12V AGM battery cycle

[IanN]

Here is an interesting insight to the 12V AGM battery. Vehicle is a 2013 Model S, 85kwh.
This is for sharing purposes only. I love my Tesla, it makes me smile every time I drive it. I have PW2, am inline for a solar roof and a Cyber truck.

The battery is brand new, Tesla Mobile Service was great. Afterward, I installed a cheap Bluetooth battery monitor which displays history on the phone.
(Auto Battery Monitor BM2) from amazon. Installing it is a small challenge with frunk disassembly and tight quarters at the battery.

This plot is over the past 15 days. You can see that the 12V battery discharged and recharged, 32 times in these 15 days. That is a bit more than twice a day. The car was driven and or charged where the lines are squiggly (I did not keep good records).

It is easy to see why the 12V battery is short lived with this many cycles per day.

Charge begins at 12.53V, ends at 13.79V and there are short spikes at the end of charge of about 15.3V.

I may put a trickle charger on the Model S to reduce the number of charge cycles. I do this on my ICE cars as they rarely get driven.

Just thought it was interesting enough to share...Enjoy...

 

[Regaj]

Your 12v AGM is a sealed lead acid type chemistry and is very different from the Lithium-ion HV pack the powers your car. Unlike Lithium chemistries, lead-acid chemistries LIKE full charge. And those aren't "charge cycles" (not complete ones, the kind we usually mean when we discuss them) you're seeing. They're essentially the same thing a trickle charger would do... adding charge current whenever the battery discharges a few percent.

Glad you're enjoying your S! How many miles on it?

 

[IanN]

Nerding out on Battery data...

Some more battery info based upon above charts. Stuff below is for days where the car is just sitting, no driving or charging.

Longest Discharge time was 15.1 hours before recharge
Shortest Discharge time was 3.7 hours before recharge
Charging time is about 2.7 hours each cycle.

Charging:

Charge current is 5 amps average or 0.15C. This is normal for a deep cycle AGM. You can see from the chart that the charger has a 2+ stage charge cycle. There is a third stage that I have never seen before which is the 15.8 volt spike at the end of the charge. It lasts for only 2-4 minutes then the charger shuts off. Anyone know what that is?

Discharging:

DOD (depth of discharge) for each cycle is between 40% and 30%. This is normal for an AGM battery.
(based on the rate of discharge it is closer to 40%).

About 13Ah is being pulled from the battery each cycle with the battery state capacity of 33Ah.
On a slow discharge day that is about 11 Watts average (or 0.03C discharge rate)
On a fast discharge day that is about 46 Watts average (or 0.11C discharge rate)
I am also not sure why some days use 4X the power of others. Any ideas?

Typical Cycle life for a SLA deep cycle AGM battery is:
1500 Cycles at 40%DOD
2400 Cycles at 30%DOD
I got this from various data sheets for solar batteries. They have the best data for cycle life. The data sheets were very similar for cycle life vs DOD.

The 12V life expectancy is 1.9 to 3.1 years, from the measured DOD and cycles per day.

My previous 12V AGM (the one that was just replaced) lasted 26 months. Just about right for the math. However, the battery before that one lasted 4 years (above the calculated life expectancy.)

From my experience with solar batteries and EV conversion from Lead acid to LiFePO4; the DOD is fine and the Charge/Discharge rates are low.
The short the life is due to the cycles per day. For an existing car there is really only 2 ways to fix this increase the battery capacity 2X or 3X. Or switch to LiFePo4 (however the existing charger will not make full use of the battery)

Tesla’s new LiFePO4 12V batteries dramatically increase the cycle life (4X or more) and allowed 80% DOD (decreases cycles per day) but they are much smaller and will have a lower capacity (increases cycles per day).

I will be testing the effect of leaving the car plugged in. Does the 12V stay charged or still cycle...

 

[IanN]

Your 12v AGM is a sealed lead acid type chemistry and is very different from the Lithium-ion HV pack the powers your car. Unlike Lithium chemistries, lead-acid chemistries LIKE full charge. And those aren't "charge cycles" (not complete ones, the kind we usually mean when we discuss them) you're seeing. They're essentially the same thing a trickle charger would do... adding charge current whenever the battery discharges a few percent.

Glad you're enjoying your S! How many miles on it?

I have about 67K miles. Also I like you're quote. I rode motor cycles for 30+ years and the tesla gives just as big a smile. Im gona do more math on the charts.