Fuse for rechargeable pp9

[Danndad]

I have a 8 cell battery holder that I want to use to hold 7 aa rechargables.
The 8th battery is really a piece of bamboo with a couple of drawing pins in either end and a length of 5amp fuse wire soldered to either end mimicking the battery connection.

The set up will be housed in a pp9 case and used to power my transistor radios.
My query is, is that 5amp going to give me much protection if there is a short?
I have been looking at wire ended fuses on the RS website and have seen these-

https://uk.rs-online.com/web/p/non-reset...es/7649457
https://uk.rs-online.com/web/p/non-reset...es/7649466
I was hoping these might have been ok but when looking at the specs I am a little confused as to wether I can use either of these.
I thought that it would be easy to use these wire ended fuses in place of the 5amp fuse wire.

[Mike Watterson]

The fuse is so wire heating up during a short can't set insulation or case on fire. NiMH are very low resistance compared to Alkaline and extremely low compared to layer cell.
I use 6 x Alkaline AA in 2 x 3 way holders. Advantage is 5 to 10 year shelf life compared to rapid self discharge of some NiMH.

Good idea for a fuse. I use a 20mm fuse on HT packs (45V and up) and on packs using multiple D size alkaline for LT. I use small fuse holders really meant for PCBs, but wired.

I'd use a 500mA for such a NiMH pack. A 5A might protect against fire, but anything using PP9 pack will be a lower than that average draw.

How do you fit an 8 way AA cell holder in a PP9 case?

[Danndad]

Thanks for the reply Mike.  The batteries are rechargeable NMH from lidl.

The 500ma fuses in the link are the ones I really wanted to use but in the specification this has cast some doubt on whether these are suitable.

For the 500ma types it reads:
Breaking Capacity at Maximum Voltage Rating :- 300 A @ 125 V dc, 50 A @ 125 V ac

I’m hoping someone can explain what this means and if they are suitable, I enclose some pics as well.

Danndad

               

[Kalee20]

They should be suitable.

The breaking capacity refers to what the fuse will interrupt when there's a fault - if, for instance, the fuse is protecting a piece of equipment on a 125V DC supply (say ten car batteries in series) and there's a short-circuit, the fuse will melt, an internal arc will develop, and if the car batteries are new and capable of driving more than 50A into that arc and the downstream short-circuit, then the arc won't go out - it will persist. For weaker batteries which can't supply 50A, the internal sand or whatever will quench the arc. And of course for really weak batteries which can't even supply 500mA, the fuse wire won't even melt.

[ppppenguin]

The breaking capacity ratings seem odd to me. I would expect the AC breaking capacity at a given voltage to higher than for DC. Simply because the arc can extinguish as the voltage goes through zero.

[Mike Watterson]

The 500mA ones are fine. Breaking capacity would hardly be an issue for 7 x AA NiMH in series.

I was thinking of single sided 8 way holders!

Keep your existing dummy and make a new one for the fuse. Then if you decide to use Alkaline AA (say if the NiMH are flat) you can have 6.

[pamphonica]

I have recently been re-building NiCd and NIMh battery packs for various pieces of equipment.
I have taken to including a resettable "Polyfuse" in each pack.
The ones I favour currently are the Bourns MF-R090. These are rated at 60V and 0.9A. RS part number 6478538
I found some of these in packs I was re-building.
They sit pretending to be a short until they heat up on overcurrent and open. As soon as they cool down they reset - quite fast.
They hold happily at 0.9A but trip at 1.8A. More than adequate protection from shorts on the output of the battery pack.
So charging or discharging currents of up to an amp are fine.
at about 32p each, a fine investment.
-Jeremy

[Kalee20]

The breaking capacity ratings seem odd to me. I would expect the AC breaking capacity at a given voltage to higher than for DC. Simply because the arc can extinguish as the voltage goes through zero.

You're right!

And I looked at the RS link, it's the case there, not a misquoting by Danndad. I'm puzzled now...

Looking at the dimensions, 7.1mm long, I wonder if RS themselves have got it wrong? It only takes 60V DC to sustain an 8mm arc as long as 50A or more is available. Quite often, small fuses are rated for DC at no more than 28V.

[ppppenguin]

Same applies to DC switches. It's a lot harder to switch off high voltage DC. The DC isolator for my PV system is 3 or 4 poles of ordinary AC isolator in series.

[`FRANK.C]

Is rectified but not smoothed AC as difficult to switch as smoothed DC?
I would have thought it would be easier to switch than if it were smoothed as the voltage drops to zero.
The reason I ask is that there is a certain model of Beko washing machine. In it the AC that supplies the motor is rectified with a bridge before it is switched by relays and then travels on to the motor.
The relays are 16A AC types and there contacts tend to weld together.

Frank