With the battery connected to the car the problem could be either a bad battery or parasitic draw. That is why I mentioned disconnecting a cable.
Disconnect the negative cable and connect the meter between cable and battery post while setting the meter on the Millamperes scale. See what’s going on with that first. You might consider a trunk light staying on since the car was hit from behind.
Thanks, what if I switch off the trunk light? There is a switch. Would that work? I will then go ahead and disconnect the cable. (I am a little nervous to do that since I am concerned that I may not connect it back well enough and it will come loose while driving.)
Here is a very simple test that I think you have already done.
Thank you, I have disconnected the negative cable, and started charging the battery. I realized that i have a DieHard automatic battery charger (DH-136), not a maintainer. Btw, I noticed that there was some residue aroung the negative post of the battery. I don’t know if this is expected after only 10 months. Btw, I took the car recently to a mechanic: he said that all Interstate batteries nowadays are of poor quality (ever since they bought Exide, he claims) and he wanted me to buy a new battery from him ($220 with installation) before he would look into possible electrical issues. I decided against it because I did not find negative reports on the battery and it is still under the full 2-year warranty. (The battery model is Interstate MT47/HP5.) Anyway, should know sometime tomorrow mid-morning if the battery has charged. I will let you know.
Well I’ll go out on the limb and say I think you need a new battery. A 2 year warranty kinda says its not the best in the first place and a jolt could have some effect on it.
Thanks, it looks like it because the charger says Bad Battery this morning, don’t Interstate batteries have 2-year full and prorated batteries for 6 years after that? Btw, my negative is disconnected from the car, not the positive. This should be fine to decide that the battery is the problem, correct?
Doesn’t matter which post is disconnected to isolate the battery from the car.
However, it is safer to disconnect the negative post first.
Google an explanation if you like.
Years ago I had a battery develop an internal drain after a collision.
In that case someone hit the rear quarter panel while it was parked.
Thanks! I am just trying to make sure before I show up at the battery shop. Is it possible for some electrical parasitic drain in the car to have damaged the battery? Here is the battery I have: https://www.interstatebatteries.com/products/mt-47-h5
If your charger says Bad Battery and your voltmeter also does (not holding its voltage, you need to replace the battery. It may resolve all your problems. And if it doesn’t, at least you won’t have a bad battery confusing all the systems. Let us know if there is a second problem after the battery is replaced.
Thanks, the reading is now 9V. Btw, the reading is also 9V when I connect the negative lead of the multimeter to the negative post of the battery and the positive lead of the multimeter to the cable of the car (that is disconnected now, but normally connects to the negative post of the battery).
Here is the explanation for your “disconnected” reading: Draw yourself the simplest circuit diagram, with a battery connected to a resistor, then erase the negative connection and place a voltmeter between the battery and that cable. A voltmeter will have very high resistance, so essentially no current flows. So if you look at the voltage drops, start at the plus side of your battery which is 9V, next there is no drop across your positive cable, since it has no resistance Next there is NO drop across the resistor, since there is no current. Then there is no drop across the negative cable, so one end of your voltmeter is at 9V and the other is at zero, the negative side of your battery. So,your reading is what is expected, but gives no information. If your battery dropped to 9V while disconnected, the battery is bad.
OK, thanks for the explanation. I was just wondering if there was any indication of parasitic drain in the electrical system of the car from the readings.
If you had just set your voltmeter to an ammeter, its low resistance would then allow the parasitic drain current to flow. And then the removal of fuses one at a time would help locate it (if you have it). One caution: even if your ignition is OFF, there may be items still drawing a current, so I personally do not know what current to expect, and what would qualify as parasitic. But, if your disconnected battery has a falling voltage, the parasite is inside the battery.
Thanks! Overnight, the battery left on its own (with positive connected to the car, negative disconnected) did not drain. It is still at 9V. I will see what happens if I set to charge it again. The problem appears to be that the battery does not charge or is at 9V when the red light (bad battery light comes on).
Why are you not just getting a new battery to see if that is the problem ? It seems that this one is not really good .
It doesn’t surprise me that the bad battery stops falling at 9V. The battery is made up of multiple cells. Some are good and at least one is completely discharged. The good ones total 9V.
You are asking good questions!
Didn’t you say many days ago that the battery is under warranty?
Thanks! Yes, the battery is under full warranty. I am just not very knowledgeable on these matters, so was trying to get as much information before I went back to Interstate Batteries. Thanks a lot again!
Really no need . They will test the battery and either replace it or prorate according to Interstate warranty guidelines .
I learned what I know about batteries exactly the same way you are learning…by asking questions and experimenting. Batteries can fail in odd ways, giving odd symptoms.
The last time I bought one, I found a good price at Costco.
Maybe I missed it, but OP, why aren’t you simply disconnecting the battery from the car, charging it overnight, then monitoring the charge with your multimeter? If it holds at 12.6V, it’s a problem somewhere else (most likely), if it falls it’s a bad battery.
It seems that this suggestion has been given several times, but then the thread goes off in another direction. You’re asking about taking the battery to an auto parts store to test it…why? You’ve got the tools in your possession. You’re asking about battery warranties and parasitic drain…why? You haven’t done the one simple test that has been recommended to see if the battery is bad.
The very next thing you do should be to test the battery on its own to see if it holds a charge. Anything else is a waste of time.