1 Minute per Second Battery Start Rule

[MrAl]

Permanently installed battery tender? Plug in when not driving?

Hi,

Well if you are lucky enough to be near a power outlet you could easily do that. My car is parked where there is no outlet so that is not an option for me or it would be plugged in right now

[MrAl]

I think the better approach (than idle a cold engine just to charge battery) is to try for longer trips when you do start the engine. Sitting without starting for a couple weeks should be OK for a normal battery. As I understand charging systems, at idle with a weak battery, the alternator may not do much charging at idle. That may not be true.
volvolugnut

Hi again,

Yes it is true and that's another reason why i always recommend measuring the charge current. It's well worth the little effort it takes to do.

There is an old quote in electronics not sure who first said it:
"One measurement is better than a thousand expert opinions".

I've found that to be very true in many cases for various reasons although there are other quotes too that suggest that many experts have enough experience to be able to judge certain things pretty well.
It's hard to beat a measurement though because that tells you right then and there with that very system if anything might be wrong.

[MrAl]

How do we factor in the potential negatives, idling a car several minutes, especially in cold weather, with nice thick engine oil? Another question altogether I guess. Or did I misunderstand the original question...

Hi there,

Well you can measure the charge current, and that takes into account many of these cases being asked about. You do have to get a decent DC clamp on ammeter and you do have to be able to find a wire you can clamp it around, that's the only catch. I was able to do this on my car though and there is little room in there. You do also have to make sure the polarity of the ammeter is right too or you might actually be measuring discharge current (ha ha) which would give you misleading results.

I think you can get a DC clamp on meter now for about $40 USD but beware the AC ones wont do any good at all they cant measure DC current. They are often very cheap and the DC ones are higher priced.

If you are good at measuring voltages you can measure the voltage across a length of the wire going to the battery but that takes a lot more care and if you get that wrong you will get very misleading results. That's why a clamp on ammeter that can measure DC current is still best.
Most of them will do 100 amps or better and of course it should go at least that high.

[MrAl]

My thinking is that the resting voltage (when the battery has sat idle overnight) in combination with the charging voltage tells you everything you need to know. If the charging voltage is 14 or more, but the resting voltage is 11.8 or less, the battery is getting near the end.

Hi there,

Yes there is a lot to be said about the 'resting' voltage but there is a variation due to temperature too so you'd have to be careful about the measurement temperature of the battery. In cold weather it would be different.

Get yourself a decent DC clamp on ammeter and you will never regret it

I would offer to measure anyone's charge current but you'd have to drive to my house which is probably inconvenient for most members here. You also should do it several times over months to get a good idea what is going on with your battery and charge system so it's best to have your own current meter or perhaps share one with a good neighbor.

[RickHaleParker]

A 20 - 30 watt solar panel mounted in the window that faces south when the car is parked and a PWM charger controller will go a long way toward keeping the battery topped off.

I got both sitting on a shelf. Think I will install them in my XC90 which I am using less since I got the headlight done on my C70.

If you want to monitor the SOC more accuracy then looking at the voltage, a Coulomb counter might be the answer.

[abscate]

I don’t think I ever got to that project of measuring the house current of a typical p80.

Current flow of the engine with no other accessories.

ETM
ECM
Coils
DDM
Relays

I’m going to guess 14.13 Amps

[MrAl]

A 20 - 30 watt solar panel mounted in the window that faces south when the car is parked and a PWM charger controller will go a long way toward keeping the battery topped off.

I got both sitting on a shelf. Think I will install them in my XC90 which I am using less since I got the headlight done on my C70.

If you want to monitor the SOC more accuracy then looking at the voltage, a Coulomb counter might be the answer.

Yeah thanks, meant to do that maybe this winter, would help a lot in the cold weather if i can find a good angle that doesnt block a window.

[pgill]

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

[scot850]

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.

[BlackBart]

- The rate of a chemical reaction is controlled by the Arrhenius Rate Law
Paul

Whoa I'm outta my league.