Power supplies

RobW · #1 10-05-2016

Thinking about replacing my 20 year old power supply. It is still working ok but starting to get a bit noisy so rather replace before it fails at a meeting.

Current one is only 10amps output which does not really pose a problem as I currently charge at 1C so max I need is 7.2amps for my 7200mah 1S i use in my GT12. Use a lower rate for 1/10th off road.

Only thought is new chargers seem to be coming onto market with 30/40A outputs and many new lipos seem to be able to be charged at 4/5C. So if I want to future proof myself a bit, what is the maths involved? Is it as simple as needing a 40A power supply to power a 40A charger IF I ever wanted to use the charger to its full capabilty in some time in the future or do you need more to cope with inefficiencies in supply and charger or does it depend on number of cells?

Any advise on the maths or recommendations on power supplies appreciated.

Rob

badger5 · #2 10-05-2016

Hi Rob, if you're charging at 1S or 2S then the amps going to the charger will be less than to the cells. From memory charging 2S at 5A my power supply is giving approx 3A. You could say that it would be easy to work out using the VIR equation but I'm not sure how efficient chargers are as they can get a bit warm.
When I get chance I'll test my SkyRC charger with my power supply and let you know.

RobW · #3 10-05-2016

Thanks, Richard

MiCk B. · #4 10-05-2016

Hi,

Best to think about it in watts, rather then amps (and volts).

Watts is volts by amps.

12v at 1 amp is 12w of power, same as 6v at 2amps.
Both provide the same amount of power.

So charging to 8.4v at 6 amps is 8.4 x 6 = 50.4w
So you charger is supplying 50w to the pack, your supply, will need (ideally) to supply 50w to the charger, in reality it will be more due to losses.
So for some headroom take charger needs 60w to supply 50w.

So 60w at 12v ~ 60/12 = You need a supply giving out 5a at 12v.

So if you want to charge at 10a (8.4x10 = 84w)
You'll need a supply to give 7.5a (at 12v)(90/12)

A 15 to 20a supply with give you lots to play with.

A 20a supply will give you 240w at 12v.

This will allow you to charge at 27a at 8.4v (230/8.4)

You can also use the above to see how many chargers you can run off your supply.

(Always safe to assume there will be some loss in conversation, mains to 12v to charger.)

MiCk B. :-)

JonyNitro · #5 10-05-2016

Hi few people I know use cool ice psu and rate them very good and good decent option, I use a efuel Psu bit pricey but dam good.

Thanks above for the Math

RobW · #6 10-05-2016

Thanks guys, for making maths simple and recommendations. Got to say efuel ones look pretty smart. 30a one for £100-ish looks like plenty.

Rob

badger5 · #7 10-05-2016

Do t forget the Prolux 30A, £80 and adjustable voltage so you can increase a bit if your charger will let you.

Hog · #8 22-08-2016

Just a note of caution with the 30a efuel supplies. Bought one new from eBay. Lasted just under a year before voltage went haywire. Schumacher repaired / replaced it at a price as it wasn't under warranty as it didn't come to the UK via them.

New one lasted about 8 months and went pop yesterday and is now stuck on 19.8v. Luckily my charger is undamaged. Voltage selector dial feels very rough. I look after my gear so I'm narked this has happened again.

Cutting my losses and buying something else rather than pay to have it repaired again.

mark christopher · #9 22-08-2016

Got the 30 amp efuel, from. Skyrc. Never had it miss a beat in 6 months I have owned it.

Charging at 40 amps on 1s draws just around 25 amps

fidspeed · #10 23-08-2016

seen a few r/c ers with modified computer server PSU cheap to buy

blistering output and smooth power output apparently easy to modify usually std "kettle lead" 3 pin input socket

not done it myself but wil be looking when my current unit fails

regards dave

reg · #11 23-08-2016

I can give you details of a chap on FB that makes 100amp ones,the best ones Iv seen,Iv got one and they are very good and really well made

mark christopher · #12 23-08-2016

You have to be care full with the server power supplies, most mod old units that have passed their service life, some last, some fail.they also need earthing properly.

If you don't know electrics, my advice don't mess with them. With out modding they would fail a pats test

tonydevon · #13 23-08-2016

charging at 40A, shit the bed.... I thought I was being a bit naughty winding my charger up to 6A LOL

Quote:

Originally Posted by mark christopher

Got the 30 amp efuel, from. Skyrc. Never have OT miss a beat in 6 months.I have owned it

Charging at 40 amps on 1s draws just around 25 amps

mark christopher · #14 23-08-2016

Quote:

Originally Posted by tonydevon

charging at 40A, shit the bed.... I thought I was being a bit naughty winding my charger up to 6A LOL

My charger will go to 70 if I ever want to using both channels , I don't charge and 40 bit want to test my psu would should I need to.

D20MCK · #15 23-08-2016

I use one of these Fusion 200W adjustable supplies in my man cave. It's a good bit of kit and it's cheap. If you wanted to charge a 1S lipo at 40 amps on this you could do without a problem.

http://www.wheelspinmodels.co.uk/i/2...EYYaAkf18P8HAQ

mark christopher · #16 25-08-2016

Wed

Quote:

Originally Posted by D20MCK

I use one of these Fusion 200W adjustable supplies in my man cave. It's a good bit of kit and it's cheap. If you wanted to charge a 1S lipo at 40 amps on this you could do without a problem.

http://www.wheelspinmodels.co.uk/i/2...EYYaAkf18P8HAQ

Fail to see how 15 amps output from a psu, will let you charge at 40 amps, as above my charger pulls 25 amps from my 30 amp psu when charging at 40amps on 1s

D20MCK · #17 25-08-2016

40 amps @ 4.2 volts is 168 watts, 168 watts @ 15.8 volts is 10.63 amps , its a 200 watt supply mark.

Having never charged @ 40 amps myself or have a charger capable of doing so I'm only going by the above numbers.

Assuming your supply output voltage is 12 volts with a charge rate of 40amps @ 4.2 volts the process must be really inefficient, what is happening to the other 132 watts? That's a lot of power to loose in heat.

Hog · #18 26-08-2016

Quote:

Originally Posted by mark christopher

Got the 30 amp efuel, from. Skyrc. Never had it miss a beat in 6 months I have owned it.

Just keep an eye on the voltage selector dial - both of mine have felt rough when they've failed so I don't know if it's overheating and melting? And both of mine have failed between the 6 and 12 month window. The most I have hooked up to the supply at any one time would be 3 lipos @ 4.5A, TX on charge at 1A, and possibly a soldering iron @ 3.5A, so even worst case it's only 18A.

Bought a Fusion 30A twin output supply for £50 instead. No frills, does the job. Only thing I'm missing is the USB port but that's no biggie.

Robocop · #19 26-08-2016

I had a few power supplies when starting out,then bought a much more power supply that's got to be 10 years ago at least,if it went now would go out and buy another without any hesitation

mark christopher · #20 27-08-2016

Just noticed I put 1s
I did both 1s and 2s and 2s was prob the higher amp