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I need to build a batteryguard

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supern

New Member
Hello! Thanks for a nice forum! Very much interessting to read :)

I've just bought myself a minifrigde for my car. This is on allmost all time, even if my car is standing still (during a lesson, work ect). Yesterday i slept over at my buddys house and forgot about the frigde being on. This resulted in dead battery. Therefore do i need a circuit between the battery and frigde who brakes if the input-voltage gets under 11.5V.

Can anyone help me with schematics? I've got 1 year experience from school, but have allmost forgot everything about parts ect. My current circuit painting uses one zenerdiode to control the whole thing. The problem is that i don't remember a thing of how to use it. I have free access to parts. My neightboorhood is a teacher (don't dare to ask him about helping with the schematic hehe.. he's mad after work)

Car: 12v battery, 65AH/310A DIN
Frigde: 12v, 4AH (no brand on outside, and bought it as a bulk. no documents or package with it).
The circuit will have to handle 5A (securitydifference hehe) and have a green diode for "on" and a red diode for "off".

Anyone who can help me? :)
Greetings, Anders, NORWAY!
 

Hero999

Banned
The best way is to buy one, either that or leave it connected to another car for a few miniutes to jump start it.

11.5V isn't too low so it should be alright after a charge and I'm surprised it doesn't start, there again the starter draws so much current I would think it's pretty important to have a full battery to start the car.
 

boxer4

New Member
Yeah, if you flatten your battery, it'll be hard to start.

What power control do you have to work with on the 'fridge? Does it have soft control or has a "big old switch"?

Any inductive load on it?

How much loss are you willing to take to have this auto cut-off? Relay design? Huge MOSFET bank design?

Probably the solution will be a comparator, a voltage reference, and the electronic switch. I have this really cheap battery voltage detector that has three LEDs on it that determines voltage. It probably would be a good place to start with something like that.
 

chemelec

Well-Known Member
I Designed one up Years ago for my Truck, But I never put it on my website.

Possibly I still have the Artwork for the PCB.
I check to see if I can find it.

You can Email me if I Forget to get back to you here.

Gary
*********************************************
supern said:
Hello! Thanks for a nice forum! Very much interessting to read :)

I've just bought myself a minifrigde for my car. This is on allmost all time, even if my car is standing still (during a lesson, work ect). Yesterday i slept over at my buddys house and forgot about the frigde being on. This resulted in dead battery. Therefore do i need a circuit between the battery and frigde who brakes if the input-voltage gets under 11.5V.

Can anyone help me with schematics? I've got 1 year experience from school, but have allmost forgot everything about parts ect. My current circuit painting uses one zenerdiode to control the whole thing. The problem is that i don't remember a thing of how to use it. I have free access to parts. My neightboorhood is a teacher (don't dare to ask him about helping with the schematic hehe.. he's mad after work)

Car: 12v battery, 65AH/310A DIN
Frigde: 12v, 4AH (no brand on outside, and bought it as a bulk. no documents or package with it).
The circuit will have to handle 5A (securitydifference hehe) and have a green diode for "on" and a red diode for "off".

Anyone who can help me? :)
Greetings, Anders, NORWAY!
 

chemelec

Well-Known Member
I Found it, I did put it on my Website.

http://www3.telus.net/chemelec/Projects/Low-Bat/Low-Bat.htm

The PCB is Available from me if you can't make your own.

Take care......Gary

supern said:
Hello! Thanks for a nice forum! Very much interessting to read :)

I've just bought myself a minifrigde for my car. This is on allmost all time, even if my car is standing still (during a lesson, work ect). Yesterday i slept over at my buddys house and forgot about the frigde being on. This resulted in dead battery. Therefore do i need a circuit between the battery and frigde who brakes if the input-voltage gets under 11.5V.

Can anyone help me with schematics? I've got 1 year experience from school, but have allmost forgot everything about parts ect. My current circuit painting uses one zenerdiode to control the whole thing. The problem is that i don't remember a thing of how to use it. I have free access to parts. My neightboorhood is a teacher (don't dare to ask him about helping with the schematic hehe.. he's mad after work)

Car: 12v battery, 65AH/310A DIN
Frigde: 12v, 4AH (no brand on outside, and bought it as a bulk. no documents or package with it).
The circuit will have to handle 5A (securitydifference hehe) and have a green diode for "on" and a red diode for "off".

Anyone who can help me? :)
Greetings, Anders, NORWAY!
 

Sceadwian

Banned
Keep in mind even with a low voltage cut off circuit you're cycling your battery more than is recommended which will shorten it's life, and maybe make it impossible to start during severe cold weather. You may want to think about adding an extra dedicated battery (marine deep cycle) just for the fridge or your car battery is going to wear out twice as fast as in a normal car.
 

chemelec

Well-Known Member
Sceadwian, You may want to think about adding an extra dedicated battery (marine deep cycle) just for the fridge or your car battery is going to wear out twice as fast as in a normal car.
And adding that extra battery causes your alternator to fail quicker.
 
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