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Discussion Starter #1
Hello gentlemen,

I ordered an Apex HO alternator, 390 amp for the car, and was planning to add a second battery for reserves. Correct me if I'm wrong, but I understand that one battery will consume the other without an isolator? The 390a should leave me with a constant surplus of available power; am I correct in assuming that the HO alternator size should alleviate that concern? If there is a surplus, does this also remove the need for an isolator? Lastly, any input on battery size?

The equipment is as follows:

Car: 2003 BMW 325xiT (stock alternator was 120a)
Zapco: Z1.600 LX
Tru Technology B-2110
Airlift Performance Air ride system: (Draw from the compressor It's fused at 30a)


Thanks for any insight you can provide.

Mike
 

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1) Correct me if I'm wrong, but I understand that one battery will consume the other without an isolator?
2) The 390a should leave me with a constant surplus of available power; am I correct in assuming that the HO alternator size should alleviate that concern?
3) If there is a surplus, does this also remove the need for an isolator?
4) Lastly, any input on battery size?
1) Yes, but it's not quite that simple. 2 identical batteries of the same age are theoretically identical in electrical properties, so they would not need an isolator. That is an ideal situation that doesn't exist in real world usage, differences in voltage and/or internal resistance will cause one battery to drain the other. The only question is "How much is acceptable?".
2) No, it doesn't matter how much amperage your alternator provides when charging, the battery problem occurs when the alternator isn't charging.
3) See number 2^^
4) Biggest battery you can physically fit.
With a large enough alternator you may not need the extra battery. (Just in case I misunderstood question 2)
 

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Lithium batteries are often used for low current draw.
 

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I just got a Northstar AGM to replace my stock battery. I don't have a spot or inclination to add a second battery, so I decided to replace mine with the best AGM battery I could find. I put an Odyssey Extreme in my Tahoe, but the Northstar was the right choice for now in my Z.

Even with an isolator, I kept going through batteries (regular parts store batteries) in my truck, so I isolated the stereo electrical from the stock electrical, each with their own battery AND alternator. No problems since then.
 

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Even with an isolator, I kept going through batteries (regular parts store batteries) in my truck, so I isolated the stereo electrical from the stock electrical, each with their own battery AND alternator. No problems since then.
I have been thinking about doing that exact same thing, but I haven't had the same problems as you.
 

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Discussion Starter #7
1) Yes, but it's not quite that simple. 2 identical batteries of the same age are theoretically identical in electrical properties, so they would not need an isolator. That is an ideal situation that doesn't exist in real world usage, differences in voltage and/or internal resistance will cause one battery to drain the other. The only question is "How much is acceptable?".
2) No, it doesn't matter how much amperage your alternator provides when charging, the battery problem occurs when the alternator isn't charging.
3) See number 2^^
4) Biggest battery you can physically fit.
With a large enough alternator you may not need the extra battery. (Just in case I misunderstood question 2)
Thanks for the detailed response. So I will be needing the isolator, and with at I can use the battery that best suits my space constraints bigger obviously being better. I appreciate it, sir!
 

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Discussion Starter #8
I just got a Northstar AGM to replace my stock battery. I don't have a spot or inclination to add a second battery, so I decided to replace mine with the best AGM battery I could find. I put an Odyssey Extreme in my Tahoe, but the Northstar was the right choice for now in my Z.

Even with an isolator, I kept going through batteries (regular parts store batteries) in my truck, so I isolated the stereo electrical from the stock electrical, each with their own battery AND alternator. No problems since then.

Thanks for the input. My battery is about a year old and an AGM, but I dont recall its CCA or Ah. Opttima has started making a drop in H6 repacement for my stock battery which I'm thinking of using. I need to compare its values to my current one to see how much better it actually is. I have my air system in my spare tire well and my amp rack floating above that. I'm struggling to find space for an extra battery.
 

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As far as battery goes, I don't know that it really matters beyond terminal/post type and physical size. Measure your spot and get the largest high quality AGM battery you can physically fit in there. And get an outboard charger than can charge an AGM battery properly.

I got this one not too long ago (NOCO Genius 7200), along with an outboard plug to wire in for quickly connecting in the garage with the hood down. I'll just wire it up inside the grill, one in my Tahoe, one in my Z, then pull whichever in the garage I'm not using and let the Noco charge it overnight to top up the AGM battery.

Oh, and Optima isn't your best choice in AGM batteries, if you ask me. Their quality has arguably gone downhill in the last few years, although they aren't trash by any means... just that they put a whole lot more into marketing than any kind of new technology. I did some pretty exhaustive research for myself and decided on Northstar and Odyssey Extreme as the top performers.
 

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You could pass without battery isolator that will also present 0,7-1 V drop (dual battery isolator, solenoid, diodes), with two identical batteries - capacitance and age,... installed near side by side. if it is a daily driven car then it would be probably without problems. If you drive this car only on occasions, then it might be a problem with discharged battery.
 
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