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Old 09-29-2017, 06:03 PM
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Default two batteries parallel - draw?

My wife has a 97 Mustang GT with two batteries. One is in the stock location under the hood, and one is in the trunk for use on a hydraulic pump system that raises the car. The battery in the trunk is connected from the front battery in parallel, and has an electronic device that ensures charging from the alternator without reverse draw towards the front battery. The battery under the hood has a lower cold cranking amperage than the battery in the trunk. My problem is, the front battery that actually starts the car keeps dying on me after a couple days of being parked. I'm wondering if the higher amperage battery int the trunk is draining the lower amperage battery under the hood, or if the issue is something else entirely. We thought it was a faulty battery under the hood last year, so we bought a new battery. It lasted a little while, but eventually died on us. The battery in the trunk hasn't given us much trouble.

Do two car batteries in parallel have to be the same amperage? If the draw is from the factory wiring, how do I isolate what is draining the battery?

Thanks in advance,
Luke

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Old 09-29-2017, 07:41 PM
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Bill Hanlon Bill Hanlon is offline
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Unhook the rear battery while "a couple of days of being parked" occurs. Does the front battery still go down?

If yes, then disconnect the device that prevents the "reverse draw" and try again. Should tell you something.

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Old 09-30-2017, 05:51 AM
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At the time the engine is switched off, if one battery is at a higher voltage than the other, the high voltage battery will try to charge the other battery until the voltages are the same. If you have some sort of one way device in that is supposed to prevent this, then maybe it has failed. The amperage rating or CCA of the battery does not come into it.

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Old 09-30-2017, 07:38 AM
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Quote:
The battery in the trunk is connected from the front battery in parallel, and has an electronic device that ensures charging from the alternator without reverse draw towards the front battery.
Is the alternator only charging the rear battery?
Does it have a separate wire from alternator to the front battery?


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Old 09-30-2017, 05:41 PM
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Quote:
Originally Posted by johnta1 View Post
Is the alternator only charging the rear battery?
Does it have a separate wire from alternator to the front battery?

The alternator charges the battery under the hood like usual. The two batteries are connected in parallel. The trunk battery is connected directly to the front battery clamp. So, the alternator charges the rear battery at the same time.

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Old 09-30-2017, 05:54 PM
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Quote:
Originally Posted by Geoff View Post
At the time the engine is switched off, if one battery is at a higher voltage than the other, the high voltage battery will try to charge the other battery until the voltages are the same. If you have some sort of one way device in that is supposed to prevent this, then maybe it has failed. The amperage rating or CCA of the battery does not come into it.
I charged both batteries fully with a battery charger. They were equal voltage. The battery in the trunk is bigger and has more amperage, but has the same voltage as the front one. The device seems to be working properly. The front battery died while sitting, but the rear battery stayed charged.

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Old 09-30-2017, 06:50 PM
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Quote:
The trunk battery is connected directly to the front battery clamp. So, the alternator charges the rear battery at the same time.
Where does the alternator charging wire connect at?

For such separation, probably should use a 12V power post somewhere.
(like a horn relay or something on firewall)


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Old 09-30-2017, 06:51 PM
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Quote:
The front battery died while sitting, but the rear battery stayed charged.
Probably has a drain on that circuit then.


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Old 10-01-2017, 03:55 AM
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When the batteries are connected in parallel, as the OP says, any current drain will be on both batteries.
The one with the higher amperage rating will maintain a higher a higher voltage for longer, but at the same time, it will try & charge the lower voltage battery. Condition/age of the batteries also comes into this. A can of worms.

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Old 10-01-2017, 08:52 AM
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Sure, but he has a battery in the trunk and one in original place.
(I think)

He also has a device to keep the rear battery from trying to 'charge' the front battery.
(at least supposed to)

The long cable to the back has to have some resistance to it compared to the front one.

If the charging wire is connected at the rear battery, it would be what is determining the charge to the system.
(and where the sensing wire is located also)

With the device on the rear it may keep the front from getting much of a charge?

There are many scenarios probably on what is going on.
Without seeing the whole picture, hard to determine what is happening.


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Old 10-01-2017, 08:58 AM
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I agree that there isn't much info to go on here. I suspect the (electronic device) is a battery isolator. It seems to be working and the problem is probably in the forward original wiring, but without a lot more to go on this is a guessing game. Time to look for battery drain and start pulling fuses.

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Old 10-01-2017, 08:45 PM
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Quote:
Originally Posted by stellar View Post
I agree that there isn't much info to go on here. I suspect the (electronic device) is a battery isolator. It seems to be working and the problem is probably in the forward original wiring, but without a lot more to go on this is a guessing game. Time to look for battery drain and start pulling fuses.
Can you explain how I can use a multi meter to check for the drain, while pulling fuses?

Thanks,
Luke

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Old 10-02-2017, 04:47 AM
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Can you give us a drawing showing the batteries, isolating device, how/where the alt is connected etc.

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Old 10-02-2017, 07:38 AM
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Quote:
Originally Posted by Geoff View Post
Can you give us a drawing showing the batteries, isolating device, how/where the alt is connected etc.
I'm not sure why you are asking where the alternator is connected. It is a factory, stock connection. Nothing special there. Nothing changed from factory.

There is a cable that goes from the positive battery clamp under the hood, to the "isolator" mounted in the trunk, by the rear battery. There, a short cable that goes from the isolator to the positive post of the battery in the trunk. The negative side of the battery in the trunk is grounded to the body of the trunk.




[HOOD BATT]POS-->------------>-------->(ISOLATOR)--POS[BATT TRUNK]



I hope that clears it up for you

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Old 10-03-2017, 03:38 AM
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A factory, stock set up does NOT include two batteries, with one in the trunk, so there IS something special there....

You keep talking about an 'isolator'. Could be many things: a switch, a relay, a large diode.
Which is why I asked for a diagram.....

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Old 10-03-2017, 07:21 AM
stellar stellar is offline
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It is starting to sound like the (isolator) is a large diode. Search the forums for testing for a battery drain, to see how to do it.

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Old 10-03-2017, 12:55 PM
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Battery isolators are usually used in the recreational vehicle market to enable charging more than one battery and blocking discharging from say, the rv side battery when using the vehicle battery for starting etc. Could be as simple as a large diode, but they typically had additional circuitry.

George

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