SLA charging circuit

Discussion in 'The Projects Forum' started by ShaunManners, Oct 26, 2009.

  1. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
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    Hi All,

    You have all been really helpful before, so before I go into my problem, here is an update of my project (almost 6 minute video) for you... please feel free to browse through my gallery.

    I have a 3.5hp lawnmower engine driving a 100A alternator. I have removed the rectifying diodes and replaced them with some external 100A diodes.

    I then have two 12v SLA batteries in series which I then apply to the field using PWM. The output from the rectifier goes straight to the motor.

    So to drive, you set the power with the engine throttle, and then increase the duty cycle on the field therefore increasing the traction current.

    What I'd like to do is use the output from the alternator to also charge the batteries, the problem is, the output goes down to 0v and I don't know what the maximum voltage is (I did get 125V when it wasn't under load). Does anybody have any ideas how I could tackle this? I don't mind having an LED on the control panel to indicate that the batteries aren't being charged.

    Any thoughts, comments or suggestions welcome!

    Cheers
    Shaun
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Hi Shaun,
    Long time no hear from!
    If I remember correctly, you were originally wanting to use the 100A alternator as a motor to start the 3.5hp engine by feeding synthesized 3-phase power to the field windings.

    SLA batteries usually use some kind of a gel for the electrolyte. If you charge them too quickly, bubbles will form in the gel, and that will permanently reduce the battery capacity. Maximum charge rate is 1/8th the battery capacity. If you attempt to charge them more quickly, you will ruin them.

    Automotive batteries are designed for quick momentary discharges (large bursts of current for starting, perhaps 200A), and then to be charged right back up.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Just get a solid-state automotive Voltage Regulator (junkyard), or build one yourself. I can send a schematic if you want it.

    They are basically a voltage-controlled switch; fully ON if the battery voltage is < 14.xxV, and fully OFF if the voltage is > 14.xx+ΔV, where xx is typically 0.25V, but could be as high as 0.5V, and where Δ is small, like 10mV.

    The resistance of the field winding (rotor) in a typical automotive alternator limits the max current to about 2A. During running, the average field current is less than 2A (load dependent). Think of the alternator as a current amplifier, output current = k*Ifield, where k is about 40, i.e. 2A in makes 80A out.

    To regulate, the VR rapidly switches the field ON/OFF. The inductance of the field winding smooths the pulsed current. Practically, the VR, and field inductance turns the system into a PWM, where the switching frequency, and ratio of field current On time to OFF time dynamically is adjusted to hold the battery at the optimum charge voltage.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    What is the motor voltage when you are running at a typical speed?
     
  5. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    Hi Sqt!

    Yes it has been a while hasn't it! Hows things going for you?
    A friend of mine who has made a loco in a similar way wanted to come and visit in the summer, so I had to work flat out on all the other things to get it working well enough for him to see. And yes, good memory! I got the engine turning over, but it kept getting to the point where the alternator would stall. It (the engine) did try to fire a few times, but it never started. One theory I had was that it would try to fire, but the alternator would stop it which in turn made the alternator stall. it was either that or the fact it reached the highest frequency where it could produce enough torque at that voltage.
    I still want to go back and look into it, but it was because of my friend I decided I couldn't spend any longer trying to get that bit working, so in the end I got the cassette out of the gears from a pedal-bike with a socket welded to it and put it in a cordless drill. I could then put it on the nut on the top of the flywheel and start it that way... when the engine started, the cassette ratchet would stop the drill from being taken out of my hand!

    Thanks for this...the batteries I have are 12v12Ah so it needs to be charged at about 1.5A yes?

    As it is at the moment, they aren't being used to start the engine, so it's only the field which is drawing the main current... at full duty cycle this is about 3A. I'm probably wrong here, but I was assuming that two batteries in series, would give 24v 12Ah is that correct? in which case I'd get 4 hours use out of them in the worst case.

    I rarely use full duty cycle and then there are the times when sat in the station that I could turn my reversing relays off (they just swap the polarity of the motor) therefore disconnecting the motor, and I could apply a little current to the field which would then charge the batteries.

    My problem is that I need some sort of charging circuit - I found one fairly simple trickle charge one online, which is fine when you have a relatively stable power source, but when mine can go from 0v to whatever it's maximum is.. I think I need some sort of voltage monitoring circuit that turns on the charging circuit when the alternator is generating over a certain voltage. The charging circuit would also need to be able to deal with potentially high voltages.

    I could just charge the batteries after each days running... but they are hard to get at, and since I have a generator, it should be possible to charge them from that?!

    Cheers
    Shaun
     
  6. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    Thanks for this Mike, if I am correct, you mean regulate the field using PWM to keep the output from the alternator at the correct charging voltage for the batteries? or have I got the wrong end of the stick?
    This is how the regulator normally works in in alternator isn't it... unfortunately I cannot do this as my alternator is being used to provide traction current for the motor... I'm applying 24v to the field using PWM and then the output from the alternator drives the motor.

    My friend is designing a much longer loco, which means he can have a second much smaller alternator for charging, but I don't have the room for that, hence why I am trying to figure out a way of charging from the one I do have already.

    Cheers
    Shaun
     
  7. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    This is a good question, I didn't actually measure the voltage, although I did have an ammeter on it so I know it was averaging about 15-20A. This was with two people on the train, and so it could possibly go up to about 60A+

    I thought I'd measure the motors resistance and then I'd be able to tell you what the voltage was... but my meter said 4Ω, and as I rotated the shaft, it went over 350Ω...

    You might be able to explain this to me? at the moment the chassis of the loco is apart for repairs to the coupling rods, so I cannot give it a run to check the voltage unfortunately.

    Cheers
    Shaun
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, anywhere from 0.6A to 1.5A maximum. It's better to charge them at a fairly slow rate. The faster you charge them, the more the battery heats up internally, which causes more chemical activity. A cool battery will last many times longer than one which is kept hot.

    Yes.
    No; you might get 12 hours use if you discharged them at a 1A rate. However, the higher the rate of discharge, the more power is dissipated in the battery itself causing heat. Discharging SLA batteries more than about 30% will shorten their life significantly. If you discharge them beyond 50%, they will have a very short life.

    Well, it seems to me that you should really only need battery power until the alternator is producing output; then the alternator can provide current to the field (rotor) from the stator windings via a regulator of some sort. Some sort of "buck" regulator should be able to do that for you. You would need a separate "buck-type" regulator to charge your SLA batteries from the alternator's output.

    However, at the moment we have very little of an idea how your alternator/motor is wired; except that you've removed the internal rectifier bridge (and apparently the regulator as well) and are using external rectifiers.
     
  9. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    Ok :)

    I see... so using them until they are completely discharged is not a good idea... I thought they were deep cycle batteries as used in electric wheelchairs etc. and were designed to be fully discharged and charged over and over again. but you know more about this than me, hence why I'm asking questions! ;)

    neat idea... I'll look into buck regulators..
    how would you suggest switching between the batteries and the regulator?
    I'm not talking specifics, just a general idea.

    It's basically just that... no regulator, just a 24v PWM signal on the field, and external 3 phase bridge rectifier, with the DC output going to the motor via 4 relays... they just switch the polarity for forward and reverse.

    I'll draw a diagram tomorrow if thats any good, although it's rather simple (I probably should have done it in the first place... sorry, I know what it's like as I moderate web design forums, and it's always helpful to have an example or a diagram.)

    Cheers
    Shaun
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it's not good to completely discharge even deep-cycle batteries. SLA batteries do fall into that category. People who want the best service life and performance from deep-cycle batteries will size their batteries so that they won't drop below 70% charged until they can be recharged, and discharge them at not more than 1/8th their AH rating. You're really pushing things running your little 12AH batteries with a load of 3A.

    The general idea is to transfer the source of the rotor's current from the battery to the generator output as soon as the alternator is capable of generating a self-sustaining output level. The batteries could then be recharged from a buck regulator on the alternator's output; providing that there was enough voltage being generated.

    I guess I don't understand why you're using four relays, as two single-pole double-throw relays would be enough for an electromechanical H-bridge circuit.

    Yes. Generally, schematics are considered a necessity around here; that way there is little chance of being misunderstood.

    It would also be most helpful to know what voltage is being output from the alternator when the engine is running and the electric motor is providing power.
     
  11. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    I'm really surprised that 3A is pushing them, but thanks for the info. Would it be better if I sourced some other batteries? I know you can get small automotive style ones... would that be a better idea before I go an design a charging circuit?

    Yes I can see what you mean... it seems quite a complex solution... having to monitor the output voltage and then switch between sources. but if this is the best solution then thats what I'll have to do I guess. Although, if I was able to find a different battery type, maybe I wouldn't need to? or is it just wishful thinking? ;)

    Indeed they are... except all of my components are rated at 100A... including the relays, and I could only find SPST relays at that rating, so I needed four of them.

    Unfortunately I haven't had chance to sit down and draw a schematic today... but I'll post it as soon as I do.

    and yes, wish I had monitored it (the voltage) when I was testing... I thought it'd be a case of measuring the resistance of the motor and I'd be able to work it out from the current, but it would appear it's not that easy to measure the resistance for some reason. I'll see if I can devise a way of testing it, but at the moment I don't have it all attached to the chassis, so the motor wouldn't be driving anything.

    Thanks for all your comments though, it's been a great help :)

    Cheers
    Shaun
     
  12. ShaunManners

    Thread Starter Active Member

    Feb 16, 2008
    72
    1
    I have just been trying to find the technical datasheet for my batteries, but not having much luck... they are: Yucel y12-12 C20 which is made by Yuasa I think.

    I did find this which is similar... is it because it is made diffrently that it has those specs, or is it me misinterpreting the info? :confused:

    Cheers
    Shaun
     
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