Author Topic: Cable Current and DoD for car batteries.  (Read 2225 times)

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george65

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Cable Current and DoD for car batteries.
« on: November 29, 2016, 02:50:37 PM »
I'm playing with a couple of panels, a controller and a couple of car batteries.  All gerry rigged up with bits of whatever wire I have and as heavy as I have.  Just want to learn and play atm before going on to bigger and more expensive things.

Each panel does about 8.5A nominal at around 30V and I have them wired in parallel to keep the voltage at minimum and the amps higher. 
As this is just playing around with low voltage stuff, I'm just using the wire I have which is AC mains type with earth which I am not using.  As it happens it's 2.5MM.

What I'm unsure of and hope someone can fill me in without getting too precise or others having a typical electrical argument, is which rating on a cable matters, amps or watts? I know the correlation with voltage but I seem to recall it's not linear when applied to DC over AC type wiring.

The electrical cable I'm using is rated at 16A. Not sure if this is at 240 or 440 but lets say it's 240.  That gives me a total wattage of 3840W.
Now on my 24V fool around solar system, what will that wire carry?
Is it 16A @24V or any volts up to 16A,  OR is it a total of 3840W at any voltage which means I could put 160A @ 24V giving me the 3840W limitation.
Pretty sure that's not right so what is the principal or law here ( in simple terms, A or W?)
I seem to recall there is some non linearity in the way this works and without getting into all the formulas and technicals, what's the story?

Is my cable limited to 16A at any voltage giving me a total 16 amps or am I able to pump through any volt/amp combination up to the 3840W?

If it's a 3rd situation, could someone please explain that as simply as possibly how I pick up a piece of wire ( most likely going to be household type) and know what I can put through it for my playing around.

Other question is what should my min voltage on these standard lead acid batteries be.
I know they are not designed to take much discharge so what can I safely discharge them to Volt wise to avoid damage? I know I can't take a lot off them as the are not deep cycle types but wondering what do I set the load cut off voltage to in order to be able to use them without shortening their life.

I understand the amount I can draw off them with regards to the total capacity is very low but for my tiddlywink purposes, this is irrelevant.
All I want to know what's the limit I set on my controller cut-off on the load to keep the batteries safe?
 


joestue

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Re: Cable Current and DoD for car batteries.
« Reply #1 on: November 29, 2016, 05:43:22 PM »
watts lost in the cable is proportional to amps squared.

so yes, you can run a 2.5mm wire at 160 amps, provided you cool it off with water.

so for a more realistic example, 10awg copper is .001 ohms per foot. for each foot at 8 amps you will loose 8 millivolts, multiplied by 8 amps is .064 watts. the usual rules are 30 amps for 10 awg copper, which works out to about .9 watts per foot of cable. note that there are usually two wires side by side, so 2 watts per foot, in cables of approximately 30 square inches per foot.

the limit has always been temperature rise, not volts or amps or watts.

you will have to look up the resistance per foot, i can't recall what it is for 2.5mm^2 cable.


the usual advice is don't discharge lead acid batteries below 12 volts (2 volts per cell), particularly so if you are discharging them slowly.
« Last Edit: November 29, 2016, 05:47:23 PM by joestue »

george65

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Re: Cable Current and DoD for car batteries.
« Reply #2 on: November 30, 2016, 04:29:26 AM »

To keep it simple for the simple, IE, Me, What would a " comfortable" working limit be in watts or amps for 2.5MM Cable @24V be?

DamonHD

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Re: Cable Current and DoD for car batteries.
« Reply #3 on: November 30, 2016, 06:08:16 AM »
I use this calculator:

http://www.calculator.net/voltage-drop-calculator.html

and the handy table below suggests that you should not use such cable for >20A-ish; below that you can decide is the losses are too high.

Rgds

Damon

george65

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Re: Cable Current and DoD for car batteries.
« Reply #4 on: November 30, 2016, 08:01:07 AM »

Thank You very much for that Damon!
That is extremely helpful and even more interesting.  Very easy to punch in different numbers to see the effect they have.

Seems I was for this setup, going completely overkill. Even with the 2.5mm wire I'm losing negligible power.  The losses are far less than I anticipated so I can stop worrying about getting heavier and more expensive cable. I can see I really overdid it with the car battery cables I made to supply the inverter from the battery's.  :0)

One big lesson there is the cable in the inverter itself. I thought it was rather thin to be taking the current the inverter must draw at full load. Using the calc you linked, I now understand that as it is only about a foot long, it can take that current easily and still have minimal losses.  It seems the length of cable is more significant than most other factors within reason.

I was also a bit surprised to see there is no difference between AC and DC with all other factors being equal. I always thought that DC had a much higher falloff over distance but clearly that's not right.
I read there that voltage drop should never be more than 3% so I surmise if I stay lower than that, then there will be not heating issues. Very handy to know.

This also answers something I was pondering. I have a roll of this 2.5mm Cable and I was wondering about tieing all 3  conductors together to boost capacity and use 2 lengths, one for pos and one for neg.  According to the calc, That would REALLY make things more efficient for longer cable runs particularly and out perform much more expensive wire far better than I would have expected.

Looking at what I am doing and would like to do ultimately, the 2.5 cable will handle it easily. Most solar cable here is 4mm. Seems overkill for what an individual panel can produce or in fact an array at higher voltages. They must have had some very long runs in mind is all I can think.

Thanks very much again.  I have learned a lot from that, put my mind at ease and you have saved me money I would have spent for no gain.
Much appreciated!

Bruce S

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Re: Cable Current and DoD for car batteries.
« Reply #5 on: November 30, 2016, 12:52:53 PM »
george65;
Now that you've got the answer for the cables, let's talk about the car batteries.
IF those are like the car batteries I learnt on; they are probably 650CCA? that's about 45Amps at 12V nominal before total death.
Most car batteries don't like being below 12.8 and mine fully charged read about 13.1 after I let them sit ~~1 hour. This is the main reason why I jumped to NiCds (other than I got the NiCds for free  ;D )

Since mine was setup for working with my plastic E100 and now hydroponics they're still going 3 years on with little issues.
I set the LVD to cut out at 12.8 and left it at that. I of course had GHURD holding my hand while I used one of his nifty devices.


Hope this helps 
Bruce S
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george65

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Re: Cable Current and DoD for car batteries.
« Reply #6 on: November 30, 2016, 06:01:53 PM »
Bruce,
Mine are a bit over 105 AH.  Being I have a diesel and kept the new one out of the wifes car before replacing it with the correct size one, I like plenty of capacity.

I have been letting charge go to 28V but tried to drop it down. The UPS I am using keeps beeping constantly and showing low charge in the batteries. Took the charge up to 30 and got the same thing. Yesterday was very cloudy so the panels didn't quite keep up and it dawned on me when the voltage fell to 25 it shut up. I'm not sure if I can reset the alarm threshold. The thing has a USB and RS port but I haven't looked on the APC site to see if there is any software to change parameters on the thing yet.  I don't know what the cutout voltage on the UPS is but I fear it's probably in the 11V region or maybe 10.

Still not sure what's going on though because when  put the power to the inverter last night, it charged the batteries to 27 and the charge indicator was 3 lights instead of 1 I get under solar power and it seemed happy.  I'll go out and reset the cutoff to 25.6 and see how I go.
I have gone to pains to clean all terminals and checked there is good contact.  Even overloaded the UPS to make sure there was no sparks from the connections and they didn't get hot. All was fine.

I had dinner with my brother in law last night and he wanted to know if I could set up something on his boat for him. He's had a LOT of trouble with the onboard generator which is powerful enough to run a house as they tend to be on boats and thought he could get a few more batteries and some panels and do away with the generator.  I asked what loads he had and was told about the little fridge, LED lights he's fitted, TV, Radio etc.
He has a separate 24V Alternator on one engine so could use that for top up when cruising or running short.
I said this sounds doable.

Then his wife who has a reputation in the family of not being the smartest tool in the shed asks " What about the air conditioner and the fan heater?"
Say what??  I ask about those and he says yeah I'd want to run them too.  I said well everything else is fine but if you want to run AC and heaters, game over.  Seems she is more electrically savvy than him! Or just more mindful of her comfort.

Anyway, seems I was good for the voltage afterall and thank you for your input.  This is just learning and playing around so I realise the shortcomings of car batteries but they are what I have lying around and probably better to put them to use like this than have them just lying round getting a charge up every couple of weeks.

Now what I'd like to find is a load diverter. That way I can connect say 2 fridges and not have one try to come on while the other was running and overload the system or have long times when I'm not making the most of the generation.  Other thing is a change over from solar to grid power when the solar drops like at night.

Maybe when I get to learning this Arduino stuff I can make something?  Might take a while to get up to that speed though.

OperaHouse

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Re: Cable Current and DoD for car batteries.
« Reply #7 on: November 30, 2016, 09:59:51 PM »
I might be doing two fridges next year.  The 5CF lost freon two years ago and we just bought a 7CF to replace it.  We wanted something larger anyway. I have some quick taps around somewhere. Thought about using it to store fruit at about 60F. 

Two fridges wouldn't be hard to do.  If they have their own temperature control, two SSRs and two current sensors.  Both SSR would be on all the time. When current is detected in one, the other turns off.  Throw in some voltage monitoring so neither will come on when the voltage is low and a delay to allow battery to recover before other fridge is enabled.

It could also be done with one current sensor and one mechanical relay.  One fridge is powered all the time with the NC contact.  If no current is detected, it switches the other fridge on with the NO contact every 5 minutes for 30 seconds.  If not drawing power it switches back.  There would be a run time limit when it would switch back. This would give both fridges equal chance to run.

Bruce S

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Re: Cable Current and DoD for car batteries.
« Reply #8 on: December 01, 2016, 01:30:19 PM »
Bruce,
Mine are a bit over 105 AH.  Being I have a diesel and kept the new one out of the wifes car before replacing it with the correct size one, I like plenty of capacity.

I have been letting charge go to 28V but tried to drop it down. The UPS I am using keeps beeping constantly and showing low charge in the batteries. Took the charge up to 30 and got the same thing. Yesterday was very cloudy so the panels didn't quite keep up and it dawned on me when the voltage fell to 25 it shut up. I'm not sure if I can reset the alarm threshold. The thing has a USB and RS port but I haven't looked on the APC site to see if there is any software to change parameters on the thing yet.  I don't know what the cutout voltage on the UPS is but I fear it's probably in the 11V region or maybe 10.


Maybe when I get to learning this Arduino stuff I can make something?  Might take a while to get up to that speed though.
If this is an APC unit, there is DOS based software, that will allow you to run that will do just what you ask. It's also good for telling the system how many batteries and the AH of them (I think) it's been ~ 6 years since I played with it, but I do know that the Matrix, and some of the series 3000 & 1500's had the software on both 3 1/2 floppy and CD.
Also came with a serial cable to do all the configurations. Last time I played with it; it would even allow you to test the system for run times.

 Hope this helps
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