Battery charging questions on meters

[Dave B]

I don't quite get it. What does the DC volt meter indicate as battery charging starts and rises ? If I had a DC volt meter and Amp meter connected in the 24 vdc charging circuit after the rectifiers of my 3 phase wind generator, does the DC meter continue to read higher than 24 volts as output increases above cut-in ? Obviously the current will rise if the output increases but this bit about the battery clamping the charging voltage to the battery voltage makes me wonder what I'll see at the DC meter ? I searched this and it all sounded sketchy, no direct answers. I have 2 nice analog meters I plan to use and may have to change the scale faces and range to match. Thanks for any replies,  Dave B.

[Flux]

If you connect a dc voltmeter to the battery it will read the battery voltage.

At cut in current will flow when the generator voltage exceeds the battery voltage. If the battery is fully charged and not on load then it may take about 28v to start charging. If the battery is low and on load then you may see current from perhaps 22v.

If you consider a battery that has been standing with no load for some time then its voltage will drop when on discharge and rise when on charge. If the current is small in relation to the battery size and the thing is in a reasonable state of charge then the variation when charging or discharging will not be that great.If it is near flat then the volts will drop rapidly as you add load. Similarly if it is fully charged then the voltage will rise rapidly with charging current.

If you look at a good battery site you will find charge and discharge curves of voltage against time.

Over the main part of the curve the charging part runs parallel with the discharge part but the charging condition will be at higher voltage. Towards the extremes the charging volts rise rapidly as you reach gassing point and similarly at the discharge end the voltage falls sharply below about 20% capacity. I don't have time to find and upload these curves, I have them in ancient books where people understood such things. I am sure they are on the web somewhere.

An analogue dc meter is quite handy to see at a glance if you are running flat or reaching full charge. It is not sufficiently accurate to determine state of charge but unless you know your battery and can leave it standing for hours I have no real faith in using volts as an indication of state of charge.

As to your question, yes the battery does clamp the volts but the clamp voltage depends on battery history and state of charge. under normal conditions when charging you can expect something between 24 and 28v. Below this you are either taking more than you are charging or the battery is flat. Above 28v you are reaching gassing point and unless you are equalising you need to cut the charge manually or with a charge controller. The best place to work with wind is in the well clamped bit.

Diverting excess charge is wasting wind power, better to use lots of load on those days and go easy with load on windless days. Man is more intelligent than a charge controller that can't alter your load patterns.

I hope this is what you were looking for, I didn't entirely follow the question.

Flux

[FishbonzWV]

Hi Dave,

Yes, all you'll see is the battery bank voltage.

I have analog meters on my small system. I like being able to see the status at a glance. If I want a more precise reading I have a HF digital that I modified with a momentary push button in line with the 9v battery.

BT Humble wrote a nice little program for the meter faces.

I created new faces for my E and I meters.

http://www.otherpower.com/images/scimages/236/MeterDial.zip

Bonz

[Dave B]

Thank you guys for your reply,

 OK, so let me see if I've got this. If I am charging a 24 VDC system then with my meters properly connected I will not (or should not) see too much over 24 VDC when charging. So if I wanted to set my volt meter to read say 30-35 VDC full scale this should be fine as long as it's loaded by the battery bank, correct ? I realize voltage is not an accurate way to determine battery condition but I'd like to indicate 24 VDC reasonably high on the scale with some head room. I just couldn't get it that the DC voltage will not indicate higher as the current increases. DC open voltage could then be say even 3 times the battery voltage but when loaded by the batteies will not indicate this, right ? Thanks again,  Dave B.

[SparWeb]

Dave, this might help you out, too.


Battery charging article from Homepower magazine

The diagrams in the article make it much easier to visualize what happens at higher/lower charging or discharging rates.  Only really intended for flooded lead-acid batteries, but the principles are similar in other types.  

[Flux]

That's right, with battery charging from a current limited source the battery determines the terminal voltage. The battery is a strange form of load and involves chemical changes and electrolysis.The voltage is roughly set as the difference in potential of hydrogen and oxygen. It is a non linear load where the voltage is not following the current, that is why battery charging mills behave very differently from things with linear load ( heating).

Only when the current supply is virtually unlimited ( current far too high for the battery bank)does the voltage try to follow the current. A normal working battery would not let you get 3 times its nominal voltage, it would gas violently and probably spark over inside and explode.

The voltmeter does in fact indicate higher as current increases but only in the range up to about 30v and even that high is only during equalising, when accepting reasonable charge it will be below 28v.

Your 30 to 35v fsd is quite suitable, 35 is a bit of an awkward scale value and normally 30 would be high enough for normal use. Perhaps if you want to do significant equalising on wet banks and you want to keep the meter on scale you could live with the awkward 35v or choose 40v.

If you use a hydrometer then 30v fsd is adequate.

Flux

[Dave B]

Steven,

  Thank you for the info. this is helpful battery info. for sure. It still seems to me that if you have 3 times the DC battery voltage for charging that with your volt meter leads across the battery you would read this higher voltage some how. I know, it's magically converted to be read as current while charging and the the DC is clamped to read the battery voltage. Magic. Thanks again,  Dave B.  

[Dave B]

Thanks again Flux,

   This is all good stuff and I am learning as I move from heating with the wind to charging batteries. A different animal for sure seeing the battery load as opposed to the heating elements. I've rewound a new stator for 24 volts and it will be interesting again for sure to tweak the blade performance to the load and the furling will act different too I am sure. Any guesses on this ? I'll be posting updates as I move forward. Thanks again,  Dave B.

[ghurd]

Depending on who's equalization charts you choose to follow, a 24V bank can be 32V or even a bit more.

The battery voltage will come up some with higher current.  Not a lot, but enough to maybe be a bit confusing with a digital meter.

Even 100ma into a 12V 35AH will jump the voltage 0.1V in a few seconds.

If the wind is gusty, and the battery is seeing times with 20A of output power to loads, and a minute later 50A of charging current, the battery voltage will swing around quite a bit.

I believe the speed at which it changes is also related to the battery's SoC.  Personal observations.  No charts or anything to back it up.

Just expect it to change some related to the current.  Maybe the opposite of how the voltage bounces around with varying loads?

I see others posted before I finished typing, but I'm posting it anyway!

G-