I know someone here knows how to do this.

Bret4207

At the casting bench in the sky. RIP Bret.
Could someone please explain to me how I measure current draw off a 12v electric fence charger? I want to get a solar charging system to keep a deep cycle battery up to snuff, but have no clue how to judge how many amps/watts,whatever it draws. Presently a fully charged battery will last 10-14 days of 24/7 operation. I tried a little 1.5 watt charger but it didn't seem to help and I didn't like it not having a charge controller.
 

S Mac

Sept. 10, 2021 Steve left us. You are missed.
If you got your multimeter up and going hook it in the circuit (inline). That's where you pop the fuses in the meter, if the draw exceeds the fuse. :angry:
 

Ian

Notorious member
1.5 watts is pretty useless for anything other than maintaining a fully-charged battery. The 120 mA is somewhat useful for keeping up with the off-draw of a typical computerized vehicle battery, which is 30-75 mA 24/7/365 including cloudy days. 250 mA in full sun is better. Little panels like this don't need regulation on a big wet-cell battery, there just isn't any way they can push enough current to hurt anything even if their open-circuit voltage can sometimes be over 20.

If you knew what the reserve capacity (RC) of the battery you're using is it would be simple enough to derive the amp/hours (watts) needed to keep up with the battery. Battery should be marked with RC or tell me exactly what it is and how old it is and maybe I can figure it out. Days of sun per year, hours of usable sun per day all factor in to the size of the panel bank you will need. A good guideline for around here (MUCH further south) is to count on 4-6 hours a day of sun on average. You may only be able to count on three hours a day averaged over a year, maybe less.

An Interstate Marine battery has about 140 amp/hours reserve capacity. 14 days is 336 hours, so it can push 2.4 amps for 14 days before being run down, not counting the tiny .120 amp charger's input, so that comes out to 2.385 amps per hour or so of draw you're dealing with in a perfect world and if your battery is good and has similar RC.
 

Ian

Notorious member
If you got your multimeter up and going hook it in the circuit (inline). That's where you pop the fuses in the meter, if the draw exceeds the fuse. :angry:

Problem is it pulses, and if the pulse duration is shorter than the sampling rate of the meter, it won't register. A graphing multimeter or 'scope with inductive lead would be needed just like when checking PWM fan clutches for excessive draw.
 

Bret4207

At the casting bench in the sky. RIP Bret.
Hmmmmmm. Shoulda known this would be more complicated than it seemed. I'm looking at 7 watt/460 mA charger in the latest Northern tool catalog as a base line. Looking at Premier1's solar fencer set ups they use a minimum of 10 watts and up to 40 for a 2 Joule and I'm running a 6 Joule. Maybe I'd be better off going larger and using a controller? Better safe than sorry?
 

Ian

Notorious member
If I'm figuring this right, your six Joule charger would peak at 120 watts output at 12.6VDC input and is going to pull nearly 10 amps DC from a battery. That means 40 amps input during charging hours to keep up, but you'd need two deep cycle batteries to have enough RC to carry it through the night. 40 amps from solar is a lot of panels. Something doesn't add up.

Are you looking for a new fence charger, or just to add solar to what you have? Try to find the input specs from an on-line PDF for what you plan to use and we can figure it from there.
 

fiver

Well-Known Member
that'd be 480 watts.
not really a lot, but for a 12 volt system that's like running a pretty rockin' stereo system on full power, with an alternator supplying clean straight power it's okay.
or your horn, headlights, heater, interior lights, blinkers, air conditioner, and brake lights all at the same time..... on 4 cars.
you'd need at least 2 deep cycle battery's to run that for like 10 minutes.
 

Bret4207

At the casting bench in the sky. RIP Bret.
Yeah, but it's not a continuous 6 Joules. It's a MAX potential of 6 Joules once every 2 or 3 seconds and that's only if it's grounded somewhere. This much I do know, the ground is variable. That much I can prove with a simple neon light tester.With a solid ground I get 5K volts. With a poor ground I might get 3. I believe Joules relates, sort of, to amps as a measurement of potential ability to shock. So I'm thinking that's why the rates sound like it would drain a battery pretty fast, yet the battery will sometimes go close to 2 weeks and still bite you.

Does that help at all or just confuse it more?
 

Pistolero

Well-Known Member
A joule is one amp flowing through one ohm for one second. That is the same as saying
1 volt and 1 amp for 1 second. So your six joules, with a voltage of 5,000 V is about 1.2 milliamps
for .....how long? Need the exact duration and duty cycle to go farther.

I am understanding that a single power pulse is rated at 6 joules, is that correct?

If less than 10 Amps, an ordinary multimeter should measure it .....if it were constant draw, which it
isn't. Doesn't the maker give any info on average input power draw?

If the solar folks are right, a 40 Watt solar panel for a 2 joule charger would IMPLY, a 120 watt
panel to support this one. But remember, the 120 W is peak output, and you gotta figure that
you get, at best, this time of year, half sunlight, and a bunch of that is shadowed, typically. If
it takes a 120 W solar charger, probably something like one third of that continuous charging
from mains power should about equal that.

Bill
 
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Ian

Notorious member
Even with a sun tracker at the equator you don't get half sunlight. There are charts of the US showing useable daylight factors, my gate opener instructions came with one for selecting panel requirements.

I think the main thing to go off of here is actual use, which we know. The reserve capacity of the battery (whatever it is, probably not more than 150 amp/hours and this diminishes with age and charge/dishcharge cycles) is being consumed in as little as two weeks under some conditions and that equates to something near a 3 amp average draw at somewhere around 12.5 volts. At 25% daylight the panel needs to produce 12 amps to keep up. That's 150 watts of solar, right?
 
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fiver

Well-Known Member
even if a 120 didn't keep it charged continuously it would for sure extend it another 50%.
seems to me you could pull up to the battery with the truck every two weeks for like 15 minutes and keep the battery charged.
 

Bret4207

At the casting bench in the sky. RIP Bret.
"...seems to me you could pull up to the battery with the truck every two weeks for like 15 minutes and keep the battery charged."

Pretty much what I do now unless it get way down (charger has a battery charge gauge). Using net fence I have miles of wire and every balde of grass that touched it is a potential ground/voltage loss. I'm going to research it more by contacting company. I think I know where to head now. Thanks guys.
 
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