Using hybrid to jump start non hybrid car

[Ala Larj]

We have an old Honda jazz and a Lexus GS300h. Due to lockdown neither have been used regularly. Took my eye off the ball and have had to jump start the Lexus using the Honda. I now run the Lexus once a week to keep the Battery topped up. However, the jazz doesn’t get used much and has struggled to start during this cold spell. Luckily I’ve had no problems so far but if the Battery dies why can’t I jump start it via the Lexus?

I’ve seen a few posts on here saying that you shouldn’t use a hybrid to jump start a normal car. I may be naive but but what is the problem?

if I attach jump leads from my Lexus to the Honda surely it is only taking current from the Lexus 12v Battery and not from the hybrid Battery?

Therefore what is the problem?

 

 

[ColinBarber]

For one the Lexus Battery in general and the CCA rating isn't that large. Secondly the current would be pulled from both the Battery and the hybrid system if the vehicle is in Ready mode and the DC-DC converter can only supply 120 A (on the 300h). You therefore run the risk of overloading the system.

Now given that it is only a Jazz the Battery itself is probably big enough but I wouldn't have the vehicle in Ready mode so the hybrid system isn't supplying any current.

In order of preference:

1. Just don't do it.
2. Disconnect the Battery from the vehicle and just jump from the Battery.
3. Jump from the hybrid vehicle but don't power it up to stop current being supplied from the hybrid system.

[Ala Larj]

Thanks. I didn’t realise current would also come from the hybrid Battery. 

[Herbie]

2 hours ago, Ala Larj said:

Thanks. I didn’t realise current would also come from the hybrid battery. 

In all cars, whether hybrid or conventional, the 12V Battery is only used to start the car. Once the engine is running or the hybrid is in READY mode, the alternator or the hybrid's DC/DC converter will supply the entire electrical demand of the car and also keep the 12V Battery topped up.

When jump starting a conventional car the advice has always been to have the donor car's engine running before attempting to start the recipient car. This is so that the recipient car draws on the donor alternator, not the Battery.

This isn't a problem because alternators can squirt out 300A or more but, as Colin said above, a hybrid's DC/DC converter can't supply anything near that. I would hope that there would be some form of current-limiting circuitry or fusing arrangement that would protect the converter but if not, there's likely to be a big bang and an escape of magic smoke.

[nicnac]

10 minutes ago, Herbie said:

In all cars, whether hybrid or conventional, the 12V battery is only used to start the car. Once the engine is running or the hybrid is in READY mode, the alternator or the hybrid's DC/DC converter will supply the entire electrical demand of the car and also keep the 12V battery topped up.

When jump starting a conventional car the advice has always been to have the donor car's engine running before attempting to start the recipient car. This is so that the recipient car draws on the donor alternator, not the battery.

This isn't a problem because alternators can squirt out 300A or more but, as Colin said above, a hybrid's DC/DC converter can't supply anything near that. I would hope that there would be some form of current-limiting circuitry or fusing arrangement that would protect the converter but if not, there's likely to be a big bang and an escape of magic smoke.

Its an interesting one ................ is there any info out there I wonder in Google Land ?. The magic smoke escaping from the black squares ..........I have seen that a few times in my life .

Personnally I would not try it !

[ColinBarber]

3 hours ago, Herbie said:

I would hope that there would be some form of current-limiting circuitry or fusing arrangement that would protect the converter but if not, there's likely to be a big bang and an escape of magic smoke.

🙂

Yes it is all fused. Toyota normally do a good job of making their vehicle electronics bulletproof. But these aren't your normal 5A mini blade push in fuses, these are 120 A and 140 A fuses (output of DC/DC converter and directly off the Battery +ve). I'm sure for most people changing those is an expense and an inconvenience they could do without.

[Ala Larj]

Thanks all. I will not be using the Lexus to jump start the Jazz.  I have a small Battery charger which came with my lawn tractor. The Battery in that is 12V, 18Ah and 200A EN. The Jazz Battery is 12V, 33Ah and 270A EN. Could I use the lawn tractor charger to charge the Jazz Battery? 
Sorry for my ignorance but I don’t really understand electricity!

[Britprius]

24 minutes ago, Ala Larj said:

Could I use the lawn tractor charger to charge the Jazz battery? 

The lawn mower Battery is an ideal candidate for jump starting the Jazz.

John.

[Britprius]

I should also have added the mower Battery is also suitable for jump starting the Lexus hybrid if the need arises.
Keeping the mower Battery charged on a regular basis will keep it in good condition while not being used, and give you in effect a starter pack for your cars in an emergency.

John.

[Ala Larj]

Thanks. That’s good news.