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Thread: Question on building DIY power supply.

  1. #1
    Gold Member IMHO's Avatar
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    Question on building DIY power supply.

    My son have a question that is out of my league. I hope there is one of you professionals that can help me answer him. I am putting his question as a quote.

    Hi guys.

    I have a question for the electrical people among you.

    I fly RC helicopters, and need to charge my lipo batteries.
    The problem I have is getting my battery charger the power it needs.

    It can take input voltages from 12v to 18v.
    My bats that I am charging are 25v, so when giving the charger 12v, it needs to ramp it up to 25v. That seems inefficient.
    So what I would like is 17V input.

    The charger can use 200W, so I need a supply that gives 17V @ 20A.
    Can any of you think of an easy and cheap way of doing this?

    I know off the shelf applications are available but they are very expensive for a student.

    Thanks in advance!
    Cheers
    Quintin

    PS. All voltages mentioned are in DC.
    ~Expenses will eat you alive! - My first Boss~

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    Diamond Member adrianh's Avatar
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    Look up sarfly on google. Sarly is the best local RC group

    DO NOT F*CK AROUND WITH LIPO BATTERIES. nicd and nimh batteries are fairly forgiving to incorrect charging methodologies, LiPoly batteries explode if they are overcharged. Many RC planes and devices have been burned to a crisp because the batteries got cooked.

    Looking after the batteries is as much a part of electric RC as learning how to operate the radio. This is a fair warning, LiPoly battery chemistry does NOT like DIY rigs, why do you think you get LiPo charging bags...to contain fires and explosions. Lipo Battries also need to be balanced
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    Diamond Member Justloadit's Avatar
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    Today's switch mode supplies a re efficient, and there is no reason to worry about it supposedly not being efficient. The switch mode power supplies will always supply the same amount of power to the batteries , irrespective of the voltage being 12 or 17V. What is important, is the charging circuits within the unit must comply to LiPo battery specifications.

    They are costly because of what is required of them to ensure that all the required specifications for LiPo are adhered too.

    Currently the LiPo batteries are the best there is against size/weight to power ratio, instantaneous current availability and time to charge.
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    Gold Member IMHO's Avatar
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    adrianh, he has a Turnigy professional Lipo charger, that take care of all the aspects and balancing of said batteries. The charger is not the problem. He has spend the money on that and got the best, because of the dangers you mentioned. He is in electric RC for 5 years already and knows the stuff. The problem is the power supply to power the charger. He is using a PC power supply at the moment, but it does not deliver sufficient power. He is playing with linking two PSU's now, in series, the get more volts/amps to the charger. I understand this is very dangerous however. He has cut the earth wires to make it work. It sound like he just popped the one psu!

    When he use the one only PC psu, it takes 45 min to charge 1 battery. With the two in series, it takes 25 min. The time to charge is therefore the problem. He has 4 batteries to charge, for two flights of about 5 min each.

    The best solution would be to just go buy a big ass power supply, but funds is a problem.

    The charger and batteries can handle 200 watt. The psu delivers 200 watt, but the voltage drops to 11 v, causing the charger to draw more amps at 11v, generating excessive heat. At 17v the charger would be pulling considerably less amps to get 200 watt.
    ~Expenses will eat you alive! - My first Boss~

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    Gold Member IMHO's Avatar
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    Spec of the charger.

    Input Voltage: 10~18v
    Circuit power: Max Charge: 200W / Max Discharge: 25W
    Charge Current Range: .1~10.0A
    Discharge current range: .1~5.0A
    Ni-MH/NiCd cells: 1~18
    Li-ion/Poly cells: 1~6
    ~Expenses will eat you alive! - My first Boss~

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    Gold Member IMHO's Avatar
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    Lets make it simple. He needs a power supply of 17/18 v 60 amp. This power supply will then power four of mentioned chargers.
    ~Expenses will eat you alive! - My first Boss~

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by IMHO View Post
    He is using a PC power supply at the moment, but it does not deliver sufficient power.
    .............The psu delivers 200 watt, but the voltage drops to 11 v, causing the charger to draw more amps at 11v, generating excessive heat.
    Linking 2 x PC PSU's together will end up with scrap. 400w and 600watt PC power supplies are basically entry level nowadays and easily available at low cost.

    A '17/18 v 60 amp' powersupply is going to be massive. At 60A the wiring alone will need to be 10-16mm2.
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    tec0 (28-Dec-12)

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    Diamond Member adrianh's Avatar
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    Sounds like you need a small welding machine. I'm not joking...Those machines are happy to supply the current that you need, the only question is whether you can get the correct voltage.
    How easily someone is offended is directly proportional to how stupid they are.
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    tec0 (28-Dec-12)

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    Diamond Member tec0's Avatar
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    Have a look at Manson Bench Power Supply 30V 15A sadly the datasheet wouldn’t load but I would give them a call and find out if you can set the voltage to your desired level. It appears that it can but I hate it when you don’t have access to the datasheets…

    sixty amps is a hell of a lot of power does it really need that much? It is more than enough to kill you sixty times over…

    Not to mention your power bill

    I double checked my oven uses a 50 amp breaker now normally when it comes to amps it doesn’t matter if it is AC or DC 50 amps is a $hit load of juice
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    Quote Originally Posted by tec0 View Post
    Have a look at Manson Bench Power Supply 30V 15A sadly the datasheet wouldn’t load but I would give them a call and find out if you can set the voltage to your desired level. It appears that it can but I hate it when you don’t have access to the datasheets…

    sixty amps is a hell of a lot of power does it really need that much? It is more than enough to kill you sixty times over…

    Not to mention your power bill

    I double checked my oven uses a 50 amp breaker now normally when it comes to amps it doesn’t matter if it is AC or DC 50 amps is a $hit load of juice
    That was my reaction too. Then he reminded me it is at 17v and not 220v.

    The 60A is necessary if he want to get tree more chargers. So he is planning ahead. 60A can actually drive 5 chargers, but he is leaving headroom.

    The problem with PC psu is that you get a voltage drop. It is also just 12v and the charger must then do adjustments, making the charger work harder and take longer to charge. When he connected the two units, he took the 12v and the 5v supplies, ending up with 17v but it drops to like 9v while charging.

    He joked about looking at my inverted welder. Is that an option?
    ~Expenses will eat you alive! - My first Boss~

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