Battery decline while driving

[eddieoggy]

Hi,
I have owned an MG4 EV SE from new in October 2024 and have been mostly very happy with it bar the LKA which eased slightly after getting it adjusted on the first service last October.
The claimed range in summer is supposed to be 218 miles at 100% charge and I sometimes travel to an address in Wales from Tamworth which is 118miles. I did this trip in January 2025 and my average speed was averaging 65 mph. Getting to my destination with approx 20 miles left .
However I did the same trip a few days ago and had a very different outcome.
I noticed the percentage dropping rapidly and slowed down from my 65 mph to approx 40mph about 30 miles from my destination but the guage showing the percentage dropping rapidly and about 15 miles to my destination it had reduced to a hair raising 4%!
I reduced speed to a crawl at 20 mph and limped to the local Morrisons store at Ebbw Vale where I plugged into a rapid charger costing £4.29 for 10 mins . Which got me to my destination easily.
Please don't make the same mistake I did and assume all will be well on a drive in winter that you have previously undertaken without problems because it may not be!

 

[gazza487]

Have you balanced the battery recently and / or done this after flattening to below 10% and letting it fully charge (plus some extra time to balance until complete?).

 

[eddieoggy]

I may well be mistaken, but I was under the impression that the type of battery that is standard on this model, ( MG4 SE, ) is such that it doesn't require balancing? I thought it was ok to let it dwindle to a low percentage and recharge it to 100,% with no problems. Unlike other batteries recommended to only charge up to 85% from 25%

 

[gazza487]

I may well be mistaken, but I was under the impression that the type of battery that is standard on this model, ( MG4 SE, ) is such that it doesn't require balancing? I thought it was ok to let it dwindle to a low percentage and recharge it to 100,% with no problems. Unlike other batteries recommended to only charge up to 85% from 25%

The SE with LFP is an odd beast. Yes it will take a 100% charge with less degradation than stuffing the other type full.

It has a quirk which I believe you may have just found. The manual does allude to it but here goes - weekly charge it full to balance is the advice (balancing is shoving the bulk of the charge in until some of the cells reach their peak charge then it will systematically sit there drawing lower power to top the cells up individually so it is properly full). The next bit is flattening the thing - let it sink to below 10%, sit at low charge for a bit then charge it full plus let it sit there on charge to fully balance every 3 months. Both of these need to be done AC charging.

The reason for these are because the LFP basically charges to full (the battery or cells reach a voltage) and it stays there until the charge in the battery is getting low, the voltage drops. To give you a display the car counts what goes in and what goes out and can’t properly know until it has seen both low and high. It sounds like you may have encountered the car getting low and it’s “guesstimating” is out a bit, sees the voltage dropping and plays with your %. Some have seen rapid 5% drops in this region in similar circumstances I believe. I haven’t but have been meticulous with the charging and have only ever rapid charged twice.

 

[eddieoggy]

Right thanks for the comprehensive reply. I will bear your comments in mind for future reference.
Much appreciated!

 

[Rolfe]

I have an SE SR and do the calibration charge regularly. Whenever I come home at under 10% I let the car sit for an hour or two (to let it get a good look at the bottom of the well) then charge (AC, obvs) to 100% in one run - don't let it break or it will lose its place, something something hysteresis curve - and let it balance as long as it wants to.

Certainly, its winter range is not what its summer range is, but the GOM is absolutely reliable and has never once played any nasty tricks on me.

 

[eddieoggy]

That's very interesting,thanks Rolfe.

 

[Tig170]

I'm 50,000 miles into our MG4 LFP SE (Sept 2002), I'm convinced that it will do a million miles, in winter I can get upto 160 miles, but in the summer can get over 200 miles. Also use it bi-daily (Nov-Feb) as a V2L charger to our ESS Victron Energy Quattro system. Just charge it to 100% every few weeks and you will be fine.

 

[Tight Git]

You've had the best advice from @Rolfe and I'm another one with an SR with over 65k miles that finds the GOM very accurate and predictable.

One additional proviso is that in cold weather if it starts off dry then starts raining mid way through a long journey it very quickly destroys your calculations and range.