How is the fuse amp rating determined for a 12V AGM battery bank?

Question

I would like to put 3 x 130Ah AGM batteries in parallel and attach a fuse to the positive terminal of each battery. What size fuse should I use? How is the amp rating of the fuse determined?

The batteries will be connected to each other and the inverter will be connected to the battery using heavy duty copper cable. Smaller gauge cable will possibly be connected to the output terminals with Anderson connectivity. An inverter of up to 1000W will be connected.

EDIT:

Even though an inverter of up to 1000W will be connected, the maximum power draw likely to be less than 500W. I would like to have this safety margin in case an appliance of higher wattage needs to be used so I want the system to be able to cope with a current of as high as 80A. The batteries will be charged either via solar panels or via a 20-30A 240V AC charger.

Answer 1

The fuse amperage should be less than

• the maximum discharge current rating of the battery
• the maximum current rating of the cables and connectors

Answer 2

The fuse rating for the wiring to the inverter should be determined by the inverter's requirements. The inverter manual probably specifies an appropriate fuse size and type.

For smaller gauge wiring, the fuse should be sized to protect the wiring.

Answer 3

I realize this is an old question. I decided to answer it anyway. OP is probably gone but others may benefit.

Fuse size cannot be discussed without discussing wire size. The purpose of the fuse is to prevent wiring from getting too hot. If the wire gets too hot it may cause burns to people who touch it or even cause the insulation to smoulder or burn. It can even start a fire. Fuses protect against that.

To figure out how large the wire needs to be consult an ampacity table (use a search engine). From the cerrowire website ampacity table I see that for 80 Amps you would need 3 AWG wire. But I have never seen 3 AWG wire. So I recommend you use 2 AWG (equivalent to 6.5mm diameter) wire. You can use a 100 Amp fuse.

There is one more thing. Fuses have a maximum interrupt rating. Fuses do not operate instantly. The fault current must be in excess of the fuse rating for some period of time before the fuse will blow. The higher the fault current, the faster the fuse will blow. But if the fault current is extremely high, a physically small fuse may explode when it blows. Theoretically it may even fail to open the circuit due to arcing or something. With lead acid batteries, the potential fault current may be several hundred Amps. So you should select a fuse with an interrupt rating of at least 1000 Amps.

If the wire has to run a long way, you may need to use heavier wire to avoid excessive voltage drop under maximum load. That is fine. You can still use the 100 A fuse with larger wire. If you want to learn about voltage drop, pose a separate question.

Note: the wire to the solar panels will likely be much smaller diameter. So you need to put a separate fuse in series with those smaller wires, sized based on their diameter.

Answer 4

I guess that you use 12V car batteries. The voltage can drop by large currents to 11V.

P=U*I 1000W / 11V = 90A 90A /3 car batteries = 30A. You can use a 32A fuse for each battery. Use real thick wires! and as short as possible. One voltage drop over a wire uses 83 watt, that will fry your wire for sure.