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Discussion Starter · #1 ·
Hi everyone. I just had a second battery installed in series and I noticed that my voltage now drops to from 14.7 to 12.5-12.7. Whenever I start my car it does start there and then drops within a few minutes. This drop happens regardless if I’m playing my system or not. The shop said it was the alternator which is a brand new ho 320 amp one. I had it tested at numerous spots and they said that the alternator is ok. This has been going on for a couple weeks now and my car is still running. If it was the alternator, wouldn’t the batteries have died by now?
 

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So the same alternator was yielding a higher voltage at idle before adding a second battery? When the voltage drops, does raising the engine RPM change anything?
 

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Hi everyone. I just had a second battery installed in series and I noticed that my voltage now drops to from 14.7 to 12.5-12.7. Whenever I start my car it does start there and then drops within a few minutes. This drop happens regardless if I’m playing my system or not. The shop said it was the alternator which is a brand new ho 320 amp one. I had it tested at numerous spots and they said that the alternator is ok. This has been going on for a couple weeks now and my car is still running. If it was the alternator, wouldn’t the batteries have died by now?
Theoretically, yes, the batteries would have died by now.
We'll need more info on the make and model of the car to help, and info on the alternator and how the batteries are wired.
 

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Discussion Starter · #4 ·
Theoretically, yes, the batteries would have died by now.
We'll need more info on the make and model of the car to help, and info on the alternator and how the batteries are wired.
It’s a 2014 xts vsport. Batteries are wired in series and the alternator was made by a local shop. 320 amp.
 

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Discussion Starter · #5 ·
So the same alternator was yielding a higher voltage at idle before adding a second battery? When the voltage drops, does raising the engine RPM change anything?
Yes. It was consistently at 14.7 volts. And raising the rpm’s usually doesn’t change it much. Maybe .1-.2 volts
 

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It’s a 2014 xts vsport. Batteries are wired in series and the alternator was made by a local shop. 320 amp.
You mean parallel? 2 12-volt batteries wired in series makes 24 volts.
As for how the batteries are wired, I meant "Do you have a battery isolator?", "Where is the second battery grounded?", etc.
 
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Series? You sure? If they are in series, you should be getting 24-30 volts. I think you mean in parallel: positive terminal of both batteries connected together, and negative terminals going to ground.

Disconnect the new battery, and measure the voltages on both batteries with the car off. Then, with the new battery still disconnected, start the engine, and measure the voltage. Probably obvious, but be careful that the battery cables don't get shorted during this process.
 

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Discussion Starter · #8 ·
You mean parallel? 2 12-volt batteries wired in series makes 24 volts.
As for how the batteries are wired, I meant "Do you have a battery isolator?", "Where is the second battery grounded?", etc.
Yea sorry parallel. I’m not sure if there is an isolator. Second battery was grounded in the trunk I think. Could be wrong. Sorry this is kinda new to me. But if it’s wired in parallel, if I remove the fuse under the trunk, won’t that disconnect all power to the car? Or if I disconnect the negative cable from the battery under the hood? I ask cause i did this and the car still runs.
 

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Discussion Starter · #9 ·
Series? You sure? If they are in series, you should be getting 24-30 volts. I think you mean in parallel: positive terminal of both batteries connected together, and negative terminals going to ground.

Disconnect the new battery, and measure the voltages on both batteries with the car off. Then, with the new battery still disconnected, start the engine, and measure the voltage. Probably obvious, but be careful that the battery cables don't get shorted during this process.
Yes sorry. Parallel. Can you disconnect the rear battery from disconnecting something from the front? I did disconnect the fuse and the car and system still turn on. Also tried disconnecting the negative terminal from the battery under the hood and car still started but I forgot to see if the system was still on.
 

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Discussion Starter · #10 ·
Yes sorry. Parallel. Can you disconnect the rear battery from disconnecting something from the front? I did disconnect the fuse and the car and system still turn on. Also tried disconnecting the negative terminal from the battery under the hood and car still started but I forgot to see if the system was still on.
Sorry I ask cause I can’t get to the rear battery. I have no idea what I’m doing and am scared I’ll mess something up with all the wiring.
Luggage and bags Bag Automotive design Grey Outdoor shoe
 

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Post a pic of the battery under the hood.

I am willing to bet the alternator is just not sensing the need for a charge due to the extra batt.

BTW when you initially start a car the starter is going to cause a voltage drop.

Maybe what you are trying to convey is the alternator used to consistently keep the voltage at 14vdc. Then after the extra batt was installed you now see 12vdc stable. I say ride it out for a bit and wait till the alternator kicks in. In all seriousness by the looks of your setup you would of been just fine nixing whatever start bat under the hood you have and upgrading to a great AGM.
 

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I hope you didn't spend too much on the install there... that's... well, pretty pathetic. They've just piled everything into a tangle behind the amp. When a pro is doing an install, they're supposed to keep everything nice and neat - that's what you are paying them for (*)... I worry about what's hidden under the panel...

OK, so the second battery is likely tucked under the factory trim panel there that the amp & wiring tangle is screwed down to; next to the spare tire, jack, and other bits and pieces. That's where you would disconnect it to really isolate the issue.

Depending on how, exactly, it's wired in - you might be able to disconnect the fuse in the front that was part of the aftermarket stereo install; but in order to find out, you'll have to access the battery in the trunk to see how it's wired, at which point you might as well disconnect it there and take your measurements.

Those factory panels usually just pop into place; gentle pressure with your fingers or a plastic panel tool (looks like a putty knife or spatula, but it's not metal) will usually get them to pop loose. Occasionally there's a screw or a clip that needs to be popped up before it'll come loose - but it's usually pretty obvious what's there. Putting it back is just a matter of sliding the clips back into the slot that they popped out of, and applying a little bit of pressure with your hand.

There's lots of videos of people doing this sort of stuff on YouTube; and there might be a video on your specific car. GM, Ford, Toyota, Honda, etc -- they all use the same basic strategies for attaching interior panels.

Generally speaking - you should figure out how to access the new battery; as they do (even semi-sealed AGM batteries) require occasional maintenance & checking (not as often as your engine oil, but taking a peek at it every 6mo to 1yr is a good idea).

(*) I give amateurs - especially newbies - a bit more leeway with this stuff, because, well - they're not pros.
 

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Hi everyone. I just had a second battery installed in series and I noticed that my voltage now drops to from 14.7 to 12.5-12.7. Whenever I start my car it does start there and then drops within a few minutes. This drop happens regardless if I’m playing my system or not. The shop said it was the alternator which is a brand new ho 320 amp one. I had it tested at numerous spots and they said that the alternator is ok. This has been going on for a couple weeks now and my car is still running. If it was the alternator, wouldn’t the batteries have died by now?
No matter how many batteries are connected together in parallel, voltage should only ever be as high as 14.7 while the car is running. 12.5 - 12.7 is pretty normal for one or more fully charged batteries at-rest (i.e., no charge or load for at least a couple/few hours).

Could it be that what you're describing is fully charged batteries resting at 12.5 - 12.7 until you start your car and your alternator instantly brings them up to 14.7; and then after a couple minutes the voltage regulator puts the batteries into a 12.7 float cycle? I'm not up on modern automotive charge/voltage regulators, but this seems at least plausible and would explain why your batteries haven't died.
 

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No matter how many batteries are connected together in parallel, voltage should only ever be as high as 14.7 while the car is running. 12.5 - 12.7 is pretty normal for one or more fully charged batteries at-rest (i.e., no charge or load for at least a couple/few hours).

Could it be that what you're describing is fully charged batteries resting at 12.5 - 12.7 until you start your car and your alternator instantly brings them up to 14.7; and then after a couple minutes the voltage regulator puts the batteries into a 12.7 float cycle? I'm not up on modern automotive charge/voltage regulators, but this seems at least plausible and would explain why your batteries haven't died.
Goggle gm charge cycle. Mind blowing since your not up to date. Like 7 states of charge. Even a iffy high voltage desolation mode iirc.

Here ya go. It's a good read to get up to speed. 🍻

 

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Discussion Starter · #15 ·
Post a pic of the battery under the hood.

I am willing to bet the alternator is just not sensing the need for a charge due to the extra batt.

BTW when you initially start a car the starter is going to cause a voltage drop.

Maybe what you are trying to convey is the alternator used to consistently keep the voltage at 14vdc. Then after the extra batt was installed you now see 12vdc stable. I say ride it out for a bit and wait till the alternator kicks in. In all seriousness by the looks of your setup you would of been just fine nixing whatever start bat under the hood you have and upgrading to a great AGM.
Yeah I think that’s what I’m trying to say lol. Again apologies for my total lack of knowledge on this. I put a brand new yellow top under the hood. So is that a normal thing for the voltage to be “12vdc stable” after installing the second battery?
 

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Discussion Starter · #16 ·
No matter how many batteries are connected together in parallel, voltage should only ever be as high as 14.7 while the car is running. 12.5 - 12.7 is pretty normal for one or more fully charged batteries at-rest (i.e., no charge or load for at least a couple/few hours).

Could it be that what you're describing is fully charged batteries resting at 12.5 - 12.7 until you start your car and your alternator instantly brings them up to 14.7; and then after a couple minutes the voltage regulator puts the batteries into a 12.7 float cycle? I'm not up on modern automotive charge/voltage regulators, but this seems at least plausible and would explain why your batteries haven't died.
Yes that’s exactly what is happening! Is this normal?
 

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Discussion Starter · #17 ·
Goggle gm charge cycle. Mind blowing since your not up to date. Like 7 states of charge. Even a iffy high voltage desolation mode iirc.

Here ya go. It's a good read to get up to speed. 🍻

I will definitely read this. Thank you
 

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Discussion Starter · #18 ·
I hope you didn't spend too much on the install there... that's... well, pretty pathetic. They've just piled everything into a tangle behind the amp. When a pro is doing an install, they're supposed to keep everything nice and neat - that's what you are paying them for (*)... I worry about what's hidden under the panel...

OK, so the second battery is likely tucked under the factory trim panel there that the amp & wiring tangle is screwed down to; next to the spare tire, jack, and other bits and pieces. That's where you would disconnect it to really isolate the issue.

Depending on how, exactly, it's wired in - you might be able to disconnect the fuse in the front that was part of the aftermarket stereo install; but in order to find out, you'll have to access the battery in the trunk to see how it's wired, at which point you might as well disconnect it there and take your measurements.

Those factory panels usually just pop into place; gentle pressure with your fingers or a plastic panel tool (looks like a putty knife or spatula, but it's not metal) will usually get them to pop loose. Occasionally there's a screw or a clip that needs to be popped up before it'll come loose - but it's usually pretty obvious what's there. Putting it back is just a matter of sliding the clips back into the slot that they popped out of, and applying a little bit of pressure with your hand.

There's lots of videos of people doing this sort of stuff on YouTube; and there might be a video on your specific car. GM, Ford, Toyota, Honda, etc -- they all use the same basic strategies for attaching interior panels.

Generally speaking - you should figure out how to access the new battery; as they do (even semi-sealed AGM batteries) require occasional maintenance & checking (not as often as your engine oil, but taking a peek at it every 6mo to 1yr is a good idea).

(*) I give amateurs - especially newbies - a bit more leeway with this stuff, because, well - they're not pros.
The battery is literally right behind that mess. And thank you for pointing out that it’s pathetic. At first I liked it but now after you pointing this out, I’m kinda pissed. I can see a portion of it and I can see the fuse hooked up to it. And since I’m such a newb, I don’t think imma touch this mess lol. I will surely mess something up. I’m gonna take some more pics for you guys too.
 
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