December 22, 2023
Tom Rose

How do you smart charge a BYD Atto 3?

How do you smart charge a BYD Atto 3?

The BYD Atto arrived on our shores last year and quickly became one of the best selling EVs in the market. It has great capability for the price point and here at Evnex we love the quirky styling.

What we are less enamoured with is the AC charging control, which unfortunately does not play nicely with smart charging. Almost all useful smart charging features (like off-peak charge scheduling and solar diversion) involve not charging the instant you plug in your car, and instead waiting for the opportune moment - be that the start of your free charging hours or the point in the morning when your solar export reaches 88mph, or rather, 1.5kW.

Like most EVs, the Atto 3 goes into sleep mode to save power when left alone. Sadly, unlike most EVs, it doesn’t wake up when a charger offers it some power later. So how do you smart charge a BYD?


  • Buy an Evnex charger (or if you already have one, wait for firmware version 2.4 to get pushed to it)
  • Turn on “Keep Awake” under General Settings in the Evnex app
  • It won’t go to sleep because we fixed it for you!

But if you want to know how we did it…

Sleeping Beauty

So what’s the BYD equivalent of True Love’s Kiss? It turns out that there are a few ways to wake an Atto 3 and make it start charging, such as bringing the wireless key within range or unlocking it remotely via the app. We looked at ways that our chargers could pretend to be your key or integrate with your app, but weren’t happy with the security ramifications of any of them.

Having struck out there, we instead turned to ways of keeping the car awake while waiting for charging to start. Could we make it think it was going to get charged but not actually deliver any energy? And how close to actually charging do you need to get to keep it interested? Can you just pretend that the cable has been removed and reinserted, or do you actually let it start charging but pause immediately? Cue some fevered experimentation in tantric charging…

Eyes Wide Shut

In general it’s frowned upon to mount your charger on the carpet, but we have a warning sign so it’s fine.

Wiggling the control pilot lines (this is definitely the technical term) to simulate the cable being plugged in didn’t help at all - the BYD stayed asleep. So we moved on to actually offering to charge, or at least pretending to, from time to time.

Initially we were under the impression that it took an hour for the car to fall asleep, but we got some very mixed results when offering 5 minutes of juice after a 55 minute pause - once or twice it was still awake and charged, but mostly it was already out for the count and didn’t.

So we changed tack and went for a 1 minute on / 10 minutes off regime, which was enough to keep the car awake for hours. Reducing it to 30 seconds on / 15 minutes off remained successful, but 5 seconds on / 15 minutes off clearly wasn’t stimulating enough and the car fell asleep.

It turned out from the logs that 5 seconds wasn’t enough time for the car to actually consume any energy, so we rejigged the setup such that instead of offering to charge for a fixed time, we teased it by offering energy for just long enough to see it start charging, and then refused to actually satisfy its cravings. This was just enough temptation for our poor little Atto 3, and it stayed awake for hours on the promise of jam tomorrow.

Wake up in a city that never sleeps

We were pretty happy with this approach. We could keep the BYD awake for as long as we liked without having to give it any energy at all, and then when the time was right finally let it have its fill. So we deployed it to a few friendly customers to see how it went in the real world. And as with all of the best plans, it is safe to say that it did not survive first contact with the enemy.

The first problem was that the BYD app was sending notifications every time charging started, so if the driver plugged in at 5pm and charging started at 11, they were told thirty times. We were meant to keep the car awake not the driver, but luckily you can turn those notifications off so not a big deal.

What was a big deal was that one of our drivers drained his 12V battery (yes, EVs still have those). At this point you can’t even wake up the car with the key, so there’s no way to force it to charge the tiny 12V battery from the giant traction battery. And even if you could, you’ve already ruined the 12V battery because they really hate being empty. Thankfully this wasn’t our driver’s first rodeo, and he had a charger to top it up.

You know things are going badly when your battery voltage has fallen off the bottom of the y-axis

You might wonder why a BYD which is being kept awake managed to drop the ball and not top up its 12V battery once it fell below a certain level. And we wondered that too. Luckily, one of our engineers is friendly with a BYD engineer, so we asked them. Apparently Chinese law (or perhaps BYD corporate policy, we never quite clarified) prohibits official contact outside the country, but this doesn’t matter if you have a mandarin speaker and WeChat so we set up a call.

WeChat so you don’t have to

The BYD engineers told us that the Atto 3 checks every five minutes to see if an attached charger is offering it any power, and if not after three loops it goes to sleep. So our 15 minute pause was a pretty good match.

It only checks whether the 12V battery needs topping up once an hour (it used to be even less often than that), and will only charge it from the traction battery if it’s below 11V. So to avoid completely draining the 12V battery while keeping it awake, it needs to last at least an hour after hitting 11V. And to do so without ruining the battery it shouldn’t really go much below 11V at all. Also it won’t do it at all if the traction battery is too low.

Luckily there was some good news. If you charge the car, the 12V battery gets charged at the same time. So we could avoid draining it by doing little bits of actual charging. The BYD engineers would figure out how much charging would be needed and get back to us.

Only they didn’t - hopefully because they were too busy coming up with a better AC charging design for the Seal, and not because they’d got into trouble for talking to us. Once again it was time to get out the number 8 wire and figure it out for ourselves.

Charge me up

(click to listen to the unofficial Evnex anthem)

How not to do it

We need to put as much energy back into the 12V battery as the car was consuming by being awake. So we did what any self-respecting engineers do in this situation: We got our hands on an Atto 3, wired it up to lots of instruments in our workshop, and Drew. Lots. Of. Graphs.

Sometimes we zoomed in!

As EV charging experts we’re used to dealing with giant batteries that work in kW and kWh, but these tiny little ones work in Watts and Watt-seconds (enough to drive your car about 2mm). We tried about thirty charging regimes and eventually satisfied ourselves that we could balance energy in and out for a sustained period.

Sometimes we drew two!

But we were slightly worried that this might not be all that great for the battery, so we popped down the road to talk to the best 12V lead-acid experts we know, the team at Dynamic Controls. They’ve been in the powered wheelchair control business forever, so they’ve forgotten more about battery health than most experts learn in a lifetime.

Normal people do this to cars parked at the office, right?

Lead on MacDuff

The team at Dynamic Controls looked at the graphs and were concerned. Not with our charging regime, they thought that was fine and wouldn’t damage the battery (phew!). What surprised them was that the battery used in the car was not the type they expected for this application, and also that the charging voltage was never going above 14V. So perhaps this goes some way to explaining why some drivers are experiencing shorter than expected lifetimes for their 12V batteries.

Speaking of lifetimes, we found an old envelope and did some maths on the back of it to see if turning charging on and off posed any risks for the charger - in particular the relays that connect power to the car.  A plausible worst-case is that some poor Atto 3 out there is being deliberately kept awake for 16 hours every night, 7 days a week (basically the same as being a parent of preschoolers). Even under this level of sleep deprivation, the charger relay should last over 10 years.

So we were satisfied that we had a way to keep the BYD awake that wouldn’t damage the 12V battery or reduce the life of the charger. Armed with this knowledge we made some final improvements to minimise the amount of unsmart micro-charging (limit to 6A, stop if the car doesn’t need it any more, different regimes if you know charging is coming quite soon), and then reached out once again to our wonderful team of real-world guinea pigs.

Don’t sleep, I know just what you’re saying

We’ve tried this out with ten Atto 3 drivers who between them have now completed over 50 charging sessions. Thanks to their generosity, we’re confident that we can now release our method for smart charging BYDs. And even better than that, we’ve now got lots more graphs!

Believe it or not, this is quite informative.

It also works with intermittent solar, keeping the car awake under patchy clouds:

The only Atto 3 in the world that’s smart charging on solar diversion?

I could stay awake just to hear you charging

It would be fair to say that we didn’t think solving this problem would get quite so complex, but this is the sort of thing you have to untangle if, like Evnex, you are on a mission to charge every electric vehicle from clean, affordable energy.

In the end, what this boils down to for a driver with an Evnex charger is a little button in the settings page that looks like this:

I spent three months mucking around with an Atto 3 and all I got was this lousy button

But if you’ve read this far, you’ve got some idea of the effort that went in behind that button. And there are a ton of buttons like that in the Evnex driver app which do all sorts of smart things, like turn on solar diversion, show you live grid carbon emissions, set your Time-of-Use pricing and report on your charging history.

We’re committed to keep improving the charging experience of our drivers, and we’ve got a great engineering team who don’t mind tackling big challenges to make your charging smarter.

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Tom Rose
Tom Rose
Product Owner
December 22, 2023