# Low RPM alternator to charge 12V battery

by ararrati
Tags: alternator, battery, charge
 P: 10 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/Permane...d_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. Regards, Armando
 P: 10 What is my best option? getting more batteries in series or parallel? or should I get 1 alternator for each wheel and actuator?
 Sci Advisor Thanks P: 1,949 Welcome to PF. What powers the winch that pulls the cart back up the slope?
 P: 10 Low RPM alternator to charge 12V battery an AC motor, a constraint of my design is not to have any other cable connecting the cart except for the winch cable.
 P: 572 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.