Hey Paul thanks for adding some temperature data to this list.pgill wrote: ↑28 Nov 2022, 09:57 MVS Team,
I am going to look at this problem slightly differently.
- 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.
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
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.