Owners with AGM battery -- what is the typical max voltage (after resting)?

[alank2]

HF has a handy battery tester that is compatible with AGM batteries and will give a SOC as well as a health rating:

https://www.harborfreight.com/12v-digital-battery-and-system-tester-58759.html

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[sajohnson]

the one above is basicly if you shut the car off and check the voltage, before any rest period. with all the stuff going on in todays vehicles thats usaly what your going to get unless you take the time to disconect the battery and let it settle for a couple hours then measure it

Good point about the additional loads in modern vehicles.

What we're in the habit of doing is unlatching the hood when returning home. That way the battery can be checked and a BT connected without opening the driver's door and waking everything up.

Of course, even then the current draw never drops to zero. Typical parasitic draw is <50mA (no more than 85mA). While that will eventually kill the battery, there should be little effect on voltage in the ~2 hour rest period between discontinuing a charge (either a long drive or BT) and checking the voltage.

'Open circuit voltage' is what you mentioned -- the measurement with the battery disconnected for a couple hours. That would be the most accurate, but a ~50mA draw for 2 hours won't make much difference.

If the voltage is measured immediately after turning the engine off, the reading may be very high, >13 volts, since the alternator can put out ~14.4 - 14.6V. Then after resting for a couple hours, it will be something less -- ideally close to 13V, with the parasitic draw of ~50mA or less.

One last note, the OE FLA battery in our BL was always close to 12.67V (perfect) -- with the parasitic load. It's as if the charging system is unaware the recall replacement is an AGM. That said, BT does not have a separate setting for AGM batteries, they are combined with FLA.

 

[sajohnson]

HF has a handy battery tester that is compatible with AGM batteries and will give a SOC as well as a health rating:

https://www.harborfreight.com/12v-digital-battery-and-system-tester-58759.html

Thanks!

I looked into these testers (not this specific one) months ago and the consensus -- from techs who were not selling anything -- was that a decent (reliable, accurate) one cost a minimum of several hundred dollars. The ones many shops use cost thousands.

With most products there is a range of prices. Usually the best bang for the buck (aka, the 'point of diminishing returns') is somewhere in the middle. As a general rule of thumb, the price of the best value product might be 2-4+ times the cost of the least expensive item. In this case the multiplier seems even higher.

I ended up deciding that if I bought a digital battery tester I'd want a decent one, but since I don't have a shop I couldn't justify spending $400-$500+.

Maybe there are less expensive testers that work well. It's been a while since I researched them. At the time the reviews were all over the map. One thing to keep in mind is that a user has no way of knowing whether the tester is accurate, without having access to a known good tester to compare it to.

 

[sajohnson]

A bit more than 'a little while'...

I just checked and the battery measured 12.63V.

I went ahead and connected a different BT -- same exact model, just a different unit to see if that makes any difference in the final charged voltage.

Just before disconnecting the BT the voltage was 13.13V, typical.

After disconnecting the BT the voltage dropped quickly to ~13V, then continued to decrease more slowly. After a couple minutes it was down to 12.94V and still slowly dropping. That's with everything off (no doors opened).

I'll check it again in a little while.

 

[sajohnson]

Just before disconnecting the BT the voltage was 13.13V, typical.

After disconnecting the BT the voltage dropped quickly to ~13V, then continued to decrease more slowly. After a couple minutes it was down to 12.94V and still slowly dropping. That's with everything off (no doors opened).

I'll check it again in a little while.

It had dropped from 12.94V yesterday to 12.75V this afternoon.

We took it for a short 15 mile r/t with 3 stops and it's at 12.70.

 

[sajohnson]

Wasn't there a forscan parameter that determines what SOC maximum that the BMS will charge to?

I'm gonna look into this, because routinely charging to just ~80% is not good for the battery.

That begs the question, why does Ford (and other mfrs) design the charging system that way? Is is to squeeze out a bit better mileage?

FLA and AGM batteries prefer to be kept at 100% SOC as much as possible.

 

[DWG]

Curious as to why only 80% too. a Google search for Ford vehicles said this:

Ford targets an 80% state-of-charge for AGM batteries through its Battery Management System primarily to save fuel via smart alternator control, leave a buffer for regenerative braking, and reduce the risk of overcharging the battery during normal cruising.

Reasons for the 80% Charge Target

• Fuel Economy: Running the alternator at lower output during acceleration and cruising reduces engine load, which slightly improves miles per gallon.
• Regenerative Braking Buffer: Keeping the battery under 100% leaves vital reserve capacity to absorb electrical energy when the vehicle decelerates or brakes.
• Smart Charging Logic: Ford’s software uses conventional charging only when the battery dips below roughly 80%, shifting to variable smart charging above that threshold to prevent excessive gassing and overcharge wear.

A search for all manufacturers resulted in a similar response.

I have a good relationship with my Ford dealer service department so I bring my cars to my Ford deaIer for all of my maintenance and repairs. Regarding the battery, for many years now I have been replacing the battery every three years hopefully to avoid a dead battery situation. The Ford batteries are warrantied for 3 years and they seem to fail right around that time.

 

[incavulator]

Curious as to why only 80% too. a Google search for Ford vehicles said this:

Ford targets an 80% state-of-charge for AGM batteries through its Battery Management System primarily to save fuel via smart alternator control, leave a buffer for regenerative braking, and reduce the risk of overcharging the battery during normal cruising.

Reasons for the 80% Charge Target

• Fuel Economy: Running the alternator at lower output during acceleration and cruising reduces engine load, which slightly improves miles per gallon.
• Regenerative Braking Buffer: Keeping the battery under 100% leaves vital reserve capacity to absorb electrical energy when the vehicle decelerates or brakes.
• Smart Charging Logic: Ford’s software uses conventional charging only when the battery dips below roughly 80%, shifting to variable smart charging above that threshold to prevent excessive gassing and overcharge wear.

A search for all manufacturers resulted in a similar response.

I have a good relationship with my Ford dealer service department so I bring my cars to my Ford deaIer for all of my maintenance and repairs. Regarding the battery, for many years now I have been replacing the battery every three years hopefully to avoid a dead battery situation. The Ford batteries are warrantied for 3 years and they seem to fail right around that time.

The AI comment on regenerative braking does not apply to the BS since it is not a hybrid, but the rest of the info is pretty solid.

Vehicles with IBS sensors (intelligent battery sensors) monitor battery temp, voltage, current, etc. and use proprietary algorithms to calculate SOC (and sometimes SOH). The data from the IBS is used in conjunction with the smart alternator to decide when and how much to charge the battery. This is done for two primary reasons. 1) To maximize fuel economy (as stated in the last post), and 2) To reduce battery warranty by modulating the charging of the battery when it is "best" for the battery (e.g. not too hot, not too cold, , as a function of SOC, etc.).

I'm not sure how they arbitrate conflicts between actions that would increase fuel economy and those that would improve battery life. Fuel economy is mandated and there are financial penalties, and poor battery management results in increased battery warranty costs for the manufacturers.

 

[sajohnson]

Curious as to why only 80% too. a Google search for Ford vehicles said this:

Ford targets an 80% state-of-charge for AGM batteries through its Battery Management System primarily to save fuel via smart alternator control, leave a buffer for regenerative braking, and reduce the risk of overcharging the battery during normal cruising.

Reasons for the 80% Charge Target

• Fuel Economy: Running the alternator at lower output during acceleration and cruising reduces engine load, which slightly improves miles per gallon.
• Regenerative Braking Buffer: Keeping the battery under 100% leaves vital reserve capacity to absorb electrical energy when the vehicle decelerates or brakes.
• Smart Charging Logic: Ford’s software uses conventional charging only when the battery dips below roughly 80%, shifting to variable smart charging above that threshold to prevent excessive gassing and overcharge wear.

A search for all manufacturers resulted in a similar response.

I have a good relationship with my Ford dealer service department so I bring my cars to my Ford deaIer for all of my maintenance and repairs. Regarding the battery, for many years now I have been replacing the battery every three years hopefully to avoid a dead battery situation. The Ford batteries are warrantied for 3 years and they seem to fail right around that time.

As incavulator said, "The AI comment on regenerative braking does not apply to the BS since it is not a hybrid, but the rest of the info is pretty solid."

I suspected fuel economy was a factor.

One question I have is why the OE FLA battery was always charged to 100% SOC -- 12.67 > 12.7V? 80% SOC for a FLA battery is ~12.42V. It was never that low (after driving a reasonable distance).

I can just rely on the BT to top the battery off, but intentionally designing a charging system to only charge to 80% SOC seems like false economy. IDK, I guess we'd have to know how much the battery's service life is shortened vs how much fuel is saved.

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