The 12V Battery Question

[timberwolf]

Wow, TW, a bit harsh. While I agree with you to a point, I'm going to comment about what you've said.

Sorry, one would think I would get better at this? I was using the Royal "You", and not specifically at you personally or your post - I would have quoted your last post and then chopped it up into little pieces if it was personal ;)

My own experience suggests that it is not possible to predict when a Battery is going to let "me" down by measuring its voltage. One week it is fine and the next the door locks are sugglish and the relays are reluctant to engage. My scangauge has never helped me to predict that event. The best I can do with a DVM is confirm my original symptoms, i.e. car was reluctant to start, because the Battery voltage was low.

[Joseph D]

Wow, TW, a bit harsh. While I agree with you to a point, I'm going to comment about what you've said.

Sorry, one would think I would get better at this? I was using the Royal "You", and not specifically at you personally or your post - I would have quoted your last post and then chopped it up into little pieces if it was personal ;)

My own experience suggests that it is not possible to predict when a Battery is going to let "me" down by measuring its voltage. One week it is fine and the next the door locks are sugglish and the relays are reluctant to engage. My scangauge has never helped me to predict that event. The best I can do with a DVM is confirm my original symptoms, i.e. car was reluctant to start, because the Battery voltage was low.

I didn't take it personally, and no offence taken.

I did get that you were making generalised comments, but based on my experiences, what you said did not ring true so hence my comments.

If you have the 12 V Battery voltage on your ScanGauge, I'm surprised you cannot pick when your battery is bad. It stands out like a sore thumb to me.

[timberwolf]

The Scangauge II displays a voltage pid(?) read from the on-board LAN via the OBDII diagnostic socket, it unfortunately can not directly measure a voltage.

I do not drive the car during the week, the SGII is removed from the car after use because I don't like leaving it attached in plain sight and it will add to the standby current drain.

When my SGII has been unplugged and then plugged in a week later, the device won't boot up until after the car is started and then it is only reading the car's charging voltage. If I leave the SGII connected after a trip, it shows the surface charge on the Battery. The only useful thing I've noticed over the years after leaving the car for 30-40 mintues, is that when the Battery is new and/or fully charged it stays at 12.8V. The rest of its life the Battery won't hold 12.8V for very long even after a full 24 hour charge, and I have never seen a voltage that clearly says to me that my battery needs replacing.

[kevin h]

Possibly showing my thickness, but its a 12v Battery, how can it show 12.8v

[timberwolf]

I've always just accepted that was how they were made, this might be the reason...

http://www.batterystuff.com/kb/frequently-asked-questions/powersports-batteries-faq/12-volt-battery-reading-13-volts.html

[Joseph D]

Possibly showing my thickness, but its a 12v battery, how can it show 12.8v

The simple answer is that with batteries the true voltage varies, so for simplicity's sake a "nominal" voltage is used. Hopefully, the information below helps in this regard.

I've always just accepted that was how they were made, this might be the reason...

http://www.batterystuff.com/kb/frequently-asked-questions/powersports-batteries-faq/12-volt-battery-reading-13-volts.html

A nice article but I think the author has over simplified it by trying to cover all bases. The net result is some of the numbers are off.

These are the numbers as applies to AGM:

Fully charged straight off the charger @13.5 V ... 2.25 V/cell .............. (13.5 V)

With surface charge dissipated ........................... 2.18 - 2.20 V/cell .... (12.9 - 13.1 V)

AGM batteries should be charged at between 2.40 - 2.45 V/cell (14.4 and 14.7 V) initially starting with a high amperage (~4.2 - 16.5 a depending on make of battery) (till 80%) then declining amperage until fully charged. The voltage should drop to 2.20 - 2.30 V/cell (13.1-13.8 V) for float. Note that the maximum voltage that you will get straight off the charger is what ever voltage your charger floats.

BatteryStuff does make a good point, one that I try to hammer home:

The life expectancy; measured as cycle life or years remains excellent in most AGM batteries if the batteries are not discharged more than 60% between recharge. - See more at: BatteryStuff | articles | battery-articles | Gel vs AGM

60% is ~12.6 V roughly.

Without getting too technical, I hope this is of help

[kevin h]

Thanks all. Getting paranoid now, everyrime I start the car I ask myself "will she start". So, next service in 3 weeks, no long distant or urgent journeys to make. More expensive service coming up

[Joseph D]

Thanks all. Getting paranoid now, everyrime I start the car I ask myself "will she start". So, next service in 3 weeks, no long distant or urgent journeys to make. More expensive service coming up

Don't know why you need to get paranoid. It is quite easy to make a 12 V display from a couple of components from your local hobby electronics outlet or use a DVM (Digital Volt Meter). With that, you can monitor your Battery voltage in real time.

Also, if you don't own a modern smart Battery charger, you can pick up a modestly priced, but very suitable, unit from Lidl for ~£20. You will need a Battery charger (IMHO) if you own a Prius, as it tends to be very gentle bringing a partly discharged battery back up to full charge. You will see this behaviour if you monitor your 12 V battery full-time -- how long it stays at 14.4 (battery low) until it drops to 13.8 (Battery nearly full). Very rarely do you see it at 13.5 (fully charged; on float.)

You never gave any feedback on my suggestion to charge up your battery, as it was flat.