Low voltage cutoff for 12vdc gel cell battery

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tom baker

tom baker

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I need to design a low voltage cutoff circuit for a 12 VDC gel cell battery, that normally is on a charger at 13.7 volts
The circuit will open a relay before the battery voltage drops too low making the recharge cycle too long.
(common battery as used in fire and intrusion panels)

Whats a reasonable voltage for a cutoff circuit?
 
G

gar

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Ann Arbor, Michigan
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EE
110308-2252

Tom:

I can not answer your question. I suggest you contact the battery manufacturer.

The voltage will be be temperature dependent to some extent. This may be of concern.

The real question could be what is an optimum charging philosophy for maximum life of the battery.

Following is some very limited information:

A Power-Sonic PS-1270 F1 12 V 7.0 Amp. Hr. Sealed Rechargeable battery.
It has been setting on the shelf for many months since I last recharged it.
Room temperature is about 68 deg F. Current voltage read 12.25 V. Started a 200 MA charge and after a few minutes it was up to 12.68 V. This battery is probably at least 5 years old. Made in China.

By contrast a battery pair I bought to replace batteries in a UPS, but never installed had very poor storage life. These were made in Taiwan.

A Die-Hard Marine setting in the garage at about 35 deg F and not charged in about the last month is 12.82 V.

I doubt you really want battery steady-state unloaded voltage to drop noticeably as a criteria of when to charge a battery.

The particular battery manufacturer may be very important to your question.

.
 
S'mise

S'mise

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Location
Michigan
Evrything I hear about gel-cell's they do just fine with a constant charge.
Don't you mean cut-out when voltage gets too HIGH? When a battery is discharged the voltage is low and when the charge completes it is High.
Best advice is from battery manufacture. Your looking for 2.35v per cell.
13.7V sounds a little low. 14.4V is ideal and I would let it float charge rather than cut out when charged.
 
M

MichaelGP3

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San Francisco bay area
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Fire Alarm Technician
Rats! You beat me to it!

Rats! You beat me to it!

SG-1 said:
You may be looking for something like this:

http://www.homesecuritystore.com/p-424-elk-965-low-battery-cutoff-master-power-switch.aspx

I do not think it has a voltage adjustment. You may be able to modify this easier than building from scratch.
Click to expand...

There is also this:

http://products.boschsecurity.us/en....125.829-CATMb07c5306b1b21288e10e4836d4a0a649

The Bosch D135A cuts off the battery at 9.5 volts, as opposed to 10 volts for the Elk model.

S'mise: 14.4 volts may work as a 'fast charge' rate (or as the starting charging voltage that later tapers off), but as a constant trickle charge 14.4 volts will reduce the service life of a sealed lead-acid battery. 13.65 - 13.67 volts is ideal for a constant trickle charge.
 
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tom baker

tom baker

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Location
Bremerton, Washington
Just to clarify the low voltage cutoff is when the equipment is running on battery with no ac power.If I can cut it off before the voltage gets too low, then it will recharge faster when AC returns.
 
Electric-Light

Electric-Light

Senior Member
The cut-off voltage should be based on parameters desired, such as maximum battery longevity or run time, as well as the load current.

The cut-off though, maybe limited by the device's minimum operable voltage.

For applications that draw enough current to cause more than a few percent of voltage drop, I don't like conventional cut-off voltage sensing, because it causes premature dropping before device's minimum voltage has been reached to "protect" the battery. Voltage drop is significant at lower voltage and there should be a second pair of sensing wires that go directly to the battery.
 
G

G._S._Ohm

Senior Member
Location
DC area
Using either of these ICs
http://www.national.com/mpf/LM/LM10.html#Overview
http://www.national.com/mpf/LM/LM311.html#Overview
is a good starting point.

You'll need hysteresis because the battery voltage may rebound once the load is removed. You may also want an indicating LED telling you that the thing has fired. You might find a way to power the circuit from the battery being monitored.

Once you post a circuit from the Web, changing one or two resistor values will probably make it work for your application.

This thing is so simple it is almost guaranteed to work the first time, but just in case you get weird readings on your meter during troubleshooting it helps to have an o'scope with a relatively wide bandwidth nearby.

You can get the parts from Hosfelt, Allelectronics or similar places.
 
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G

G._S._Ohm

Senior Member
Location
DC area
For your basic circuit try searching this
http://www.national.com/apnotes/
Usually you need to add bypass capacitors and pay attention to single point grounds to get these basic circuits to work in the real world.
Pick the lowest speed, lowest bandwidth comparator or opamp that does the job.
 
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