Newbee with some basic electrical questions

[Tfisher309]

I have a 50ah battery that can supply 150a continuous and 300a peak and a controller that can handle 120a continuous and 200a peak.

Here are a few things that I don't understand:

The cables from my battery to controller are 8ga. Shouldn't they be as big as the wires from my motor?

I have a 72v dc to 12v dc converter for my lights. Can I run my lights directly off of this converter or should I have a small 12v battery between the converter and lights?

My battery maker suggested a 150a circuit breaker between the battey and controller. This sounds too small. Shouldn't it be at least 200a?

Most of the circuits breakers I see are for 12v to 48v. Can I use one for a 72v system?

I have learned a lot from this forum. Thanks for all the information.

 

[flob]

Hi,
i use a little 12v life bat to "straighten" the voltage, especially as some controllers won`t like the rippled voltage a dcdc produces.
A circuit breaker is there to safe your equipment, if your controller can handle 200A that would be the right number then.
the breaker should rated at least your fully charged packs voltage, no way round it.
look for them at electric motorsports or ev-west just to name a few ev-conversion suppliers.. not on eb..y and the like.

wire size ... sorry we do not use ga8 ..ga 1 .. here so i cannot comment on that.
i use 35mm2 for a setup of 150A cont. and supposedly 400 peak, on 2 conversions so far and these do not get hot or warm under normal conditions...

try this site to get an impression:
https://electronicbase.net/wire-size-calculator/

scroll down to the calculator

you wont have to size for the max if you are planning a commuter or fun bike, as you wont see full pwr for more than a few sec... for a racer though up the size towards the suggested nbrs.

have fun
flo

 

[Tfisher309]

There is nothing I can do about my 8ga cables. I just don't understand why they aren't the same size as the ones coming from the motor.

Is it OK to use a 48v circuit breaker in a 72v system? It seems it would be OK because amps are what is being measured.

 

[john61ct]

That is nuts to say you "can't" use proper wiring, basic 101-level stuff there.

And a real expert just posted the answer to the question you then responded with.

You say you are a noob, only one way to climb that learning curve...

 

[Tfisher309]

Sorry to bother you.

 

[amberwolf]

. I just don't understand why they aren't the same size as the ones coming from the motor.

Motor currents are higher than battery currents because of the way motor controllers transform the power from one to the other, so motor phase wires are generally thicker than battery wires.

Is it OK to use a 48v circuit breaker in a 72v system? It seems it would be OK because amps are what is being measured.

A breaker doesn't actually measure anything, but it does trip once it's rated current has been exceeded by however much and however long it's manufacturer's spec sheet indicates. (if there isn't one, you won't know without testing it under your normal load and your overload conditions whether it will work or not).

However, a fuse or breaker can only guarantee current flow interruption without a fire from internal arcing if it is rated higher than the voltage of the circuit you need it to break. If you use a fuse or a breaker rated for less than the voltage of your circuit, it may or may not break the circuit at all, and if it does break it it may arc-damage it's contacts (for a breaker) every time it opens such that it doesn't connect correctly when re-closed. It may even weld itself shut upon reclosing. On opening when tripping from overcurrent, it may continue to arc across the open contacts, which is the same as having an arc welder running inside it--if it does not extinguish it will get VERY hot very quickly and can set fire to the breaker, wiring, and anything around it. (same with a fuse except there's no contacts, just the fuse holder and ends).

 

[tomjasz]

Sorry to bother you.

Don’t mind Hans. He’s big on comments but short on demonstrating or showing his builds. Welcome to anonymous internet’s.

 

[amberwolf]

I have a 50ah battery that can supply 150a continuous and 300a peak and a controller that can handle 120a continuous and 200a peak.
<snip>
My battery maker suggested a 150a circuit breaker between the battey and controller. This sounds too small. Shouldn't it be at least 200a?

Depends on the breaker. Each one will be rated for a certain amount of overload current for a certain amount of time (usually a curve on a chart). A breaker wont' pop instantly once it goes past it's rated current; it takes time for this to happen, so if your controller peak currents are short enough, it will not pop the breaker. But if there is a chance the peaks could last longer than the breaker "timeout" and pop while you're trying to accelerate in traffic, it could leave you with no power to do anything and you're left at the mercy of the rest of traffic, which could have unfortunate results. Or other things may happen to pop the breaker, depending on your riding conditions, terrain, etc.


The main thing is to ensure your battery and wiring isn't damaged by overcurrent--if the battery has a BMS it should shutdown it's output if current exceeds it's maximum for longer than it is capable of. The breaker is there in case that doesn't work or the load is beyond what it or the wiring can handle.

I have a 72v dc to 12v dc converter for my lights. Can I run my lights directly off of this converter or should I have a small 12v battery between the converter and lights?

Depends on the converter, and your specific needs.

If the converter is rated for more current than you will ever need for the lights, then it will run them fine. If you have incandescent lights, it may take significantly more current at turn-on than during running, so you may need twice the converter capability than with all LED lights, for instance.

Note that if you're using automotive lighting, your converter needs to be 13.6v nominal output to provide the same voltage the lights are actually intended to run on. (they'll be noticeably dimmer at only 12v). Most automotive lighting will run on up to about 14.4-15v safely, sometimes higher, but 13.6 is the nominal voltage of the typical car lighting / starter "12v" battery.

If you're going to use a battery, too, it should be of similar voltage. A 4s LiFePO4 works well for this (but a 4s non-LiFePO4 doesn't).


To have a battery between them, if the battery is to be charged by the converter under any conditions, ever, the converter must be a CC/CV type (like an LED PSU, like the Meanwell HLG or ELG series, etc), or it will shutdown (or fail) when connected to the battery if the battery is below the converter's output voltage. So if the battery is there so that if the converter shuts off (like your traction battery runs out but you still want lights working), then if the battery drains down significantly lower than the converter's voltage, the converter has to supply a lot more current than normal to try to charge it, so if it doesn't operate like a charger does (most can't) it will just shutdown at best, until the excessive load (battery) is removed.

To avoid that and just use a plain converter you'd need to recharge the battery separately before powering the system back on, under the conditions above.

 

[flob]

Hi
amberfwolf did give a very good explanation but i want to get back to the breaker and fuse for another reason:

So first off i distinguish the breaker and fuse: fuse for an emergency only disconnect (technical problem), while breaker is used regulary on my bikes( i can lock mine shut with a little lock) when bike is not in use- servicing...... no one playing around ...

So even if you are under the impression a lower volts unit might hold up- there is other things to consider:
In a nutshell: safety
spending 100 heck 200 bucks more than you wanted but saveing your bike, your garage, your home or your wife and kids from a fire is money well spend.
You only park and charge outside? well what about a first-responder in case of an accident?
Let alone the poor guy trying to haul your bike back after its been potentially wrecked?
These innocent bystanders, merely trying to be of help have no knowledge whatsoever if your breaker might be welded and, as we are talking voltages above 42V DC!! might get a terminal shock!! Simply because they thought "Yeah breaker shut - all safe"

So - whatever you do: consider safety first and also consider the innocent bystanders.

flo

 

[Tfisher309]

Thank you, very good information. The learning curve where I'm at is pretty steep, but things are gradually making sense. This forum is awesome. I would not have been able to what I'm trying to do without it.