Low RPM alternator to charge 12V battery

  1. Hi guys,
    I am working on a school project. I have an inclined cart mounted on rails moved by a winch. I would like to install an alternator on one of the cart axis, so I can charge a 12V car battery - 20Ah. The wheel turns at 12 RPM. At that speed the alternator will not work, so I am also adding a 1:20 gear ratio to turn up the RPMs to about 240. This is the alternator I am thinking of buying ( http://www.windbluepower.com/Permanent_Magnet_Alternator_Wind_Blue_Low_Wind_p/dc-540.htm ). There is an RPM - Volt-Amps graph which shows that at 240 RPMs it yields 3 amps and 25V. The battery will power 4 actuators, which will use 1.2 Ah. The wheel will only charge for 5 minutes@240RPM, every cycle. Will this work or will the battery discharge after 20 or so cycles. What else can I do to improve my design.

    Last edited: Aug 4, 2014
  2. jcsd
  3. What is my best option? getting more batteries in series or parallel? or should I get 1 alternator for each wheel and actuator?
  4. Baluncore

    Baluncore 2,436
    Science Advisor

    Welcome to PF.
    What powers the winch that pulls the cart back up the slope?
  5. an AC motor, a constraint of my design is not to have any other cable connecting the cart except for the winch cable.
  6. What is the voltage needed for the actuators?

    Let's say it's 12 V, then you will need a voltage regulator for your alternator to convert the current from 3 amp@25V to 6.25 amp@12V (= 3*25/12). If you run the alternator at 6.25 amp for 5 min, you will make 0.52 A.h of charge (= 6.25*5/60). Not enough for your 1.2 A.h requirement.

    Similarly, with your 20 A.h battery, let's say you can use half of its charge, 10 A.h, before the voltage drops too much. That means that the battery can support 8 cycles (= 10/1.2). If you have the previous alternator charging as well, you will support 14-15 cycles (= 10/(1.2-0.52)).

    As for battery in parallel or series, adding a battery in parallel to a similar one will double the current at the same voltage; adding it in series will double the voltage but give the same current.
  7. Baluncore

    Baluncore 2,436
    Science Advisor

    How long will each cycle take before it can be repeated ? What is the duty cycle?
  8. jack action the actuators require 12V
    thank you for your reply, so If one alternator gives me 0.52A.h of charge on one wheel. Can I install 1 alternator on each of the four wheels of the cart to get 2.08 A.h?. I have been reading about the 12V batteries and it has come to my attention that they are not very efficient at recharging. From information I got from starter chargers, there is an efficiency of 22% when battery is charged at 30 Amps and 40% when charged at 10Amps. Extrapolating at 6.25 Amps I should be getting an efficiency of 43.38%. So I would end up getting .902 A.h
    To get 1.2 Ah at 43.38% I need an input of 2.8Ah. 8.4 Amps@ 12V, but I would need a bigger gear reatio 25:1. My concern is that at 25:1 gear ratio, will be produce a slippage problem by overcoming the factor of adhesion of the cart (cart is 12ton when full, 2ton empty)
    Also, if I am worried about the A.h hours available then I should add the batteries in parallel, correct?

    My duty cycle is 13.5%, active for 40 seg every 5 minutes
    Last edited: Aug 5, 2014
  9. The battery world is a very specialized world of which I'm not an expert. Batteries have different charging methods & characteristics depending on their construction. There are certainly a lot of inefficiencies (electrical & mechanical) to consider that are not in my previous calculations, but one thing is for sure, you can't do better than that.

    Yes, the charge of different alternators will add up.

    If you have 12V batteries and actuators then you have no choice but to put the batteries in parallel to keep the 12V voltage. Otherwise putting two 12V batteries in series will give you 24V, 3 will give you 36V, etc.

    I'm not sure how your system works, but if your actuators work when the alternator is charging, then the current will go directly to the actuator and the battery will not be charged (or discharged for that matter). In a car for example, The concept behind the alternator is to give all the electrical power that is required (ignition system, lights, etc.) such that even if the battery was disconnected, the car still function as usual. The battery is really only needed to start the engine and is then charged only for the amount of power used by the starter such that it can be re-use for the next engine start.
  10. Hi Jack,
    When actuators are operated the cart will always be at rest, so all the current is drawn from the battery. That is why it may not work. A car battery in normal conditions is under much lower usage than in this application.
    However, I am now thinking of using a rotary actuator and using mechanical advantage to lower its energy consumption.
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