Model S + other EVs with 12V Lead-Acid starter batteries

franknesss · Sep 10, 2014

I was just wondering how the Model S and EVs keep their 12V batteries charged and prevent them from dying. How long can you NOT drive your EV before you get a dead starter battery like ICE cars. Is there a small alternator or does the 12V Lead-Acid get charged when the EV is charging?

Thanks in advanced

Saghost · Sep 10, 2014

franknesss said:
I was just wondering how the Model S and EVs keep their 12V batteries charged and prevent them from dying. How long can you NOT drive your EV before you get a dead starter battery like ICE cars. Is there a small alternator or does the 12V Lead-Acid get charged when the EV is charging?

Thanks in advanced

Most EVs, including the Model S, use a DC-DC converter to power the 12V accessories and charge the 12V battery from the big battery. It serves the same functions as an alternator, but has no moving parts (except sometimes a cooling fan.)

Some of them, including the Volt (not sure about the Model S) have a separate circuit in the charger module that provides 12V power while the car is charging, and newer Volts also have a "battery tender" mode that checks the state of the 12V battery and charges it periodically when left plugged in and off for extended period.
Walter

franknesss · Sep 10, 2014

Saghost said:
Most EVs, including the Model S, use a DC-DC converter to power the 12V accessories and charge the 12V battery from the big battery. It serves the same functions as an alternator, but has no moving parts (except sometimes a cooling fan.)

Some of them, including the Volt (not sure about the Model S) have a separate circuit in the charger module that provides 12V power while the car is charging, and newer Volts also have a "battery tender" mode that checks the state of the 12V battery and charges it periodically when left plugged in and off for extended period.
Walter

Thanks for that answer!

tezco · Sep 10, 2014

The S wakes up from time-to-time to check on the 12V. If power in the 12V is low, the DC-DC converter is turned on to charge the 12V. When this occurs too often (at the end if the 12V's life span), you will see a message on the dash informing you that the car needs service, (or service may be calling you on your phone).

franknesss · Sep 17, 2014

tezco said:
The S wakes up from time-to-time to check on the 12V. If power in the 12V is low, the DC-DC converter is turned on to charge the 12V. When this occurs too often (at the end if the 12V's life span), you will see a message on the dash informing you that the car needs service, (or service may be calling you on your phone).

So from what it sounds like, having a low charge on your 12V starter battery can have an effect on range, since it only charges when the DC-DC converter is on or has to turn on?

jerry33 · Sep 17, 2014

franknesss said:
So from what it sounds like, having a low charge on your 12V starter battery can have an effect on range, since it only charges when the DC-DC converter is on or has to turn on?

When the Model S is idle, it still wakes up every so often to check and charge the DC battery. The only time it would have a significant effect on the range is when the battery is about at the EOL and it has to be charged a lot. What's not known is how close to EOL does the message appear.

franknesss · Sep 17, 2014

jerry33 said:
When the Model S is idle, it still wakes up every so often to check and charge the DC battery. The only time it would have a significant effect on the range is when the battery is about at the EOL and it has to be charged a lot. What's not known is how close to EOL does the message appear.

Well, from what it seems, that can't be the case either. Typical Lead-Acid batteries don't have a very long shelf life if not constantly charged. So, if the Lead-Acid is receiving it's charge from the DC-DC converter, that means it's drawing it's charge from the main battery, which i think could reduce the overall range. I'm not sure on how the Model S does it's 12V battery management, but I was just wondering if someone had a more definitive answer.

WarpedOne · Sep 18, 2014

Regarding reducing range ... typical 60Ah Lead-Acid battery in a diesel car holds about 60x12V = ~700Wh. That is about 3 miles of range for a 100% charge.
Battery in MS is smaller as it does not have to crank a heavy ICE also "range reduction" for one complete fill-up should be less then 3 miles.

cynix · Sep 18, 2014

Why don't they just use a Li-ion 12V battery?

tom66 · Sep 18, 2014

cynix said:
Why don't they just use a Li-ion 12V battery?

Cost & simplicity.
Another thing to safety test and qualify - additional expense.
Lead acid is fine for this application, eventually we'll probably see LiFePO4 batteries used.

breser · Sep 18, 2014

tom66 said:
Cost & simplicity.
Another thing to safety test and qualify - additional expense.
Lead acid is fine for this application, eventually we'll probably see LiFePO4 batteries used.

Another reason is it probably makes the 12V battery fairly easy to replace with 3rd party service if the car isn't near Tesla service. A dead 12V means you're not going anywhere. If the car needs a unusual battery you may be stuck until you can source that part.

franknesss · Sep 18, 2014

breser said:
Another reason is it probably makes the 12V battery fairly easy to replace with 3rd party service if the car isn't near Tesla service. A dead 12V means you're not going anywhere. If the car needs a unusual battery you may be stuck until you can source that part.

Actually, i was curious if anyone has tried LiFePO4 batteries yet.

spaceballs · Sep 18, 2014

franknesss said:
Actually, i was curious if anyone has tried LiFePO4 batteries yet.

I would connect voltmeter (with min/max function) up and see what is the highest voltage the 12v system charges the lead acid, i.e. connect up and leave running during drive. That will determine if you can use LifePo4 (btw always have BMS for it).

TonyWilliams · Sep 19, 2014

Folks have already put lithium 12 volt batteries in the Nissan LEAF, and I'm sure there are others. It's just very expensive for hopefully longer life and lighter weight.

franknesss · Sep 19, 2014

TonyWilliams said:
Folks have already put lithium 12 volt batteries in the Nissan LEAF, and I'm sure there are others. It's just very expensive for hopefully longer life and lighter weight.

Do you know if they have any results?

TonyWilliams · Sep 20, 2014

franknesss said:
Do you know if they have any results?

My Nissan Leaf Forum View topic - Does anyone have or use a 12v Automotive Lithium battery?

My Nissan Leaf Forum View topic - Replaced 12v Lead with 12v Lithium

Lithium Pros

Total $841.05

wycolo · Sep 20, 2014

> Typical Lead-Acid batteries don't have a very long shelf life if not constantly charged. [frankness]

Not the case. Once a month topping off is all a good quality 12v battery needs when not being used. The problem arises when people forget to do this so they leave a charger connected all the time. Better to use a charger that monitors the voltage and only charges when necessary. The newer micro-processor chargers do that. They also pulse charge which is good for battery health (reduces sulfation etc). Tesla does all these good things on both the MS and Roadster. If you monitor 12v power outlet it is usually between 14.3 and 14.7vdc.
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