How many amps can I safely draw from the 12v battery?

Hi all,

I'm planning on hard-wiring in a 12vdc to 110vac inverter into my Prius. (I know drawing off the HV pack would be more efficient, but that's more confusion & tinkering than I want to mess with right now.)

The inverter will only be used when the Prius is ON and can recharge the 12v battery. But I need to know how many continuous amps can I safely draw from the 12v battery without overloading/overheating the 12v charging circuit.

I vaguely remember somebody (maybe Wayne?) saying that the 12v charging circuit is rated at 100 amps. Is that correct? Can I safely & continuously pull that much off the battery?

Thanks!

 

Hi Rick,

Yep, all of those things suck 12 volts, at various amps (if I had to guess, I would think they would draw under 10 amps combined). The next time I'm messin around, I'll hook up my ampmeter and get a better estimate of how many amps all of these things take...

In the meantime, I *think* I have found in the repair manual where it says 80 amps, but I'm not sure.

I did one experiment so far:
I hooked up a 120v 750 watt heater to my inverter and tested it while driving to work. It worked fine, and I had the A/C blower on high, radio on, and headlights on...

So that would be 750/10 = 75 amps, I think, in addition to the built-in 12v devices. If indeed the DC/DC can only put out 80 amps, then 75 amps is the limit!

Notes:
The wimpy prius 12v battery can't even power the 750 watt heater at all (the voltage drops to 10.5 volts pretty fast and sounds the low-voltage alarm on the inverter). When the car is in ready mode, I noticed the voltage would sometimes jump up to 17 volts as the DC/DC tried to charge the battery!


So far now I would say the limit is 75 amps...

-Tim

 

Ok, let me re-state how many amps I think the built-in devices can take.

The PTC can draw 25 amps, I guess, at 300 watts (I don't honestly know if this PTC heater is real or imaginary). I suppose all of the lights could draw 10 amps, and other motors could maybe draw a lot too.

So maybe these could take a lot of amps (40 or more?) for short periods... Maybe my 75amp load was pushing my luck?

I was once able to drive an old corolla, with a "standard" 12v battery, for 40 min with a dead alternator, but I suspect our prius battery would be shot after a few minutes without the DC/DC circuit...

-Tim

 

Hi Rick,

You are right, there are2 of them, each on a 30 amp fuse. So that's 50 amps right there, which when added to my 75 amp load would be 125 amps. Yow!

Luckily, the PTC heaters are only used if the temperature is set to MAX, and apparently they won't come on if the current drain is too high (see below).


Interesting info on PTC heaters from the repair manual (my emphasis):

CIRCUIT DESCRIPTION
The PTC heater is installed in the radiator in the heater unit on the driver and passenger’s FOOT sides and
operates when cooling water temperature is low and normal heater effectiveness is insufficient.
The air conditioner amplifier assy switches the circuit in the PTC relay and operates the PTC heater when
the operating conditions (cooling water temperature is Below 55C (131F), setting temperature is MAX.
HOT, air outlet damper position is FOOT or FOOT/DEF and blower switch in not OFF) are met.

HINT:
* PTC operation may stop when the electric load on the vehicle is high. Perform the inspection under conditions with low electric load (such as with headlights turned OFF).
* Since the two PTC heaters are gradually turned to the ON position, perform the inspection 30 seconds after the blower switch is turned to the LO position.

-Tim
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