Shangrila

I would like to buy an multimeter and measure the amps my charger is using from my power supply when charging. Reason is I want to get a smaller UPS to carry around and don't want to get one that will slow down my charger.

Can someone explain what I need and how to do this?
I'm weary of shorting out charger and/or ups.


Thanks

phildoggg

iTrader: (24)

they make multimeters for measuring amps also. It will have a clamp on one end. just clamp over the power wires going to charger and it will tell the amps being pulled.

JayL

you measure the amps you pull by the amp setting you charge at:

for example: you have a 5200 lipo, you charge it at 1c 5.2 amps, thus you need 5.2 amps from the power supply


usually give about 10% overhead, meaning go over what you think you need 10%

crusey_aus

iTrader: (3)

Charge amps x volts = power required from your charger

Chargers lose a small amount in effifiency so add 20%

5000mah 2s pack, charged at 1c (5.0 amps)

5.0 amps x 8.4v = 42 watts

Add 20%

So lets say 50 watts

If you are using a 12v supply

50 watts divided by 12 volts = 4.2 amps

Cheers

shultz

iTrader: (52)

Not sure you can get a clamp amp meter to read DC amps... my fluke and ideal clamp meters won't... just AC amps.

Anyway, what you need is a meter that will read DC amps up to at least the amount you're trying to measure... many less expensive meters will only read 10-20 amps. Once you have the meter put it in-line with the positive lead of your charger and measure the amp's.

This may save you some trouble... Having never felt the need to check this before you got me interested in finding out myself... I tested the input amps on one of my chargers and found the following.

1A charge rate drew about .9A from the supply.
2A charge rate drew 1.9 from the supply.
5A charge rate drew 4.8 from the supply.

So... assuming the charger is not not as accurate as my fluke you can figure if your drawing X amps from the charger to charge your battery the draw on the power supply will be about the same.

crusey_aus

iTrader: (3)

shulz,

Those figures would be correct is you were using a 12v supply and charging 3s lipos to 12.6v

Charger input amps changes relative to the charger output amps / volts

If you were charging a 2s pack the input amps would go down

If you were charging a 4s pack the input amps would go up


A 50 watt charger will only charge a 3s pack at around 4 amps

Same 50 watt charger will be capable of charging a 2s pack at 6 amps

Both will require 12v @ 5 amps (60 watts) or thereabouts input


Work out what size batteries you plan to charge and its easy to figure out what size charger / power supply you need

Cheers

Duster_360

iTrader: (21)

I don't know what cell count you're planning on charging with the UPS or how many times you'll try to charge off the UPS. If your needs are small, its prob something thats possible. This link was a thread for multiple 6S lipo charges using a 600amp car booster that was shown to be inadequate for the OP's needs - may want to review this thread and make sure what you want to do is doable with a UPS type power source to begin with -


http://www.rctech.net/forum/radio-el...-question.html

Dave H

Crusey’s sample power calculations are pretty much spot on, assuming a reasonably modern switch mode based charger. Which most are these days. Need to consider the voltage level of the battery in order to predict the power supply needs.

With older linear based chargers the supply amps were indeed the same as the charge amps. The charger was simply a controlled current voltage step down device. And couldn’t charge more than about seven 1.2 volt cells with much amperage (or perhaps 2 lipo cells if such a charger was made). With lots of heat and associated inefficiency.

Back to measuring the amps with a meter, as far as I know any modern digital type multimeter that can measure high enough DC amps will be fairly expensive. Some older inexpensive analog type multimeter units could measure fairly high DC amps, but they generally weren’t al that accurate.

The clamp thing is typically a transformer based deal, generally only good for AC currents. An exception is for highly transient DC currents, like a car starter current meter for example, but not very accurate, more of an on/off - is it working type measurement. But not useful for a constant DC current like a battery charger uses.

Back on track again, suggest just using Crusey’s calculation method for an estimate. Post up your battery, charger, and UPS details if you like and we can help double check you calcs. Unless you want to pony up for a good meter, like a Fluke, which is nice to have of course. I think they start at over $200.

Duster_360

iTrader: (21)

On a fwiw basis, forget about measuring amperage with a cheap DVM to answer your question- mine which is typical of the cheap model, doesn't go higher than few hundred ma.

If I was trying to do this I'd use my wattmeter/data recorder.

Shangrila

Wow - thanks for all the great replies!

Given what's been posted i) sounds like my needs of 7.5amp max charging of nimh and lipo will draw about 8amp ii) the cheap type multimeters I was imagining are not going to cut it.

I have a Robitronic overloader 2 charger. My typical charge amp is 5amp for 3900mah 6 cell nimh packs, and 4000mah lipos. I do however use my charger to discharge at 15amps at home after races.



My UPS is a Graupner 13.8v, 23amp big black box. I am tired of humping it to races and am thinking of replacing it with the new muchmore powermaster mini which is only 13.8, 6.5amp.


The advertising for the mini says its for tire truers and the like tho and I don't want to have longer charge times at the track. But I have seen picts on UK sites of Ellis and others apparantly using it as their main UPS.

So will this UPS be slow my charge rates down or is it the answer?
Its small and lightweight!



Not worried about home use, post race discharging as I can hook up to my existing Graupner ups for that.

jiml

iTrader: (3)

You may have your acronyms confused.

UPS is short for Uniterruptable Power Supply. These are generally used on computers so they don't crash in case of a power failure. It's meant to be a battery backup, not a power supply

What you're talking about is a power supply which sometimes is abbreviated to PS. One letter makes a difference (al least that's what she said).

Competition Electronics makes a 20 amp power supply that is small and light. they sell for around $120 and it's worth it. There are others that look the same and a little cheaper which will work just as well.

A 20 amp power supply is more than enough to run a charger.

Oh, and when your discharging a battery you're not drawing any current from the power supply, the current is coming from the battery.

Shangrila

Sorry, that was a typo with U. Ofcourse I meant PS.

The Comp Elec one is similar to my graupner. Tired of lugging such a hefty item when compared to the MM mini. Based on the above comments it is sounding like it might lengthen the charge time when I charge at amps above 6.5 . Pity, as that is right on the margin.

JayL

I have the much more mini and I charge 10 amps with it all the time, it is underrated

also you don't use PS amps to discharge so the amps you discharge at have no meaning for choosing a PS

Dave H

Been awhile since I’ve used nimh, what does a 6 cell peak at, something like 9 to 9.5 Volts? Lets try 9.5.

5 Amps x 9.5 Volts = 47.5 Watts
47.5 Watts + say 20% to power the charger/inefficiency = 57 Watts
57 Watts / 13.8 Volts of power supply = ~ 4.1 Amps needed from the power supply
4.1 Amps is about 64% of the power supply rating of 6.5 Amps. Which is just about perfect for a nice safety margin for reliable, cool, and stable power supply operation.

If you are charging a 2s lipo at 1C, or 4 Amps, then:
4 Amps x 8.4 Volts = 33.6 Watts
33.6 Watts + 20% = ~ 40.3 Watts
40.3 Watts / 13.8 Volts = ~ 2.9 Amps
About 45% of ps rating, should be cruising.

So assuming you only charge one battery at a time, at these rates, you should be good to go.

And yes, as a couple guys have mentioned, discharging requires very little from the power supply.

Shangrila

Brilliant - I love the amount of knowledge members of this board have!

Thanks very much all for posting up and doing that maths
It is much appreciated.


So I will save on buying a multimeter and put that money towards a MM mini!

Result!!