Calculating Battery Charge From TV in Amps

jkjenn · August 2015

How would I determine how many amps per day (9 hours of driving) that I am getting from my TV. If my TV if putting out around 14v (+/-2) at the 7 pin connector? I am trying to work out my usage versus how much I am adding back into my battery. Does it make a difference if I have 2, 225ah, 6v batteries instead of 1, 12v battery?

Normfun · August 2015

You would need a DC amp meter to know for sure.

jkjenn · August 2015

normfun said:

You would need a DC amp meter to know for sure.

Thanks, @normfun - imthink my voltmeter has a mode for this.

wizard1880 · August 2015

Install a battery monitor, like a Bogart TriMetric, and you'll never worry about battery charge and capacity again.

Normfun · August 2015

@Jenn . Carefull most meters will only take 10 amps @<15 seconds. Clamp Style will read much higher amounts.

jkjenn · August 2015

normfun said:

@Jenn . Carefull most meters will only take 10 amps @<15 seconds. Clamp Style will read much higher amounts.

If I don't blow something up, the credit goes to @normfun.

mash2 · August 2015

On my multimeter, the manual indicates to take no more than 10 seconds to take a reading AND to wait at least 15 minutes between measurements (and don't do more than 10 AMPS). They also indicate that if you are testing a circuit, to shut the power off to the circuit before removing the test leads ....

ChanW · August 2015

Jenn, the amps coming out of your alternator, going into your Tab will vary, depending on the charge that's on the battery, and the draw of items that are being powered, at that moment.

ie: If your battery is fully charged, you'll not see a very high draw measurement with your ammeter. And conversely, if your battery is greatly discharged, you'll see a high reading.

Your TV puts out what the Tab needs, up to the alternator's max rating.

If you're trying to get an idea of the maximum that your alternator can put out at any one moment, you'd probably need to get the specs for the alternator, or have it tested under load.

I guess I'm not sure what your asking.

DurangoTaB · August 2015

My guess is it ain't much...like 2-3 amps, similar to a trickle charger.

jkjenn · August 2015

DurangoTaB said:

My guess is it ain't much...like 2-3 amps, similar to a trickle charger.

I used some other formula I found on the web and it appears to be about 2ah. I think a solution could be to add a small solar panel to the mix.

T@Buhura · August 2015

jkjenn said:

DurangoTaB said:

My guess is it ain't much...like 2-3 amps, similar to a trickle charger.

I used some other formula I found on the web and it appears to be about 2ah. I think a solution could be to add a small solar panel to the mix.

I'd be curious to know what formula/calculator you use to compute TV output current? It's based on power requirements so it would be interesting to understand their methodology.

Since I had been wondering the same thing and what I did was put my multimeter in series between the XC90 and the T@B using patch cables with alligator clips on the pin that provides power to the trailer to get a true measure under load.

Image: https://us.v-cdn.net/5021717/uploads/editor/er/eh55sc8pso60.jpg

The XC90 puts out nearly 7A which is great for offsetting a little more than half of the fridge's 13A on 12v while towing.

jkjenn · August 2015

T@Buhura said:

jkjenn said:

DurangoTaB said:

My guess is it ain't much...like 2-3 amps, similar to a trickle charger.

I used some other formula I found on the web and it appears to be about 2ah. I think a solution could be to add a small solar panel to the mix.

I'd be curious to know what formula/calculator you use to compute TV output current? It's based on power requirements so it would be interesting to understand their methodology.

Since I had been wondering the same thing and what I did was put my multimeter in series between the XC90 and the T@B using patch cables with alligator clips on the pin that provides power to the trailer to get a true measure under load.

The XC90 puts out nearly 7A which is great for offsetting a little more than half of the fridge's 13A on 12v while towing.

I am impressed.

I have no idea where he got his formula from or if it is even right, but I found this on a Jeep forum:

"

see what the fuse size is protecting

the circuit the trailer connector is on and multiply it by 13.2. If it is a 20 amp fuse x 13.2 volts = 26.4 watts then figure you are good for 80% which is around 21.12 watts.

I then converted watts to amps and accounted for an 20% loss. Probably horrible logic, but I am not going to buy one of those tools right now to measure.

T@Buhura · August 2015

jkjenn said:

I am impressed.

I have no idea where he got his formula from or if it is even right, but I found this on a Jeep forum:

"



see what the fuse size is protecting the circuit the trailer connector is on and multiply it by 13.2. If it is a 20 amp fuse x 13.2 volts = 26.4 watts then figure you are good for 80% which is around 21.12 watts.
I then converted watts to amps and accounted for an 20% loss. Probably horrible logic, but I am not going to buy one of those tools right now to measure.

Wow! Yeah, I wouldn't go by that logic at all. All he's really calculating is the amount of power the fuse can support and then applying an arbitrary engineering margin (unless he knows for sure). Even if it is a known margin for the JK it's still a broad range which the circuit could support.

The reason I went all out to actually measure is the tech at our local T@B dealer told us the fridge would kill the battery on 12v while towing since the car would only put out about 2A. Knowing Volvos I seriously doubted that applied and also knowing the fuses in the panel were 10A to 30A depending on pin for the 7pin connector, I suspected it would be much higher (I think the e-brake pin is 30A just didn't recall what the one for "charging" was). I was happy I verified, especially since we were preparing for our big Alaska trip. I had hoped it would match the draw from the fridge. Having heard 2A is the norm (not just the tech but other posts here in the forums) and seeing ours at 6.7A, I was really happy!

You should measure your JK's max output charging current to be certain. If your Voltmeter has a setting for current (A) then you've got a multimeter and you can do the same test. You'd only need the patch/jumper wires since current is measured in series and has to be in the circuit being measured. Voltage is in parallel so typically you don't insert your multimeter into the circuit like you would for current (like I did).

Actual current cannot be calculated. A perfect example are the converters you'll find for electrical devices (like to power/charge a laptop); they're all rated for a wattage which is really only meant to mean maximum power. In reality the device will only draw a portion of that. Meaning at 120v if you have a power supply rated at 60W, it will provide a max current of 0.5A. The device in question may only draw 0.4A (48W) and never max out the brick/power converter. If it were to need more than 60W, then that's when problems start (overheating and even fires).

ChanW · August 2015

TabUhura, did you turn on the fridge on 12v while you were measuring with your multimeter? Also, if your car was idling, rather than higher RPMs, it would limit the output.

I'd be surprised if your TV was limited to only 7A output. A typical alternator will put out much higher amperage than that if it's needed - it's common to have a short 20 or 30 amp 'boost' immediately after the starter does its workr, because the starter has just drawn a lot of power out of your battery.

You can see this in any vehicle that has an ammeter on the dash. 2A is more like a maintenance or trickle charge.

ChanW · August 2015

Not sure this is going to help you with your calculations, but this article is pretty good at describing TV alternator output (clearly better than me! Note a typical automotive alternator is rated at 80A)
http://cartech.about.com/od/Power/fl/Understanding-Alternator-Output-Ratings.htm

jkjenn · August 2015

ChanW said:

Not sure this is going to help you with your calculations, but this article is pretty good at describing TV alternator output (clearly better than me! Note a typical automotive alternator is rated at 80A)
http://cartech.about.com/od/Power/fl/Understanding-Alternator-Output-Ratings.htm

Thanks, @ChanW. I would assume that the this won't translate to what is actually at the 7 pin connector, but it dies help me understand better. I have wondered if it would help me to disconnect my winch from the battery when not in use.

ChanW · August 2015

Unless the winch switch is faulty, it should not be drawing any power if you aren't using it, so there's no need to disconnect it.

I'd be sure to disconnect the Tab before you use the winch though! Or at least turn off the fridge...

You got me wondering, with this thread, so I ordered a simple 12v watt/hour amp/hour meter, which's designed for these types of measurements (RV, solar, RC models, etc.)

It's called 'Watts Up'.

There's a cheap one on Amazon, and a more expensive one too, with the same capabilities. Looks like the cheaper one is less reliable, from the reviews. The better one comes highly recommended from those who know those things.

It'll measure your amps, watts, voltage, as well as accumulated consumption over a period of time, ie: amp/hours and watt/hours.

It would have to be installed in-line wherever you're trying to measure, like between the TV and the Tab, or between your solar panel and the Tab, etc.

The cheap one: http://www.amazon.com/Digital-Battery-Balance-Voltage-Analyzer/dp/B00EYZS6R6
The better one: http://www.amazon.com/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK

I'll let you know what I learn (if I learn anything B-)

ChanW · August 2015

Also Jenn, as long as your 7-pin connector has been wired correctly, with a large-gauge "charging" conductor (should be #8 or #10 directly from the battery/alternator to the 7-pin with an in-line breaker), and with a good ground, the amps output at your 7-pin would be the same as at the alternator.

Amps are always the same throughout the circuit, but there might be a voltage drop, if your wire isn't heavy enough for the load on the circuit.

jkjenn · August 2015

ChanW said:

Unless the winch switch is faulty, it should not be drawing any power if you aren't using it, so there's no need to disconnect it.

I'd be sure to disconnect the Tab before you use the winch though! Or at least turn off the fridge...

You got me wondering, with this thread, so I ordered a simple 12v watt/hour amp/hour meter, which's designed for these types of measurements (RV, solar, RC models, etc.)

It's called 'Watts Up'.

There's a cheap one on Amazon, and a more expensive one too, with the same capabilities. Looks like the cheaper one is less reliable, from the reviews. The better one comes highly recommended from those who know those things.

It'll measure your amps, watts, voltage, as well as accumulated consumption over a period of time, ie: amp/hours and watt/hours.

It would have to be installed in-line wherever you're trying to measure, like between the TV and the Tab, or between your solar panel and the Tab, etc.

The cheap one: http://www.amazon.com/Digital-Battery-Balance-Voltage-Analyzer/dp/B00EYZS6R6
The better one: http://www.amazon.com/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK

I'll let you know what I learn (if I learn anything B-)

Funny, this has been on my wish list for a couple of weeks, Let me knw how it works!

jkjenn · September 2015

ChanW said:

Unless the winch switch is faulty, it should not be drawing any power if you aren't using it, so there's no need to disconnect it.

I'd be sure to disconnect the Tab before you use the winch though! Or at least turn off the fridge...

You got me wondering, with this thread, so I ordered a simple 12v watt/hour amp/hour meter, which's designed for these types of measurements (RV, solar, RC models, etc.)

It's called 'Watts Up'.

There's a cheap one on Amazon, and a more expensive one too, with the same capabilities. Looks like the cheaper one is less reliable, from the reviews. The better one comes highly recommended from those who know those things.

It'll measure your amps, watts, voltage, as well as accumulated consumption over a period of time, ie: amp/hours and watt/hours.

It would have to be installed in-line wherever you're trying to measure, like between the TV and the Tab, or between your solar panel and the Tab, etc.

The cheap one: http://www.amazon.com/Digital-Battery-Balance-Voltage-Analyzer/dp/B00EYZS6R6
The better one: http://www.amazon.com/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK

I'll let you know what I learn (if I learn anything B-)

@ChanWdid you ever get that installed?

ChanW · September 2015

I have it wired, but I have yet to take a trip with it, so I can't tell you anything. Should be soon though! I'll let you know. (I wish I were planning a trip to the Rockies... Have a great trip!)

ChanW · September 2015

Y'know, I can do a quick test with it in my yard... Run the battery down a bit with the fridge, then hook up the truck and see what the Watt's Up meter tells me, with a low idle and then a higher rpm.

I'll see if I can get to it tomorrow, if DW and DG don't get my time.

ChanW · September 2015

@jkjenn, I tried that Watts-Up 12v watt meter on the seven pin connector between the TV and the Tab, but couldn't get it to work right, so I have no news. I'll have to try it later with a connection at only the fridge.

I did do a test with the "Kill a Watt" 120v watt meter. I measured the 120V usage with the fridge in both 120v mode and 12v mode. It showed what we already knew, that the fridge draws about 11-12Amps in either mode.

At the time I tested it, the fridge was near empty, I had pre-cooled it down to 35° beforehand, and the days were in the 80s, with nights in the 70s. In these conditions, the fridge had a 50% - 60% duty cycle in both 120V and 12V modes (I've installed the 12V thermostat relay mod that was mentioned earlier, so the 12V mode is not on continuously).

So that says to me that, in 12V mode, in those specific outdoor conditions, the fridge will draw at around a 7Ah rate (with minimal opening and closing of the door, and no adding any warm stuff to the fridge). (@mawebb feel free to correct me on all this, please!)

HTH

Have a great trip, Jenn!

jkjenn · September 2015

Thanks, @ChanW. The colder weather should help and I am pre-chilling the fridge, now.

The thermostat relay sounds like a great idea. You have to wonder why Norfolk don't build that into the fridges.

Calculating Battery Charge From TV in Amps

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