Did autozone test my batteries incorrectly?

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T

The motor guy

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So I had some ac work done on my truck like a month or two ago and some genius didn't tighten one of my battery cables enough. Something still doesn't add up here though. The truck in my sig has two 850cc amp batteries and I would think it would still start off of one battery. All it did was click when I turned the key and I had to be jump started. Anyways, I got to thinking that since the batteries are wired up in parallel that you should unhook them and test them individually, which they didn't do. Opinions? I'm glad my 1/0 gauge jumper cables finally came in handy.
 
I always test my batteries individually. Did they perform a load test? What were their test results? Do you have a battery charger? If so then isolate each battery and fully charge. Then test.
 
They should have tested the batteries one at a time. One battery could be going bad and discharging killing the other one.
 
Last time my batteries were going bad the guy just gave me new ones. They couldn't even get the tester to work at first.
 
How old is the batteries? Usually, they last around 3-4 years. Do yourself a favor, go to Harbor Freight and buy a battery load tester around $20. It should help you determine if you need to change your batteries or not. Doing it yourself, you should take the time to do it right like one battery at a time.
 
Originally Posted By: JMJNet
How old is the batteries? Usually, they last around 3-4 years. Do yourself a favor, go to Harbor Freight and buy a battery load tester around $20. It should help you determine if you need to change your batteries or not. Doing it yourself, you should take the time to do it right like one battery at a time.
I won't load test a battery. One almost blew up on me one time and I promised never again.
 
If they truly are parallel connected lead-acids (without a dual battery isolator) then the cells have to match in voltage and essentially condition overall. If not (for example one battery in poorer shape than the other) then the bad battery will drain the good one until the voltages match (which might be too little to start the vehicle). If you have an isolator then the vehicle will charge them individually and the start battery will get priority. A good isolator should allow you to switch both in parallel for starting for extra current. Don't forget to switch back to isolated after the start.
 
Originally Posted By: motor_oil_madman
You haven't lived until you see smoke coming from a battery.
On these I prefer to take the negative cable off one and test individually if possible. Dual 850s usually will say good on a conductance tester even if one is marginal Ive noticed. Maybe the newer ones are better than my old model but I'd think it would give a better idea of how each one is working if you split it. Whats nice is the out-of-vehicle testing mode is more accurate and faster too, so if I want to get really in depth, Id pull the negative off one battery, test it, reinstall and then do the same on the other one. Then none of the electronics lose power during testing either.
 
Last edited:
Originally Posted By: Johnny2Bad
If they truly are parallel connected lead-acids (without a dual battery isolator) then the cells have to match in voltage and essentially condition overall. If not (for example one battery in poorer shape than the other) then the bad battery will drain the good one until the voltages match (which might be too little to start the vehicle). If you have an isolator then the vehicle will charge them individually and the start battery will get priority. A good isolator should allow you to switch both in parallel for starting for extra current. Don't forget to switch back to isolated after the start.
Dpesn't his diesel already use both batteries to start the vehicle? He does not have an isolator from the factory and very few diesel pickups ever do. Its just a dual battery system and both are always functioning together. Good you mention they need to work together and be in similar condition. They should be replaced together even if only one is faulty and hopefully with an identical product.
 
Get a battery load tester . You're always going to need one anyways. Amazon, under $20. It's one of those gotta have tools for anybody who does their own work.
 
Originally Posted By: Johnny2Bad
If they truly are parallel connected lead-acids (without a dual battery isolator) then the cells have to match in voltage and essentially condition overall. If not (for example one battery in poorer shape than the other) then the bad battery will drain the good one until the voltages match (which might be too little to start the vehicle). If you have an isolator then the vehicle will charge them individually and the start battery will get priority. A good isolator should allow you to switch both in parallel for starting for extra current. Don't forget to switch back to isolated after the start.
I guess that explains it then. The battery with the loose cable must have lost charge long enough and then caused it to bring the other battery down with it.
 
If you can, the following is worth doing: 1 Remove one Neg battery cable off a battery so the 2 are not connected - wrap the end in a rag, can't touch anything else. 2 Load test each as-is, note which is weaker. 3 If you have a DMM, measure and record the battery voltage. 4 Charge each battery equally 5 let sit overnight - with batteries isolated form each other. 6 measure battery voltage again, are they real close? (less then .1V apart?) 7 load test now, note weaker battery The problem is that a weaker battery with a lower voltage will draw power from the other until both are low!
 
A 100 amp load test (common) for a specified time to maintain a minimum voltage should not blow up a battery. Starting amps are higher at 150A or more.
 
I was told those cheese grater load testers don't work well. I think i'll bring it back to autozone and disconnect a battery for them and retest them. It sat for a couple days and kind of hesitated for a second before cranking. It just doesn't turn over like it used to. They also tested my alternator and it passed. It showed 13.86 volts which imo is a bit low, but maybe not for just being jump started 20 minutes ago. I thought it's supposed to show in the 14-14.5 range.
 
I brought it back to autozone today and they tested them correctly and came up with one bad battery. So I got both batteries warranteed for free.
 
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