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Discussion Starter · #1 ·
Hi everyone,
I've owned my (2000 MT) car for about 5 years. I just replaced my 12v battery. This is the 3rd battery for this car. The previous owner had just replaced the battery prior to sale. I replaced that battery in 2018. My "new" battery I just replaced tonight. I always keep it on a battery tender when I know I'm not going to drive for more than a week or two. Just a habit for all of my vehicles.

My IMA battery has been removed. I'm still using all of the electronics to maintain the 14.1v to the small battery. I have a digital display that monitors the battery. It matches the OBD readout on my cell phone.

The electrical charging system seems to work normally. Any rpm's above 4200 pops the battery light on for about a minute then 14.1v comes right back. i can hear the relay kicking in and out during hi and low rpm's. The only odd thing is I can hear the electronics make a weird "electrical" noise during charging. I've read this is normal. Just an odd sound.

Any opinions that the way the electronics maintains the small battery actually slowly damages it?

Thanks!
 

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All sounds normal to me for a non IMA car.

You are using the 12V much more than an insight normally would.
So you need decent quality battery for a start, with perfect engine earth leads etc. .

Have you done the green/white wire DC-DC cut mod. That will help a bit.
Details are on here.
 

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With the IMA bypassed, I think the 12V will end up getting cycled a lot more. Every time you come to a stop, engine speed will be too slow to uphold DCDC output. Every time you go high RPM, DCDC will disable, as you've seen. These are times that 12V system power will be coming only from the battery - that wouldn't be happening most of the time were your IMA not bypassed. Once you get going again, the battery will then charge, until the next stop or next high RPM event, etc.

Not too familiar with lead acid's ability to cycle, but my understanding is that typical automotive lead acid isn't good at it. Even deep cycle lead acid doesn't seem that good, say, compared to lithium or NiMH. I used a deep cycle battery with a 12V inverter in a camper-like situation for a while, and the battery didn't last nearly as long as I thought it would/should.

BTW, pretty sure DCDC voltage as measured at battery terminals is only 13.85V most of the time, not 14.1V... You might want to check out this thread I made about hacking and adjusting DCDC output: Mod to Adjust DCDC Converter Output Voltage

There's probably some useful info and links there, even if you don't want to do the mod.
 

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Discussion Starter · #4 ·
All sounds normal to me for a non IMA car.

You are using the 12V much more than an insight normally would.
So you need decent quality battery for a start, with perfect engine earth leads etc. .

Have you done the green/white wire DC-DC cut mod. That will help a bit.
Details are on here.
With the IMA bypassed, I think the 12V will end up getting cycled a lot more. Every time you come to a stop, engine speed will be too slow to uphold DCDC output. Every time you go high RPM, DCDC will disable, as you've seen. These are times that 12V system power will be coming only from the battery - that wouldn't be happening most of the time were your IMA not bypassed. Once you get going again, the battery will then charge, until the next stop or next high RPM event, etc.

Not too familiar with lead acid's ability to cycle, but my understanding is that typical automotive lead acid isn't good at it. Even deep cycle lead acid doesn't seem that good, say, compared to lithium or NiMH. I used a deep cycle battery with a 12V inverter in a camper-like situation for a while, and the battery didn't last nearly as long as I thought it would/should.

BTW, pretty sure DCDC voltage as measured at battery terminals is only 13.85V most of the time, not 14.1V... You might want to check out this thread I made about hacking and adjusting DCDC output: Mod to Adjust DCDC Converter Output Voltage

There's probably some useful info and links there, even if you don't want to do the mod.
 

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Discussion Starter · #5 ·
I didn't think about the other grounds. Not sure if that would make a difference but its worth a check. Also I have thought of clipping the grn/wht wire but figured the engineers knew what they were doing. I'm concerned about the over voltage at higher rpm's.
I'll closely read up on your links.
Thanks to both of you for your input!
 

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I didn't think about the other grounds. Not sure if that would make a difference but its worth a check.
If by "other grounds" you mean the two below the airbox, rather than just the 12V neg, those have been known to make a big difference in a variety of ways, and they often corrode and break. It's usually one of the first things people around here suggest to look into if there's various electronics problems, especially charging-related...

Also I have thought of clipping the grn/wht wire but figured the engineers knew what they were doing. I'm concerned about the over voltage at higher rpm's.
If these two sentences are a contiguous thought, the WHT/GRN wire doesn't impact high RPM DCDC disable, that would probably be the BLK/GRN (or grn/blk) wire, if anything. The WHT/GRN controls whether DCDC is allowed to drop into a 'low power' mode, where output voltage drops from the normal ~13.85V to ~12.2V under various circumstances (such as low 12V system load, like headlights OFF, in auto-stop, etal)...

I think disconnecting the WHT/GRN at the DCDC connector is probably like mandatory for those going IMA bypass. Otherwise, it seems like the 12V battery could easily be kept perpetually undercharged. My recollection is that lead acid should be kept at least above 50% (and actually, I'm thinking I read they should be kept almost fully charged most of the time), and that 12.2V is below 50%...
 

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IIRR when the lead acid battery is filled, it will be around 70-72% charged.
Battery V: 12.84=100%
12.50=75%
12.18=50%
All % readings are capacity.
 

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^ I had briefly googled for some voltage vs. SoC graphs, saw a few, they all had slightly varying values. I'm looking again... Meanwhile, I found this about cycle life for lead acid - it's awful:

Depth of discharge
Starter battery
Deep-cycle battery
100%
50%
30%​
12–15 cycles
100–120 cycles
130–150 cycles​
150–200 cycles
400–500 cycles
1,000 and more cycles​
Table 3: Cycle performance of starter and deep-cycle batteries. A discharge of 100% refers to a full discharge; 50% is half and 30% is a moderate discharge with 70% remaining.
https://batteryuniversity.com/learn/article/lead_based_batteries

Same site, info about charging lead acid, lots of voltage recommendations with pros and cons: Charging Information For Lead Acid Batteries – Battery University

Here's a table for specific gravity and voltage vs. SoC for starter lead acid:
Approximate
state-of-charge
Average
specific gravity
Open circuit voltage
2V6V8V12V
100%1.2652.106.328.4312.65
75%1.2252.086.228.3012.45
50%1.1902.046.128.1612.24
25%1.1552.016.038.0412.06
0%1.1201.985.957.7211.89
Table 2: BCI standard for SoC estimation of a starter battery with antimony.
Readings are taken at 26°C (78°F) after a 24h rest.
https://batteryuniversity.com/index.php/learn/article/how_to_measure_state_of_charge

So this table says 12.65V for 100%, 12.24V for 50%. But, I recall S Keith, who seemed to know a lot about lead acid, saying that the charge state values, like 50%, are way misleading with lead acid, because you need to keep them charged a lot higher and more frequently than say NiMH or lithium. I think it was the 'sulfation' idea, that letting lead acid drop below say 50% will quickly lead to a lot of sulfation, that you need to charge high to prevent that. This article/page says something along these lines, or the one on charging does, specifying higher charge voltages to reduce sulfation...
 
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