Fuses and Battery power meter

[rosjen]

Hi, I just ordered and inline fuse holder rated at 20A for my 10ah 36V battery, but the fuses and holder are only rated to 32v 20A, will the fuses go pop right away? The website didn't show the voltage just the amps i thought the amps would cover the requirement.

Also, I am not sure if my battery meter is reading correct, it still says full power after 20 miles of use! I do provide a lot of assistance but still let it take most of the initial load getting to top speed. Is this normal for a new battery?

 

[NRG]

You should be fine, the fuse wont pop unless there's an issue. What battery meter do you have? Voltage is not a reliable indication of battery capacity, especially with Lithium, the voltage will stay more or less constant over most of the discharge.

 

[rosjen]

Not sure what the battery meter is, it came with the kit from Frank. Thanks for the fuse tip, I just keep reading you should never have a lower voltage fuse than the circuit voltage - not sure why?

 

[wurly]

Think of electricity like water flowing down a pipe.
The gradient of the pipe would be voltage(V, volts), flow of water would be current(A, ampere's), any restriction in the pipe would be resistance(R, ohms)including the pipe itself.
Very simplistic i know, but it helped my daughter understand how electricity works.
Current is what will blow a fuse. A short circuit would be like removing the pipe (resistance). All the current will want to go positive to negative...like real quick! this is what will blow a fuse, usually indicating something very wrong.
Putting in, say a 10Amp fuse, and you would be expecting the current flow under normal conditions to also 'blow' as your setup could be drawing more current than the fuse was rated for.
20A should be fine, if it isn't, up it to 25A.
Of course, electrickery is far far more complicated than that oh yes, don't mix water and electricity.

To see how much power (W, watt) is being drawn from your battery a Turnigy Power/Wattsup meter would be a good investment.
I can see how much power i have used and can work out what percentage is left. It saves draining the battery dangerously low.

 

[Uningles]

The rated values for the fuse-holder will indicate at what voltages and currents the insulative qualities of the component are expected to perform to (no doubt with some hidden safety margin).

For the fuse itself, the rated voltage is important to ensure that, when the current exceeds the fuse rating and the fuse actually blows, the fuse will actually break the circuit and that current will not continue to flow through arcing inside and across the fuse - i.e. that the fuse will do its job and protect against the short-circuit/overload condition.

A 10 cell 36v Lit-ion pack with a nominal voltage of 3.6v per cell probably charges to 4.2v per cell for a total of 42v - so your fuse components are running 10v (or approx 28%) over rating, at worst. Not too much over tolerance, but not without some small risk for the really unlucky, I imagine.

 

[Uningles]

You should be fine, the fuse wont pop unless there's an issue. What battery meter do you have? Voltage is not a reliable indication of battery capacity, especially with Lithium, the voltage will stay more or less constant over most of the discharge.

Most battery level meters that are full/half/empty affairs and matched to the battery chemistry and number of cells in the pack will be fairly accurate, under normal temperatures.

A lit-ion 10 cell pack could be considered "full" between 42v to >38v, "half full" from 38v to >34v and "empty" from 34v to 32v. However, the pack voltage will vary somewhat depending on load, resulting in some transition between these arbitrary states. Lithium-ion has a more constant voltage under varying load than say Ni-cads which actually have a flatter discharge curve (given a constant load), but a less constant voltage under the same varying loads.

So, yes, the pertinent question is what sort of battery meter is it? Is it matched to the battery chemistry and pack size?

 

[rosjen]

Wow,

I actually don't know what to say other than, is it safe to run a fuse and fuse holder rated at 32volts and 20 amps on my 36 volt 250watt system? Can I expect my bits to be protected, without the fuse blowing sooner or later than needed?

Very informative posts, but beats the hell out of me

 

[NRG]

As already mentioned, yes.

 

[NRG]

Most battery level meters that are full/half/empty affairs and matched to the battery chemistry and number of cells in the pack will be fairly accurate, under normal temperatures.

A lit-ion 10 cell pack could be considered "full" between 42v to >38v, "half full" from 38v to >34v and "empty" from 34v to 32v. However, the pack voltage will vary somewhat depending on load, resulting in some transition between these arbitrary states. Lithium-ion has a more constant voltage under varying load than say Ni-cads which actually have a flatter discharge curve (given a constant load), but a less constant voltage under the same varying loads.

So, yes, the pertinent question is what sort of battery meter is it? Is it matched to the battery chemistry and pack size?

Indeed, but even matched 'level meters' are notoriously inaccurate and do not provide the granularity for accurate measurement of capacity.

 

[z0mb13e]

Power = Current x Voltage (or P=IxV) therefore

Current = Power / Voltage (or I=P/V)

So I = 250/36 = 6.94 Amps

You would need a fuse capable of 10 amps continuious rated power at 36v

That doesn't take into consideration the peak and in rush currents which will likely be several order of magnitude higher.

Fuses often have instantanious current ratings of several hundered amps for a second or less which is usually enough to cope with the surge in power when things first start up like the motor getting the bike moving.

On a pratical level, all you really want to do is protect the battery from short circuiting so the 20a fuse should do the job and avoid blown fuses from high power starts and long hill climbs.