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XC60 Battery Replacement, State of Charge (SOC), Alternator Control

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jgibbs2273
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Joined: Sun Aug 13, 2017 9:51 am
Year and Model: 2012
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XC60 Battery Replacement, State of Charge (SOC), Alternator Control

Post by jgibbs2273 » Wed Sep 20, 2017 8:28 pm

I was not aware how difficult it is to replace the battery on my 2012 XC60. Normally, I would go to big box retailer or local auto shop but I've found those batteries do not meet Volvo's specs. From the Owners manual, the requirements are;

Cold Start Capacity (A): 520-700
Reserve capacity (min): 100-135

Most of the available replacement batteries either have CCA greater than 700 or an insufficient reserve capacity. In America, battery capacity is typically specified in reserve capacity where as in Europe it is measured in Ah. The battery monitoring system looks for "H6 70Ah 600 Cold Cranking Ampere" and measures the "State of Charge" as a percentage of the actual Ah. Reserve capacity is not related to Ah by any algorithm so it is difficult to know if a particular reserve capacity meets or exceeds the required 70Ah.

Where all this matters is that the alternator control algorithm changes when the battery state of charge is above 80%. Above 80%, the alternator control algorithm can be fast or slow regeneration. Regeneration uses engine breaking to charge the battery via the direct geared alternator (it is not belt driven) and increases mpg by running alternator at a lower voltage. However, this only happens when battery SOC is above 80% or 56 Ah (.8*70Ah). During regeneration, battery charge voltage is slightly above battery running voltage (12.3V) and well below 14.2V typical conventional charging voltage.

For a battery with 615 CCA and 95 min reserve capacity (slightly below the OEM requirement), the measured battery state of charge for a new battery (reset with the VIDA tool) is only 74% and uses "conventional charging" instead of the regeneration modes. After buying a brand new battery, I would have expected to get access to all the energy savings possible on this vehicle.

Most battery manufacturers list their batteries as direct fit, not direct OE replacement. I'm wondering if I should have purchased a volvo replacement battery which was only $20 more.



jimmy57
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Re: XC60 Battery Replacement, State of Charge (SOC), Alternator Control

Post by jimmy57 » Thu Sep 21, 2017 7:49 am

Having more CCA is not a problem. Battery selection by a car manufacturer is cost and weight vs need. The extra pounds can make the vehicle get into a next incremental class for EPA MPG and emissions testing and that can make MPG rating go down. A higher capacity battery will have a lesser drain to do the same task and will live longer as recharge percent is another factor of lifespan.
If you put a voltmeter on the car and drive you will see the 14.2V mentioned is exceeded every time you coast after the initial start charge cycle is done (10 minutes or so depending on several variables I'm sure). The algorithm may be optimized for 600 CCA/70Ah but the battery monitor sensor is the REAL determinant. The SOC is driven by current absorption measured by amps in and resulting voltage. That will not be very different with a slight increase in battery capacity.



jgibbs2273
Posts: 5
Joined: Sun Aug 13, 2017 9:51 am
Year and Model: 2012
Location: Virginia
United States of America

Re: XC60 Battery Replacement, State of Charge (SOC), Alternator Control

Post by jgibbs2273 » Thu Nov 16, 2017 6:52 pm

The battery monitoring system always assumes the battery is 70 Ah capacity. During operation, the BMS senses the current into or out of the battery and increases or decreases the SOC assuming the battery is 70 Ah. It does not dynamically measure the actual capacity of the battery.

The initial SOC is reset based on the battery voltage after the car sits overnight. My battery measured 12.53V at 75 degrees F after sitting overnight and this corresponded to 73% battery capacity. A 100% battery would be closer to 12.63V. To measure the voltage, I left the hood open and the battery cover removed so I could measure the voltage in the morning before I opened the car door. The battery was fully charged since the charge current quickly dropped below 3 amps on my way to work and was under 1 amp by the time I arrived.

It stills remains a challenge to buy a replacement battery since many battery manufacturers do not specify Ah. They may specify reserve capacity but this is only loosely correlated to Ah. From what I could tell, in the absence of an Ah specification, a heavier battery weight corresponds with higher battery capacity. CCA is not particularly useful for modern engines that start the first time. It is easy to design a battery with high CCA but much harder to design one with a good capacity rating.



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