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Thackeray

Charging the 12v battery - no need to run the engine?

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I've been trying to understand what's needed to keep the 12v Battery charged while the car is not being used frequently. I don't have an easy way to connect a charger so I wanted to try and understand how the car charges the 12v Battery in normal use. I'm sure this was obvious to a lot of people but for anyone else who was as unclear about it as me, I thought it might be useful to post what I've found out.

I've found this video from Weber Auto, which has also done lots of other useful videos on how hybrid cars work, focussing mostly on the Toyota Prius.This one is about the 12 volt system.

 

From what this clip says, it seems clear that you don't have to run the engine to charge the 12v Battery. As soon as you turn on the car to the Ready state, the 12v Battery is being charged with power from the high voltage Battery via the inverter/converter.

This, of course, will gradually run down the high voltage Battery but then the engine will start to charge it up again. But apparently it makes no difference whether the engine is running for the 12v Battery to be charged continuously at around 14.5 volts all the time the car is in the Ready state.

 

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3 minutes ago, Thackeray said:

But apparently it makes no difference whether the engine is running for the 12v battery to be charged continuously at around 14.5 volts all the time the car is in the Ready state.

Correct, the DC-DC converter will take the voltage from the hybrid Battery and convert to 14.5volt to run the low voltage system, including recharging the aux Battery whenever the system is in Ready mode. The petrol engine will cut in and out as required to keep the hybrid Battery charged.

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Toyota's own advice on their website says that to keep the 12V Battery topped up during the lockdown period, put the car into the READY state for about 60 minutes each week (making sure that other things like the radio, the climate control etc., are off) with the transmission in Park, and the engine will run as and when necessary to do the job.

Obviously, don't leave the car unattended whilst doing this  :laughing:

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The difference is that if you keep car with engine ready, if hv Battery charge goes low it is charged by engine, else you risk to discharge it and you can no more restart your car. So, better keep engine ready to run if required by system. 

 

 

 

 

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From my observations, the car sat in 'park' with the engine running isn't very good at charging the hybrid Battery. It's better when being driven, charging via regenerative braking.

Feels like the engine would need to be running quite a lot to keep the hybrid Battery charged. And that means fuel consumed.

Is the best (cheaper, cleaner) means not a trickle charger to keep the 12v Battery healthy? Not practical for everyone (like OP William said), but a better solution if it is so, perhaps?

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Of course, a trickle charger is the best way of all but as the OP said, he can't do that.

I would imagine that wasting petrol is the worst way of keeping the 12V Battery charged but if there's no other way then it has to do.

The traction Battery will be alright for a couple of months at least, I would imagine.

We are going out to buy food and as we only have the one car, it gets topped up by proper running anyway.

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Remember that, in hybrids, after  warming up, engine remains off most of the time.

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7 minutes ago, Zotto said:

Remember that, in hybrids, after  warming up, engine remains off most of the time.

Depending on load, speed and accelerator position

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14 minutes ago, NemesisUK said:

Depending on load, speed and accelerator position

And cabin temperature. The engine is the only source of heat so if you have the cabin nice and warm, the engine will be firing more often.

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Of course, but if the purpose is to keep 12V Battery charged you don't need t move car and make warm cabin.

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16 minutes ago, Zotto said:

Of course, but if the purpose is to keep 12V battery charged you don't need t move car and make warm cabin.

Indeed, it will remain off until the traction Battery requires charge and depending on the original charge level that could be a long time..

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4 hours ago, matt8 said:

From my observations, the car sat in 'park' with the engine running isn't very good at charging the hybrid battery. It's better when being driven, charging via regenerative braking.

Regen braking doesn't produce much power at all and it doesn't get dumped into the 12v Battery. Much more power available from the hybrid Battery.

Not sure what current the vehicle will supply to the 12v Battery to charge it, as with a conventional vehicle the charging system is really only designed to keep the Battery topped up, not charge it up from a semi-discharged state - you use a Battery charger for that.

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1 minute ago, ColinBarber said:

Regen braking doesn't produce much power at all, much more available from the hybrid battery.

 

It produces enough in a steep 1/2 mile descent to my house to bring the charge level from 1/3rd to nearly full. 

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54 minutes ago, Zotto said:

Of course, but if the purpose is to keep 12V battery charged you don't need t move car and make warm cabin.

I know, but Nemesis talked about load, speed and accelerator position, which would all imply a moving car.

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18 hours ago, matt8 said:

From my observations, the car sat in 'park' with the engine running isn't very good at charging the hybrid battery. It's better when being driven, charging via regenerative braking.

I'm sure that's right that it's better to drive the car to keep it in good operating condition, mostly because the engine is running most of the time and generating electricity while it runs.

But when the car is moving, some of the electricity has to be used to power motor/generator 2 (MG2) as the car won't move unless MG2 is turning. If MG2 has no power, the engine would just spin to no effect. While you're driving, the engine is also turning MG1, which can generate the electricity needed by MG2 and any spare power can be stored in the high voltage Battery. Some of the engine power is also used to add torque to MG2's torque to drive the road wheels. So only some of the engine's power is left over to be stored as electricity in the high voltage Battery.

By contrast, if the car is stationary, all the engine power is used to drive MG1 and virtually all the electricity MG1 generates can be stored in the high voltage Battery.

So if you wanted the high voltage Battery to be charged, it might be quicker to do it by driving the car, with the engine running most of the time, perhaps up to around 2,000 rpm or more. But in terms of petrol used, it would be more efficient to keep the car stationary.

While the car is stationary and the engine is spinning at around 1,100 rpm, MG1 is turning at around 4,000 rpm. But when you drive the car at lowish speeds, as MG2 goes faster to drive the road wheels, MG1 goes slower generating less electricity. Until eventually at some point it will stop altogether and then start spinning in the opposite direction. So at town speeds MG1 is probably spinning more slowly and generating less electricity than when the car is stationary.

Having said all that, I wouldn't have thought there's much need to charge the hybrid Battery at all unless you're going to store the car for six months or more. Even on one bar of the gauge, the Battery still has a 40% charge and most reports on this forum that I've seen say that it loses very little power over long periods of time.

 

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My car is left unused for 4 months during the winter. The traction Battery never loses it's charge and the 12 volt Battery is left connected to a Ctek smart charger the whole time. Never had a problem.

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