12V Battery Voltage Management

Hello,

I was going over the following page that describes deep cycle batteries and a couple of
things they mention intrigued me:

Deep Cycle Battery FAQ Capacity

A couple of statements they make are:
"

Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%."

"

For longest life, batteries should stay in the green zone."

The "green zone" in the second statement refers to 40% to 80% SOC.

Are the above statements applicable to the Prius 12V battery and if so, is there any voltage management logic employed by Prius to make sure the above are satisfied. My understanding
is that the HV battery will supply a steady 13.6V to charge the 12V battery from the point the car
is in Ready state until switched off. Would it not cause it to run afoul of the above statements?

Thanks.

BUMP TO MAKE SURE THIS STAYS ON TOP OF THE DUPLICATE.

The only 12V battery management that I am aware of for the Gen III is a temperature sensor on the battery that allows the Converter module to charge around 14.2V as long as temp is below a certain setpoint. Above this temp it lowers recharge voltage to about 13.6V.

Now the HV battery has a Battery Management ECU whose main purpose in life is to maintain the SOC between 40% and 80% SOC.

No, the GIII has a proper battery management system. I've seen the voltage as high as 14.1 and as low as 13.4. Haven't driven it long enough to discern a pattern yet.

Keep in mind that there are quite a few different kinds of "deep cycle" batteries. There are pure lead ones, calcium grid ones, lead antimony ones, glass mat (the one Prius has), glass mat pure lead, gel cell, etc. Each has slightly different "float voltage" characteristics for long life.

The Prius battery is a "float type", that is, it isn't used most of the time you're driving, and is kept on float. It never sees high current discharge either. Just a few 1/2 sec. 50 Amp pulses is all it takes to get the car in "ready".

The GII battery enemy was loss of water, either by constant electrolysis or high temperatures. Not sure what the common eventual killer will be for the GIII. Certainly deep cycling of it will shorten its' life, no matter that they call it a "deep cycle" type. It's still hard on it.

Thanks for the info. So, I wouldn't be inadvertently hurting the battery life by putting it on a charger once every three months or so?

I recently checked my 12V and it was measuring 12.53V in the morning before I started it. I checked using a Digital Multimeter directly at the terminals. Since the manual says fully-charged would be 12.6-12.8, I got a CTEK 4.3 charger (new model) and hooked it up for overnight charging. By morning the charger went all the way up into the float mode and since then morning measurements have been around 12.73V. My daily commute is 3.7 miles each way so I suspect the 12V wasn't having enough of charging time from the HV battery.

So I gather from the above replies that the specific type of the 12V we have is designed to be cycled less than 5% or so, or be under constant float charge and still hold up long time. I wonder if the Optima is designed to hold up in this fashion?

A couple of things ....

In your first post you said, prius charges the battery 13.6V not exactly that is the float as someone else pointed out .. the charging is about 14.3-14.5 V and that will stay there for as long as the charging circuit detects "full" charge then it goes down 13.6 or about... but it will be raised again if you put load on the 12V system .. but the MAX current is 4.3A ( if I remember right) so you have to be careful if you use any "external" charger ... most would supply higher current at least in low SOC. Because most of them designed for "normal" sized car batteries. ... And that COULD very easily damage the battery. So you need to check the specs for the charger and if the current is significantly higher then 5A I would not use it.

Second thing ... as you correctly assumed 3.7 miles commute will not be long enough (not in mile but in time. if it takes 20 minutes than you have NO PROBLEM ) to charge even a mildly discharge battery ... But the good news is that you can just leave the car in Ready when you got home, perhaps, and the traction battery will charge it ... in my experience even in cold weather about 20-25 minutes the 12 V battery is charged and charging voltage drops to float level ...

And 12.53 off load voltage is healthy !!! And if you measured right after disconnecting the charger that is NOT the real off load voltage there is some "static" charge there for a while ... you would need to wait an hour minimum to measure ... (if that thing is on a timer OK you got it ...)

cheers

The 12.53 I measured was before I ever charged it externally, after it sat overnight. According to the manual, 12.2-12.4 is half-charged and 12.6-12.8 is fully charged. So I grew concerned and got the charger.

Now for the current, the CTEK 4.3 uses 4.3Amps (steady I think) while the Prius manual says 4.2 Amps or less. I figured 0.1 Amps extra probably would be within tolerances.

I am still wondering/curious if the Optima that's being touted by many as a replacement for the OEM, is designed to hold up to this constant float-charging scenario in the Prius long term.

Anything I read and looked Optima would be waste of your money ..... only gives you a somewhat better performance if you fully discharge the battery (guess by accident) then the Optima will perform better .. BUT with your use it will have the same issue that slowly never re-charged .... Optima claims that it is OK for the prius ... but never really recharging will be the same issue as the OEM.... different issue .... but again this just MY OWN conclusion ... you have to make your own.

cheers

Keep in mind that starting the car DOES NOT drain the battery much. It's the "parasitic drain" of all the cars systems overnight that drains charge the most. So short trips actually have less an effect than on "normal" cars.
Also keep in mind the voltage levels you posted are not -that- low. Measurement accuracy is significant at the levels you measured compared to desired.
I would watch the voltage over a few week time period. If it keeps getting lower each day then you have a problem. If it is steady not so much.
If it keeps getting lower and you want to do something other than driving it for an hour once a week you could get a "Battery Tender". As long as you have someplace to plug it in.

Thanks for the replies so far in regard to my specific situation.

I am still wondering as to how the OEM battery/SOC-management handle the generic case:

Discharge caused by:
a. Parasitic drain
b. The small drain in the morning from starting the HSD.

If this combined discharge is less than 5% of battery capacity, wouldn't such repeated
cycling less than 5% DoD (Depth of Discharge) cause even a normal AGM battery fail prematurely?

Please see the cycles Vs. DoD chart in the below thread as well as the 2 comments from the
Battery page in my first post above.

Optima vs. OEM 28800-21171 | PriusChat

Wouldn't it have been better for the Prius SOC Management (for the 12V) to wait for
a DoD of 10% or more before starting to charge it back up, thus assuring battery longevity?

I am sure there are good reasons/tradeoffs for it not to be done like that, I am
just trying to understand if anyone out there knows and can explain in layman terms if possible.

Thanks.

This is the first time I've heard that. This must be deep cycle battery specific. General car starting batteries are not deep cycle and they do very well with very superficial cycles under 5%. I really don't know whether the prius batery is a deep cycle type (doubt it) and what % discharge is Prius starting cycle (this could be measured easily).

No the OEM battery is NOT a deep cycle type ... I can only give you my experience. Car is almost exactly 3 years old... one battery accident ... something was left on once overnight and in the morning when put it into ready all alarm went on traction battery relay did NOT engage and all kinds of warning lights. I also saw how low the headlight was ... So I knew what must have happened. Opened the hood put a charger on it (with 5A rating which is just a bit higher than the car does it) waited a minute then put the car in Ready ... it worked no problem. Turned off the charger disconnect ALL GOOD.

Car is driven to work every day ... it was left the longest 15 days (trip abroad) (SKS is turned off own driveway). It was a year ago. Car started without any incident next day. If you turn off (if you can) the smart key system (it will turn off after not sure 6 day anyway) 2 or even 3 weeks should be fine. The battery self discharge should be OK for at least a months (unless extreme temps).

I regularly measure the battery (after the car sitting overnight) and it measures 12.4-12.5 which is good .... so I would say the car manages the battery quite well if you drive the car . It stands up at least as well as a regular car battery. IMHO

IMHO, 12.4-12.5V is low for SLA. This would be only 75% SOC for starting battery and lower for sealed: Measuring State-of-charge - Battery University

But, the Optima Yellowtop is marketed as a Deep Cycle Battery, right?

Thanks very informative ...

One problem with prius that when you open any door the silly car immediately energize the breaking system so that puts a "big" discharge right there so I can not really measure undisturbed battery ... but the manual states that 12.6 and above mean fully charged battery ... and suggest the measurement taken several hours after parked or overnight. I guess if you have a garage you can leave the hood unclosed in the evening and measure it there ... unfortunately my garage is occupied right for now.

But I am generally do not understand the concern ... if the 12V battery would be a real issue we would see a lot of complaints here which we do not of course accidentally fully discharging the battery will damage it but again that is not a design issue ... and not prius specific. Average car battery last 5 year in ideal circumstances but usually less than that right ?

Deep Cycle battery only makes sense if you "plan" having many full discharges.

OK, if you measure SOC after opening doors, then the measurement is not valid. However, this brings interesting point. If only the brake pressurizing brings battery SOC to 75% or lower, then one is outside the harmful 5% threshold for deep cycle and deep cycle battery makes more sense. I need to do some testing myself.

I -believe- the 12V battery IS a deep discharge type. I looked in both the owners and the service manual and couldn't find any confirmation (or refutation) of this however. But it makes sense. It is certainly not a "starting" battery (one optimized for high current discharge). As far as I know there are only the two types commonly available.
This does bring up one other point for newbies however. NEVER use the Prius 12V battery to "boost" another car! You may "get away with it", but I'd bet good money you would shorten its life noticeably. You would also put some very expensive electronics in danger from voltage spikes caused by charge dump during the process of boosting. So "just say no".

Now, much has been made on this forum on the desirability of a "deep discharge" battery. Yes, they are optimized to resist damage from deeper discharge events, BUT, they still suffer damage! So try to avoid deep discharge events.

The amount of charge removed during the brake pressurization when you open the door is minimal. It's a few 50 Amp 1/2 sec. pulses. Ditto for when you put the car in "ready". Measuring the battery voltage after opening the drivers door, once the pre-pressurization routine is finished should give you accurate information of the state of the battery after overnight "resting". Keep in mind the car does other things overnight that draw power from the battery, so worrying about the brake system is superfluous. To get an accurate sense of the state of the battery you just want to ensure it sits for a while -after- it was last charged. Overnight is a good time duration for this. Some minimal discharge events are probably good for this measurement. So as long as it doesn't get put in "ready" you should be fine.

Per the manual:

I typically check mine directly on the terminals using a cheap Digital Multimeter.

I think that is not really a good conclusion from all the data remember the 75% is not a hard number just an estimate from the Voltage this is a range with as much as 10% error interval.

Also the SKS is running let just say 10 hour around 100mA (that is a guess but I am sure this is in the range) this would mean 1AH charge lost close 3% loss overnight (assuming 100% SOC of the spec 36Ah). If someone actually know what is the nominal power consumption of the SKS we could better estimate this. If twice that you are out of 5% rate by just that !!!

Like what? If I may ask other than the security system which I tried to estimate above. Yes the lights are stay on for what 30 seconds after turning off the car that can not be much either.

OK this is helpful so I will measure tomorrow... I think my commute is long enough to fully recharge ... I see the charging voltage drop to floating level usually less than half way (I monitor with ScangaugeE). But I do not know if that means full SOC. I have a Y cable on the ODBC port so easy to measure voltage in the morning ... I was under the impression that the break pressurization is significant load and could alter the voltage.

I did some more searches and the startup current load on gen 2 amounts to only 0.03 Ah (if my math is correct): Prius Startup Current

I assume gen 3 should be similar.

The SKS draw is supposed to be only 15 mA, so after 12 hrs we are looking at only 0.18 Ah.

That combined is under 1% of total battery capacity.

I've got to do some testing this evening.

I would estimate that at under 0.1 Ah.

Wow that is very low considering the radio receiver is on and some type of processing unit...

So what else is drawing power then ???

If that is the case the car should survive a whole lot longer without any charging than a lot of people experienced.