The Prius 12 volt charging system.

If the 12V battery is not holding a charge most of that 500 watts would have to be dissipated as heat. Has anyone noticed a failing 12V battery getting hot?

 

^^ That would also reduce the heat buildup, as some of that power goes to producing gas instead of heat.

 

Your findings and assumptions are CORRECT Britprius.. nice job.

A scanguage showing high voltage amperage can show the same results with a little special math..

Hv current as view from my scanguage shown normal daytime current at about .8 amps (12volt battery fully charged).

At night with headlights on low beam scanguage shows about 1.7 amps draw from hv battery.
Thats about .9 amp difference.. .9 amps multiplied by 225 volts is 202 watts... factor dc to dc converter effiency is about 50%
your left with 101 watts being consumed by 12 volt system... Divide 101 watts by 13.8 volts you get 7.31 amps..

Perhaps 7.31 amps is a little low for headlights, parking lights, and tail lamps if so. Then dc / dc converter is maybe 55 % effecient..

I have seen scanguage with 70 to 80% discharged 12v battery show 4.5 amps draw from high voltage battery.

Doing the math suggest (55% effecient dc-dc converter) 33 amps of charge into 13.8 battery..(orignal 12v 35ah battery).
Just realized something 70 to 80 % discharged battery is probably down to 12.0 volts..So when scanguage shows
4.5 amps hv current... 12 volt battery is experiencing 38 amps of charge based on 55% dc converter effiency.

Stepping down voltage steps up current with the same ratio. Provided stepdown transformer can handle the full current.!
And no stepdown transformer is 100% efficent..so there is some current loss..Also a stepdown transformer at high frequencies can be quite small compared to a 60hz type.. as most high frequency transformers require only a few turns of coil in the primary and secondary windings..The prius stepdown transformer probably has 16 turn of coil in the primary and 1 turns in the secondary.
A 16 to 1 step down of voltage but a 16 times step up of current. The secondary winding must be of large diameter wire to handle the dc to dc converter output of 100 amps at 13.8 volts... I have not located said transformer in inverter/converter teardown pictures on the net...Have you? OOps I found it..see attached pic..
its upper center and to the right is high current choke that looks similar..

Also of interest my old 12 volt battery had blown seals from the high charge currents.
the orignal battery which i kept.. the label on the top of the battery is the vent seal that forces gases through the valve-vent..
My label had bubbles and tunnel crevices leading to the open air..
With my new battery now about 3 years old I never had a discharged battery yet.

 

12 amps sounds very accurate for headlamps, parking and tail lamps..
If memory serves correctly each headlight is 60 watts = 5amps@ 12v ?...Or = 4.34 amp @ 13.8v
Anyway that does bring up dc converter effiency to approximately 84 % efficent...NICE

I found stepdown transformer and posted picture to my orignal post..Transformer looks to have feroite core which is very
efficient.

That all brings up my estimated 12volt discharged battery,, charge current up to 58 amps...when scanguage reports hv
current at 4.5 map....

I did post all of this years ago on prius chat with lots of disagreement....In that post I said discharged prius 12 battery would pull 60
amps I still stand by that... While charge current drops quickly it is greatly above the 3 amp hour recommended charge rate.

From my recollection The coolant transfeer pump pulls 8-10 amps, the ABS boost pump that usually operates shortly after opening
the driver door pulls 13 - 16 amps..

The prius ign system and all ecu's power up I think around 8-10 amps.
So your 8 amps or so that you mentioned sounds accurate to me..
When I turn off the multifunction monitor I see no differenct on hv current...It must be under 1 amp at 12volts..

I havent looked at scanguage hv amperage with prius powered up in non-ready mode.... I suspect 10 amps at
12 volts...

Your choice of wheel chair deep cycle batter sounds like good-one...provided It doesn't pull over 100 amps fully discharged..(initial
recharge current)..

To be safe should you have a discharged deep cycle 55 ah battery.. just dont use all the accessorys, for the first 5 minutes of
recharge .

I seen agm 80ah 12v battery pull 90amps for 3 minutes (charge rate) following a 15 amp discharge for 20 minutes..years ago..
You should be ok I think if you should ever have a discharged battery (unlikely)..

What type current meter are you using and how high will it read? ...I have a craftsman amp clamp type that will measure dc current..
(rare animal..most read only ac current) I think it will read over 100 amps dc... but it is not high quality...
I have not used it on prius.It would require girlfriend riding in trunk hehe.. scan guage does all I need..

 

John its been about 3 years since I observed the high charge voltage due to a completely discharged battery.
I'm thinking 14.85volts I have a cigaret lighter volt meter purchased from walmart for 15.95 that agrees very closely to my fluke vom.
Second best thing I ever bought next to the scanguage.. The cigarette volt meter swivels up against dash for some consealment when
away from vechicle..Its backlighted light green and only draws .015 amp. but stays inserted always...After many hours of viewing
voltage at a glance I can actually determin inverter/converter temp high medium or cold as normal readings battery fully charged
converter cold = 13.75 inverter warm = 13.71 inverter hot (153 degrees ) = 13.67.. each summer the fully charged voltage drops more
and more.. It might be time to change inverter coolant...inverter/converter getting hotter each summer.. or cooling pump maybe getting
weaker and weaker...?

Yea discharge testing with deep cycle battery.. Thats my kind of fun... my next battery will be deep cycle type.

My next fluke meter dc amp clamp .. awesome..

Yea ccfl backlight is probably the only thing turned off (when turning off mfd) but doing so does register on by cigarette volt meter
(about .02-.03 volts difference.. backlight probably consumes 300 milliamps..(better guess).

Have fun John.

 

Interesting data. I usually see ~1.1 A as opposed to your 0.8 A. Do you have numbers before you changed the 12 V battery?

I noticed your car is 2009 and you said the new battery was put in 3 years ago. So the original one failed pretty early?

Thanks!

 

My orignal battery experienced 3 total discharges.
Also about 26 half capicity discharges.
And about 14 quarter capicity discharges.
It was still good when I replaced it.. that orignal battery was no sissy!

The dealer sold me a new one for 159.00 it is marked as no pb (lead).
Its morning voltage is 12.96 volts as opposed to 12.6 of lead acid type.
The new battery looks identical to orignal .. What is it made of ? ?

Fresh out of sealed box it measured 13 volts..What is its chemistry?
My brand new prius had a confused abs ecu..abs2 relay was energized
24 by 7 by 300 days.. it was pulling half an amp continously for 300 days
I believe kthis is why there are so many prius battery posts.

Confused ecus wont go into cpu-sleep-mode after dead battery, jump starts, low voltage encounters, etc.

 

The said new battery has been installed for closed to 3 years.
I dont think much is visable in battery compartment and Im out
of town. It must be a gen 3 battery as a gen 3 exibits these higher voltages.
I think the nonlead battery was similar in weight but I didnt actually weigh it.. I still have the very nice douhle box it came in which might show some specs ... Im sure its same ah rating. Ill check box tomarrow

Before I cleared the half amp discharge problem on the old battery the scanguage probably never went below 1.4 hv amps.
After I fixed discharge problem it did drop to .9 amp on old battery.

with cabin vent blower on low and hv battery cooling fan on scanguage shows 1.1 amp.

 

The 1.1 A I measured was with both fans off. Now I am curious to see what happens if I put in a new 12V battery. Hmm...

By the way I would be interested in the picture of the box as well. Thanks!

 

Wow! I suspected the same, but 43-20 amps might not seem like much in the big hybrid picture, but it does explain why I got low mileage on my gen1 until I finally replace the OEM battery. Once replaced and fully charged, the main traction battery stayed with 9 green bars most all the time, unless sneaked in some EV driving in town (I have 3 different school zones to drive thru to get to the store. Good time to go EV and save a $180 ticket!)

 

I take it back..
Seems my new (3year old) battery does contain lead.. I mis interpeted there picture-symbol shows trash can with big X over pic
with Pb bottom.. see attach pict.... the label say limit charge to 4.2 amp within 10 hour period...also see pic...

I will check box when I get home this afternoon... If there is any other info on box I will picture and post..

sorry for any confusion....

OOps I should have checked my hi resolution camera settings ...I didn't realize i was sending 1meg pixel pic..
Hope hi-rez doesn't cause any problems...I will double check in the future..