I have a basic circuit: battery, fuse off (+)lead, 4 lights in parallel.

My question is, when charging is a fuse needed?

Also in the battery/charger circuit, which side is considered positive?


Ive obtained a 2amp glass fuse, as recommended, but it is a high voltage fuse. Checking the voltage off the plugged-in charger(12V/1AMP), with its 2amp high-voltage glass fuse on, reads at 14.5V.

I attach the charger to a 12V battery, which has a 10amp blade-type low-voltage fuse, off the positive cathode.

The charging light doesn't come on when connected; however the charged-complete led came on when I it was being checked with the voltmeter.

here is my system:


simulate this circuit – Schematic created using CircuitLab

charger + fuse: charger+fuse a weather-proof connector between battery and charger: battery to charger connector the battery with text: battery top of battery can see fuse off cathode and a quick-connector type: battery blade fuse


I'll assume you have something like this:


simulate this circuit – Schematic created using CircuitLab

If you don't, then do provide more information.

When charging is a fuse needed? I'd say yes! Basically, fuse is there to protect the battery from being damaged by the load. Batteries can often provide large current when shorted and if somehow the load gets shorted, battery would be protected by the fuse from the short. If the battery is charging, and the load somehow gets shorted, then you'd have even worse situation. Not only would battery provide current for the short, but charger would as well. The result would be melting cables and damage to the battery and the charger due to overcurrent.

Next, about the "positive" part. That, as always, depends on perspective since voltage needs a reference point. I'd call positive terminal of the battery positive and I'd call the terminal of the charger connected to the positive terminal of the battery positive as well. They're both more positive than their own negative terminals and are commonly marked as positive, which means that others will easily understand about which terminal you're talking.

If you explained to us why you want to know which terminal would be which, we could provide more information. For example if you measured voltage between the positive terminal of the charger and the positive terminal of the battery, while the charger is running, you'll see that the charger's terminal is more positive than the battery's positive terminal (making the positive terminal of the battery negative). In such situations I'd still keep the designations I introduced in previous paragraph since changing them could add confusion into conversation.

  • \$\begingroup\$ I would put the fuse directly on the battery terminal, between the battery and the positive bus, or add a second fuse between battery + and charger +, both fuses to be as near the battery as possible, to protect all wiring (and the battery) in case of a fault anywhere. \$\endgroup\$ – Peter Bennett Sep 5 '13 at 2:51
  • \$\begingroup\$ @Peter Bennett Me too. \$\endgroup\$ – AndrejaKo Sep 5 '13 at 8:03
  • \$\begingroup\$ Hi, the diagram is correct. However, when charging the battery there will be no load. It will just be a charger and battery. I wanted to know if fuses are needed if just charger and battery? (sorry I was not more specific) Also, in looking at your diagram, I guess I was confused about how to connect the charger to battery- ie positive to positive. (again sorry and thankyou) \$\endgroup\$ – syntax Sep 5 '13 at 13:29
  • \$\begingroup\$ Is it ok to use a 10amp fuse on a 1amp charger/12V battery with no load? Would a 10amp fuse allow for faster charge/less safe? Would a 1amp fuse be slower charge but safer(ie blow at lower threshold)? I am using 18G wire. \$\endgroup\$ – syntax Sep 5 '13 at 13:47
  • 1
    \$\begingroup\$ @syntax: The fuse should be rated a little higher than the expected current, so it won't blow under normal use - for your 1 amp charger, I might use a 2 amp fuse. The fuse rating will have no effect on the rate of charge. \$\endgroup\$ – Peter Bennett Sep 5 '13 at 15:37

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