Hi,

Driver looks fine. One minor thing, though:

On Mon, Apr 03, 2017 at 03:23:48PM -0500, David Lechner wrote:
> [...]
>
> +	batt->psy = power_supply_register(dev, &lego_ev3_battery_desc, &psy_cfg);
> +	err = PTR_ERR_OR_ZERO(batt->psy);
> +	if (err) {
> +		dev_err(dev, "failed to register power supply\n");
> +		return err;
> +	}

There is devm_power_supply_register() and it makes sense to use it,
since that means you can drop the remove function completly :)

> +	return 0;
> +}
> +
> +static int lego_ev3_battery_remove(struct platform_device *pdev)
> +{
> +	struct lego_ev3_battery *batt = platform_get_drvdata(pdev);
> +
> +	power_supply_unregister(batt->psy);
> +
> +	return 0;
> +}

-- Sebastian