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Old 11-05-2018, 06:02 PM   #1 (permalink)
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Default Parasitic discharge of 12v battery

I recently fitted a new 12v battery as the old one was about 9 years old and not holding charge.
On installation, the voltage was showing 12.8v on my plug-in voltmeter and went up to 12.9 after a 40 mile drive.
The car then stood for 4 days and when I checked the voltage again it was down to 12.1v - 50% charged?
The car alarm was off and interior lights all off. Car is completely standard except for a grid charger harness and cooling fan wiring.
I doubt the battery is at fault so where is the power going?
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Old 11-05-2018, 08:10 PM   #2 (permalink)
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When were the ground last replaced?
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Old 11-05-2018, 11:51 PM   #3 (permalink)
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I too have noticed that the Gen1 seems to have a modest level of parasitic discharge. On top of that, the dc-dc converter doesn't always leave the battery fully charged. There have been threads on that subject. The results are that the battery can get pretty low after a week or so.

I keep meaning to measure the parasitic loss. It shouldn't be hard to do. Just hook an ammeter in the ground side of the battery to find the total. A guess one would have to make a fuse adapter and plug the ammeter into various circuits to figure out where the actual source of the drain really is. The ECM would probably be a candidate.

Also, it takes a while for a flooded battery to reach rest voltage after use. Since you had been charging while driving, you probably didn't actually have 12.9 when you shut down. If you had given it a couple of hours, you may have had only 12.6-12.7V. Just the way flooded batteries work.
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Last edited by jime; 11-06-2018 at 12:01 AM.
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Old 11-06-2018, 05:58 PM   #4 (permalink)
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Thanks for your replies.

The ground straps are the originals and they look fine, or at least they are not broken. I have read that there may be high resistance in otherwise sound looking cables so I will check this out.

Jime, You make some good points about "rested" voltage and I like the idea of a fuse adapter to measure current drain.

The dc-dc converter works in mysterious ways. When I start the car it charges at a steady 13.9v at fast idle speed and maintains this charge rate at normal idle for as long as I leave it running.
But when I drive off the voltage drops to 12.2v and stays there for about 50-70% of my journey.

I have searched the forum and there is conflicting advice regarding the dc-dc converter's output. Some say it's constant and others say it's varied, it's powered by the IMA battery or it takes power from the IMA motor/generator.

A bit confusing, but it works!
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Old 11-07-2018, 02:15 AM   #5 (permalink)
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DCDC output voltage is controlled (only) by the following three devices:
-engine temperature sensor, which varies output voltage slightly based on how hot the engine (and by extension the battery) is.

-ECM, which CANNOT disable the DCDC, but can only change it's output voltage to either 12.x volts or 14.x volts (where x is based on engine temperature). You can prevent the ECM from dropping down to 12.x volts by cutting the WHT/GRN wire that goes to the DCDC. You don't need to do anything else but put tape over the cut leads.

-MCM, which CAN disable the DCDC, or enable it if the IMA is in good shape. You can prevent the MCM from disabling the DCDC by cutting the GRN/BLK wire that goes to the DCDC. Again, you don't need to do anything else but put tape over the cut leads.

...

My recommendation is to cut the WHT/GRN wire only, which will still allow the MCM to disable the DCDC if something bad happens to the battery.

I have a more detailed thread somewhere on this forum.
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Old 11-07-2018, 04:46 PM   #6 (permalink)
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Quote:
Originally Posted by mudder View Post
DCDC output voltage is controlled (only) by the following three devices:
-engine temperature sensor, which varies output voltage slightly based on how hot the engine (and by extension the battery) is.

-ECM, which CANNOT disable the DCDC, but can only change it's output voltage to either 12.x volts or 14.x volts (where x is based on engine temperature). You can prevent the ECM from dropping down to 12.x volts by cutting the WHT/GRN wire that goes to the DCDC. You don't need to do anything else but put tape over the cut leads.

-MCM, which CAN disable the DCDC, or enable it if the IMA is in good shape. You can prevent the MCM from disabling the DCDC by cutting the GRN/BLK wire that goes to the DCDC. Again, you don't need to do anything else but put tape over the cut leads.

...

My recommendation is to cut the WHT/GRN wire only, which will still allow the MCM to disable the DCDC if something bad happens to the battery.

I have a more detailed thread somewhere on this forum.
After grid charging, the dc-dc converter does seem to charge more frequently so it looks like my ancient IMA battery is causing the problem.
I'm not too keen on cutting wires but if that's what it takes....
Thanks for the info.
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Old 11-08-2018, 02:32 AM   #7 (permalink)
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You can just always remember to drive with your headlights ON - I've never seen the DC-DC drop into low power mode (~12.2V) with the headlights ON. Otherwise, it can go into low power mode quite frequently and for long durations - like almost all the time except for when you're regen-ing...
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Old 11-08-2018, 10:30 AM   #8 (permalink)
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Quote:
Originally Posted by eq1 View Post
Otherwise, [the DCDC] can go into low power mode quite frequently and for long durations - like almost all the time except for when you're regen-ing...
This is why I cut the WHT/GRN wire (which disables the 12.x output mode). I've had that modification in place for maybe two years and I haven't had any problems. Others who cut the WHT/GRN reported a drop in fuel economy, but mine hasn't changed (granted, I rarely average above 70 mpg).
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Old 11-08-2018, 07:47 PM   #9 (permalink)
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That mod to keep 12v battery charged thread is here: https://www.insightcentral.net/forum...y-charged.html
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Old 11-10-2018, 06:12 PM   #10 (permalink)
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Quote:
Originally Posted by Foos View Post
That mod to keep 12v battery charged thread is here: https://www.insightcentral.net/forum...y-charged.html
Thanks for the link.
The switched option might be best as it allows you to choose between float charge or 'economy' mode depending on the distance of the journey and how long the car has been standing.
Interesting thread.
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