Battery Low Cut Off (Equipment under voltage protection)

[Sean J]

If anyone's got any ideas (schematics), for a 12V Lead acid battery cut-off
circuit (10,5V - so I've heard). The equipment draws approx.100ma.

I've seen a circuit which runs a transistor in series with the load ??? I
just can't seem to locate it. What ever the circuit, I think its going to
require some hystersis .... maybe I can drive the base of a series
transistor with an op-amp (Voltage comparator) .... Just let me know if I'm
dreaming.

Any advice would be appreciated. Thanks

Sean

 

[Ken Smith]

If anyone's got any ideas (schematics), for a 12V Lead acid battery cut-off
circuit (10,5V - so I've heard). The equipment draws approx.100ma.

10.5 is a tad high if you only draw 100mA. You may be able to go down to
10.2 without risking the batteries.

I've seen a circuit which runs a transistor in series with the load ??? I
just can't seem to locate it. What ever the circuit, I think its going to
require some hystersis .... maybe I can drive the base of a series
transistor with an op-amp (Voltage comparator) .... Just let me know if I'm
dreaming.

Any advice would be appreciated. Thanks

How does the user turn the device on and off?

In a system that had a keypad, I made the power button turn on the power
but not latch it on. The latch on was done by the RESET/ signal which
came from a comparitor.

When the voltage is high enough RESET/ goes high and via a small N mosfet
holds on the main P mosfet. When the voltage goes too low, the comparitor
pulls RESET/ low. This shuts off the voltage the comparitor is sensing so
there is no way it can come again.

This circuit also gives a bit of short circuit protection. If the P
mosfet can't pass the current, the voltage goes low and the power shuts
down.

 

[Tam/WB2TT]

If anyone's got any ideas (schematics), for a 12V Lead acid battery cut-off
circuit (10,5V - so I've heard). The equipment draws approx.100ma.

I've seen a circuit which runs a transistor in series with the load ??? I
just can't seem to locate it. What ever the circuit, I think its going to
require some hystersis .... maybe I can drive the base of a series
transistor with an op-amp (Voltage comparator) .... Just let me know if I'm
dreaming.

Any advice would be appreciated. Thanks

Sean

I did something like that a couple of years ago, but I used a flip-flop
arrangement to keep the switch energized. The monitor circuit was on the
load side of the switch, so when it tripped, there was 0 load on the
battery. The switch got reset when the AC power came back. As I recall, it
tripped at 11V.

Tam

 

[MikeM]

If anyone's got any ideas (schematics), for a 12V Lead acid battery cut-off
circuit (10,5V - so I've heard). The equipment draws approx.100ma.

I've seen a circuit which runs a transistor in series with the load ??? I
just can't seem to locate it. What ever the circuit, I think its going to
require some hystersis .... maybe I can drive the base of a series
transistor with an op-amp (Voltage comparator) .... Just let me know if I'm
dreaming.

1. Download and install LTspice/SwitcherCAD III from

http://www.linear.com/software/

2. Cut and paste everthing below the ____________ in this message
into a text file named "DisCon.asc".

3. Start up LTSpice

4. File/Open DisCon.asc (from where-ever you saved it)

5. Click on the LittleRunningMan button.

6. Select V(Load) for plotting. It shows the Load voltage vs time as
the simulated V(Bat) goes from 13V to 9V. (You can add V(Bat) to
the plot pane by clicking on the wire labelled "Bat".)

This circuit does a good job of disconnecting the load when V(bat)reaches
the set point controlled by R5. The voltage drop across the PFet is only
a few mV while the load is on. The Battery current after the load is
disconnected is <1uA.

What I didn't work out for you was an automatic start up circuit when a
charger is connected to the battery. I'll leave that as an exercise to
the student .

Prof. Mike M

_____________________________________________________________________________________

Version 4
SHEET 1 880 680
WIRE 80 -80 128 -80
WIRE 352 64 352 32
WIRE -192 64 -192 32
WIRE 0 48 0 -32
WIRE 208 304 208 240
WIRE 128 304 128 272
WIRE 0 64 0 48
WIRE 64 112 208 112
WIRE 128 -80 208 -80
WIRE 128 -32 128 -80
WIRE 128 48 128 176
WIRE 128 176 0 176
WIRE 0 176 0 160
WIRE 208 -32 208 -80
WIRE 208 -80 352 -80
WIRE 208 112 208 48
WIRE 208 160 208 112
WIRE 352 -80 352 -48
WIRE 128 208 128 176
WIRE -64 -48 -64 -80
WIRE -64 -80 -16 -80
WIRE -64 32 -64 48
WIRE -64 48 0 48
WIRE -192 -48 -192 -80
WIRE -192 -80 -64 -80
FLAG -192 64 0
FLAG 352 64 0
FLAG 208 304 0
FLAG 128 304 0
FLAG 352 -80 Load
FLAG -192 -80 Bat
FLAG 128 176 z
SYMBOL pmos 80 -32 M270
SYMATTR InstName M1
SYMATTR Value IRF7406
SYMBOL res 336 -64 R0
SYMATTR InstName R1
SYMATTR Value 100
SYMBOL Misc\\battery -192 -64 M0
WINDOW 3 -93 172 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PWL(0 13 10 9)
SYMBOL res -48 -64 M0
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL res 192 -48 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL res 192 144 R0
SYMATTR InstName R5
SYMATTR Value {Var}
SYMATTR SpiceLine tol=1 pwr=0.1
SYMBOL npn 64 64 M0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL zener 144 272 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 144 -48 M0
SYMATTR InstName R3
SYMATTR Value 4.7K
SYMATTR SpiceLine tol=1 pwr=0.1
TEXT 260 282 Left 0 !.tran 10
TEXT 136 -152 Left 0 !.NODESET V(Load)=13
TEXT -256 288 Left 0 ;Voltage dependant \nLoad Disconnect\nMike M\n4/8/2004
TEXT 8 344 Left 0 !.STEP param Var 82K 100K 2K
TEXT 16 376 Left 0 ;R5 (80K to 100K) adjusts trip point

 

[Tim Wescott]

10.5 is a tad high if you only draw 100mA. You may be able to go down to
10.2 without risking the batteries.

I thought it was just the opposite. The smaller the load WRT the capacity
of the battery, the higher the shutoff voltage should be. For example, if
you are only drawing 100 mA from a large, deep-cycle battery, and the
voltage is 10.2 Volts, the battery is probably ruined. Much better (in
that case) to cut out right at around 12 Volts, or 11.5 or something.

[snip]

--Mac

Come to think of it, that's my understanding too, at least with NiCd and
NiMHs. When you draw lots of current there's some resistive voltage
drop, so the battery gets to a low voltage with more charge. You can't
take this too far, of course, but with NiCd batteries you go down to
0.9V per cell for "normal" discharges and 0.8V per cell for way heavy (5
minute discharge) kinds of current.

 

[Mac]

10.5 is a tad high if you only draw 100mA. You may be able to go down to
10.2 without risking the batteries.

I thought it was just the opposite. The smaller the load WRT the capacity
of the battery, the higher the shutoff voltage should be. For example, if
you are only drawing 100 mA from a large, deep-cycle battery, and the
voltage is 10.2 Volts, the battery is probably ruined. Much better (in
that case) to cut out right at around 12 Volts, or 11.5 or something.

[snip]

--Mac

 

[Tam/WB2TT]

If anyone's got any ideas (schematics), for a 12V Lead acid battery cut-off
circuit (10,5V - so I've heard). The equipment draws approx.100ma.

10.5 is a tad high if you only draw 100mA. You may be able to go down to
10.2 without risking the batteries.

I thought it was just the opposite. The smaller the load WRT the capacity
of the battery, the higher the shutoff voltage should be. For example, if
you are only drawing 100 mA from a large, deep-cycle battery, and the
voltage is 10.2 Volts, the battery is probably ruined. Much better (in
that case) to cut out right at around 12 Volts, or 11.5 or something.

[snip]

I looked at the Panasonic web site for their VRLA batteries, which I think
are gel cells. They consider a 12V battery to be discharges at 10.5V.
However, they also point out that the battery will be good for many more
discharge cycles if you don't discharge all the way.

Tam

 

[clive]

I did a similar circuit for a LiPolymer battery which really don't like
exterme discharge, they explode on recharging sometimes!

I used an LM10 op-amp which also has an internal voltage reference, so use
the voltage reference to set up the threshold voltage into the inverting
input and the non-inverting input is connected to the equipment side of the
hexfet, via a voltage divider. The output of the LM10 drives an O/C
transistor which drives the gate of the hexfet(with pullup to battery). When
the threshold is crossed, the gate is turned OFF, and the battery is
completely isolated with zero current until there is an input/charging
voltage which exceeds the threshold to turn the circuit on again. The
reverse diode inside the hexfet performs this role nicely. Depends of course
how much current you need as to device specs. I used an IRF7424 or
equivalent for my needs.

Good luck, Clive