Battery Percentage vs Range

[ncdlloyd]

We had our MG4 at the dealer’s yesterday to look into the issue where our 12V battery was flat shortly after delivery. They found no fault and put it down to a requirement for the initial dealer to completely charge and completely de-charge the main battery before delivery.

When we picked it up I noticed that while the battery was around 30%, the range was only 40 miles. I charged it last night to 77% and the range increased to about 120 miles.

If the max range is 220 miles, these percentages don’t make sense. Do they line up with what other people see?

Thanks,

Nick

 

[meicd]

Range is based on driving style. If you reset all the trips it starts guessing again. Give it a few miles and it will settle down.

 

[PristDean]

Your 12v battery will be topped when you charge. It's likely your car may have been locked / unlocked, iSmat app used and left standing a while for the 12v to go flat.
If you drive the car around for a while the GOM (Guess 'o' meter) will update and provide a more accurate range as above. Your current GOM likely has not had much to go on.

 

[ncdlloyd]

Thanks.

 

[PristDean]

As a note, if you use the iSmart app, you see after the scan the 12v battery reading at something like 12.4v. When you put the car on charge and do the scan again, it will read something like 13.8v or higher as it charges.

 

[ncdlloyd]

We’ve had the car for six weeks now. The 12v battery problem happened in the first week and hasn’t happened again. I’ll see what happens with the range over the next week or so.

 

[Yandards]

Normal use for us on a Trophy sees about 4 miles to the kWh, so at 60kwh that's about the 240 miles.

Motorway running reduces that significantly to around 20 miles per 10 percent of battery - max 200 mile range.

Oh and it's summer not winter, that'll really crimp your range, used to double the cost per mile from 2p to 4p over the colder months. (Expectation management piece, not an EV moan).

 

[Rolfe]

We had our MG4 at the dealer’s yesterday to look into the issue where our 12V battery was flat shortly after delivery. They found no fault and put it down to a requirement for the initial dealer to completely charge and completely de-charge the main battery before delivery.

When we picked it up I noticed that while the battery was around 30%, the range was only 40 miles. I charged it last night to 77% and the range increased to about 120 miles.

If the max range is 220 miles, these percentages don’t make sense. Do they line up with what other people see?

Thanks,

Nick

Do you have the SR battery? I'm guessing you do, because you said the max range was 220 miles.

I'm catching on to a wrinkle with these things. They really do need to be charged to 100% and allowed to balance at 20 watts for about half an hour quite frequently so that the GOM is reasonably accurate. The LFP battery has a very flat charging curve and it's hard for the battery management system to figure out the state of charge when it's in the middle of its charge range. Then near the end as the battery begins to run dry it becomes obvious and the BMS will recalibrate to the truth, which might be a nasty shock.

Two or three YouTube videos where people ran out, relying on the GOM and the % charge, because the range dropped suddenly from maybe 15 miles to about four. I can't be sure, but I think these were LFP batteries, and they may not have been balanced recently enough.

I took my own car down to 4% with no nasty surprises at all - the range and the % charge decreased evenly as expected with no sudden change, but the battery had been allowed to balance at 100% the last time it was charged.

There's a lot of talk flying around about only charging to 80% routinely, for battery health. This is true for the NMC LR battery. It's fine about partial charges and cycling between 20% and 80%, only being balanced once a month, and it doesn't like sitting for days at 100%. The LFP in contrast is absolutely fine about sitting at 100%, and really really likes to go there and balance as often as it's charged.

I manage mine by running it down to wherever I feel like going before I plug it in (maybe 30%, from a several days or a week's accumulated driving) and then take it all the way to 100% and let it balance. I never do a partial charge and then start driving again unless I'm on a long journey and using rapids. I've never had a problem with the GOM showing me unrealistic numbers, or suddenly recalibrating and changing the estimated range.

I've only had the car since April, I'm no expert, but it took me a wee while to work this out against all the talk about "only charge to 80%" from the NMC drivers. So something to think about.

 

[ncdlloyd]

Thanks Rolfe, interesting points. What do you mean by balance at 20 watts?

 

[Rolfe]

The car does it automatically once it gets to 100%, so long as you don't stop the charge. The first couple of times I charged I was confused about what was going on, because once the clock reached the time the app estimated for the end of the charge, the car continued to charge, and if I refreshed the app the estimated end time just changed to the current time. I think I stopped it once or twice, not realising what was happening. Then someone told me to look at the wattage being reported.

While the car is on the granny charger it shows about 1.85 Kw. Can't remember exactly what it did when I was on a type 2 charger, but more than that. Nevertheless the wattage stays more or less constant while the car is charging, until it's at 100%. Then the figure changes to a much lower power level, usually 20 watts but I've seen it go up to 80 watts sometimes. This continues for about half an hour and the app then reports that charging is complete.

The battery needs this period to balance the charge between its cells and keep track of what they're doing. The LFP in particular needs to do this. It's recommended to let it do it once a week, but it will do it every time it goes to 100% if you let it. (The NMC only needs to do it once a month or so, and can be cycled between 20% and 80% the rest of the time.) The downside of not doing this is that your % charge and the GOM may start lying to you, and this may not be helpful if you're on a motorway and missed the turn-off for the charger, let's say.

 

[ncdlloyd]

Thanks again Rolfe. We’ll start doing some 100% charges and see how we get on. I’d been staying away from them for battery health, but it sounds like it’s needed in this case. The car is also a lease, so the long-term health is less of a concern.

 

[Rolfe]

You do have the SR battery then? Yes, the message is to ignore all the exhortations to charge only to 80% for the LFP batteries. MG has deliberately made the app so that you can't set a charge limit for the SR. It's meant to go up to 100% every time. These batteries really don't mind being at 100%, and as they have a longer life than NMC batteries anyway, the car will probably have rusted away before the battery suffers any significant degradation.

Best to let it go to 100% and sit on the charger whenever you charge, unless you're on a rapid in the middle of a journey, obviously. When I use the rapid at the end of my street, just to save time on the granny charger, I always take the car home and finish off to 100% on the granny charger. This is how these batteries are meant to be used. And I think it massively reduces your chance of suddenly finding yourself on a motorway with 10 miles less on the GOM than you thought you had.

As regards % charge and range, a decent rule of thumb is 2 miles for every 1% charge on the SE SR. That assumes about 4 miles/KWh so it's going to be a bit of an overestimate, but the point is that if you're getting low you can cut your speed down to the point where you can get that level of efficiency (unless you're climbing a long hill of course), so it's a fair representation of how far you can realistically go on what you have. So long as the battery was 100% charged and balanced before you set out.

 

[ncdlloyd]

Yes, we have the SR model. Based on normal usage we don’t need to charge every week and have an Ohme charger. Charging to 100% will be easier given that the Ohme and the car don’t talk to each other.

 

[Rolfe]

That's exactly what you need to do. Whenever you decide you need more range on the car, just let it run, and leave it plugged in until the app shows it's completely finished, that is, not drawing any power at all. They recommend balancing once a week for the LFP, but if you don't need to charge as often as that, I think it's probably fine just to be sure to charge to 100% every time you do it.

MG really meant it when they removed the facility to set a % charge target in the app for the SR. They don't want you to do that!

There was a really really good video about this posted a week or two ago, here, watch. Very reassuring. The guy really knows his stuff.



He skips over the balancing thing in about one sentence near the end, but from this and other sources I've realised that the LFP struggles if it's not balanced every chance it gets, and the main issue is that the GOM starts lying to you.

He's actually quite reassuring about the NMC as well, seemingly trying to get owners a bit less paranoid about charging to 100%. It's good practice not to do this every time, but it's not going to cause massive degradation if it;s not done too often. Just don't slap a SORN on it and leave it like that!

 

[ncdlloyd]

That's exactly what you need to do. Whenever you decide you need more range on the car, just let it run, and leave it plugged in until the app shows it's completely finished, that is, not drawing any power at all. They recommend balancing once a week for the LFP, but if you don't need to charge as often as that, I think it's probably fine just to be sure to charge to 100% every time you do it.

MG really meant it when they removed the facility to set a % charge target in the app for the SR. They don't want you to do that!

There was a really really good video about this posted a week or two ago, here, watch. Very reassuring. The guy really knows his stuff.



He skips over the balancing thing in about one sentence near the end, but from this and other sources I've realised that the LFP struggles if it's not balanced every chance it gets, and the main issue is that the GOM starts lying to you.

He's actually quite reassuring about the NMC as well, seemingly trying to get owners a bit less paranoid about charging to 100%. It's good practice not to do this every time, but it's not going to cause massive degradation if it;s not done too often. Just don't slap a SORN on it and leave it like that!

Wow, he really does know his stuff!

 

[Rolfe]

Wow, he really does know his stuff!

Yeah, he's the real deal. And I realise I was wrong about something. I had read that the reason batteries shouldn't be left at 100% was that all the lithium ions were crammed into the cathode, and the physical space they occupied was bad for the battery. Now if that was true, then it would surely follow, or so I thought, that the LFP would also be susceptible to the same problem, even if it was to a lesser extent. So that gave credence to the guys who were stroking their chins and saying, well now, you people with LFPs, don't get complacent, you should be following the same routine. (Even though MG don't just specifically tell you not to do that, they've jemmied the app so you can't do that even if you've fallen for this schtick and want to.)

Indeed, we had a new member come on here with a second-hand SR which was reporting some odd range anomalies, and he was worried about the health of the battery. The car was only a year old, I don't know what you can do to a battery in only a year to damage it. But anyway, someone immediatly piled in telling him that probably it had been charged up to 100% too much and that had damaged it. Because that's what NMC owners are always being guilt-tripped about. But even in an NMC, you'd practically have had to leave it at 100% for the entire year to do any measurable harm. The car had done 8000 miles, so that probably hadn't happened.

As we discussed the issue, it became fairly clear that the opposite was almost certainly the case. The battery hadn't been charged up to 100% nearly enough. Either the previous owner had believed the 80% mantra and taken the trouble to stop the charge most of the time, or he'd been using rapid chargers a lot, maybe not having home charging. The battery was taken to 100% and balanced a few times and everything started to look fine. (The member rejected the car anyway because of other problems, and was offered a different one.)

But anyway, that's not what causes the degradation at all. It's not the ions cramming into the cathode, it's the high potential reached at high charge that causes the electrolyte to degrade. A completely different process. Now, look at 4 min 18 sec on that video. LFP has a very different chemistry which doesn't reach such a high potential, and so the electrolyte doesn't degrade. So what the hell, as he says, whack the LFP up to 100% whenever you choose.

He also seemed to say that the problem of "shallow cycles" which he said wrecked some Leaf batteries, doesn't apply to the LFP. People would buy a Leaf (NMC) and just go down the shops then come home and plug the car in and charge it again, so it was spending almost all its life at close to maximum charge. (They should really have been doing several supermarket or school runs between charges, and only charging to 80% most of the time, but it's human nature to want to keep the tank full, isn't it?) He implied that the LFP wouldn't mind if you did that - he interrupted his description of the problem to state that he was just talking about NMC.

So with the SR we have a smaller range, and slower maximum charge speed on rapid chargers (88 Kw as opposed to 140 Kw for the LR battery), but we do have an easier battery to manage, and we really can keep it fully charged ready to go all the time if that's what we want to do.

I still do up to a week's worth of driving on one charge and then think, well I'm under 40%, probably ought to charge, because I've internalised all the mantras about NMC battery care too much, but the message seems to be, if you want to bring the car home and plug it in every night no matter how short your journey was, just do it.