question

roya-taylor avatar image
roya-taylor asked

What size fuse on the 12v side of a bluesmart 12v/25A charger?

Hi

I have a bluesmart IP65 12v/25a charger running off 240v. I'd like to connect this to the busbar rather than using the aligator clips onto the battery & shunt terminals.

On the 12v side, what fuse should be used between busbar and charger?

Preferred Set up on the Positive cable: 150amp LiFePo4 battery > Class T 300amp Fuse > Switch > Lynx PowerIn (modified to accept Megafuses) > IP65 12v 25a charger (plus MPPT, Inverter, DC loads)

Preferred Set up on on the Negative cable: 150amp LiFePo4 battery > Shunt > Lynx PowerIn (modified to accept Megafuses) > IP65 12v 25a charger (plus MPPT, Inverter, DC loads)

A M8 eyelet connector with inline fuse (30a) can be purchased separately but product descriptions say they are only suitable for up to 20a chargers. Would this suit the 25a charger?

Thanks

fuses
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2 Answers
old-joe avatar image
old-joe answered ·

A couple of issues -

1. At 25A charger output current, the 20A fuse will blow! Maybe not immediately, but it will soon after charging is initiated. Fuses have an inverse time/current curve, so dependent on several factors (primary being load current), the time to fail may vary from instantaneous to sometime later.

2. Typically, the output fuse is not installed to protect the charger, It is installed to protect the charge circuit wiring (bus) from short circuit faults and load induced over-heating causing damage, possibly even a fire. I imagine that the charger has internal current limiting as protection.

3. Mounting and connection of any DC output fuse needs to be well considered to prevent false fails. The fuse is a 'fault limiter'. It is a deliberate weakness inserted in the circuit designed to heat up and 'fuse' on SC faults and sustained over-current faults. Alas, if you have a poor connection at the fuse, the IR losses at the connection will create heat that soaks into the fuse element, thereby de-rating the fuse from its nominal rating. The connections to the fuse need to be made with cable or solid bar of a size that will act as a heat sink to minimise this occurrence,

Hope this is helpful.

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old-joe avatar image
old-joe answered ·

Just read your question again and note the fuse in question is 30A. My error. I am guessing that the 30A fuse and 20A charger combo is to allow for initial inrush or surge current, and possibly thermal issues to which I have alluded to. One thing is certain, you won't do any damage by running the 25A charger on the 30A fuse, except possibly to your wallet if it needs to be replaced.

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