Multi Battery Charger for Lead Acid

[John Fields]

On Sun, 23 Dec 2007 09:11:07 -0800 (PST), gearhead
<nospam@billburg.com> wrote:

All right guys, staring at all that spice has gone to your heads.

---
Yup, good call.


--
JF

 

[gearhead]

On Dec 23, 10:32 am, Chris W <1qaz...@cox.net> wrote:

John Fields wrote:
I also tried to simulate the circuit posted by gearhead.  I couldn't
find a NTD110N02R in LTspice, NTLMS4504N was the closest I could find.
With the .3 ohm resister the current was 2.7A so I tried changing it and
the simulation kept giving me errors.  Also the voltage seemed to be all
over the map but the current was very flat.

SNIP

I tried it and it worked OK with any N MOSFET and a resistance of
0.37 ohms except that there's no end of charge and the supply will
keep on pumping nearly 2 amps into the battery no matter what its
voltage rises to.  

Unless it somehow increases the voltage to the battery above 15V I don't
see how it could keep pumping 2 amps into the battery.  
Let me explain why I think that.  

Chris, your thinking is correct.

I have used that 0-15V power supply as a kind of
manual charger in the past.  I have a DC watt meter that shows current
and voltage.  I just plug it into the power supply and adjust the
voltage so it is about a tenth of a volt over what the battery measures
at.  Then I plug the battery in.  Then I adjust the voltage up till the
current is at the point I want for the battery I am charging.  

Yes. Avoid the manual stuff by implementing what you asked for
in the original post: current limiting.

As the
battery charges the current goes down and I keep turning the voltage up
to get the current back up.  When I finally get to a little over 14V I
stop and just wait for the current to drop.  If it is a big battery,
once the current is below about 1A I stop.  If it is a small battery, I
stop when the current goes down to about .1A

The circuit in a smart charger would do exactly that, except
it would probably just reduce the voltage
to about 13.5 instead of turning completely off.

If the current the battery pulls from the voltage regulated power supply
reduces as the battery charges why would it not do the same with the
circuit from gear head?

Also as a side note if you stop charging the battery as soon as it
reaches 14.7V at full charge current, it will not be fully charged.  To
fully charge it you need to leave it at your stop voltage till the
current drops off, how much it needs to drop off depends on the size of
the battery for a car battery after it is down to around 1A it is done
for a little 7Ah you need to wait till it is more like .1A see the
charge graph in the pdf below.http://www.bb-battery.com/productpages/EP/EP7-12.pdf

It's basically an accurate 2 amp constant current source with a
battery disconnect when it charges up to 14.7V with only 300
millivolts of headroom to the 15V supply.

Recent experience has shown that charging to 14.7V is doesn't add much
more charge than charging to 14V.  

If John's circuit continues to apply 14.7 volts, then the battery will
continue charging, and top off; no problem there.

I couldn't tell for sure from the
spec sheet for the NTD110N02R suggested by gearhead how much voltage
would be dropped and as you can see from the graph posted above that
wasn't much help either.

The mosfet has .0047 ohms Rds, according to the data sheet.
At two amps it'll drop only about a hundredth of a volt.

I haven't looked closely at John's circuit, but it seems to me that
in the end the battery will get almost the same treatment as it would
with a mere limit current. His circuit theoretically could charge the
battery
very slightly faster than mine, because at 14.26 volts (with a .37 ohm
resistor), the resistor in my circuit would cause the current to start
to
tapering off a bit, whereas with his it would continue getting 2 amps
until it hits 14.7 --
but ONLY if the battery actually still is drawing 2 amps above 14.26
volts.
Might or might not do so, but won't make much difference anyway.
Pretty much
a wash between the two circuits.

I'm not a big spice user. I'm wondering if it models true battery
behavior.
Perhaps it just uses an ideal voltage source to represent the battery.
If that was the case, then spice would majorly misrepresent the
charging
behaviour of lead-acid batteries or any battery type for that matter.
You would be better off using a breadboard, multimeter and pencil and
paper.

Mike Robinson