MHEV 12V battery supply system

[EngineerNewbie]

Hi guys. I'm new to hybrids and was wondering why the 12V car battery in MHEVs doesn't need a smart battery system, and it's ok to always charge the battery 100%.In ICE, conventional cars the battery is maintained at 80%, but don't know for mild hybrids is ok 100%.
Thanks in advance!

 

[BrianPetersen]

I'm not sure what you mean. The normal charging system in a normal car charges a normal sealed flooded lead-acid battery to 14-and-change volts whenever the engine is running. Electric vehicles that use a power supply to supply the (nominal) 12-volt system do exactly the same.

 

[EngineerNewbie]

Mild hybrid cars use a dcdc converter in order to supply power from 48v to 12v battery.

 

[GregLocock]

You've made two statements

1) Conventional cars only charge their 12V batteries to 80%
2) MHEVs use DC-DC converters and charge their 12V battery to 100%

Could you find any evidence that that is the case? 80% charge is only 12.42V which seems a bit on the low side for a lead acid, and even 100% resting is only 12.8V. Most alternators are set to >13.0 V



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[EngineerNewbie]

This I don't know. I think they recharge with more than 12V, getting energy from dcdc.
I was thinking at 100% because gets the energy directly from other source, so no need for battery energy management, so every time it charges to 100%.
Does this makes sense?

 

[GregLocock]

You don't seem to know much about either ICE charging systems or DC-DC converters or lead acids. Your original question was based on two statements which may or may not be true. Do you actually have a point?

"I think they recharge with more than 12V"

I'm sure they do. Charging a lead acid at 12.0V would be ineffective.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[EngineerNewbie]

Yes, this is true. I'm new to hybrids and studied metallurgy in the past. If a had more knowledge, I wouldn't ask for help here. Can you help me with this topic or are you just showing off here?

 

[BrianPetersen]

Well, it's unclear what you are asking, and what statements made by others you are accepting as gospel and what you are rejecting.

The statement that a conventional car only charges the battery to 80% is flat-out wrong. With modern batteries (sealed, maintenance-free, flooded lead acid) all normal charging systems maintain 14.0 - 14.4 volts to fully charge the battery.

You introduced an acronym "MHEV" without explaining what it means. I am going to ASSume that this means "mild hybrid electric vehicle" in which case it is entirely possible that there is still a conventional alternator, although it might rely on the high-voltage charging system and a DC-to-DC converter to maintain the 12-volt system charged.

If it uses a DC-to-DC converter from the high voltage hybrid battery to the 12-volt system then you can be assured that said DC-to-DC converter will maintain 14.0 - 14.4 volts charging voltage for the nominal 12-volt system just like a conventional charging system does.

It's also unclear what you mean by a "smart" charging system, because most of them are pretty dumb. If the engine is running, it maintains 14.0 - 14.4 volts. Some of them apply SOME logic to try to make better use of coasting and deceleration by regulating to a slightly higher voltage under those conditions and a slightly lower voltage the rest of the time (at a cost of potentially not having the 12-volt battery charged at the time of stopping the engine). But that's about as "smart" as they get.

I don't see any reason why a mild-hybrid + DC-to-DC converter based charging system would work ANY differently.

Now, ask the specific question that you have.

 

[EngineerNewbie]

Why does a mild hybrid electric car uses a conventional charging system of the 12V supply system(meaning a 100% charging of the battery), and not a smart system in order to manage the charge and discharge of the battery?

 

[BrianPetersen]

Again I'm unclear what you really mean by a "smart" system.

When someone switches the car off and walks away from it, the car has no knowledge of whether that owner is going to return to the car the next day to drive to work, or is leaving it in an airport parking lot with an expectation that the car will be ready to go upon their return a month from now. If the 12-volt battery wasn't fully charged, it might be fully DIScharged by that time, in which case ... no keyless entry, and if it uses a conventional 12-volt starting motor for cold starting ... no start!

It's also impossible for the car to predict the end of a journey. If it doesn't keep the battery fully charged, that journey might end at a time when it isn't ... and then. see above!

The logical countermeasure is to keep the 12-volt battery fully charged all the time when the car is running, which is the same as what a conventional charging system does!

I believe the Hyundai Ioniq PHEV is the first one to dispense with the 12-volt battery and rely fully on the car's DC-to-DC converter (probably with a small capacitor). It is a prerequisite for this, that the starting motor used for hybrid start-stop operation also be strong enough for cold-engine start under worst-case winter conditions.

Even a Tesla (full electric vehicle) has a conventional 12-volt battery for operating all the "usual car stuff" ...

 

[EngineerNewbie]

I'm talking about the BMS(battery management system) on the battery.
What you said there is correct, you have more experience in this than I do. I only know some of this stuff for more than a week now, so not pretty sure how the wholse system works.