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What's proper battery charge voltage?? Shunt Regulator Resistors?

M

mike

Jan 1, 1970
0
What's proper battery charge voltage??

I've been asked to design a shunt regulator for
a wind/solar powered system on a mountaintop at 5000 feet.
It's a 12V Sealed Lead-Acid battery system that is cooking
the batteries when the wind is high.
The wind generator has internal limiting, but it still lets
the batteries go over 15V.
The brute force solution is a shunt regulator that
holds down the volts and keeps the wind generator
from over-revving in high winds.

What's a good way to shunt 14.4V at 40 amps?
By the time it's derated for reliability, including for altitude, I need
over a KW worth of resistors.
My initial thought was to cut some resistors out of
sheet metal and blow a fan on them.

But maybe it's better to use nichrome and let it radiate
most of the heat. Pros? Cons?

It didn't take long to realize that I don't understand
the difference between fast charge and float charge.
I'd always used the rule of thumb that I could fast charge
lead acid batteries at 14.4V, but should use 13.6V for
continuous float charge. Given a particular chemical state,
state of charge, the battery shouldn't care, within limits,
whether it's been fast or slow charged???

Sounds like the right thing to do is measure the energy out
of the battery and replace it (+ some efficiency percentage)
at 14.4V then switch the charger to 13.6V.
That make sense? Increase battery life? Overkill?

mike


--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
E

Ecnerwal

Jan 1, 1970
0
What's a good way to shunt 14.4V at 40 amps?
By the time it's derated for reliability, including for altitude, I need
over a KW worth of resistors.

Water heater elements, stuck in the hot water tank; Don't fuss about
finding 12V ones, just feed the power you have into one, or several in
series if you can't get enough resistance from one (even a single
element should have plenty of power handling). If that's too much
bother, go to the local junque shoppe and pick up a toaster, or
resistance heater, and use the nichrome to dump heat to air.
 
M

mike

Jan 1, 1970
0
Ecnerwal said:
Water heater elements, stuck in the hot water tank; Don't fuss about
finding 12V ones, just feed the power you have into one, or several in
series if you can't get enough resistance from one (even a single
element should have plenty of power handling). If that's too much
bother, go to the local junque shoppe and pick up a toaster, or
resistance heater, and use the nichrome to dump heat to air.

Help me with the math here.
Power is proportional to the square of the voltage.
If I have a 4KW 240V water heater element and a 12V supply (just to keep
the numbers easy) that's a voltage ratio of 20 or a power ratio of 20^2
= 400.
So I'll be able to dump 10 watts into the element from my 12V supply.
Seems a little shy of the ~ 600W I need to get rid of.
Where'd I go wrong???
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
E

Ecnerwal

Jan 1, 1970
0
Danger of replying too early in the morning - brain not fully engaged.

You would indeed have to put a lot of elements in parallel to get the
power dump you want with standard water heater elements.
 
V

Vaughn Simon

Jan 1, 1970
0
Ecnerwal said:
Danger of replying too early in the morning - brain not fully engaged.

You would indeed have to put a lot of elements in parallel to get the
power dump you want with standard water heater elements.

Or use an inverter to step up the voltage, you would lose some
efficiency, but in this case who cares? Are water heater elements center
tapped so you can run them on 110V?

Vaughn
 
B

bw

Jan 1, 1970
0
mike said:
Help me with the math here.
Power is proportional to the square of the voltage.
If I have a 4KW 240V water heater element and a 12V supply (just to keep
the numbers easy) that's a voltage ratio of 20 or a power ratio of 20^2
= 400.
So I'll be able to dump 10 watts into the element from my 12V supply.
Seems a little shy of the ~ 600W I need to get rid of.
Where'd I go wrong???
mike

You just about have to use a temperature compensated charge. Not that
complicated.

Use Power = Amps squared times Ohms.

600 watts divided by 1600 is 0.375 ohms. Maybe try a few electric range
heating elements in parallel.

Brent Wegher
 
M

mike

Jan 1, 1970
0
bw said:
You just about have to use a temperature compensated charge. Not that
complicated.

Ok, but temperature compensated at what nominal voltage?
Use Power = Amps squared times Ohms.

600 watts divided by 1600 is 0.375 ohms. Maybe try a few electric range
heating elements in parallel.

Ok, but the snipped part of the orignal post asked about the relative
merits of low temperature convection vs high temperature radiation.
mike
Brent Wegher



--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
F

Fred B. McGalliard

Jan 1, 1970
0
Why not just get a bank of some large resistors with cooling fins? It should
be easy enough to dissipate a few KW that way.
 
M

mike

Jan 1, 1970
0
Fred said:
Why not just get a bank of some large resistors with cooling fins? It should
be easy enough to dissipate a few KW that way.

snip

Thanks. Whoda thunk you could buy a resistor.
Guess I shoulda posted the question to alt.homepower.frugal.
mike


--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
TEK Sampling Sweep Plugin and RM564
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
D

daestrom

Jan 1, 1970
0
mike said:
What's proper battery charge voltage??

I've been asked to design a shunt regulator for
a wind/solar powered system on a mountaintop at 5000 feet.
It's a 12V Sealed Lead-Acid battery system that is cooking
the batteries when the wind is high.
The wind generator has internal limiting, but it still lets
the batteries go over 15V.
The brute force solution is a shunt regulator that
holds down the volts and keeps the wind generator
from over-revving in high winds.

What's a good way to shunt 14.4V at 40 amps?
By the time it's derated for reliability, including for altitude, I need
over a KW worth of resistors.
My initial thought was to cut some resistors out of
sheet metal and blow a fan on them.

Uh, if this is a problem because of high winds, why not just mount the
resistors somewhere on the tower. When you need to cool them, its windy.
When its not windy, you don't need to cool them.

daestrom
 
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