A $14.99 Toyota battery app: built a LeafSpy for one car platform and sold the first batch to owners in their group chats

A Danish LEGO software engineer built an app after work that does just one thing: it connects to a Bluetooth OBD dongle and reads the battery of the Toyota bZ4X and its sister models on the same platform. No subscription, no in-app purchases, $14.99 one time. Seven weeks after launch it has only 5 ratings in the US store, so the volume is small; but it walks an old path again, and that path can keep going with another car platform. Below: who it sells to, why people pay, where users come from, and whether you should build one for a specific car too.

An Austrian owner used ChatGPT to find its bug

In 2026 Oct 2, on the German bZ4X owners forum (bz4x-forum.de), an owner from Bamberg posted: "Has anyone tried this app?" Below it were links to BZ-Battery on the App Store. Replies quickly split into two camps: one said "I don't have a bitten apple"; another glanced at it and said "it needs an OBD dongle, not for me"; others asked whether there was an Android version.

A few days later, an owner from Salzkammergut, Austria bought it, and the next day replied: it shows detailed trip logs, and battery temperature tells you whether to preheat; "it's just a toy." Later he fed the app's exported CSV to ChatGPT and checked three trips: on the 125 km trip the app computed 22.14 kWh, which matched the power integral; on the 60 km trip the app computed only 7.80 kWh while the integral gave 13.41 kWh, 40% off. The conclusion: the app's cumulative energy counter sometimes falls behind.

This forum page shows the whole shape of the product: users are owners willing to tinker, who plug in OBD dongles and export CSVs; what they complain about is whether the numbers are accurate, not whether it's expensive.

What it is

The first line on BZ-Battery's store page: "Reads the high-voltage battery of the Toyota bZ4X and shows you what the car keeps hidden, on your phone and CarPlay screen." You buy a Bluetooth OBD dongle yourself and plug it in under the dashboard (the author develops with OBDLink CX; vLinker and Veepeak also work, but the cheapest ELM327 clones can't read this car). Then you can see:

• The voltage of each battery cell, with the weakest one flagged; all battery temperature sensors
• The battery health (SOH) the car reports itself, plus measured capacity logged monthly
• The voltage of the 12V small battery
• How much energy each trip and each charge used; five dashboards, also viewable on CarPlay

The cars it supports are a whole family on Toyota's e-TNGA platform: bZ4X (2022–2026), bZ, bZ Woodland, C-HR+, Subaru Solterra / Trailseeker / E-Outback / Uncharted, Lexus RZ, and even China's bZ4X listed. When it launched in Sep 19 it only supported 2026 bZ4X models; over two weeks four versions expanded it to more than a dozen models (App Store, 11-06).

Price: US $14.99, Canada $19.99, Germany €17.99, Denmark 129 kroner. The end of the store page says: read-only, can't change any settings or clear fault codes; no account, no ads, no tracking; you can open a demo mode to look around even without a car.

The author, Andreas Halse Gravesen, is in Aarhus; by day he's a software engineer at the LEGO Group, under the company HGSoftware ApS. His earlier apps were small utilities unrelated to cars.

How many people are buying: a few hundred, estimated

First, the numbers, so you don't get fooled by "charted":

• Apple US store: 5 ratings, averaging 5.0 stars (11-06); across all stores there are fewer than ten, and in the last 7 days it added only 1 worldwide (Canada, 11-05).
• It has never been on any country's overall chart, only on each country's Top Paid Utilities category chart: #2 in Latvia (10-28), #5 in Luxembourg (11-06), #6 in Finland, #7 in Denmark, #20 in Norway, #33 in Canada (11-06); its best US rank was #104 (11-01), and on most days it sat between #150 and #196. In small-country paid category charts, a few sales a day is enough to chart.
• The number of countries it charted in went from 10-28 to 4 to 11-01 to 8, then fell back to 11-05 2. The wave from the early-October CarPlay version is receding.

Working backward from the review rates typical of paid tools, it sold roughly a few hundred copies, an inferred few thousand dollars in revenue. That's side-project scale, not business scale.

Compare two companies that use the same model: EV Dashboard, built for the Hyundai / Kia E-GMP platform ($12.99, launched late April). Six months in, it has 65 ratings in the US store and peaked at #49 on the US Top Paid Utilities chart. LeafSpy Pro ($19.99), the originator of this model, built only for the Nissan Leaf, has been selling for ten years. It has 115 ratings in the US store and is still ranked #4 on the US Top Paid Utilities chart as of 11-06. The ceiling is clear: the owners of one car model, times the small slice willing to plug in an OBD dongle.

Why people pay $15

The manufacturer hides the data. The Toyota and Subaru apps only show charge percentage and range, not SOH, individual cells, or the 12V battery; Toyota's remote features are free for three years, then require payment (bzforums owner thread, 2026-04). For a $40,000 car, you can't see whether the battery is healthy; for $15 you can, and owners do that math. Buyers of used bZ4X also ask on r/BZ4X "how do I check SOH."

It hit the 12V stall at the right time. The 2026 bZ and C-HR's 12V small batteries kept draining, the owner groups argued about it daily, in Sep 22 a law firm began collecting owner information for an investigation, and Toyota only pushed an OTA fix in mid-October. BZ-Battery's 2 Oct 1.4 update, the second line of its release notes was "you can now read the 12V battery directly." On r/trailseeker, an owner wrote a long review; the first thing they praised was "the biggest benefit is being able to see the 12V status now."

More cars on the road. The updated bZ sold 8 units in the US over its first 22,563 months (carbuzz, 09-08); the bZ4X was Norway's top-registered EV in July this year. The author is in Denmark, where the bZ4X is one of the densest markets.

Where users come from: the author posts in every owner group himself

There are no ads, no TikTok, zero discussion on X, no press, and the YouTube videos on bZ4X battery use other apps. Its users are almost all brought in by the author posting himself:

• The same text posted across: r/BZ4X, r/Toyota, r/trailseeker, r/ToyotaCHR, opening with "This app needs an OBD dongle; I use the OBDLink CX myself, and it was also developed around it. Unfortunately the app isn't free..."
• The CarPlay version got another round: r/BZ4X, r/Solterra, r/LexusRZ, plus the Facebook group "2026 Toyota BZ owners", with the line "needed Apple's special approval, and it finally came through."
• For the 12V version, in another FB owner group: "Many of you have been asking for this..."

The playbook is just one thing: each time it adds support for another car, post in that car's subreddit and groups; each update that delivers "the feature you asked for" gives a reason to post again. After that, owners share it on their own, as the German forum thread shows. In App Store "customers also bought," it's linked with evDash, Carista and OBDeleven, which suggests a small share of organic traffic comes from people searching the store for OBD.

Who it's competing with

It's currently the only iOS app dedicated to battery diagnostics for e-TNGA cars. But plenty of things can do much the same job:

Several parallel products already run this playbook: EV Dashboard for E-GMP, Voltage for Chevy Volt / Bolt, Battery Health – For Tesla / For Rivian for Tesla and Rivian. None of them copied BZ-Battery; they arrived at the LeafSpy route independently.

There's also a cost-changing factor: Clutch Engineering's open-source project OBDb (Pelican) has already published 4 parameter definitions for the bZ99X. Reverse engineering used to be the hardest step for this kind of app; now many cars have been reverse engineered for you.

If it were you

B is worth a shot, but this is a side-income ceiling business. The playbook is clear, acquisition costs are close to zero, and nobody is crowding in yet. The catch: each car platform has only a small group willing to plug in an OBD dongle, and one app selling a few hundred to a few thousand copies is about the ceiling. What can get big is "many platforms, one app each."

Who it suits: a solo developer who can write iOS (Android too, ideally), who owns or has access to a particular EV, who can hold an OBD dongle and go parameter by parameter, and who is willing to hang out in owner forums and Facebook groups and reply to posts.

Who it doesn't suit: anyone who wants one app to pay their bills; anyone who wants to build for the bZ4X (it's already thoroughly covered, and its author updates often); anyone who doesn't want to touch hardware or debug in the car; anyone who wants subscriptions (this audience buys one-time, and neither LeafSpy nor BZ-Battery has in-app purchases).

How to do it: pick a high-selling platform that doesn't give out battery data and has no dedicated app yet. VW MEB (ID.3 / ID.4 / Skoda Enyaq), GM Ultium, and BYD export models are worth checking first (my inference, go verify on the stores and forums whether anyone has built one). Check whether OBDb has this car's parameters first. If it does, you've saved half the work. Copy the feature set from this one: weakest cell, SOH, monthly capacity curve, 12V, trip efficiency, CarPlay. Read-only, no account, and a demo mode shown on the first screen of the store page. One-time purchase at $13–20. The bug from the German forum is a reminder: cumulative energy must be reconciled against the power integral, because these users will export CSVs and check you themselves. Build the Android version too. The first question on the German forum is about exactly that, and BZ-Battery doesn't have one yet.

How to acquire users: free. Post honestly in each model's subreddit, Facebook owner groups, and local owner forums (what OBD dongle is needed, not free, read-only). Watch for whatever owners are arguing about in the groups right now (recalls, 12V dead batteries, OTA updates, used-car battery checks), ship a feature that addresses it as fast as you can, then come back and say so in the group.

Timing: the earlier the better, depending on the car. The best window is the first month or two after a new platform launches, when owners start their first collective complaints. Once general tools like evDash or Car Scanner handle a given car well, the case for a dedicated app gets thinner.

Might be wrong: (1) The car models you're targeting already have ready-made dashboards in general apps (Car Scanner, evDash), and forums keep recommending them; (2) Once OEM apps start showing SOH and 12V (when Toyota or VW put battery health in their official app, this business is dead); (3) BZ-Battery still had single-digit ratings in the US store through spring 2027, with zero to two countries charting, which suggests even the first batch didn't sell through, so this model has topped out on e-TNGA; (4) Automakers are blocking OBD reads (one already blames third-party OBD devices for 12V dead batteries). If any of them disables these parameters in firmware, the whole path narrows.

Data and sources

• App Store product page and Apple's public API (US store, 11-06): price, rating, version, release notes, supported models; daily paid-category chart rank and rating changes by country, 10-28 to 11-06.
• German forum bz4x-forum.de thread "bz-battery" (opened 10-02, two pages): owner replies and CSV energy reconciliation.
• Posts by the author on r/BZ4X, r/Toyota, r/trailseeker, r/ToyotaCHR, r/Solterra, r/LexusRZ and two Facebook owner groups; long-form user comments on r/trailseeker. Reddit and Facebook post bodies couldn't be opened directly; quotes come from search-result snippets.
• Competitors: App Store pages for Car Scanner, evDash, EV Dashboard, LeafSpy Pro, Carista, Voltage (11-06); the OBDb / Pelican bZ4X page.
• Background: carbuzz (2026-09-08) US bZ sales; carsguide (08-17), fuelcellsworks (08-03) Nordic sales; classlawdc (09-22) 12V dead-battery investigation; bzforums owner threads.
• Apple doesn't publish sales or revenue. The "a few hundred copies" and "a few thousand dollars" figures in this piece are estimated from rating counts. Several auto-generated download-estimate sites online don't match Apple's numbers, so they weren't used.