# Automotive low battery disconnect

#### thedrive

Joined Sep 23, 2016
32
hello,
I'm looking to create a circuit to control another circuit that I have built for vehicles. I need the a automatic low battery shutoff. The main idea is before battery power gets below a threshold where the car would not have the power to start the car it would throw a relay to disconnect my circuit. I was thinking of a compairing circuit to monitor voltage. Problem is after about 30 minutes of my system running with the vehicle off my battery voltage drops to 11.99v my understanding is that at 12v my car battery is considered "dead". However my car starts with no issues at this 11.99 volts. Now I know that this voltage isn't really accurate because the battery is under load. Hence my problem...anyone have an idea on how to monitor a battery under load and then turn my circuit off if battery power drops to a point where it won't have the power to start the car?

Any ideas are appreciated!

#### crutschow

Joined Mar 14, 2008
27,010
How much current is the circuit using?
It sounds like the wire resistance to the battery is causing incorrect reading of the battery voltage.
One way around this is to momentarily shut off the load to measure the voltage, if that's a feasible option.

#### thedrive

Joined Sep 23, 2016
32
How much current is the circuit using?
It sounds like the wire resistance to the battery is causing incorrect reading of the battery voltage.
One way around this is to momentarily shut off the load to measure the voltage, if that's a feasible option.
I'm testing the battery voltage with my multimeter on the battery terminals themselves so I **think** the voltage reading is correct. My circuit is running some of the cars electronic systems (mainly the blower motor). I will try to figure out how to get overall amp draw and report back

Joined Jul 18, 2013
22,860
my battery voltage drops to 11.99v my understanding is that at 12v my car battery is considered "dead". However my car starts with no issues at this 11.99 volts.
Where did you get that info? You would have to monitor the voltage under a starting load to determine if it was 'Dead'.
My battery read out can show about 10-10.5 and still start the car, it depends on what is drops to when the starter engages, there are other factor such as temperature etc.
There is a load tester to determine the condition of the battery which provides a very heavy load while indicating voltage over time.
A simple comparator will do what you want, but you need a local supply for the circuit to achieve this.
Max.

#### crutschow

Joined Mar 14, 2008
27,010
.....................
A simple comparator will do what you want, but you need a local supply for the circuit to achieve this.
Max.
Why can't you power the comparator circuit from the voltage you are measuring?

#### tranzz4md

Joined Apr 10, 2015
310
You might consider using a second battery with an isolator/charger like RV people use. That'd get you the longest run times.

#### thedrive

Joined Sep 23, 2016
32
Why can't you power the comparator circuit from the voltage you are measuring?
I can...I guess the main question Im asking to the forum is at what voltage is a car battery considered to low to crank the engine? Reading around on the net the general idea is 12.6v is fully charged and 12.0v is dead. However my car will start just fine when the voltage reads 11.9v with a slight load of about 4.0 amps on the battery. I want this circuit to shut off before the battery is to low to start the car.

#### crutschow

Joined Mar 14, 2008
27,010
I would not run the battery below 12V, even if it will start the engine below that voltage.
Discharging the battery to too low a voltage will reduce its life.

Note that the "12V is dead" voltage is after the battery has sat for some time with no load.

#### thedrive

Joined Sep 23, 2016
32
Where did you get that info? You would have to monitor the voltage under a starting load to determine if it was 'Dead'.
My battery read out can show about 10-10.5 and still start the car, it depends on what is drops to when the starter engages, there are other factor such as temperature etc.
There is a load tester to determine the condition of the battery which provides a very heavy load while indicating voltage over time.
A simple comparator will do what you want, but you need a local supply for the circuit to achieve this.
Max.
I didn't monitor cranking volts. I had my multi-meter connected to the battery terminals while the engine was not running. I had the cars heater fan running for about 45 minutes to see how many volts would drop after 45 minutes. The cars heater blower motor draws around 3.0 amps. let's say I want to have the cars radio on for 1 hour while the car is off. I want to be able to monitor the battery somehow to automatically shut the radio off
You might consider using a second battery with an isolator/charger like RV people use. That'd get you the longest run times.
Im not really worried about run times but more that I want to know when the cars battery is getting to low to crank the engine. I know that some of the newer cars will alert you if you are sitting in the car without it running and you are listening to the radio to long it will alert you when the battery starts to get low. I want to make a circuit like that for my older cars. If I'm sitting in the car to long with the radio or lights on and the battery is getting to low to crank
the engine I want the circuit to alert me. I just don't know how go about it. If I monitor the battery voltage how do I know what voltage is to low to crank the engine?

#### thedrive

Joined Sep 23, 2016
32
I would not run the battery below 12V, even if it will start the engine below that voltage.
Discharging the battery to too low a voltage will reduce its life.

Note that the "12V is dead" voltage is after the battery has sat for some time with no load.
I see, thanks for the input.

Joined Jul 18, 2013
22,860
Why can't you power the comparator circuit from the voltage you are measuring?
I was thinking the reference would also vary, but a zener could clamp it off.
Max.

Joined Jul 18, 2013
22,860
If I monitor the battery voltage how do I know what voltage is to low to crank the engine?
You don't really, not for sure, there are many other factors such as engine and ambient temperature.
You have to decide a voltage value for worst case scenario and design for that.
Max.

#### bwilliams60

Joined Nov 18, 2012
1,392
I have to agree with tranzz4md. A second battery with an isolator or magnetic switch is in order. Deep cycling a conventional SLA battery will result in decreased life. More importantly, the alternator is being damaged by repeated abuse each time you start the vehicle. It's purpose is to replenish the battery as soon as you start the vehicle meaning it will go to near 100% duty cycle to do that. Do it enough and it will overheat and fail. 12.00 VDC in technical terms is deemed a dead battery. 75% charge is 12.4 and 100% is 12.6VDC. 75% is the minimum you ever want to let your battery get to. Below that and the alternator begins to work. It may start your vehicle but you are creating other problems.

#### tcmtech

Joined Nov 4, 2013
2,867
In a more realistic note,

How often is running your battery too low to start the engine a problem?

Daily?
Weekly?
Monthly?
Yearly?
Maybe 2 -3 times in the many year life of the battery itself?

#### thedrive

Joined Sep 23, 2016
32
I have to agree with tranzz4md. A second battery with an isolator or magnetic switch is in order. Deep cycling a conventional SLA battery will result in decreased life. More importantly, the alternator is being damaged by repeated abuse each time you start the vehicle. It's purpose is to replenish the battery as soon as you start the vehicle meaning it will go to near 100% duty cycle to do that. Do it enough and it will overheat and fail. 12.00 VDC in technical terms is deemed a dead battery. 75% charge is 12.4 and 100% is 12.6VDC. 75% is the minimum you ever want to let your battery get to. Below that and the alternator begins to work. It may start your vehicle but you are creating other problems.
the timeout limit. I also want the system to shut off if battery power
In a more realistic note,

How often is running your battery too low to start the engine a problem?

Daily?
Weekly?
Monthly?
Yearly?
Maybe 2 -3 times in the many year life of the battery itself?
gets to low. Don't want to be in a situation
In a more realistic note,

How often is running your battery too low to start the engine a problem?

Daily?
Weekly?
Monthly?
Yearly?
Maybe 2 -3 times in the many year life of the battery itself?
Normally I agree...but I have a device that runs the cars fan for up to 1 hour when the car is off. This device draws about 5.5 amps and is used daily about 7 months out of the year. I want to make a circuit that will shut this device off should battery voltage get to low. problem is figuring what what "to low" is. I know that some of the newer cars give an alert when the battery gets to low after you have been sitting inside listening to the radio with the lights on to long I just dont know what they are measuring to determine that a battery is getting low

#### tcmtech

Joined Nov 4, 2013
2,867
Any decent quality vehicle battery should have no issues with holding a 5 - 6 amp load for an hour or even several given they are typically at least 30 Ah or larger. If it does the battery is already shot and needs replacing.

#### thedrive

Joined Sep 23, 2016
32
I appreciate your input on this and I agree. I do want the circuit to hopefully eliminate a need for a jump start on a battery that is on it last leg. I dont want my system stranding people with dead batteries. Hoping I could come up with something that will turn my system off before it gets to a place where the car cant be started. I will post back if I find a good solution.

Joined Jul 18, 2013
22,860
As earlier, a simple comparitor, LM311 etc.
Max.

#### tcmtech

Joined Nov 4, 2013
2,867
I appreciate your input on this and I agree. I do want the circuit to hopefully eliminate a need for a jump start on a battery that is on it last leg. I dont want my system stranding people with dead batteries. Hoping I could come up with something that will turn my system off before it gets to a place where the car cant be started. I will post back if I find a good solution.
I follow the reasoning but with a battery that is too weak to run a 5 - 6 amp load for an hour its standing voltage when charged will be too low to begin with making the disconnect false trip all the time.

A new battery when fully charged will stay around 12.5 - 12.9 volts for a long time and even if ran down to around 10% charge will still hold 12+ volts while having enough amperage generating capacity to start the engine.

Whereas with a old worn out battery it may not even hold 12 volts at full charge and will drop off fast from there so even if you cut it off at 11.5 volts it may very well not have enough reserve capacity to produce the amperage needed to start the engine without dropping well below the cut off circuit tripping point if it can start it at all.

Battery protection device work well on new good condition batteries but on old worn out ones they are pointless and just make a nuisance of themselves.

That and very few vehicles have radios that hold their programed settings so every time the battery cut off trips you have to go in and reset everything plus possibly put up with your vehicle's engine computers having to reset and relearn their settings as well.

Personally if it was me I would be looking at why the fan runs after the ignition is off and fixing that design issue. I don't know of any vehicle that does that on purpose.

#### bwilliams60

Joined Nov 18, 2012
1,392
I wasn't able to watch your video that you attached. It apparently won't open. Can you send me the link?