17

This morning I had starter battery troubles with my 2001 VW Eurovan. As it happened, my father-in-law arrived just at the right time in his fancy new Toyota Camry hybrid. So we opened the hoods, dug out the jumpstart cables and then got stuck.

Under the hood of the hybrid Camry is very unfamiliar territory. I think I was able to identify an internal combustion engine in there, but there was nothing that remotely resembled a good, ol' fashioned lead-acid battery with red and black posts to clip the jump leads to.

Now by definition, hybrid cars have oodles of battery power, but is any of this whizzy new battery power available to jumpstart hapless internal-combustion vehicles whose century-old technology has given out on them?

In other words, can a I use a hybrid vehicle to jumpstart a non-hybrid vehicle? If so, how?

Case in point is a Toyota Camry hybrid, but I'd also be interested in the general case (any hybrid or full electric vehicle).

  • I just used my Toyota Camry Hybrid to jump a Toyota Solara. + on Solara to + on Hybrid, - on Hybrid to discharge metal on Solara. Started right up, but we did not crank the Solara engine until after disconnect. – JoshDM Sep 29 '16 at 16:45
13

There are two power systems on all electric cars (fully electric and hybrids alike). This is in conjunction with the much higher voltage battery packs which normally power the electric motors. The reason for this extra power system is to power things like lights, electronics, and HVAC systems (to name a few). Most hybrid vehicles (and never a fully electric vehicle) do not have an alternator. Instead they have a DC to DC converter which steps down the power from the battery pack to recharge the 12v lead-acid battery.

You can use this 12v system to jump other vehicles. The key is to know where the battery is located. In some cars, it is located in the trunk area. In other cars it is located up front. In yet other cars the battery is not easily accessed, but there are power provisions (studs) which allow you to clamp onto with jumper cables. It is good to know exactly where your 12v battery is located not only for jumping other vehicles, but also in case your vehicle ever needs to be jumped.

I have seen noted to be careful when jumping your car or jumping to another car. You need to use the cables to charge the dead battery. Don't let there be large surges of electricity or you can damage electronics. If you leave the jumper cables in place for a while, this will allow some electricity to build up in the dead battery before you try to actually start the other vehicle. As small as this auxiliary battery is on a fully electric car, I don't think I'd use it to jump another vehicle, though you would still need to know where it's at in case you ever need to get it started.

  • 1
    Apparently even the Tesla Model S has a 12V lead acid battery somewhere. – timbo Sep 21 '15 at 22:35
  • @timbo - It makes sense on several levels to have an industry standard 12v system even in electric cars. The main reason is you don't have to redesign a lot of subsystems within the vehicle to run off the higher voltage. Plus, even with having a step down to a 12v source (the dc-to-dc converter I talked about), you would still want a battery to act as a buffer/capacitor type object which will help keep power spikes to a minimum and to help condition the power coming from the converter. – Pᴀᴜʟsᴛᴇʀ2 Sep 21 '15 at 22:39
  • 2
    @Paulster2, You would need a 12v for the ODB system if nothing else (got to love government regulation), but I disagree with your assertion, "you would still want a battery to act as a buffer/capacitor type object" first of all, a capacitor is what you want for that it's even in the name, second any DC-DC converter you use should have an output capacitor that takes care of that for you. – Sam Sep 21 '15 at 22:50
  • @Sam - it isn't the output you are looking for ... it's the reserve power which I was talking about. You wouldn't want to over draw your DC-DC converter or you could burn it out (or fry a fuse) in the process. – Pᴀᴜʟsᴛᴇʀ2 Sep 21 '15 at 23:04
  • 2
    @Paulster2 I'm an electrical engineer and I've never heard of such sillyness, all power sources should have output capacitors and all sinks have input capacitors. I think you are working off bad data. – Sam Sep 21 '15 at 23:17
7

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.

  • What starts the ICE in a Prius, if the 12v battery isn't a starter battery? – HandyHowie Dec 24 '15 at 11:48
  • Actually, ICE is started by the HV battery by the motor-generators. Prius has no conventional starter motor, the HV motor-generators do its job. The 12V battery is there merely to boot up the computer that subsequently connects the HV battery that is disconnected when the hybrid system is off. – juhist Dec 24 '15 at 12:00
  • So if your HV battery is low, you may not be able to start the ICE either? – HandyHowie Dec 24 '15 at 12:11
  • 1
    Yes, indeed. Not even by trying to jump start it, as the jump start in Prius merely provides boot-up current. You need to contact Toyota to charge the HV battery with a special charger. I don't know how large self discharge rate the HV battery has, but if I had a Prius, I would start it every month or so to charge the HV battery if otherwise unused. And, also to charge the LV battery as well. There is a small continuous current drain on the LV battery. The HV battery is disconnected when power is off, but then it may self-discharge. – juhist Dec 24 '15 at 12:16
  • Paragraph 3 is the key - and it's good advice for all jump starting. It's much better to get some charge in the recipient. If you try jump starting a vehicle that has no battery at all, you'll feel the cables get hot! – Toby Speight Jun 1 '18 at 10:50
5

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!

2

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!

  • Note you lose the radio settings and perhaps some other information such as whatever the ECU may have learned about the condition of the engine when doing this. Note also if the CT200 battery is old, it may have low capacity, and thus cranking a 3 liter car and its huge battery may take so much juice out of the tiny CT200 battery, that you won't boot up the CT200 computer anymore and require a jump yourself! So, if you do it this way, better to attempt to crank the other car quickly and very quickly disconnect your battery. – juhist Jul 8 at 16:28
1

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.

-2

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).

  • 4
    -1 because most hybrids do have a 12V system to hook into. You're right about the higher voltages, but your speculation about the "unbalanced" batteries is just completely wrong and misleading. – Zach Mierzejewski Sep 21 '15 at 21:52
  • 1
    Tapping onto a vehicles Lithium batteries is an incredibly bad idea for any reason. Go look up videos of lithium battery fires. Don't touch lithium with anything but an IC designed to deal with them. – Sam Sep 21 '15 at 22:52
  • @Zach Mierzejewski ... if two batteries are series-connected, and of one is discharged and the other is not discharged, can you count on the charger being able to charge the discharged one without overcharging the charged one? Also, is a hybrid's 12V system capable of delivering - say - ~150 amps of jumpstart current? – TDHofstetter Sep 22 '15 at 17:30
  • @TDHofstetter Yes, I think I can count on the charging module of my $10,000 batteries being able to handle a slight imbalance. And it is just a slight imbalance; those massive batteries move a 6000 lb machine, they won't even blink at cranking a starter motor. I don't know why I'm arguing this because jumping from the battery pack is just asinine in the first place. Also some hybrids 12V systems can deliver enough current because they have their own battery and 12V engine starter. – Zach Mierzejewski Sep 22 '15 at 17:47
  • @ Zach Mierzejewski... is that starter near the size of one for, say, a car with an I6? My motorcycle is worthless for jumping my truck. Given that a hybrid's nominal voltage is between 144V and 300V, at most you'd be tapping at the 1/12 point of the battery pack to do the jump. Since you may be drawing 150A (or even more) for the jump, that's a pretty significant drain on one small portion of your battery pack - very bad situation for the charging system to try to handle. Frankly, I have very serious doubts about the practical value of an electric or hybrid car for jumpstarting an IC car. – TDHofstetter Sep 22 '15 at 20:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.