24V System Question....

[sangesf]

I have this current setup....

I have this 24v battery on my electric bicycle and connected to it is an inline Digital Volt/Amp Meter (with shunt) and of course it connects to the controller (for the motor) and I want to connect a DC to DC converter.

The dc to dc (24to12) converter would be for Turn Signals (LEDs), radio, lighter, lighter sockets and a few other low watt 12v items that I would like to hook up. (total watts to be used/drawn = 100 watts) I'm hooking up an illuminated rocker switch (12v kind, which I already bought and can install no problem.) to turn off all those 12v accessories when I'm not using them.

Now to the problem/issue..
What I also wanted (needed?) to do was to setup an illuminated rocker (24v) switch, after the battery, to disconnect the battery when not in use (e.g. I'm in the store, at a library, etc) and which would effectually disconnect the DC to DC converter when I'm ready to charge the battery...

(The switch's most important use would be when charging, because it would disconnect the DC to DC converter from the circuit while I charged the battery - {The same power wires that go to the controller to power the bike, are the same wires that I (have to) use to charge the battery} and I didn't want to have the DC to DC converter still connected when charging)
Trying to charge the battery with the DC to DC converter still connected would cause problems, right?

Anyone's .02?

 

[cooltoy]

Not sure but with "balanced" charging at low amps and your converter always being on seems like a bad idea. Does a converter that is not powering anything suck juice?

 

[loquin]

Does a converter that is not powering anything suck juice?

Some. Depends upon the converter as to how much. Some converters HAVE to pass a minimum amount of power; in these designs, there will usually be a built-in shunt resistor to load the converter with the minimum load.

 

[sangesf]

When I have to charge the battery, I will have the power lead "open" on the 12v side of the converter, so I'm not worried about that.... I was just curious if the converter would be either a hindrance to the charging or would it be damaged or be a damaging component to the charger/battery DURING charging.

The point being, either way, I need to get a 24v at least 25a switch. My question is now, where do I find one, ESPECIALLY one that has illumination too.

 

[duivendyk]

If you did not have the illuminated switch, which requires 12 V ,you could use an ordinary switch and use it to disconnect the dc/dc convertor from the battery when you want to cut the load,this is also more efficient.You could rig up an indicator light to tell you that the 12V is enabled.If you worry about the dc/dc convertor being connected while charging your battery,why worry?, essentiaLly the same situation already arizes when you use your 12V switch to turn off the 12V loads !I don't think it will do any harm at all to leave the dc/dc convertor permanently connected to the battery,but it would take a bit longer to charge it,that's all.Does it have an on/off switch?BTW 100 Watt ,4 Amp at 24V seems like a lot of power to me/

 

[sangesf]

If you did not have the illuminated switch, which requires 12 V ,you could use an ordinary switch and use it to disconnect the dc/dc convertor from the battery when you want to cut the load,this is also more efficient.You could rig up an indicator light to tell you that the 12V is enabled.

That's what I'm looking for.. a 24v switch... I need it to be rated for at least 25a. (preferably illuminated)

If you worry about the dc/dc convertor being connected while charging your battery,why worry?, essentiaLly the same situation already arizes when you use your 12V switch to turn off the 12V loads !

I haven't hooked up anything yet, I'm asking questions before I start

I don't think it will do any harm at all to leave the dc/dc convertor permanently connected to the battery,but it would take a bit longer to charge it, that's all.

I don't think it will create a problem either, but asking just to make sure.

Does it have an on/off switch?

No it doesn't have an on/off switch.. that's what I'm wanting the 24v 25a switch for, to control power to the converter and to a couple of other 24v electronics.

BTW 100 Watt ,4 Amp at 24V seems like a lot of power to me/

100 watts is the MAX load I would have at any one time on the 12 v side of the converter and at that point it's actually 8.3 amps. The converter is rated at 15a and I'm using a 24v 60ah LiFePO4 battery to run everything on my bicycle. It's not a problem, because I can get 50 amps continuous (70amp max) from the battery and the most my Bike uses at ANY time is 35a so, 35a max for motor and 15a max for DC-DC converter so I'm not worried about power. (Or should I?)



On a side note, I'm currently getting 54 miles distance on the bike and am expecting to get ~40-45 miles once all of the electronics are hooked up and working. (but i'm also getting an additional 24v motor(250w) and battery for the front at a later date.)

 

[duivendyk]

What if you used a 30 Amp breaker, like you find in house entry panels,they have a bimetal spring device that you cock,a large enough slug of current will heat it up enough to trip the thing.It's a quick disconnect so I quess it would survive diconnecting 24 V dc since it depends on heating,although it's normally for ac use (with ac any arc can only persist until the voltage goes through zero).

 

[sangesf]

I don't know enough about AC breakers to say either. (the other thing, i need something that I can mount outside of my battery box and also is weather resistant)

 

[loquin]

As long as the switch is rated to switch at LEAST 24 volts DC, at your rated current (15A), it would be fine. A switch which is rated at 15 amps will work fine at any amperage up to that point. You wouldn't need to get it oversized.

The only time that the current would EVER be that high is if the dc-dc converter output was at max current. Your operational load is going to be in the less-than 100 watts range.

As others have said, add a 15 Amp slow-blow fuse (or breaker) to the feed circuit. A slow-blow fuse handles short-term surge currents that you can experience when the converter starts up without blowing. That way, if you developed a short, the fuse would blow, the feed circuit would be disconnected, and nothing would fry.

And, as far as switch ratings go, that's the current that they are tested to handle correctly over thousands of on-off cycles. In an emergency, they can handle more current that that. (at higher than rated currents, switches can experience 'erosion' of the contact faces. One or twice at double the current shouldn't destroy the switch - but, it will shorten its life somewhat. If you repeatedly exceed the current rating, it would lead to a short switch life, though)

 

[duivendyk]

I looked around on the web, 24V dc switches, rated at 30 A are not hard to come by,used in RV's etc