Hi, I have disconnected the battery from the negative terminal of the car, and charged it overnight. Does the positive need to be disconnected also? However, after a while, the red light comes on (“Bad Battery”) and the multimeter reads 9V. It is not clear to me of course, if the battery was charged and then eventually lost charge during the charging (assuming that this is even possible). Thanks!
In that case, it sounds like you simply need a new battery. Assuming you have a working charger, putting it on the charger overnight, and then the voltage drops to 9V, seems to me like it’s not holding a charge.
As far as I know, you shouldn’t need to also remove the positive; the disconnected negative should prevent any discharge into the vehicle.
If it were me, I would go buy a new battery of a different brand…I’m also not a big fan of the Interstate brand. But if that’s not workable, remove the battery, Uber to the store where you bought it, and insist that they give you a replacement.
Dropping to 9V suggests to me it has 2 bad cells.
I would not try to charge it again.
If a cell decides to develop a “hard” short the result could be unpleasant.
At any rate, it’s a good idea to wear eye protection when handling a lead-acid battery.
So, I went and got the battery replaced. I had to jumpstart the car and it was only 11V when I switched the car off at the store, so the agent (for want of a better word) said that he could not test anything but fluid levels. One cell had low fluid levels and so he replaced the battery. I will let you know how things go from here. Oh, btw, he said that he replaces a lot (7 out of 100, he said) of these batteries so perhaps going to Interstate in the first instance was not that great an idea. Thank you all for your help, I learnt a lot of things (obvious to many) from this thread. Thanks again, and I will keep you posted!
Keep up your voltage measurements for a few days, so if the alternator is also bad (or its regulator), you will know immediately. For instance, what does it read across the battery when the engine is running and revved up vs idling.
You might take that 7 out of 10 batteries statement with a grain of salt.
It was 7 out of 100 batteries.
7% vs 70%, still a lot and a pretty good chance of getting a bad one.