Power Consumption

My camper has 100amp deep cycle battery. 5 led lights, 1 small tv. (no built in dvd) Occasional kid device.
Over the course of a week camping, I noticed, using a multimeter it only used about quarter of a volt out of the battery.
Is there a way of measuring amps left on a battery with multimeter?
While not a lot of consumption, I have always been told you should really only go down to half of the battery, while fully draining occasionally is ok.
Im guessing flat is when it gets below 12v, and half way between the 12 and round figure of 13.8v is the half drain.

With that consumption, do I really need a 120 solar panel, or is that over kill? Shops keep trying to push one on me.
Last trip I tried a 20watt, but forgot to take readings, so above figures is without solar.

Hope this makes sense.
Thanks
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Reply By: Kelpie D - Wednesday, Dec 10, 2014 at 14:23

Wednesday, Dec 10, 2014 at 14:23
That must be a very small TV.

In short, the answer is no. You cannot measure how many amp hours are left in a battery. You can get a thing called a watt meter, which can be used to measure how many AH have been used compared to how many AH have gone in which gives a rough guide as to how many are left.

Yes, about 11.2 volts indicates a flat battery. Yes, 13.8 is close enough to fully charged.

Have a look on the back of the TV. See what watts it is. Watts divided by volts = amps. Work out all the amps for everything you want to use. Then double that figure so that you have room to spare. Then the solar panel you want will be twice the amps of your doubled amp usage figure...If you do that you have plenty for later if you want to add a fridge or a fan or a lap top etc.

For example, if all the things added up to say 1.5AH. Then your double is 3AH. So your panel would want to be capable in perfect conditions of making 6 amps, or if you like an 80 watt panel.
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Follow Up By: Kelpie D - Wednesday, Dec 10, 2014 at 15:40

Wednesday, Dec 10, 2014 at 15:40
Just to make me sound a little clearer. Cos, when I read my post it was not quite clear.

Say for example, you have 12watts of LED lights and your TV is 48watts. You would have 60 watts of usage. Say you run both lights and TV for 3 hours each day. 60 watts divided by 12 volts = 5 AH per hour times 3 hours = 15AH used.

Now, you need a solar panel capable of putting that 15 AH back into the battery. Most days you will get a good 5 or 6 hours of good charging sunlight and a few more hours of less productive light. So lets say you need 3 hours of sun producing 5 amps, which is the same as doubling the double above and gives you that 15 amp hours. 5 amps times 12 volts = 60 watts...A 60 watt panel would fit the bill just nicely.
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Follow Up By: SDG - Wednesday, Dec 10, 2014 at 16:03

Wednesday, Dec 10, 2014 at 16:03
Just had a look at my things.
No info on globes, but 24watts on tv. If I use your example of 12watts for lights, and my 24watt tv, that leaves me with a 40watt panel(rounding up) needed. As I generally only have one light used at a time, and tv long enough for a movie via usb, for the kids before bed, looks like a 60, in perfect conditions will keep it going.

Thanks
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Follow Up By: Kelpie D - Wednesday, Dec 10, 2014 at 16:13

Wednesday, Dec 10, 2014 at 16:13
24 watts ? That IS a small TV ;)

When it comes to solar over kill is best.

Yep, it looks like 40 watt panel will do the job. So if it fits the budget and the boot space a 60 watt will see you through many years.

Another thing worth looking at. Next time you replace your car battery, get a deep cycle cranking battery. I don't know how many brands there are but I have a brand called Allrounder. That way you always a fully charged deep cycle battery as back up if needed.
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Follow Up By: SDG - Wednesday, Dec 10, 2014 at 16:20

Wednesday, Dec 10, 2014 at 16:20
Think I already have one of them. When my previous car died, I pulled all the batteries, Dual battery wiring, etc out of it before I scrapped it. I was just happy when it went into the new one.


I've been told I'm tighter than a frog when it comes to my home bills, so I think I must have carried it over to my camping as well. :)
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Reply By: Rangiephil - Wednesday, Dec 10, 2014 at 14:56

Wednesday, Dec 10, 2014 at 14:56
A fully charged rested battery is 12.7 volts not 13.8.

13.8-14.5 volts is the output of most alternators that charge a battery but the voltage will reduce with no load to 12.7 volts.

12volts is about 50% of fully charged.

11.4 volts is generally accepted as the lowest you would wish to drop a battery to for longevity and the lower you go the fewer cycles the battery will be capable of.

I believe you will need a 120watt solar panel as the realistic output is just over 6 amps.
Have a read of this and his other articles which I believe to be accurate.

http://www.campertrailers.org/collyns_page.htm#Lead
Regards Philip A
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, Dec 10, 2014 at 15:16

Wednesday, Dec 10, 2014 at 15:16
Just adding to Phil' response................

It is NOT OK to ever fully drain a battery. Do not discharge below 50% of capacity, that is 12.0 volts when resting (at no charge or discharge)
Cheers
Allan

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Follow Up By: SDG - Wednesday, Dec 10, 2014 at 16:06

Wednesday, Dec 10, 2014 at 16:06
Whoops. that sounds better. from memory it was .7and a few other numbers. I rounded up as it was closer to 8. The 13 must have been thinking about charge.
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Reply By: Gronk - Wednesday, Dec 10, 2014 at 15:42

Wednesday, Dec 10, 2014 at 15:42
It seems like you are pretty right for a week camping..

Next time you go camping, check the voltage after running one of the appliances for approx 1/2 hr ( with nothing connected when you check the voltage ) and that will give you a starting point.....then once a day, check the voltage and see how far down it gets over a week..

11.8 to 11.9V is generally accepted as the lowest you want to go on a regular basis..

Next trip, see how you go without a solar panel, but if you can borrow one, keep it as a backup just in case..
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Follow Up By: SDG - Wednesday, Dec 10, 2014 at 16:11

Wednesday, Dec 10, 2014 at 16:11
Looks good to me as well for a week, maybe a few days longer. I guess a lot of people after that time would also move on to the next camping spot, thus topping up the battery via the car.
Also looks like I might have room to add a few extra power devices if I want.
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Reply By: HKB Electronics - Wednesday, Dec 10, 2014 at 15:43

Wednesday, Dec 10, 2014 at 15:43
This may be of use, it gives a rough idea of the SOC of a battery with a small
load connected to it.

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Follow Up By: HKB Electronics - Wednesday, Dec 10, 2014 at 15:44

Wednesday, Dec 10, 2014 at 15:44
Forgot to add, courtesy of Traxide Electronics

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Follow Up By: SDG - Wednesday, Dec 10, 2014 at 16:14

Wednesday, Dec 10, 2014 at 16:14
Thanks for that. My math reasoning was way out. According to this I only used around 10%
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Follow Up By: ABR - SIDEWINDER - Wednesday, Dec 10, 2014 at 16:51

Wednesday, Dec 10, 2014 at 16:51
If you bring temperature into the test it looks more like this.



Regards

Derek from ABR
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Reply By: Trevor P4 - Friday, Dec 12, 2014 at 09:38

Friday, Dec 12, 2014 at 09:38
SDG

As advised by all the others 12.7 volts is a charged battery.
With the draw your talking about you could get away with a 60 watt panel but i'd advice an 80 watt panel for those dull days.

I see no mention of an elec fridge in your post (you don't have to have one) and the 80 watt allows for future upscaling.

The important thing is to keep the volts as high as you can for as long as you can in your batt so don't fall in to the trap of trying to work out how long you can drain your batt before you connect the solar panel.

The moment you get to camp set up the solar panel. You cannot over charge the batt with a quality panel and regulator. The Regulatoer will only provide that power that is required by the batt based on usage and it is free to do so.
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Follow Up By: SDG - Friday, Dec 12, 2014 at 12:38

Friday, Dec 12, 2014 at 12:38
Thanks for that. No mention of a elec fridge as I have a 3way, so unless I'm in a caravan park with power site, I use gas.
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Follow Up By: Gronk - Friday, Dec 12, 2014 at 14:28

Friday, Dec 12, 2014 at 14:28
Good advice, but I suggested going for a week camping to get an idea how long that battery would take to get down to approx 12V...

That way, if it rained for a whole week, and he knew the battery would last for a whole week, he's got a good idea of battery performance without worrying about maybe stuffing his battery..

Once you do a battery test like that ( and all people should do it once ), then as said above, keep it topped up as much as possible, then hopefully you'll never need to find out again..

But some people mightn't want to buy a solar panel, so it still gives them an idea of battery performance..
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Follow Up By: oldtrack123 - Friday, Dec 12, 2014 at 19:01

Friday, Dec 12, 2014 at 19:01
HI
When looking at a solar panel output& panel requrements ,forget about dividing the watts by 12
You will get NOTHING like that in AMPS, EVEN ON THE BEST DAYS,although ,it is often the way those who know no better think it works
The panel MAX Amps, even on the best days will be Panel Watts / peak power point volts[typically 17V]
for a100Wpanel /17V =5.88A

If you are using a PWM controller it will not matter what the voltage of the battery is [SOC voltage], the current will not change, the charging current will be the same as the panel current
A MMPT will give an encrease in chargeing current ,how much depends on many factors, including batterySOC

PeterQ
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Follow Up By: oldtrack123 - Friday, Dec 12, 2014 at 19:06

Friday, Dec 12, 2014 at 19:06
Hi
Further to the above:
Panel output:
80W /17V=4.7A on the very best days!

PeterQ
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