Jump leads: which size?

[welland]

I want to buy some jump leads for emergency purposes.  They must be capable of starting my RX400h from another Battery.  

The Battery on my RX is rated under 400 CCA, so will 400A capacity leads be satisfactory?

Could I save a bit of money (and boot space) by getting lower-rated leads (eg 300A)?  

[sorcerer]

A traditional starter motor can draw anything upwards of 300A when starting but the hybrids don't have a traditional starter motor. The batteries are much smaller because all they do is to power the security/entry/exit system and the computers to get the car into the ready state - the engine is actually started by the traction Battery.

However, although you don't strictly need a big beefy set of jump leads I would still be tempted to go for them as the small ones can often give problems.

We've had nothing but automatic cars for almost 30 years now so I've always carried a big beefy set of leads in the boot, but about a year ago I also invested in one of these units. I've never actually used it yet so I can't give you any anecdotal evidence of how good they are, but there are loads of videos on Youtube showing them in action and they seem to perform very well. Also very handy for helping other people because I'm not sure if a hybrid can actually be the car to provide the jump start if someone needs it.

[ColinBarber]

30A would be more that enough for the 400h.

Don't use your 400h to jump another vehicle.

[Britprius]

Probably the most important aspect of jump starting your RX400H is that you make sure that reverse polarity connection of the leads is avoided at all costs. The current needed to boot the computers, and run a couple of pumps is minimal compared to none hybrid cars. However reverse connection of the leads even for a fraction of a second can be very costly.

It is therefore recommended that leads with polarity protection are used. The current rating is of little or no consequence. Even professional mechanics get this wrong on occasions, and as already has been stated do not use your car to jump another vehicle.

John.

[welland]

10 hours ago, ColinBarber said:

30A would be more that enough for the 400h.

Don't use your 400h to jump another vehicle.

Did you really mean as little as 30A?

[ColinBarber]

If you make sure the auto lights and wipers don't come on you could get away with 10A. All the 12v Battery is for is to power up the control systems until the hybrid inverter is able to start generating power and the vehicle becomes 'Ready', at which point the inverter takes over providing 12v to the vehicle and starts charing the 12v Battery.

There is no starter motor, the hybrid motor is used to turn over the engine to start it - which is all powered by the hybrid Battery, not the 12v Battery.

[welland]

4 hours ago, ColinBarber said:

If you make sure the auto lights and wipers don't come on you could get away with 10A. All the 12v battery is for is to power up the control systems until the hybrid inverter is able to start generating power and the vehicle becomes 'Ready', at which point the inverter takes over providing 12v to the vehicle and starts charing the 12v battery.

There is no starter motor, the hybrid motor is used to turn over the engine to start it - which is all powered by the hybrid battery, not the 12v battery.

Thanks, Colin, that's really helpful.  

What can be done to start a car that has a flat hybrid Battery? Can it be charged by a regular 12v charger that is connected to the 12v Battery?  Can jump leads provide the power to start the engine through the 12v Battery?

[NemesisUK]

From my handbook..

Charging the hybrid Battery (traction battery)
As the gasoline engine charges the hybrid Battery (traction battery), the Battery does not
need to be charged from an outside source. However, if the vehicle is left parked for a
long time the hybrid Battery (traction battery) will slowly discharge. For this reason, be
sure to drive the vehicle at least once every few months for at least 30 minutes or 16 km
(10 miles). If the hybrid Battery (traction battery) becomes fully discharged and you are
unable to start the hybrid system, contact any authorized Lexus dealer or repairer, or
another duly qualified and equipped professional

[Chris Skelton]

I found that the weak point of jump leads is the clips themselves, which are often made of rather weak pressed metal that can easily break. I found a set of strong clips (welding? I wonder now!) and adapted the cables to fit (had to hammer the clamps on tight).

There's a vid on Youtube somewhere about supercapacitors as a Battery substitute......you learn stuff every day!

[ColinBarber]

12 hours ago, welland said:

What can be done to start a car that has a flat hybrid battery? Can it be charged by a regular 12v charger that is connected to the 12v battery?  Can jump leads provide the power to start the engine through the 12v battery?

You really don't want that to happen. As stated above you shouldn't leave the vehicle for months without charging up the hybrid Battery every couple of months.

The Battery charge indicator in the vehicle isn't the true state of charge - there is quite a bit more in reserve - so it isn't a problem in normal situations.