Low voltage disconnect (kinda)

[Barwick]

Hey, I'm fairly new to electronics, and I'm looking to put a 12V electric cooler in my car to keep some drinks cool all the time. I've looked into it, and I think I can keep them fairly cool without requiring a spare battery (although I might want one, but that's a whole other story).

The problem is, the car battery isn't a deep cycle, so I don't want to draw much power from it while the car is off. So I want a circuit that controls the cooler as such:

When car is running (electrical system is around 14.3 volts), power is allowed through to the cooler (max 4A @ 12V DC).

When car is not running, one of two things can happen:
If battery is fully charged, allow power through to the cooler. As it sits right now, it's at I think 12.36 Volts with the car not running.
If battery is discharged slightly, but still in the "safe" zone (I don't know what voltage to use for this), don't allow any more power through to the cooler (this will prevent the battery from discharging too much).

I was thinking, probably the best way is to have some sort of circuit that will send a small power signal to a 10A automotive relay to tell the relay to close. Then, when the voltage is below X volts (say 12.2 or something), it will stop sending that power signal to the relay, and the relay will automatically open, disconnecting power from the cooler.

My problem is, how do I design this circuit that does that? And if you know anything about batteries, what voltage should I set it at to keep my battery alive?

 

[akg]

to make the control signal u need a comparator. feed one i/p with the reference voltage(based on the min: batt voltage) and the other i/p with the batt voltage. the o/p of the comparator must be given to a flip flop to keep the o/p disconnected once tripped.

 

[bryan1]

Hiya Barwick,
Siliconchip did a project a few years ago for a 12 volt cutout and jaycar,DSE and altronics all carry kits for them. Now I'm telling you this from experience, I've got a snapon cooler made by wayco that uses a peilter device as the cooler. When I first used it in my 4wd it took only 3/4 an hour to flatten a N70 heavy duty battery. My advice would be to use a second deep cycle battery and a circuit that will cut the main battery when the engine is off and switch to the second battery. Once again I made bought a kit for the battery switching and these 2 kits were one of the first electronic kits I made. The 2 battery setup was still working 4 years later when I sold the ute and the buyer was impressed I made up a system to keep the stubbie's cool and not flatten the battery. If you need I can point out the dates siliconchip did those projects.

Hope this helps

Cheers Bryan

 

[zevon8]

This can also be done with an off the shelf "battery isolator". All that is inside are 2 stud package diodes, connected to a metal bar, each diode has one end poking out from the potting, the other connected to the metal bar - which has a bolt on it poking out of the potting also. What happens is the 2 diodes allow both batteries to be charged by the alternator, but the "accessory" batteries load cannot drain the cranking battery. The diodes and heatsink are sized to suit the full load rating of the alternator, since full charging current will pass through the device at all times.

There is also a model variation that is designed to be used with some alternators that do not "self-excite" that is , they do not provide their own field current. These alternators are usually the ones with more than one battery connection.

Here is some info on how they get installed ( both types ):

**broken link removed**

More ciruit description:

http://www.aeroelectric.com/articles/bat_iso2.pdf

I have also seen them made using the diode pack from a heavy alternator, with a large heatsink attached.

For the ultimate in "cheap and dirty, but works" you can get a constant duty rated automotive contactor ( solenoid switch) and connect the second battery to the alternator through it. Use a line from the vehicle that is powered when the motor is running to operate the contactor.

 

[Barwick]

Hiya Barwick,
Siliconchip did a project a few years ago for a 12 volt cutout and jaycar,DSE and altronics all carry kits for them. Now I'm telling you this from experience, I've got a snapon cooler made by wayco that uses a peilter device as the cooler. When I first used it in my 4wd it took only 3/4 an hour to flatten a N70 heavy duty battery. My advice would be to use a second deep cycle battery and a circuit that will cut the main battery when the engine is off and switch to the second battery. Once again I made bought a kit for the battery switching and these 2 kits were one of the first electronic kits I made. The 2 battery setup was still working 4 years later when I sold the ute and the buyer was impressed I made up a system to keep the stubbie's cool and not flatten the battery. If you need I can point out the dates siliconchip did those projects.

Hope this helps

Cheers Bryan

Man, that must've been one heck of a cooler. The cooler I'm looking at is maximum 4 Amp draw, and don't most car batteries have at least 30 AH of capacity?

Mine will be going in the trunk, and I've witnessed that when it's cold at night, the drinks in the cooler get cool, and then stay cool through most of the day, so hopefully I can get it cool while I'm driving, and it'll stay somewhat cool for a while, maybe having to turn on for an hour.

I'll probably make the system that akg suggested. Although I don't fully understand how to do it yet, I'll figure it out. Don't know how to supply both 14.3V all the time on one side, and battery voltage on the other.

 

[Barwick]

Ok, I looked a few of the diagrams up for a low voltage disconnect, but couldn' really understand them. It's been too many years since I've taken a microcomptuers class, and it didn't really go into much detail on any circuits (although I got a 98% in the class)...

 

[justDIY]

where does the 4a rating come from, if it's a measurement, how did you measure it?

it just sounds really small for a 12v peltier ... I have a few different pelts, the smaller 40x40mm take around 8 to 15a... the big ones are well on their way to 30a

 

[zevon8]

If it is like the Koolatron ones, the module is pretty small, not much more than an inch square, mounted on a decent sized heatsink. Those ones came with a 120VAC brick adapter and did not draw too much power.

 

[Barwick]

where does the 4a rating come from, if it's a measurement, how did you measure it?

it just sounds really small for a 12v peltier ... I have a few different pelts, the smaller 40x40mm take around 8 to 15a... the big ones are well on their way to 30a

The specs on the outside of the box it came in says 4A @ 12V DC

It's not much power, but I just used it yesterday in the cigarette lighter, and it got pretty cool.

 

[Barwick]

I'll probably make the system that akg suggested. Although I don't fully understand how to do it yet, I'll figure it out. Don't know how to supply both 14.3V all the time on one side, and battery voltage on the other.

Anyone have any ideas on this?

 

[justDIY]

Anyone have any ideas on this?

don't compare battery voltage to 14.3v since you'll be comparing the battery to itself ... use a voltage divider to whack your battery voltage down to something like 4v when it's fully charged ... and compare that against the output of two AA batteries ... as the car battery sags, so will its divided voltage and eventually it will fall below the level of the AA batteries and toggle the op-amp

of course, you'll need to replace your AA's from time to time, their own internal resistance will be the only real load they see, since the op-amp is a high-z input, use some carbon zinc aka 'heavy duty' (or lithium) batteries instead of alkaline, they hold up to heat and cold better

 

[Nigel Goodwin]

don't compare battery voltage to 14.3v since you'll be comparing the battery to itself ... use a voltage divider to whack your battery voltage down to something like 4v when it's fully charged ... and compare that against the output of two AA batteries ... as the car battery sags, so will its divided voltage and eventually it will fall below the level of the AA batteries and toggle the op-amp

of course, you'll need to replace your AA's from time to time, their own internal resistance will be the only real load they see, since the op-amp is a high-z input, use some carbon zinc aka 'heavy duty' (or lithium) batteries instead of alkaline, they hold up to heat and cold better

Using AA's is a horrible crude solution (although it would work!), the far simpler option is a resistor and a zener to give a stable reference voltage, and if you want it more stable, use a precision voltage reference IC instead of the zener.

 

[Barwick]

Using AA's is a horrible crude solution (although it would work!), the far simpler option is a resistor and a zener to give a stable reference voltage, and if you want it more stable, use a precision voltage reference IC instead of the zener.

So, you're saying just get a 12V voltage regulator unit for a dollar, and then use resistors on the battery voltage so that when it's actually 14.3 V (car's running), the comparitor sees something like 13 V, and when it's not running, it sees something like 11 V?

 

[Nigel Goodwin]

So, you're saying just get a 12V voltage regulator unit for a dollar, and then use resistors on the battery voltage so that when it's actually 14.3 V (car's running), the comparitor sees something like 13 V, and when it's not running, it sees something like 11 V?

No, far lower than that - use a zener to give a reference of perhaps 5V to one input of the comparator, and use a potential divider (two resistors) to divide the battery voltage down to slightly above that and apply that to the other comparator input. As the battery gets lower the second one will drop and the first stay constant, once the second drops lower than the first the comparator will switch states.

For an example check , figure 14.

Notice the zener is shown the wrong way round though, and the circuit needs a few simple improvements.