DC Charging Question

[savedbygrace]

I'm new to all of this, and have been researching like crazy.  This is fascinating stuff!!

I'm planning on starting out a simply as I can, to get my feet wet!  I design and build baseball/softball pitching machines, and they use PMDC motors...so I have a bunch of them lying around...mostly high RPM models...of varying HP, sizes, shapes and colors.  Most of them are 90V or more.  I want to use one as a DC generator to charge batteries.  I'm going to use gears/chains/sprockets to turn one of my motors (I have a lot of gears/chains/sprockets lying around too!).  I know there will be friction loss, but that's OK with me for now.  Obviously, I'll be generating more than slightly over 12V with these motors.  

Here's the questions:

1.  What's the most Voltage I can input into a 12V battery without damage?
   
2.  Will the battery use only the voltage it can and just ignore the rest?  Somehow, I doubt that, but thought I'd ask.
   
3.  If I have to step the voltage down, what's an inexpensive and easy way (assuming there is) to do it?
   
   

Thanks in advance,

Randy

[kurt]

i don't have time to answer your questions at this time but they are covered here  http://www.fieldlines.com/faq/re_faq

[nothing to lose]

Basically I geuss watts is watts and they say it all converts to battery voltage at what ever the amps.

So if you have high volts lower amps at the gennie the batteries will hold down the volts to it's own level and the amps will increase. Need to make sure you don't over charge, use a dump load, as the batteries charge the voltage will rise I think. You don't want it to be pumping in 20V to a fully charged 12V bank!

So lets say the gennie puts out 90 volts at 5 amps, that's 450watts. The battery bank holds the volts to it's own level, lets says 12volts, so 450/12= 37.5amps into the battery.

If you had a 24V bank then 450watts/24volts= 18.75amps.

You get the idea, but if you reach full charge then the volts may start rising to high and damage the batteries so that's where the dump load comes in. Various ways, but basically you set it up to turn on a load like a heater element at around 13.7V to burn off the extra power and keep a load on the gennie, it should shut off the dump load maybe 13.5v so when the genny slows and excess power stops you don't drain the batteries. Never want to just disconnect the gennie, with out a load it will free wheel and over spin in the wind that was causing the excess power.

If your manually watching it, you could short the gennie wires with a switch and that shuts down the gennie but you have to be watching the voltage all the time and such, but good for storms if you want.

[commanda]

Typically, the amps won't increase much. The above statement only holds true if using dc-dc converter technology. In the above example, the current will not increase in direct inverse proportion to the voltage. As an educated guess, it might go from 5 amps to 7 or 8 amps. I can't seeing it go to 37.5 amps. If it did, there would be no need for me to be designing a dc-dc converter for just such circumstances.

Amanda

[ghurd]

You may not need as much gearing as you think.

Don't think so much like 'stepping down' voltage,

think more like 'using up' extra amps. (easier for me)

G-

[nothing to lose]

Hmm, you may be correct, but I think that goes against most of everything said around here on the subject somewhat.

Rather charging direct from the grid only converting to 120VDC and limiting the amps or charging a 12V bank with a 48V gennie, about everything I read here so far says watts is watts and the batteries hold the volts to their own level and convert the excess to more amps. Problems may arise when batteries are full charged and keep charging letting the volts rise too high and burning out the batteries though, like if charging direct from the grid using only Caps to control the amps. Batteries should charge at 50amps (using the correct Caps to limit amps) with 120Vdc, battery banks holding down the actaul volts to 12V 24V etc.., but once charged the voltage will begin to rise too high untill it hits 120Vdc (or a meltdown occures somewhere)

That's the grid charging which is unlimited amps nearly so must be limited with caps.

For gennie charging just connect the gennie DC direct to batteries and you get the same watts (minus losses) at what ever volts your bank is at. So 480 Watts is 40 amps@ 12V or 20amps@ 24V regaurdless of the genie being wired up for 48Vdc output. Macthing output and banks is best I am sure, but it seems to be said most often you still get the same watts into any volt bank and the amps cahnge with the volts to equal those watts.

Using a 180Vdc treadmill motor I get more watts at higher volts the faster it spins, but anything over 12V charges a 12V battery, 60V is just more amps into the battery at 12V. 1 amp at 12V or 5 amps at 12V still 12V batteries and charging even thought the motor is producing 60V.

 One of my little motor conversions puts out any volts if it spins fast enough, open volts I have seen near 100V a couple times. under a 120V 100 watt bulb load I lit it about 60 watts with a bicycle, I geuss thats 5 amps at 12V or 2.5amps at 24V, it charges batteries either way.

I think I said that right, maybe not?

This is the theory behind my building and so far seems to work that way, if I said it right. Sometimes I think I think something right but type it wrong.

 If I have misunderstood what has been said many times about batteries holding the volts down and amps increasing and it all being basically the same watts then I really need to know my mistake!

 My thinking on that is sort of like connecting batteries , you still only have 6 100amp 6volts to play with, but you can either have 100amps at 36volts or 600amps at 6Volts, either way it's still a total of 3600watts. And any voltage above the banks voltage (like 50V) at 3600watts will charge either bank, minus losses of course like running 3600watts at 6V ain't too good.

[commanda]

Generators have an open circuit voltage & an internal resistance. When the open circuit voltage is high enough (several orders of magnitude greater than the battery voltage), the current approximates the open circuit voltage divided by the internal resistance. Close enough to a constant current source.

So, if the open circuit voltage at some rpm is, say, 100volts, and it puts 5 amps into a 6 volt battery, it will also put 5 amps into a 12 volt battery, or even a 24 volt battery.

If it puts 5 amps into a 24 volt battery, it won't magically put 20 amps into a 6 volt battery.

If you have a drill press and several batteries which you can switch in series, please try this & prove for yourself.

Amanda

[drdongle]

There is a general recommendation to avoid gears and belts, and many of the PM DC motors will work fine with out then, a 180 V 1800 RPM motor will produce 30 V at 300 RPM, which is a sweet spot for some small machines.

 

[nothing to lose]

I must not be reading this right or have some things confused in what we are both saying here. Not argueing with you, but confused none the less what we are talking about :{

"So, if the open circuit voltage at some rpm is, say, 100volts, and it puts 5 amps into a 6 volt battery, it will also put 5 amps into a 12 volt battery, or even a 24 volt battery."

How would it do that, 5 amps into a 6V is 30 watts, so at 100volts it must be about 1/3 amp to equal 30 watts. If all else is the same, then 5 amps at 24V is 120 watts? How can I use the same genny at that same speed producing the same power and go from 30 watts to 120watts by just upping the volts? So if I go to 48 volts that same 30 watt gennie we sarted with at 6v 5 amps would be also put 5amps into the 48V bank and be 240watts? I don't think so?

"If it puts 5 amps into a 24 volt battery, it won't magically put 20 amps into a 6 volt battery."

I think it does, but not magically, mathmatically though. 5amps at 24V is 120watts, 120watts / 6V is 20 amps. Raise battery volts to 12V and drop to 10 amps, either way the gennie is producing it's max 120 watts. It's never producing more watts just because we up the volts so the amps must go down right. Since it IS producing those watts then if we lower the Volts the amps must go up. Minus losses.

Amps X volts = watts

watts \ volts = amps

watts \ amps = volts

If the above is correct, and we have a constant of 120watts, then the other two are variables and the formulas show when one raises the other must lower.

Otherwise instead of building bigger gennies and blades to produce higher watts we could just double our battery voltage to get them

But that doesn't work that way.

"If you have a drill press and several batteries which you can switch in series, please try this & prove for yourself."

I would do that but at this time I don't have anything I can put into the drill press to test with. However I have charged 1 6 volt battery faster at 6 volts with the same gennie than it takes for 2 wired at 12V. That would make me think I was getting more amps into that one battery at 6V than at 12V. Though charging the 2 still took as long doing each seperate at 6v as it would have taken to do them as 12V.

 So it would seem that I got the same total amps and watts and took aproxmately the same amount of time charging either way. Really hard to tell like that though, but when I had the remote house I was charging various ways at home and taking them there to use and bringing home to charge again, or charging off the truck as I drove or grid etc..

 But I was draining them all down equal wired up together so they all needed about the same amps to reach full charge.

Also using a smart charger with a digital diplay (only states 12V charger, does not mention 6V anywhere) when I set it to charge a 6V trojan 360 amp L16p at 10 amps, the charger bogs down, sounds like a heavy load, display shows 20 amps charge rate. Again suggesting that cutting the the volts in half doubles the amps, of course that's not a gennie it's a grid fed charger but still. 10 amps at 12V is 120 watts, on a 6V it shows 20 amps, again 120watts. Same thing happens when I set it to 40 amps on the 6V battery, it shows 80 amps and sounds bogged down

 So far I have not burned out the charger, and those were junk batteries I was playing with trying to bring back to life and gave up on, so just doing about anything to them to see if I get any results at all.

 I did get couple up several volts from where they were, but still junk, holding maybe 4.5-5V but better than the 2v and 3V they would drop to after other normal charging methodes. Did so many things to them if they do ever work I won't know why.

[commanda]

Your fundamental assumption that a generator is a constant power device, regardless of load, is incorrect.

Please study Zubblies test results here.

http://www.fieldlines.com/story/2005/5/7/2031/07918

For any given rpm, and connection method, compare the powers at 12, 24, 48 volts. It is not a constant.

Look at the power figures for the 2 star connection. Notice also that at 400 & 660 rpm, the power is greatest into a 24 volt battery. This is maximum power transfer theorem at work. This is why I'm working on a dc-dc converter with mppt controller, so the load presented to the genny is always at the max power transfer point.

You're also introducing a smart charger. This is a different beast again, and is only confusing the issue.

I have probably contributed to your confusion by over-simplifying, for which I apologise.

The current will increase/decrease inversely with the battery voltage, but not proportionally, and certainly not for the reasons that you stated.

Amanda

[nothing to lose]

Well something happened and all my browser windows went away right after I typed a nice post where I almost kinda sort of admitted I could be wrong perhaps, maybe, but am to confused to know for sure...

 I said I would go crawl back under my rock now for awhile :}

But I had time to think more and change my mind a little... so I alrady came out from under that rock again

 Well, I'm still not trying to argue, but trying to get it right all the way around.

 I think basically we are both correct, but I probably said something wrong in the first post or so. I did study up on Zubbly's chart, which now has me confused on other things as well, even though I read it several times, and was not confused before when I had first read it, but am now!

 The amps do increase as voltage drops if the watts are the same I still beleave, but not in direct exact proportion of 2/1 etc.. maybe which I may have said wrong.

And the gennie does not produce the same watts I think is the reason amps don't double at half the volts, correct? Still if you have 3600 watts it is going to go some where somehow and follow the formulas to a point, losses etc.. to consider. Where we were bumping heads I think is the fact we are not saying the same things here, I'm saying the same watts, your saying watts change, so therefore if the watts are not the same then you can't use the formlas for watts/Volts=amps and expect it to work.

In order to compare this, something must remain a constant and 2 varibles, but we are using 3 varibles and no constant and that just does not work!

Now I also see your right, the gennie is not producing the same watts in Zubbly's chart! The gennie is producing less watts at 12V at a given rpm than it does at 24V.

However I think the chart also shows that when you drop from 24V to 12V the amps do encrease as I said too, though it is not equal since you suffer a loss in the total watts produced. Below I use the 48V and 12V because I am now confused on something in that 24V area of the chart

       connection   RPM      battery v    amp    watts

48. V    2 Star       400        64.4      6.7     431.48
    
12. V    2 Star       400        16.6     20.0     332.00
    
    

We change from 48V to only 12V, Battery Voltage went from 64.4 to 16.6 far lower! At the same time as I stated the amps also go up! From 6.7 amps to a whoping 20 amps. We still came out with a total loss though because the total watts went down from  431.48 to 332. We lose 100watts here and that is why the formula does not work, 3 variables no constant.

Now back to what I said, or meant/tried to say basically. The battery bank will convert the excess to more amps while holding the voltage to it's own level. Above we see that it does do such, though we suffer a loss in watts doing so. At 48V we charge at 64.4 volts and only 6.7 amps as I read that. A 12V bank pulls the volts down to it's own level at 16.6V and the excess is converted into more amps at 20amps.

We have divided the banks volts by 4 here. Charge volts of 16.6 x4 = 66.4V so that is not entirely 1/4, 2 volts over. 6.7amps X 4 = 26.8amps which is 6.8amps more than we actaully got since we encreased in amps to only 20amps. But the amps did increase as I said, though there is a loss.

We also see that the amps do not stay the same when you increase voltage! A generator putting 20amps into a 12V batttery does not put 20amps into the 48V battery. I think I said that amps would lower as volts were encreased, but not sure.

 In that part I think you had said 5 amps into a 12v bank would still be 5amps into a 24V bank. I would think 2.5amps myself, double the volts/ half the amps, but then if watts go up as indicated then so would the amps, but it won't stay the same if the watts stay the same, and they would have to double to keep it the same 5 amps. Above 12V to 48V we do gain almost 100watts but that is far from double. More like almost 1/3, and thats from 12V to 48V also.

So I think here we are both correct to a point and we are both wrong to apoint.

Now, still confused but,  I "think" what I origanlly meant was he could connect a 90Vdc motor to a 12V battery bank and any excess power "created" above the banks charging volts of maybe 13.5 or so would be converted to amps, which is basically what Zubbly's chart shows also. It may be better to charge 48V than 12V and get more watts, but as the volts are lower the amps will be higher, but not equal watts as the example I stated.

Original post,

"Here's the questions:

   1.  What's the most Voltage I can input into a 12V battery without damage?

   2.  Will the battery use only the voltage it can and just ignore the rest?  Somehow, I doubt that, but thought I'd ask.

   3.  If I have to step the voltage down, what's an inexpensive and easy way (assuming there is) to do it? ?"

 I think I was correct, he is using a 90vdc motor as a genny, run it though a bridge rectifier and it will charge 12V batteries fine with any volts it may produce provided the batteries do not reach full charge and continue to climb above the banks  voltage. Anything over 13.5volts or somewhere will be converted to amps. So the genny won't put 90V into a 12V battery unless it reaches full charge and continues to climb over the banks voltage. I guess the way I said that too is also not really right, the DC motor probably never really puts out higher than battery volts while the battery is holding it down to charging levels? Never gave that much thought really.

 At the battery it is battery volts, but now if I connected a 48V motor next to the gennie would it get 48V and run as such while the batteries are holding it to 13.5V at the end of the line? Don't really know, I think it would being the first thing in the line.

 In my example I used the same Watts, volts, amps being produced at any battery voltage, did not account for the lost watts as shown in Zubbly's chart.

"So lets say the gennie puts out 90 volts at 5 amps, that's 450watts. The battery bank holds the volts to it's own level, lets says 12volts, so 450/12= 37.5amps into the battery.

If you had a 24V bank then 450watts/24volts= 18.75amps."

Now I think in a way that is still correct, if the max that DC motor is going to make  is 450watts then the varible here I did not cover is the RPM to make those watts. So we may lose 100 watts at 400rpm depending on the voltage we are charging at, but as shown in Zubbly's chart also if we increase that speed from 400rpm to 660rpm we may get it back again.

 So really I think I said that wrong in the example maybe to a point, but it is also still correct in a way. We just up rpm to get the max watts if the gennie will do that.

But anyway I think I learned something I had't thought of and I thank you for that.

100watt loss is alot of power, and I did not realize there was that much difference, I mostly been thinking line losses etc... at those lower volts, so thank you for pointing that out.

I hope I have this one figured out correct now

 If not I will put more thought into it.

[commanda]

1.  What's the most Voltage I can input into a 12V battery without damage?

This question is fundamentally wrong. We need current to charge a battery. As the battery charges, it's terminal voltage will increase. Once we reach float voltage, we need to cut back the current, or divert it to a dump load.

The generator has an internal resistance. The difference between the open circuit generator voltage at that particular rpm, minus the battery voltage, will be dropped across this internal resistance, plus the line resistance, rectifier diode drops, etc.

The current that will flow is simply the voltage difference divided by this resistance.

The complicating issue is that this resistance is not constant. It varies with rpm, and also the current drawn.

In Zubblies case, pushing a 12 volt battery to 16.6 volts is not good. The battery capacity is too small for the charging current. This is the crux of the answer to the question. Answer is, it depends on the battery capacity.

Batteries also exhibit an internal resistance. The reason why the terminal voltage drops as we discharge the battery is mostly due to this internal resistance increasing. This also explains why a dead flat battery can't take much current. And why a flat battery will show 12 volts on a mulimeter, buts drops to almost zero when we apply a load. A healthy, fully charged battery will have a low internal resistance, and hence can supply lots of current without the terminal voltage collapsing.

There is an IC battery charge controller, a UC3906. Google for that, and find the application notes. It explains charging batteries very well.

As I stated back before, the current will vary inversely with the battery voltage, but not proportionally, and not because of some constant power equation.

Amanda

[TomW]

Amanda;

Nice explanation. Folks seem to forget or not realize that ohms law holds true for sources as well as loads.

Thanks.

Cheers.

TomW

[nothing to lose]

Well, still totally confused, but got a better picture now, though still looking through a hazy fog at it

I other words, I think I learned something in this post, after I get some sleep finnaly, I will re-read through most of it again. Losts of good info, now I just have to figure it out when not so exhausted and can comprehend it better.

Not enough sleep, to many days.