I am trying to build a field charger for my drone batteries.

Discussion in 'Power Electronics' started by Hamish McLagan, Jun 17, 2016.

  1. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Hi experts, let it be known that I am not an Electritian and have very base knowledge about this stuff. Hence the problem.
    I am trying to build a field charger for my drone batteries. I have 2x sealed lead acid batteries in parallel. 12V 7Ah each. I want to charge with my charger with input voltage 11-18v at 5A.
    The problem is that shortly after I start the charger the voltage of the pb batteries drops to 10.? and the charger stops.
    I need a way to keep the voltage up around 12v and amp draw around 5. No ventilation possible so needs to be a cool solution.
    I can solder and get most bits readily.
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    1. Are you sure the 7Ah battery can be safely charged at 5A?
    2. The battery will need something over 13V to charge. If the source may be anywhere between 11V and 18V then the charger circuit needs to be able to boost and reduce the input voltage and it becomes a more difficult project for a beginner.
     
  3. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
    13
    2
    Charging FROM the 7ah batteries
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    If the voltage Is 10v

    Deep discharged

    You need to precharged for some minutes. I use 2x 9v batteries, or 12v pc power supply.

    Once you get 11.2 v or so, the charger will accept the battery
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the 12V sla battery voltage drops to 10.x then it is either overloaded (trying to supply too much current) or needs re-charging. You may have to limit the drone battery charging current to less than 5A. Do you have a proper smart charger for safe balanced charging of the drone batteries (presumably lithium ones)? If so, can you set its maximum current?
     
  6. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Pb batteries are fully charged at 13.2V, when I start the charger the voltage drops to around 10.2V
     
  7. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Yes I am using a proper charger. Lipo batteries for drone. Yes I can set a lower current but that would defeat the purpose of having a field charger. May as well walk back to the car and charge off the car battery.
    Is there a way to keep the voltage from dropping off? Charging at 3A would be acceptable.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Try charging at 3A and let us know the result.

    If your supply battery is aging, then the resistance in the battery is going up. This "internal resistance" causes the battery voltage to be significantly different when measuring without a load connected and with a load (your drone lipo) connected. You may need a bigger battery as your source (higher mAh, or a newer battery as source).

    Try turning charge rate down to 2.5A - 3A range and try again, this should cut the voltage drop (unloaded vs loaded) in half. You should be over 11V and all should be good, for now (until supply battery ages some more).
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    To charge sla batteries you need a constant voltage of 13.8V to 14.4V, the current will vary automatically as it charges up.
     
  10. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Sla batteries fully charged, 13.2V.
    Started lipo charger at 2.7A
    Sla batteries voltage fell down to under 11V about 20mins later. Only charged lipo's to about 4.01V per cell (4.20V fully charged).
    I could keep lowering the Amps to achieve full charge.
    Is it not possible to regulate the voltage or is the amp draw just to high?
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If your SLA battery is brand new (and fully charged), you should get 25 to 30 minutes of 5A output before voltage drops below 11v.
    Are you charging your SLA correctly?




    upload_2016-6-18_22-18-8.png

    upload_2016-6-18_22-22-13.png
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It sounds like your LIPO cells have a higher amp hour rating than your lead acid batteries. So the lead acid is drained before the LIPOs are fully charged.
     
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  13. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Sla voltage came back up slowly to 12.6V over about 4hrs after trying to charge lipo's.
    Guessing that means batteries are f@#ked right?
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    No, not necessarily. Aged but not screwed. Batteries are chemical devices and charging or discharging quickly means there is not much time for the chemicals to diffuse and react to completion. It is common to get a recovery if you let them sit for a few hours after quickly discharging them (or to see a voltage drop a few minutes to hours after quickly charging them).

    How many mAh is your LiPo battery?
     
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  15. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    I'm parallel charging 2x 3S 2700mAh batteries.
    Will adding a third sla battery make a difference?
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I doubt it. See the text in post #11.
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Just to recap here, do you want to charge your Lipo batteries from the Sla batteries,,

    OR are you wanting to charge the Sla batteries from a lipo charger???
     
  18. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  19. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
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    Charge lipo's from sla.
     
  20. Hamish McLagan

    Thread Starter New Member

    Jun 17, 2016
    13
    2
    So I'm now thinking of adding a solar panel into it to help with the load. Charging while discharging so to speak. By my figuring I'll need a 15w solar panel. 12v charging at 1.4a.
    Sound right or am I way off?
     
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