me some guidance on the subject?
 

Attachments

  • IMG_7867.jpeg
    IMG_7867.jpeg
    302.1 KB · Views: 26
I see a class-T fuse and a 120a Bussmann breaker.
Maybe another picture would help to see how the batteries are connected and likewise the inverter.
Need the inverter rating too.
 
Sorry I so late getting back. I have been out of touch for awhile.

Here’s as good of a picture as I can get of the battery hookups. It’s a simple parallel connection to both the inverter/charger and the solar charger. Is the Class T fuse what Mike Sokol refers to as a “shortstop” fuse? He recommends replacing it with a Bussman CB. Nate Yarbrough recommends a MRBF fuse for this purpose.

The inverter is a Magnum Energy MS 2000. The batteries are SOK LiFePO4s with a max charge current of 50a and recommended charge of 40a. That brings up a question for charge settings. Should I set the max charge to 40a or double it to 80a since there are two batteries in parallel?
 

Attachments

  • IMG_7925.jpeg
    IMG_7925.jpeg
    148 KB · Views: 15
You will get differing advice on overcurrent protection. Having a 'master fuse' as close as possible to the battery positive is my advice. For 12v batteries, MRBF fuse in a Blue Sea fuse holder can be mounted directly on the battery positive post, one per battery.
Positive buss bars for high current cable connections would be a more reliable setup.

Every positive cable distribuling power needs overcurrent protection suitable for the cable. The protective device, fuse or breaker, needs to be installed as close to the power source, the battery, ( or the positive buss bar), as practical.
For example there is a light gauge connected to the battery terminal, there seems to be no fuse.

note for high current inverter protection there is a lower volt drop across a fuse compared to a breaker.

Regarding charging batteries in parallel, since each battery has an independent BMS either battery could enter protection thus directing all the charge current to a single battery. If you are confident that protection will not occur, for exampleby using conservative charge voltages, then the charge current could be twice that recomended for a single battery with a robust BMS. However according to the battery data, absolute maximum charge current is 50 amps ( these 206 ah batteries unfortuantly have low cost BMS with limited current capacity).
To be safe I would set the charge current to 50amps or lower for the batteries in parallel.

Mike
 
Last edited:
You will get differing advice on overcurrent protection. Having a 'master fuse' as close as possible to the battery positive is my advice. For 12v batteries, MRBF fuse in a Blue Sea fuse holder can be mounted directly on the battery positive post, one per battery.
Positive buss bars for high current cable connections would be a more reliable setup.

Every positive cable distribuling power needs overcurrent protection suitable for the cable. The protective device, fuse or breaker, needs to be installed as close to the power source, the battery, ( or the positive buss bar), as practical.
For example there is a light gauge connected to the battery terminal, there seems to be no fuse.

note for high current inverter protection there is a lower volt drop across a fuse compared to a breaker.

Regarding charging batteries in parallel, since each battery has an independent BMS either battery could enter protection thus directing all the charge current to a single battery. If you are confident that protection will not occur, for exampleby using conservative charge voltages, then the charge current could be twice that recomended for a single battery with a robust BMS. However according to the battery data, absolute maximum charge current is 50 amps ( these 206 ah batteries unfortuantly have low cost BMS with limited current capacity).
To be safe I would set the charge current to 50amps or lower for the batteries in parallel.

Mike
I’m curious why a fuse should be put on eac battery rather than the one on the main cable attached to battery closest to the inverter/charger?

Also, the protection from the solar controller, a Victron Smart Solar MPPT 150/85-Tr VE.CAN, is provided by a 100a CB close to the controller. Any thoughts on that?

Thanks
 
Having a fuse on each battery before the parallel conection adds a further layer of protection. Although unlikely, a fault in either battery or link cable would not be protected against excess current flow. With lithium battery BMS short current protection there is a high degree of current limit, however fets failing SC is a possibility.
The Victron solar controler and its feed cable need over current protection from the most powerful power source, the battery. The power produced by the controler is limited to panel power and is thus not a risk. So fuse at the battery.
Victron controllers contain a non user replaceable fuse for a worst case situation.
 
I’m curious why a fuse should be put on eac battery rather than the one on the main cable attached to battery closest to the inverter/charger?

Also, the protection from the solar controller, a Victron Smart Solar MPPT 150/85-Tr VE.CAN, is provided by a 100a CB close to the controller. Any thoughts on that?

Thanks

Fuse can only protect the wires downstream of it. As such fusing as close the battery as possible is ideal. Imagine a short happening between pos & neg prior to the primary bus fuse. You now have an unprotected short and with low internal resistance of LFP it could be thousands of amps quickly melting the wire and possibly causing a fire.
 
I’m curious why a fuse should be put on eac battery rather than the one on the main cable attached to battery closest to the inverter/charger?
Because the risk of a short circuit will heat up the wire. The purpose of the fuse is to protect the wire, not the inverter. That is why my fuses are close to the battery. @Statistical mentioned the same concept. If the inverter has an internal short a fuse will not reverse that damage.
 
Last edited:
What's happened to the guy that would answer fuse and wire amperage questions to 9 or more decimal points?
 
Because the risk of a short circuit will heat up the wire. The purpose of the fuse is to protect the wire, not the inverter. That is why my fuses are close to the battery. @Statistical mentioned the same concept. If the inverter has an internal short a fuse will not reverse that damage.
I don’t think I was clear. Yes, put a fuse as close to the battery as possible but why put a fuse on EACH battery positive terminal rather than just the one on the downstream battery attached to the cable to the inverter/charger? The upstream battery of the two is attached to a very short cable.
 
I tried to explain.
Two batteries In parallel with a non fused link cable have no overcurrent protection if one battery fails SC. The good battery could dump energy into the cable and faulty battery.

It's a common statement that the fuse/breaker protects the cable. Its more that that, it protects the cable and whatever is on the end of that cable. For example an inverter could develop a fault in the internal DC circuits , the overcurrent protection at the battery end of the feed cable would protect.

Mike
 
Last edited:
Looks like I need to use an ANL fuse at each positive post of my 2 12v batteries in parallel. They have max continuous discharge of 100a and peak discharge of 200a(3dec). What size fuse, volts and amps should I order?
 
SOK 12v lithium batteries in parallel. Max charge current 70a, recommended 40a. Max continuous discharge current 100a each, peak 200a(3sec). I believe I will put a MBRF or ANL fuse on the positive side of each battery. What size should it be? I’ve read 120% of either max or peak but don’t know which it should be.
 
Back
Top