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What I learnt from charging my Prius - Page 4

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Posted by urod on January 4, 2010, 9:33 pm

That I understand, but I suspect there is a step-down DC to DC
hidden somewhere in the car, so that the big battery could charge the
small one.

Posted by =?iso-2022-jp?q?Hachiroku_=1B$ on January 4, 2010, 5:12 pm

On Mon, 04 Jan 2010 13:33:02 -0800, urod wrote:

Not on any Prius I ever saw. Once the 12V is dead, the car will not run.

There may be, but if the 12V dies, you can't turn the car on.

Posted by Daniel who wants to know on January 4, 2010, 10:01 pm

There is a DC-DC converter built in to the inverter unit under the hood.  It
is current limited and fused at 100 amps on the 12V side.

Posted by urod on January 4, 2010, 10:31 pm


full closed or the interior light switch [is] in [the] door position,
the lights will go off automatically after 20 minutes."

switch in the door position.  In other words, if you manually switch
lights on (not in door position) and leave them on, the battery will

Nope.  The interior light switch has 3 positions "off", "door", and
"on". I left it in the "on" position.  As far as I understand this
text, only the "door" position is protected.


battery that the user interfaces with, right?  Because that's the ONLY
battery you interface with, the ONLY battery you would be charging

Here is what I did.

(1) I bought a car battery charger (namely, Hella power-charger) and
put it to the 12V battery. Nothing happened.  I waited for 30
minutes.  Nothing happened still.  Maybe the charger was defective.

(2) With two cables, I connected the 12V battery to the 12V battery of
another car.  I hit the Power button, my car turned on.  The power
meter showed half-full 200V battery (5 blue lines).   The system did
not switch on the engine.   I waited for 1 minute.

(3) I disconnected the 12V battery.  My car turned off.  I hit the
Power button.  Nothing.  Not surprising at all, considering that the
system hadn't switch on the engine at the previous step, so it
couldn't charge the 12V battery.

(4) I connected the batteries again and turned the joystick to the
"R" (Reverse) position.  The road was tilted downwards, so the system
switched on the engine, which started to charge the battery.  Only
this way I was able to charge it.

(5) After disconnection the batteries, the power meter showed almost
empty 200V battery (1 red line).  I cannot explain the behaviour of
the power meter, unless two batteries are interconnected (via DC-DC

Posted by Daniel who wants to know on January 4, 2010, 11:13 pm

New switched mode chargers will not turn on unless they sense some minimum
voltage of the correct polarity on the leads.  If the 12v is discharged to 0
volts the charger won't turn on. In this case you need an old "dumb" charger
to bring the voltage up so that the "smart" charger will start charging.

You probaly didn't push the brake pedal far enough and the car was in IG-ON
mode instead of READY mode.

12V charge has nothing to do with the engine running as the car does not
have a 12v starter or alternator.

You must have managed to hit READY instead of IG-ON this time.

The DC-DC is only active any time the READY symbol is lit up so 12V charge
is only dependant on READY it has nothing to do with anything else. READY is
also a sign that the system main relays on the HV battery are closed and
allowing power to the inverter.

NIMH state of charge cannot be determined by voltage alone so the battery
ECU has to guess what the SoC is any time 12v power is lost.  You could
probably disconnect and reconnect the 12v battery 5 times and get a
different HV SoC reading each time.  FWIW 1 pink/red bar is actually 40% SoC
and all green bars is actually 80-82% SoC.  This 40-80% window is the main
part of why the battery lasts as long as it does.

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