Battery Charging

[Mike K]

A letter to Honest John in Saturday's Telegraph motoring section read as follows:-

 

Last volts

One of my neighbours has a Lexus hybrid that had a flat Battery after two weeks of inactivity. My other neighbour has two Toyota hybrids and is concerned that they will also get flat batteries through them not being used during present restrictions. Will he need to get a charger to keep them topped up? DP

You are referring to the 12v car system batteries, not the hybrid drive batteries. It is common for them to go flat if the car is not regularly driven distances, simply because the combustion engine is only running part of the time and therefore not always charging the 12v Battery via the alternator. The best answer is to regularly use the cars to drive distances of at least 20 miles every two weeks or so. Tell them to ask a dealer whether they can use a Battery conditioning float charger to keep the 12v Battery topped up via a power point in the garage and also ask how to connect it because there are separate jump start points under the bonnet from those on the Battery itself. They will need a smart charger such as a CTEK CT5. which is specifically for AGM stop/start batteries. 

As this seems to fit my situation i.e. low mileages, would you advise me to get a charger and put the car on charge occasionally?

[NemesisUK]

6 minutes ago, Mike K said:

the combustion engine is only running part of the time and therefore not always charging the 12v battery via the alternator.

Hybrids don't have alternators, for the very reason he gives, the petrol engine doesn't run all the time.

The 12v Battery is recharged via the traction Battery and can be recharged even when parked and the engine not running.

In fact Lexus recommend putting a little used car into the Ready state at regular intervals to allow the 12v Battery to recharge from the traction Battery. Should the traction Battery charge drop  low the petrol engine will fire up and start charging the traction Battery, providing the transmission is in Park.

To answer your question, yes you could use a CTEK, connected to the 12v Battery to keep it topped up. Left connected permanently it will maintain charge level automatically.

[Herbie]

5 minutes ago, Mike K said:

As this seems to fit my situation i.e. low mileages, would you advise me to get a charger and put the car on charge occasionally?

Yes, but not for the reasons the 'experts' at Honest John give above.

For a start, hybrids don't have an alternator and it makes no difference if the petrol engine is running or not because it has nothing to do with charging the 12V Battery. And, although the CTEK CT5 is a great charger and will do the job perfectly, we don't have stop/start cars or stop/start batteries - stop/start is a different technology.

Obviously, I don't know your level of knowledge regarding hybrid cars so if this sounds patronising, I apologise, but the following information from a different post I made a while ago may help:

In a hybrid there are two batteries - a 12V Battery that basically 'starts' the hybrid system and gets the car into READY mode (which is our equivalent of the engine running) and also a traction Battery that drives the electric motors and is around 288 to 300V depending on the car.

Conventional cars have a starter motor that, when you turn the key, will crank the engine to get it running. The starter motor can draw more than 300A from the Battery to start the car and when the engine is running, the alternator will replace the electrical energy used and keep the 12V Battery charged up but it's main job is to run all the car's electrical systems - the 12V Battery is only used for starting.

Our hybrids don't have a starter motor as such and they don't have an alternator either. Our 12V batteries only have to boot the hybrid computers and one or two other things and it takes less than 20A to do that job, which is a lot different to the 300A a conventional starter motor would need. Because of that, Toyota/Lexus thought they could save a bit of money by only installing small-capacity 12V batteries, which is fair enough, except that the downside is that the 12V Battery goes flat quicker than a larger Battery would. For instance, it's not unusual for the 12V Battery to go flat if parked up for a couple of weeks at the airport while you're away on holiday. Quite a few of us carry something like this in the car just in case it happens.

So, that's the 12V side. Remember I said that our cars don't have alternators? Well, it's because the petrol engine doesn't run all the time - the car sometimes switches off the engine and runs on electric motors, which are powered by the traction Battery. Anything that would normally be run by a belt from the engine, such as the alternator, the aircon compressor or the power steering pump, are all driven by electric motors so that they can still be used when the petrol engine is off. In fact, once the car is READY then the traction Battery feeds all of the car's electrical needs, just like the alternator does in a conventional car.

Taking the alternator as an example, the 12V Battery starts the car and gets it into READY mode. Once READY, some clever electronics in what's known as a DC/DC Converter (our equivalent of the alternator), steps down the 300V from the traction Battery to around 14.5V to keep the 12V Battery charged. Different boxes of electronics do the same (albeit at different voltages/currents) for the aircon compressor and other stuff, all run by the traction Battery.

The other main job of the traction Battery is to provide power for the electric motors that drive the wheels when the petrol engine is off. This Battery is mainly charged by regenerative braking. The regen braking system takes the kinetic energy that would normally be wasted in heat by friction of the brake pads against the discs and turns it into electricity to charge the traction Battery. Every time you lift your foot off the accelerator you'll see the power meter needle drop into the 'Charge' zone of the meter, but maybe not so far. When you actively press the brake pedal the needle will drop right down to the bottom of the 'Charge' scale, indicating maximum rate of charge.

If the traction Battery is dropping low on charge but no braking is taking place to charge it, the hybrid system will run the petrol engne, which in turn runs one of the motor/generators (MG1 and MG2) in a backward rotation to act as a generator and charge the Battery. I often get MG1 and MG2 mixed up so that may not be strictly correct but you get the idea - it's one or the other. It can be quite noticeable if the car isn't moving at the time, which is why I think that this may be what you're feeling.

However, as I said above, if the car is in neutral then this won't happen, which is why you need to keep it in drive.

Something else to bear in mind is that these are hybrid cars, not electric cars. If you run out of petrol, the most you'll get on Battery alone is maybe a couple of miles, no more, and it also opens up a whole host of other problems.

To be a fully-functional car, three things are needed as they all work together - the 12V Battery, the traction Battery and the petrol engine.

Hope that helps,

Herbie.

[Mike K]

Many thanks for your comprehensive explanation. I've learned a lot!