qsaghar

My 2009 BMW 528i has developed a peculiar problem. I can't charge the battery fully using the underhood terminals, and a battery test at those terminals show only 200 CCA from a 900 CCA battery + only 20% charge. However when I test the battery at the trunk terminals, tester readings show 901 CCA and 99% charge. Battery is fairly new, only about 4 months old. All the terminals look clean without any corrosion. I did not register the new battery when I installed it, but I don't think that non-registration would result in such drastically different readings. Can someone tell me if the underhood cables are a direct run from the battery in the trunk or do they go through some kind of junction box?

donpb

Analyzing the battery charge histogram would be incredibly helpful. If you do not register the battery, it has the potential of failing sooner than later. Have you performed a load test?

qsaghar

The battery charge histogram is fairly steady around 14.0 to 14. 2 volts. So charging is not an issue. However when I let the car sit for 10 days, battery charge was low, and I couldn't fully charge it from the underhood terminals. So I connected the charger to the terminals in the trunk and achieved a 99% charge with 901 CCA. At the same time, testing at the underhood terminals shows 20% charge with only 199 CCA !!! High resistance as well.... Obviously there's a defect in the connection between the underhood terminals and battery trunk terminals... I am trying to narrow it down to where it could be... With a fully charged battery, car starts right up, and works well at night with lights and AC on....

donpb

A charge histogram depicts the percentage of charge, not charge voltage (which has to do with alternator output). Load Test your battery, because there's going to be a difference in load between the front terminals and the battery terminals. What is your car actually not doing? Low charge notification or?

qsaghar

I don't have a histogram since I don't have any BMW tools. However at the trunk terminals my battery tester currently reads 899 CCA (battery rated at 900 CCA) at a resistance of 3.9 mohms, while at underhood terminals the same tester reads only 200 CCA at 17.9 mohms. Also when I tried to charge the battery from the underhood terminals, I wasn't able to, due to the high resistance. However the battery charged up within an hour from the trunk terminals. This is why I was asking if the cables going to the underhood terminals are a direct run from the trunk, or if they go through a junction block (which may be corroded). Currently I am not getting any messages on the speedo display, nor any check engine light. When I let the car sit unused for 10 days, a couple of weeks ago, I did get a low battery message, but once I charged the battery from the trunk terminals, it was all good; car runs and starts up without any issue. However I'm still getting a huge discrepancy between the engine compartment terminals and the trunk terminals and I will not be able to successfully trickle charge from the underhood location in the future, therefore I'm trying to solve this problem.

donpb

There are a lot of things connected inline, you'll need to isolate. Load test the battery first.

seanjordan20

Checking the cable off the terminal would be my next move from what info you're replying back with especially without having the correct tools to troubleshoot a BMW (ISTA, INPA). Seems like you've narrowed it down at least a little to proceed to a next step. Has anything changed since you first saw the big discrepancy, If not then trace the cable to see if you see anything is wrong.

gman6262

This might be useful. You have a single cable from a junction point near the battery to the jump start terminal. The jump start terminal is between the battery and the starter. If the car is starting fine then I would be surprised if you have a cable issue through the jump start point.

habbyguy

Seems to me if you can draw almost 200 amps at the under-hood terminal (with the battery tester) you have plenty of connectivity to do a trickle charger (which might only require 1-2 amps of current).

You're going to get very different readings with a full-load battery tester anywhere other than directly at the battery terminals.

What problem is the OP trying to solve here? If the charging current really isn't getting from the under-hood connection to the battery, a simple voltmeter test should confirm that there is less voltage at the battery than the under-hood terminal. Otherwise, he's chasing the wrong problem. If there IS a discrepancy in the voltage between the two, then it's just a matter of measuring across each junction / terminal / cable to find out which one has more than a small fraction of a volt (under charging conditions) across its length / boundaries.