Ok, this 12v battery thing is getting a little overblown. I ran some tests on my 12v battery to give you guys a baseline so you could check yours. Basically the history of my battery is that once I forgot the parking lights on and killed it overnight, not all the way, but down to 8.5v where the car was "dead" when I opened the door to try to use it. I charged it on my charger and watched the voltage rise from 8.5v up, the charger wasn't feeding in a lot of current and the interior lights started blinking (crazy stuff from the car due to low voltage) but the charger overcame that within a few minutes the voltage had risen to over 10.5v and then I got a text from the car asking me to plug it in due to the 12v battery being low. I did, once it started charging I disconnected my triton charger and let the car's EVSE charge up the batteries.
Since then (this was 2 years ago), I have not charged the 12v battery externally, and just let the car's HVB or the EVSE charger take care of things for the 12v battery. The 12v battery and the car are now 2.66 years old, my car was built in June it will be 3 then. Having the 12v battery drop below 11.8v unloaded is bad for it (40% charge level), so 8.5v which is below 0% (10.5v) sure didn't help, but it didn't destroy the battery either.
My car batteries last me 10 years on average, and all batteries degrade over time when I say 10 years doesn't mean the battery in years 8 9 and 10 is the same capacity as in years 1 2 and 3. Over time the battery will have less and less reserve power it can give out, but it doesn't mean its trash. At some point its going to be low enough to not be acceptable for you, at which point you mark it as dead and get a new replacement. Less reserve means you can run the accessories for less time or leave the lights on for less and less time before the battery can't start the car any more. In the case of the Cmax, it doesn't need to crank the engine only close the contacts for the HVB to start the car, hence the load on it is alot less and hence the battery is actually smaller than regular ICE cars that need CCA to start the engine.
In any case, I ran this test on my battery so you guys can compare. Note that I had not charged the battery externally, and did not charge the battery before the test. I just drove the car tonight 6 miles to the restaurant and 6 miles back, and when I got home I rolled down the window and popped the hood open. Test as follows:
First I got a voltmeter and hooked it up the battery + terminal under the hood and to a body part (the big aluminum plate to the left of the + terminal) for the negative. Initial voltage readings were 12.65v.
I then turned on the parking lights by reaching through the open window (without opening the door). Here is a chart:
0 minutes lights off, voltage showing 12.65v.
5 minutes parking lights on (headlights don't work with power off), 12.10v
10 minutes, 12.01v
15 minutes, 11.89v
20 minutes, 11.74v
25 minutes, 11.70v
30 minutes, 11.68v
35 minutes, 11.76v (not sure why this happened, but looks like something in the car that was initially taking power shut down).
40 minutes, 11.75v
45 minutes, 11.74v (voltage pretty stable now)
50 minutes, 11.64v (I made the mistake to log into MFM to check my trip driving score while online, realized this right away ran down to the garage and found the voltage drop due to this (actually saw it drop from 11.72 to 11.55v). While in the garage I heard a click and some noise from under the hood. Not sure what it was but something taking power then it shut off. Then I saw the voltage quickly rise back up to 11.63v.
55 minutes, 11.63v
60 minutes, 11.62v
I figured enough so I shut the lights off. Within a few seconds from shutting the lights off, the voltage bounced back to 11.79v.
5 minutes after lights off, 12.15v
10 minutes after lights off, 12.18v.
25 minutes (last update I'll do now) after lights off, 12.22v. Seems like this is around where its going to stabilize.
Anyway, the moral of the story is that don't let people scare you into needing new 12v batteries left and right. Try this test on your battery and just watch it to make sure the battery doesn't die on you (don't forget the lights on in other words) and if your battery can last like mine or perform similar to it then your battery is plenty good enough have no worries you're not going to get stranded.
Hope this helps,
Edited by rbort, 19 February 2016 - 08:18 PM.