Thread: An efficient 100Amp low voltage cut out?

Originally Posted by drivesafe

My point is that manic most likely does not need to cater for more than about 25 amps total, not the 100 amps he is looking at setting up for,

True, its unlikely that the relay will ever need to switch whilst on a 100amp or even 50amp draw. So you suggest the 50amp cable runs to front and back could go through a 25amp cut off relay so long as its not switching on those loads?

Originally Posted by drivesafe

Hi Toxic, and it is highly unlikely that manic will draw anywhere near 50 amps with what he is planning to use his setup for.

And something like an Optima Yellowtop could safely handle constant currents like that, but most batteries could not handle such currents for long periods of time.

Winching is usually done for just short periods of time, and starter motor currents are drawn for much shorter times.

My point is that manic most likely does not need to cater for more than about 25 amps total, not the 100 amps he is looking at setting up for, but if he was going to be drawing currents as high as just 50 amps, or more, for long periods, he could easily damage most batteries after just a few uses.

I was really more concerned with the incorrect use of a sweeping statement. We're in the technical forum, let's stop putting the marketing 'doomsday' spin on what is essentially a simple and immediate problem.

Thanks for confirming that:
1) Some batteries can handle the draw without being 'destroyed'
2) Current draw >50A IS possible (you never mentioned timeframes in your first statement)
3) His system design is outside of the scope of his immediate requirement, but this system could reasonably use this capacity in the future- no use designing today for 25A when an inverter or pie oven or camper trailer could be drawing way more in the future.

Let's face it, if the bloke is building his own voltage sensing circuit, and using the brain cells to make it all work, then surely his future power requirements will grow in proportion to the rest of his system.
Good on OP for giving it a red-hot go, I say.

Originally Posted by manic

True, its unlikely that the relay will ever need to switch whilst on a 100amp or even 50amp draw. So you suggest the 50amp cable runs to front and back could go through a 25amp cut off relay so long as its not switching on those loads?

Not sure what “50 amp” cable is.

You can use something like 6mm AUTO cable, which is actually 4.6mm2 and this has a continuos current rating of around 35 amps.

If you think you may eventually draw higher currents, then go for 8B&S ( 7.9mm2 ) and this will give you a safe constant current rating of 65 amps.

A little tip, try to locate your low voltage device as close as practical to the battery you are drawing the power from.

By setting up this way, while there can be a considerable voltage drop at the appliance you are powering, if it draws high currents, you will have a much lower voltage drop near the battery and this will allow you to draw more from the battery before your low voltage device cuts out.

This means you can power your appliances for a longer time without risking discharging your battery lower than you want to.

Originally Posted by Toxic_Avenger

I was really more concerned with the incorrect use of a sweeping statement. We're in the technical forum, let's stop putting the marketing 'doomsday' spin on what is essentially a simple and immediate problem.

Thanks for confirming that:
1) Some batteries can handle the draw without being 'destroyed'
2) Current draw >50A IS possible (you never mentioned timeframes in your first statement)
3) His system design is outside of the scope of his immediate requirement, but this system could reasonably use this capacity in the future- no use designing today for 25A when an inverter or pie oven or camper trailer could be drawing way more in the future.

Let's face it, if the bloke is building his own voltage sensing circuit, and using the brain cells to make it all work, then surely his future power requirements will grow in proportion to the rest of his system.
Good on OP for giving it a red-hot go, I say.

Toxic, I have no idea what you are on about.

Doomsday spin, I was simply giving manic a warning that batteries do not like discharge currents like he was talking about.

As for the “timeframe” that I didn’t mention, I didn’t have to mention a time frame. If you read manic’s first post, he established his own time frame when the stated he wanted to power lighting, a fridge, a stereo and uhf. All these appliances are not on for a few seconds, they are usually run for long periods of time.

Originally Posted by drivesafe

Toxic, I have no idea what you are on about.

Doomsday spin, I was simply giving manic a warning that batteries do not like discharge currents like he was talking about.

If you have no idea what I'm on about, let me elaborate for a second time.
You can't honestly say that the below constitutes sound technical advice, when you completely contradicted it in your follow up post. It was a bogus generalization, and I called it out. You are smarter than that, we all know it.

Originally Posted by drivesafe

Pulling even half the currents you are talking about would destroy batteries after just a few uses.

Let me provide some equally ridiculous platitudes:
Drugs are bad (even the cancer fighting ones)
Holden drivers are bogans (I'll never need to call a police officer for help)
...see where I'm coming from?

Hi again toxic, again, anyone who attempts to regularly place a 50 amp constant current draw on a lead acid battery will damage that battery.

I also stated that only something like a Optima Yellowtop could tolerate such use.

You then posted that you can winch or start motors and not harm a battery. The problem with your statement is that it has absolutely nothing to do with the way manic is intending to use his battery.

For your benefit again, manic has made it quite clear that he intends to use his battery to power devices that are run for long periods of time in normal use, not for a few second. If he did draw 50 or more amps for the times he is implying, he would damage most lead acid batteries after just a few uses.

When you implied that because a battery is capable of supplying high currents for a few seconds, it is then capable of supplying high currents for long periods of time. That is a “bogus” claim.

It's called a grey area mate. We are not on opposing ends of this argument, I am stating that what you claim is not particularly the case, and you backed this up when I asked to clarify.
You said it WILL (destroy a battery).
I said that is not specifically the case (and did not mention timeframes specifically because I know that the other extreme of this 'grey area' is in the affirmative).
Then you say it won't under some circumstances.

Neither of us can infer timeframes, because you, as well as I know there is no specific, to-the-second 'long period of time' that a battery either will, or will not destroy itself in your nondescript 'few uses'. You and I would agree there are too many unknown variables to make an informed decision. How the OP uses devices like a UHF or a 12V power port has nothing to do with how he might use a device connected to an anderson plug or any future devices added to the system.

The OP enquired about a low draw relay for a DIY low voltage cutout circuit which it sounds like he made himself (props to him, great work).
You shot off on a tangent with a silly and unfounded statement, inferring a whole bunch of stuff which is extraneous to his immediate concern (the relay). I can only imagine there was a subversive DBS sales pitch in there somewhere

Bear in mind we are sizing a device that isolates the battery.
I can't remember what size mine is but they only come in 200A or 100A. This does not mean I will always be drawing a constant 200A or 100A, this means I can draw up to 200A or 100A at any point in time without damaging the isolation device.
You shouldn't say "I usually draw 10A so I'll put a 10A switch in the circuit." because the moment you draw 80A, or 55A, or whatever is more than the switch can handle, the smoke escapes from the switch and it ceases to function.

If you put in a battery isolation device, it must be sized accordingly.