Short Circuit Protection of 24v SLA Battery

I have hoping you can help me identify the correct product for my application. I have 2 12v DC SLA batteries, that are to be wired in series to form a 24v battery. I am looking to short circuit protect those batteries but am having trouble identifying the correct product (fuse?, circuit breaker? ) to use

what about:

those are both ac/dc circuit breakers, but I am not sure they are sufficient to protect against short circuit.

Welcome to the forum.

As a general rule you choose a fuse for fast response and low cost, or a circuit breaker for resettability.

The part choice depends primarily on the maximum current you want to be able to provide to the load, which must be less than, or equal to, the maximum current the battery can safely deliver.

One looks at interrupt ratings for this; if the maximum current deliverable by the source, based on it’s voltage, internal impedance, and resistance of the wires connecting everything is (comfortably) less than the breaker or fuse’s interrupt rating, you’re OK.

Both of the referenced breakers have interrupt ratings in the thousands of amps, so unless you have some scary-large batteries and very beefy cables, either would likely work as long as the rest of the specifications are suitable.

Thank you both for your replies, I am going to provide a little more detail about what I am doing in the hopes that I might get a better idea if a circuit breaker would work. I am mostly worried that a circuit breaker will not react fast enough to adequately protect the battery from a short circuit.

so here is what I am doing:
I have a 24v power supply (meanwell edr150-24), it is powering a 12v “industrial” computer via a 24 to 12 DC to DC converter (meanwell ddr-60g-12). But to protect the pc from unexpected or abrupt shutdowns the 24v power supply is running through a “ups” (meanwell dups40) and there are 2 12v SLA batteries in series (Zeus PC7-12F1 7ah 12v). With the output of the ups going into the DC to DC converter. This 12v output then powers the computer. The industrial pc’s original powersupply is 12v 4A 48watt output. Also The dups40 has a battery charge current of 2 amps. All of this seems to me to indicate a 4-5amp breaker. But the pc’s powersupply is 12v 4amp, so at 24v the breaker might only need to be 2amps? if so then a 2-3 amp 24v capable breaker would be sufficient

Given that this is being used to power a pc, there shouldn’t be any large start up spikes in current, but what I would still also like clarification on if possible is if a breaker will trip fast enough, during a short circuit condition, to adequately protect the battery?

The data sheet for this product says:
“External fuse is recommended when batteries are connected”

Given the significant differences between fuses and circuit breakers, I would only use a breaker if someone from the manufacturer with technical responsibility provided an OK in writing.

They could easily have written “fuse or circuit breaker” instead of only “fuse”, but maybe they hadn’t thought about an expensive breaker substitute when they wrote the data sheet. However it seems just as likely they have technical reasons for only saying fuse.

Since I’d be contact the manufacture, if they said breaker is OK, I’d ask for part recommendations, to have a fool proof setup.

1 Like

You’re focusing on the wrong thing here; the fuse is recommended not for battery protection, but to keep the battery cables from becoming house-igniting glow wires in case of a fault.

Fuses are recommended because they’re cheap, and the resettable nature of a breaker is unnecessary; there’s no reason here for it to open in it’s lifetime, and if it does, it’s because a person has much bigger problems to deal with.

Such thoughts will give you a minimum acceptable fuse value, but actual selection would be done more on the basis of the size of the interconnect wire used. Because again, keeping those from becoming bright and shiny in case of fault is the purpose here.

Assuming that ~50W load is all you’ll ever have, a 10A fuse in an inline holder used as the link between batteries with 14AWG cabling should be quite sufficient. This will pre-empt the unit’s built-in protections and limit capacity however; 8AWG and a 50A fuse would be more appropriate if the UPS unit was to be used at max capacity.

Thanks @rick_1976 ,

Your explanation is great and helps alot.

But if you don’t mind could you clarify the following:

used as the link between batteries with 14AWG cabling

Do you mean between the 2 series batteries and the device they are connected to (in my case the dups40) or between battery 1 and battery 2?

I’d use it as the series-connection link between batteries; most inline holders will be about the right length, and doing so will save a person a splice or two. Downside is that it’s a simple matter for somebody to come along and replace it with an unfused link, defeating the protections. If that’s a possibility, placement of the fuse holder at a battery terminal is preferable.

I believe Rick was concerned on the Amp rating you maybe using Here is a chart that may help. AWG - American Wire Gauge Current Ratings so you use the correct AWG cable in our project.

Hey thanks again for the replies.

the ~50W is all I will ever have. the dups40 was oversized (I wanted the dups20 but it was out of stock and the price difference was small)

Given that i will be using din rail mounting for basically everything going in the cabinet, i will probably go with something like the following
811-310 WAGO Corporation | Circuit Protection | DigiKey

with an appropriate 10A ceramic fuse like

3061334 Phoenix Contact | Circuit Protection | DigiKey

I will try to wire with the fuse holder in between the series batteries.

It’s your project, but honestly I’d skip the DIN rail mount; every inch of cable length between the battery and the fuse is an incremental defeat of the purpose.

Something like a BF356 would be my choice.

ok that looks good, Let us know if we can be of further help.

I have accepted an answer.
I did receive a reply from meanwell

“Fuse is necessary because it opens faster than circuit breaker, which may burns your wirings.”

I will also consider the direct non din rail option.

Thank you all very much for you assistance