1 Minute per Second Battery Start Rule

[MrAl]

MVS Team,

I am going to look at this problem slightly differently.

Assumptions

- The storage of energy in the battery is a chemical process

- The rate of a chemical reaction is controlled by the Arrhenius Rate Law and can be approximated by doubling the time for every 10C reduction

- The Current flow into the battery may not all be converted into chemical energy and the storage efficiency is lower when the battery is cold

- Cranking time is 2 seconds with the engine at operating temperature and the battery at 50C

With this in mind the Rate law can be approximated as:
------The time to complete the reaction will double for every 10 degree C reduction


50C (122F) 2 seconds --> 2 Minutes (122F is reasonable for a battery in the engine compartment)

40C (104F) 2 seconds --> 4 Minutes

30C (86F) 2 seconds --> 8 Minutes

20C (68F) 2 seconds --> 16 Minutes

10C (50F) 2 seconds --> 32 Minutes

0C (32F) 2 seconds --> 64 Minutes


The one big caveat to this calculation is that the battery will be warming up due resistive heating and due to the engine bay warming up so at 0C it will likely only actually take an hour if the Hood is missing and you are driving which prevents the battery from warming up.


Conclusion

2 seconds --> 2 minutes when you stop for gas and fill up......sure that make sense.

2 seconds --> 2 minutes won't be true when you need a shovel to dig your Volvo out of the Snow



Some light reading

https://en.wikipedia.org/wiki/Arrhenius_equation



Take care

Paul

Hey Paul thanks for adding some temperature data to this list.

I can see that when we start the car and let it warm up not only is the engine warming up the battery is too. Then again when we turn the headlights on the battery warms up a little too, allowing the chemical reactions to take place faster.
It may be difficult to estimate the temperature of the battery unfortunately, but i think if we do some experiments with our own cars we can find out an empirical work around. It's not hard to estimate the outside air temperature and my car even has that in the dash. If we park in the same place all the time we should see repeatable results at least until the battery ages.
What this means is that if it is 30F outside and we run the car for 5 minutes (random time for this example only) and it continues to start up day after day or however often you want to start it, then i think we are good to go. Checking the voltage while starting is a very good way to get data on how well your particular car and routine is working. If the voltage drops more after some starts, then better to run another minute or two, then check again over the weeks to come.
This may sound difficult but it's not. Just connect a good fast acting analog meter to your battery (with an off switch perhaps) and watch the meter every time you start the car. It's easy. What i do with mine now is check it before and after starting, because i have not yet connected it to the main battery. I do sometime but i have to plug into the OBDII connector, which now that i think about it i think i will make that more permanent for testing because that is a really really good test of battery condition.

[MrAl]

How does that go for timing at -30 or -40C?

I guess we are just dead in the water/ice or just never switch the engine off!

Neil.

Ha ha, that's what it sounds like, but i think the battery warms up as it charges and even more once the engine gets hot. That's our saving grace i think
When the battery sounds like it is dead trying to start the car actually warms the battery up internally and the battery may recover when in cold weather.

The ultimate test is having an analog meter connected to the battery when starting. I think that's the best test unless you want to measure the charge current too. The meter tells you how deep the voltage goes when starting and that says a lot about the battery condition and it is perfect for the car it is being used in because it's that actual car and that actual battery, no really deep theory to resolve into reality there, it's like a go-no-go test.

[abscate]

With this in mind the Rate law can be approximated as:
------The time to complete the reaction will double for every 10 degree C reduction

This actually depends on the heat of enthalpy of the reaction , which I don’t know, for the sulphuric acid sulphate process in the battery. Said heat can be positive or negative in which case the reaction rate slows with added temperature.

Yeah, this gets complicated.

[RickHaleParker]

I think that's the best test unless you want to measure the charge current too.

Coulomb Counter keeps account of now how much energy is going out and how much is going in. Calculates the net.

[volvolugnut]

I think that's the best test unless you want to measure the charge current too.

Coulomb Counter keeps account of now how much energy is going out and how much is going in. Calculates the net.

That looks like just the thing to monitor battery state.
volvolugnut

[abscate]

I bet you could write that data to EEPROM in the fuel pump relay uP

Isn’t there a thread on that somewhere ?

[MrAl]

I bet you could write that data to EEPROM in the fuel pump relay uP

Isn’t there a thread on that somewhere ?

There is no EEPROM in my fuel pump relay, YET ha ha.
Here's the thread:
DATA~~~~~~~~~~~~~~~~EEPROM~~~~~~~~DONE
(ha ha)

That's in interesting idea though, keep the every day workings of the fuel pump relay in some memory to be looked at later to see if everything is working right. Maybe record the ECU frequency and how many times the relay turned on and off, etc.

[MrAl]

I think that's the best test unless you want to measure the charge current too.

Coulomb Counter keeps account of now how much energy is going out and how much is going in. Calculates the net.

Yes that could help too. A regular analog meter works good too if you watch it every time you start the car. I have mine in the center dash just below the ash tray.

The only thing none of these catch is a quick dead battery. I had my battery go completely dead to 0 volts one time and there seemed to be no reason for it. I didnt know until i went out to start the car.
In the past i have used a blinking light volt meter to blink out the voltage of the battery so i could see it from the house then have the computer web cam monitor the blinks and display the voltage. I havent used it though since i got rid of my Hyundai.
It's a simple circuit with a microcontroller and simply measures the voltage in bits, then converts it to a human readable set of blinks that anyone can easily interpret. Normally that would be converted to a human readable ASCII string to be displayed on an LED display, but i would not be able to see the LED display from the house unless i got a gigantic display and that would take power from the battery.

[RickHaleParker]

In the past i have used a blinking light volt meter to blink out the voltage of the battery so i could see it from the house then have the computer web cam monitor the blinks and display the voltage. I havent used it though since i got rid of my Hyundai.
It's a simple circuit with a microcontroller and simply measures the voltage in bits, then converts it to a human readable set of blinks that anyone can easily interpret. Normally that would be converted to a human readable ASCII string to be displayed on an LED display, but i would not be able to see the LED display from the house unless i got a gigantic display and that would take power from the battery.

Options:
1. Use a Tricolour led. Green = Good, Yellow = Low, Red = Houston we have a problem.
2. A string of LEDs and use a different value of series resistor for each one ... so that they cut out as the voltage drops.
3. Texas Instruments LM3914 Dot/Bar Display Driver.

[MrAl]

In the past i have used a blinking light volt meter to blink out the voltage of the battery so i could see it from the house then have the computer web cam monitor the blinks and display the voltage. I havent used it though since i got rid of my Hyundai.
It's a simple circuit with a microcontroller and simply measures the voltage in bits, then converts it to a human readable set of blinks that anyone can easily interpret. Normally that would be converted to a human readable ASCII string to be displayed on an LED display, but i would not be able to see the LED display from the house unless i got a gigantic display and that would take power from the battery.

Options:
1. Use a Tricolour led. Green = Good, Yellow = Low, Red = Houston we have a problem.
2. A string of LEDs and use a different value of series resistor for each one ... so that they cut out as the voltage drops.
3. Texas Instruments LM3914 Dot/Bar Display Driver.

Hi Rick,

Yes that sounds like a good idea too.

What i was doing was tracking the battery voltage over months to get some idea how fast it discharged. The blinks gave me 4 digits of precision.
For example, if the voltage as 12.34 volts then the blinks would go as follows:
LONG PAUSE
BLINK ON, then OFF
SHORT PAUSE
BLINK ON, then OFF,BLINK ON, then OFF
SHORT PAUSE
BLINK ON, then OFF,BLINK ON, then OFF,BLINK ON, then OFF
SHORT PAUSE
BLINK ON, then OFF,BLINK ON, then OFF,BLINK ON, then OFF,BLINK ON, then OFF
LONG PAUSE
then take a new set of samples, average, chop off some bits (oversampling), then blink out a new value.

So this gave me plenty of good data to work with.

I didnt realize before that Steve was maybe talking about the EEPROM that most microcontrollers have these days.