Can I jumpstart a non-hybrid car from a hybrid or full electric car?

Hybrid Newbie here. I got a CT200h and faced this problem yesterday (jumpstarting a dead IC car with my working hybrid).

After being somewhat overwhelmed with the dire consequences to all the futuristic computers aboard, I played it safe and solved it Old School.

I simply removed the tiny 12 volt auxiliary battery from the CT200. This tiny battery is located in the rear, passenger corner, by spare tire. Youtube has videos that show how the plastic bits snap out, easily. But breakable, google it first.

Then - with the auxiliary battery removed from Lexus and in hand I connected / jumper cabled that directly the to dead IC battery. Vroom.

Yeah the auxiliary battery is tiny, so it might not start a really big or really cold or really reluctant IC engine. In my case a 3 liter, older gas mercedes, which did turn over fine and caught on 2nd crank.

Then, of course, reinstalled auxiliary battery in Lexus and was fine.

Worked for me!

When the car is sitting, in case with all hybrids, there is a disconnected HiVolt battery, which has many cells in it. Don't worry about this getting unbalanced, it's disconnected, and will be just fine while the 12v recharges when the car is turned on. If there is insufficient 12v power to "boot up", you'll need to get jumped back amusingly because you will see a warning light and the relay won't be able to activate to the hivolt.

When jump starting from the 12v posts, you are drawing off a smaller-than-usual 12v car battery just like any other car. It is not designed for this kind of thing, and it may ultimately prove to be too much for it, especially if it isn't charged very well or is old, or if you draw to much current from it even. This is one of the reasons why people say to connect the negative to ground, also known as the frame of the car, instead of both 12v posts battery-to-battery. There is a little bit more resistance, sparks are far enough away from the battery to ignite an explosion, and some other reasons. I can't recommend doing this.

If the other person were to crank their car while attached to your car, it can damage the electronics because I've seen it happen. No I do not know why, ask someone else.

You will not cause any unequal discharges or anything, the 12v will cleanly recharge afterwards if it was not damaged by the rather deep drain it went though. Don't worry about the hivolt.

Lots of good answers, but will clarify something for what it is worth:

All hybrid cars do have a 12V and a high voltage system. In my Prius (2009, 2003-2009 model), the 12v battery is in the boot.

The standard car electrics like computers (all 13 of them) and screens and headlight and indicators and everything runs off the 12v battery. The 12v battery is often a sealed lead-acid battery, to prevent risk of hydrogen gas explosions. SLA batteries have a long lifespan but tend to have lower maximum output currents. All they need to do is provide enough power to boot up the computer, which then activates relays that connect the (220-250v ish) high voltage battery.

The high voltage battery then powers a 2000w DC-DC convertor that charges up the 12v battery, and also provides power to all the electronics of the car.

The reason for this complex isolation of the high voltage battery is due to risk-aversion; in some ways it is like fast food coffee saying 'warning, contents hot'; or peanut butter saying 'allergy advice, contains peanuts'.... However in other ways it makes sense. The HV battery is usually near the rear of the hybrid, so power cables run to the motors at the front. Both batteries are DC, and 42v DC can kill a person, whereas many of us have had 240v AC mains shocks with no harm. In a car crash, or anything that disables the computers, the relays disconnect and the high voltage battery is isolated, so you don't die if you touch the car (hopefully!)

A typical alternator is 500-700 watts, so the 2000 watt 12v DC convertor is very good, and can provide very good quality power for audio installs among other things.

However, the battery in my 80s Nissan says 320CCA - cold cranking amps - which at 12 volts is around 3500 watts. Normal car starter motors can draw well over 2000 watts, putting strain on the 12v battery (much more expensive than normal car battery) and the DC DC convertor (extremely expensive, $1000s)...

If you try to jump start a normal car, it could destroy very expensive electronics, even if you do it right. Repair costs could be $1000s. If you connect it backwards it could write off a Prius, replacing 13 computers etc!...

If you do decide to help someone with a jump start, the best way is to connect your 12v battery in your hybrid to theirs, only when your car is fully switched on. Leave it for 5-20 minutes. Disconnect before they try to crank their engine and reconnect for more charging if needed.

With all that said, I jump started a friend's 2.5 diesel truck off my Prius. The 12v battery took damage, so if the fans are on, the voltage is too low to fully start the car's high voltage relays, and I get a 'red triangle' on the dash, so I have to switch off fans etc before I start my car! I was lucky only the 12v battery got damaged, and it hasn't got worse and it is 2 years later!

Be careful if helping others; whatever you do, don't let someone try to crank their engine while your car is still connected!

The other answers failed to emphasize an important point: the 12V battery on hybrid cars is not designed to start an engine, being much smaller than typical automotive starter batteries. Thus, it may not have the cranking amps capacity to successfully start the other car. The 12V battery is merely there to provide power for lights and to boot up the computer that after booting up connects the high voltage battery. The electrical system of hybrid cars may also not be designed for use cases where there is a large current on the 12V system.

Toyota recommends that Prius should not be used to jump start other cars although other cars can provide jump start (or more accurately, boot-up) current to Prius. Probably the reason for this is that the 12V battery is not a starter battery. Another reason might be that the damage when incorrectly connecting jumper cables is probably more expensive on hybrid cars than non-hybrid cars.

If I had a hybrid car and somebody asked me to provide jump start power, I would do it by charging the recipient battery for several minutes without cranking the engine on the recipient car. During this, the hybrid system power needs to be on so that the 12V battery is recharged from the HV battery through a DC-DC converter or else the small 12V battery might be exhausted. I would also connect the jump start cables myself and not allow anyone else to connect them to ensure that expensive damage doesn't happen due to e.g. connecting accidentally negative to positive or vice versa.

Remember also the connection order for the jump start cables. The positive is connected first, and then the negative. The last connection may produce a spark, so it is made to a grounded part of the car away from the battery to prevent a theoretical hydrogen explosion hazard (although I don't really believe that this hazard would occur easily). The cables are removed in the opposite order.

After the recipient battery has been charged for several minutes, only then would I try to crank the recipient car. Probably the safest thing to do would be to disconnect the jumper cables before cranking.

However, I don't really have a hybrid. Thus, if somebody asks me to provide jump start power, I'm lazy and save my time by cranking the recipient car immediately. The heavy-duty 50mm2 cables that I have should be large enough to crank the engine.

The batteries in these electric and hybrid cars are generally set up to deliver voltages much higher than the 12V (nominal) that your car needs for a jumpstart; it may be difficult or impossible to tap into them in a way appropriate for drawing 12V for that purpose. Even if you could, doing so may unbalance the electric/hybrid car's battery system enough to cause serious trouble with its subsequent recharging because its batteries would now be charging in an ubbalanced configuration (some cells discharged much more than others).