Is this safe?? battery charging

Discussion in 'General Electronics Chat' started by RiottoiR, Dec 20, 2008.

  1. RiottoiR

    Thread Starter New Member

    Dec 20, 2008
    hello everyone!!

    I'm currently in the middle of building my own remote control boat.

    so I've made the frame, and have the servo's kind of in place and the engine mounted (50cc motor bike engine)

    so what i would like to do is install two 12v sealed batteries in series, as i have a 24v 100w 6A 2300rpm electric motor, the reason for this would be to run various electrics like lights and stuff. and well the main reason would be that if the petrol motor was to cut out the electric one could take over and slowly take it back in.

    The Question(s)!!!
    can i hook up the motor to the batteries without a circuit??
    as the electric motor will be connected to the main drive shaft and it will most likely be at a higher rpm than what it is rated (output).

    and if i did get the rpm to be just over its rated rpm (so that it will produce more than 24v to charge the batteries... say 25v or 26v) would it need a circuit then?

    I initially though it might need a diode to stop the current flowing the other direction, and if so i would program a PIC to detect once the current stops (ie the petrol motor has stalled) to bypass the diode so that the electric motor starts driving the shaft.

    also maybe I'm getting a little too ambitious here but once the batteries are fully charged i would think it would do damage to keep forcing current into them, so would there be a way of stopping this and letting the electric motor run with no resistance, as to let the petrol motor run at its best?

    okay so that was a bit more that i intended to write, but i hope whoever is brave enough to help can understand it.

    thanks to all that have and will try to help!!


    ohh, one more thing.
    i've worked out the top speed at 100% efficency (obiously it will never happen, espacially if i made the hull :eek: lol.) should be about 60mph (100km/hr)
  2. SgtWookie


    Jul 17, 2007
    50cc? :eek: That's pretty ambitious for a model boat!

    The numbers on your motor specifications don't quite add up. 24V 100W works out to 4.16 Amperes, not 6A.

    What is the amp-hour rating of your two SLA batteries?

    That would not be a good idea. You'll at least want to have an on/off capability. You also will not want to spin the motor faster than it's rated speed, as the centrifugal force may cause severe damage to the motor, even causing it to fly apart with great force. It will also present quite a load to your 50cc gasoline-powered motor.

    If you want a generator, use a generator or alternator. Motors don't work so well as generators.

    When the gasoline motor is driving the prop, the electric motor will present a hefty load, reducing available power. When the gasoline motor stops running, the gasoline motor will be a VERY heavy load to the electric motor (due to the engine compression, drag, etc) - and may cause the electric motor to stall (very high current flow).

    If the gasoline engine is 2-cycle, it will not be receiving lubrication while being driven by the electric motor (if it is even capable of turning it over). This will quickly result in damage to the gasoline engine's cylinder, piston, rings, and bearings.

    That's what regulator circuits are for. :) But, charging SLA batteries is different from charging regular lead/acid batteries. For longest life, you charge SLA batteries at 1/8 their AH rating. If you try to charge them a great deal faster, you will damage them.

    You also do not want to try to discharge them too quickly.

    Really, it sounds to me like you need some kind of transmission or clutch assembly, so that you can remotely select whether the gasoline or electric motor will be connected to the prop, and not both at the same time.
  3. Bernard


    Aug 7, 2008
    If you really need the electric motor as a back-up,could use a mechanical OR gate, like two one way clutches. " gas " engine drives prop through one clutch, electric drives via belt,gear through other.Charge batterys on shore.
  4. RiottoiR

    Thread Starter New Member

    Dec 20, 2008
    that's what it says on the label??

    haven't actually bought them yet. i thought i might wait and see what someone say's about it here first.

    it does have a centrifugal clutch, so when the petrol motor stop's the shaft can spin freely.

    where could i get one of these to suit my requirements (12v would be better than 24v, one less battery = less weight ;) ). also will a proper generator / alternator solve the rest of my problems??

    i guess you wrote this because i forgot to mention the clutch, but the motor will be spinning the shaft with minimal resistance, except for the clutch bearing, shaft bearings and water. so i dont think this will be a problem but i'm not sure.

    also i know 50cc might be a bit ambitous but what fun is it if it doesnt make a loud noise and goes really fast.

    although when i do finish it will have a slower gear ratio for testing.

    thanks for your help. hope to hear more from you :)
  5. Farlander

    Active Member

    Oct 14, 2008
    I'd like to see this...

    You could use a magneto off of a lawnmower or an airplane. A scrap yard might not be a bad place to check. A magneto doubles as a starter/generator so it would charge your bat when the petrol motor is on and act as a backup motor. The magneto is wired to act as a motor when two contacts are crossed. I'm sure some kind of relay could be activated remotely to engage the motor drive.

    Better yet, check a scooter -- I assume they have the same thing.

    How big is this boat? I'd advise a minimum 4 ft length and build it self-righting and unsinkable. You'd have a serious toy! It could even go fishing for you...

    Good luck!
  6. RiottoiR

    Thread Starter New Member

    Dec 20, 2008