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  #1  
Old 06-02-2005
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Calling Electrical Goo-Roos Power Inverter??

hey, i am going to try to run 3 battery chargers in my dads trailer, i need to find out how big of a power inverter i will need, if i can run it off the parking light power wire, or if i should run a dedicated line, and if this is worth my trouble

here are the specs on the chargers, as printed on their labels

DeWalt 7.2-18v charger:

Input: 120V 2.0A
Output: 2.8A

Makita 24v fan cooled charger

Input: 135W
Output: DC7.2v-12v 6.5A

Paslode Impulse gun charger

Input: 120V 19W
Output: 12v DC 800mA


i appreciate your help and the sooner the better, im trying to get this all hooked up today.
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  #2  
Old 06-02-2005
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Okay, first load appears to be rated at 240 watts -- but that's probably PEAK. Second at 135 watts, and the last at 19 watts.

240
135
+19
-----
394 watts total.

I'd say a 500 to 700 watt unit will do.

Lets use a 500 watt syste, and figure 70 percent efficiency in the unit, so input power will be: 500/0.7= 714 watts.

Nominal 13.2 volt input, the required current from you battery will be: 714/13.2=54 amps.

Definitely not something you can run off of the parking light wire. If it's a long run you need at least 8 to 10 gauge wire feeding the inverter.

That's based on the data you supplied. A 700 watt inverter, if it will ever be loaded to full capacity will require in input capacity of 700/0.7 = 1000 watts, with a current from the battery of 1000/13.2=75 amps.

Some inverters probably are better than 70 percent efficient, but I don't know for sure. That should be a safe assumption I think for conservative design.

Edit: It sure seems like those wattages for that first charger is on the high side. Did you get that number by just seeing the fuse rating? Or was that the actual electrical requirement. That first charger seems to me like it should draw about 1/2 what you posted. If the fuse is rated 2 amps, it probably only requires about 1 amp and I'll redo the calculations -- or you can.
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  #3  
Old 06-02-2005
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Anything over 300 watts has to be hooked straight to the battery.
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Old 06-02-2005
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I concur.
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  #5  
Old 06-02-2005
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I dare not challenge the all knowing Griggs LOL but I some how think that your calilation can not be right, I have run simular battery chargers off of a small cigerette lighter plug inverter with no problems.

Edit I am not doubting you John, I am doubting the numbers, that just sounds like a lot of power to run 3 little battery chargers.

Last edited by Ranger1; 06-02-2005 at 01:58 PM.
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  #6  
Old 06-02-2005
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Modified sine wave inverters are 85-90% eff. (normally rated at 90%)

Rand
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Old 06-02-2005
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Quote:
Originally Posted by Ranger1
I dare not challenge the all know Griggs LOL but I some how think that your calilation can not be right, I have run simular battery chargers off of a small cigerette lighter plug inverter with no problems.
Probably not 3 or them at once full load as those are. Again, my calcs are based on the data presented, which specifies almost 400 watt load. If that first one IS over-rated, a 300 watt inverter might be possible -- but you need to know that. I cannot but calculate based on the data presented and some assumptions since no other data is available.

Ben, what IS the input rating of the unit's you've run? ANY of those units, by itself, could run off a 300 watt inverter. But not all three IF the ratings presented are correct -- at least not without some potential for overload.
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Old 06-02-2005
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Quote:
Originally Posted by Ranger1
I dare not challenge the all knowing Griggs LOL but I some how think that your calilation can not be right, I have run simular battery chargers off of a small cigerette lighter plug inverter with no problems.
*gasp* You know you're not to do that! You have broken R-F law! YOU SHAL BE PUNISHED!!!!
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  #9  
Old 06-02-2005
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I don't know any of my numbers off hand but I have run 3 and sometime 4 radio control car battery chargers at the same time off of the smallest inverter wal-mart sold.
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  #10  
Old 06-02-2005
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It's okay. I'm not a muslim cleric who is considered too holy to be questioned. No fatwah's will be issued for Ben's death -- torture, yes -- death, no...
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  #11  
Old 06-02-2005
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RC batery chargers only draw at most 6 amps

"REQUIRES: Power Source Such As 110V AC Household Wall Outlet OR 12V DC Power
With At Least 6A Such As HCAP0250 Power Supply OR
HCAP0800 TorqMaster Battery."
http://www2.towerhobbies.com/cgi-bin...?&I=LXD548&P=7

So if I'm reading what John is saying right, the inverter will draw over 50amps with the three chargers, while an inverter chargeing 4 RC batteries will only pull 24amps max....? Am I even close? lol..
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  #12  
Old 06-02-2005
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I'm no expert on inverters, but in theory you should just add up the power you need. The first item says it draws 2Amps at 120V.. so that works out to 240W. The other two specify their power draw directly, so that's easy. (Although the last device's spec sounds aweful low!) Add'em up and you get 394 Watts. The inganeer in me says throw in another 25-50% for expansion and/or fudge factor and you're up to 400-600W.. Which is quite a bit, but not insane. These guys advertise a 900W for $65..

Edit: Geez! Teach me to reply to a topic like this before hitting refresh. This is the last tab I checked after opening all new topics open in firefox tabs this morning.. When I posted what you see above, there was but one post in this whole thread! .. I see John answered it though..
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  #13  
Old 06-02-2005
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Well, I prefer an efficiency number to fudge it personally. No inverter is 100 percent effiecient so it is NEVER adequate to simply add up the load requirements when deciding what happens on the 12 volt line.

Keep in mind, those of you non-electrical types, that the inverter is being fed by 12 volts (approx.) and putting out 120 volts (RMS -- equivalent to DC power, so called "heating power" sometimes). This means that with a 100% "perfect" inverter, the input current should be 10 times the output current.

So, if you have a 5 amp 120 volts AC load, you would need 50 amps on the 12 volt side, again, approximately. Since efficiency IS important, the DC numbers on the 12 volt side will be even higher. In this case, the power lost is lost as heat (predominantly) and unrecoverable electomagnetic losses (very small).
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  #14  
Old 06-02-2005
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sort of off topic but im planning on hardwiring a 500 watt power inverter in the cab of a truck. it has clips so would i be able to just cut off the clips, extend the positive lead to the batt and locally ground the neg?
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  #15  
Old 06-02-2005
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Quote:
Originally Posted by n3elz
...
If that was towrds me, the 6amps that is required for an RC charger is at 12volts, which by your calculations would be .6amps on the 120 side, which is absolutly what is required by these things. Heck they are protected by a 1amp fuse...
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  #16  
Old 06-02-2005
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Maybe the answer is not to run all 3 at the same time, here is a 400 watt continuous 800 watt peak for $27 should be plenty to run 2 of the chargers, and when you get done with the one use the other.

http://www.dcacpowerinverters.com/itemdesc.asp?CartId={1BE8D784-E0EVERESTD6-4A46-A412-CAE88974465B}&ic=PBI-400-12
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  #17  
Old 06-02-2005
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john, the second charger is the one that will take the most power, with an output that is more than double the first one. its a fan cooled 24V battery charger, the first is just a dual charger for dewalt 7.2-18v the third is not much load at all, has a very thin wire.
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  #18  
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Quote:
Originally Posted by n3elz
Well, I prefer an efficiency number to fudge it personally. No inverter is 100 percent effiecient so it is NEVER adequate to simply add up the load requirements when deciding what happens on the 12 volt line.
I don't understand. I thought we were basically saying the same thing. My 'fudge factor' is to cover efficiency and/or overmarketing factors in the unit.
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  #19  
Old 06-02-2005
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here is a dewalt single charger that plugs right into the cig lighter of the car, it doesnt show the specs though, just showing it can be run off a car. my problem is i have to run all of these at once.
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  #20  
Old 06-02-2005
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I am going with in a perfect world with inversters that run 100% effiency a 400 watt continuous would do the job for all 3, given inverts are not 100% efficent and some fudge factor on what the chargers really need I think any where around 500-700 watts would do the job. I would check out inverters around that size and hook them according to what the inverters directions say as far as wire gauge to battery and such. The inverter might come with wire and clips for batteries.
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  #21  
Old 06-02-2005
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Quote:
Originally Posted by SoundPer4mance
john, the second charger is the one that will take the most power, with an output that is more than double the first one. its a fan cooled 24V battery charger, the first is just a dual charger for dewalt 7.2-18v the third is not much load at all, has a very thin wire.
Okay, but I asked where you got that volts and amps for the first one? I'm trying to confirm what you're saying. It sounds like you could derate the first one to 100 watts effectively, and consider the whole load within the range of a 300 watt power inverter.

And, Colin, I don't like "fudge factors" when there are real calculations you can run based on real world data. Although, in this case, not having any spec for inverter efficiency can be a pain.

In the commercial world, where money is monitored, OVER designing is as bad as UNDER designing! Determining exactly what one NEEDS, and not using fudge factors pulled out of a hat, seems more rigorous and it's how I proceed as best I can with the data provided.

Everyone objects to the numbers I came up with but they would be valid if the volt/amp rating of the first one was a REAL power consumption, and not as I suspect just the fuse rating.

If his numbers had been right, and one of you told him, from the seat of your pants, "just use a 300 watt that should be fine" -- you would be doing him a disservice.

If you want a good answer, you need good data, and a method that works.

If you want a lousy answer, don't solicit an engineer for one! They will give you an analysis every time.
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  #22  
Old 06-02-2005
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Quote:
Originally Posted by n3elz
Everyone objects to the numbers I came up with
Not I, I was just saying that Ranger1 cant compare these chargers to RC car chargers because they are so much smaller

I think your right on the money, and have no reason to think otherwise.
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  #23  
Old 06-02-2005
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john you know im not arguing with you, im taking in every word you type, you were the one i wanted to answer this question. i got those numbers fromt he bottom of the charger, same place i got the rest of the numbers.

the way things are looking, perhaps ill wire these into his tool box or back seat, so i dont have to run such al ong wire. do power converters have a constant on state when hooked up? or can it use a switching wire?? i want this to turn on the chargers every time the truck is running, and off when its off, so i could use a switching wire from the radio circuit, so it wouldnt be one while starting. as far as taking the power, it is a diesel, it does have 2 batteries and a rather large alternator.

john, give it to me straight
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  #24  
Old 06-02-2005
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i should also note the first charger is a dual 18V dewalt charger, so it charges 2 at once. but i still figured the makita charger would draw more power since it has a cooling fan and fast charges a 24v battery
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  #25  
Old 06-02-2005
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They can be switched on and off, no problem. A 300 watt inverter can pull 25 to 30 amps at full load, so keep that in mind when you choose a circuit. Of course, if you don't load it much, you can use a smaller circuit.

I've met many, MANY people who don't understand how much power inverters draw from car batteries. I've known guys to run their batteries down in no time using high wattage inverters.

Your diesel sounds like a perfect candidate though with those two batteries and all.

If you intend this to be "permanent" installation, then I would run it back from the battery anyway, with a fuse near the battery, to a 40 amp relay and have the relay activated by the radio's switched hot (or any switched hot). The relay should then feed power to the inverter.

Someone asked about cutting the clips off an wiring to the battery. No problem IF you put a fuse at the battery on the hot wire. Otherwise you are a fire waiting to happen.
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