LFP: Battery heater engaged at 97% on granny charger in subzero temperatures

[fnegroni]

Second time charging my SE SR (LFP battery) to above 97% today on a granny charger outside at -2C°.
The battery heater engaged automatically and drawing around 1.5 to 1.7 kW every few seconds.
Charging to 100% will take a lot longer than I hoped.
This is the same behaviour as if I had started the battery heating myself. Difference being, when doing so manually the app says ‘heating’ not ‘heating stopped’
I don’t know how much it would draw from a 3.6 or 7.3 kW wall charger but on the granny this is what’s happening.
Debating whether to stop charging now or to let it reach 100% and balance.

 

[fnegroni]

Or even stop charging, heat the battery using the HV battery itself and then resume charging.
I need to use the car at 3.15 pm. Will see how it gets on in the next hour.
Just to show AC charging per se on a granny lead (and perhaps even a 7kW charger) isn’t enough to warm up the battery at these temperatures.

 

[fnegroni]

Notice this isn’t the balancing process, granny still pulling 2.2kW at the wall.

 

[fnegroni]

decided to stop charging now.
Start battery heater which will use the main battery and draw as much current as it likes.
Then start granny charging again and verify it’s then going to fully charge as normal.

 

[fnegroni]

Obviously the more sensible thing to do would be to stop charging and not heat the battery, since I don’t need 100% today nor tomorrow. 96 will suffice.
And I have equalised it religiously and the last time was in the heated garage where the battery was in ideal conditions just two days ago.
But for me it’s an interesting experiment to see what would happen

 

[fnegroni]

Heating stopped
Now charging from 96%.

 

[fnegroni]

Now charging with all the power of a granny.

 

[fnegroni]

Now doing its balance charging

 

[MGJimmy]

I think I would have stopped at 98% as you are using a ton of electricity to get an extra 2% you said you didn’t need.

 

[siteguru]

I think I would have stopped at 98% as you are using a ton of electricity to get an extra 2% you said you didn’t need.

Unless you're talking about battery heating ... 2% of usable battery capacity is 1.234kWh (in the MG4 LR models, 1.016kWh in the MG4 SR model). The only way you'd use "a ton of electricity" is if your charging efficiency was very low.

#Physics

 

[MGJimmy]

Unless you're talking about battery heating ... 2% of usable battery capacity is 1.234kWh (in the MG4 LR models, 1.016kWh in the MG4 SR model). The only way you'd use "a ton of electricity" is if your charging efficiency was very low.

#Physics

I’m talking about battery heating to get that last 2%, it sounds like it took quite a while to get those last 1.234kWh of power into the battery and most probably a lot more electrons for a tiny bit extra range.

 

[siteguru]

That assumes that person had battery heating enabled all the time they were charging. My experience is that battery heating turns itself off once the battery pack temperature reaches a certain value (10C?). In fact the images in this thread show that battery heating had stopped (or hadn't been started). In which case going to 100% vs 98% does not use significantly more power from the mains.

Remember, that poster was doing tests, and also wanted to balance their battery (which requires 100% SoC in the SE SR, and regular balancing is recommended anyway).

As I said ... #Physics

 

[Kevin Smith]

When it's daytime, and time or electricity are dear, skip the last few percent unless you absolutely need it (and how can you calculate that that closely?) and let it go to 100% once a week on cheap overnight electric. We have a long extension to our parking space and only granny charge at about 1.8kW. we've managed 9 months without being too concerned about getting to 100%, we try to let it happen about once a week but with 7 hours at 8.6p/kWh it takes us 3 nights if we've let the battery to get into the 20s%. If we need more range sooner we let it charge longer but avoiding pea hours mostly as we pay 56p 4-8pm. We just had our first long enough trip to need a fast charge - Northumberland to Warwickshire with one charge half way, in retrospect could have charged to 95% at the gridserve and saved some time!

 

[fnegroni]

Just to clarify: I agree I should have stopped at 97 or even sooner, possibly 90 or even 75
Which is what I did since.

But as @siteguru pointed out, I wanted to do an experiment to see exactly what would happen.

This is how we find things out: we get to the bottom of it

So now I know that when it’s freezing cold, the battery heater will come on automatically and on a granny charger it’s just not worth it but if you do need it badly then the better option is to run the battery heater for a few minutes just to get the battery up to a good temperature and then you can finish timings off.

 

[JonnyG]

............But for me it’s an interesting experiment to see what would happen

Yes, and for anyone that really needs to do an overdue battery equalisation it is useful info