Thread: Will your battery go flat if you don't start car for a few weeks?

There is a theory about charging reducing sulphation in the cells, but I'm not an expert so I'll leave it to others to comment.

Originally Posted by Young Angus

Well got the Repco one and it's charged both batteries up nicely, the isolator is on the whole time and both are sitting at about 12.5v now when the unit says they are fully charged...so that's not what they were sitting at when they were new

If I leave this plugged in for a while and it is in "maintain" mode will this maybe rejuvinate any of the batteries original properties or are they just not going to be as good as new now because I let them go flat for a while?

At 12.5v your battery is at 80-85% SOC, 12.7v is 100% SOC.
You may find this chart useful

https://www.mmbalmainauto.com.au/PDF..._batteries.pdf

Originally Posted by Young Angus

Well got the Repco one and it's charged both batteries up nicely, the isolator is on the whole time and both are sitting at about 12.5v now when the unit says they are fully charged...so that's not what they were sitting at when they were new

If I leave this plugged in for a while and it is in "maintain" mode will this maybe rejuvinate any of the batteries original properties or are they just not going to be as good as new now because I let them go flat for a while?

Firstly(don't assume!) .. how are you measuring? Is via a cheapie 'ebay' dash mounted gauge volt meter .. say like a USB port thing with volt meter .. or a proper volt meter?

Reason I ask, is I quickly added a couple of dual USB voltmeter ports into brothers D2 whilst keeping an eye on batteries. Just taped to the dash top for now, made it easy for quick glance from outside.
One says 12.5v and the other says 12.7v, but multimeter confirms 12.7v. That is, one of the cheap @$$ed devices reads 0.2v lower than true.
Also! reason I question the 12.5v you see now, is that if the start battery is actually at 12.5v with the redarc isolator it won't be on. Mine turn off at just below 12.7v. It says 12.7v in the manual, but on my multimeter the actual shutoff voltage is 12.67 every time. Close enough.
So if your batteries are showing 12.5v and isolator on, either a problem somewhere .. or accuracy should be doubted by at least 0.2v.

Secondly: I noticed the same thing with the NOCO charger. Set to AGM mode, on the two optimas. It appears that it lets them rest(or something) then charges them and after a few days it then seems to just hold them at 13.7 or 13.8v or whatever I used to see.
This was different to what it did with the two truck (200Ah) batteries I have in my shed. I set them to std mode on the NOCO charger and it charged them up(one at a time).
It then held them at 13.8v for a while(couple of days)
Actually took them up to 12.9v(multimeter) and they held there for weeks. Haven't checked now for a couple of weeks since tho.
I don't(didn't) keep a track of it all 24/7 .. only every time I was out the back for a smoko, I'd have a peek whilst 'killing myself'.

So I'm curious why when on float charge mode in AGM mode, it allows the batteries to 'settle' or something, then recharges them, but with the std (Calcium I think) truck batteries, it charges them up then keeps them at the higher 13.8 level charge what appears to be all the time.

Not sure, but the chargers adopt different profiles when set to charge different types of batteries.
It may also depend on if you are testing the batteries when they are under load or not. It may read over 13v surface charge when resting, but as soon as a load is put on it might drop to about 12.7, which is fully charged.