SOH tester: how do you measure the health of an EV battery?
An SOH tester reads the battery management system and converts that data into a percentage. What such a device can and cannot do, what types there are, and what to look out for before purchasing one.
What an SOH tester exactly does
An SOH tester is a readout device that communicates with the car's battery management system via the OBD port. That system itself tracks how much capacity the battery pack still has compared to when it was new. The tester retrieves that value, along with cell voltages, temperatures, and the number of charge cycles, and converts that back into a single number: the state of health.
That percentage largely determines the residual value. An electric car at 95 percent is in a very different position than the same car at 82 percent, even if the mileage is the same.
Reading out is not the same as testing
This is the most important distinction when choosing a tester. Reading out means that you adopt what the battery management system itself thinks. That figure is an estimate from the manufacturer, calculated differently for each brand, and it can deviate from the actual capacity.
During testing, that claim is verified against the behavior of the battery itself: cell voltages drifting apart, capacity lagging during charging, temperature differences between modules. A weak cell reveals itself especially at a high state of charge — with a half-full battery, such a thing is easily missed.
A cheap dongle does the former. Anyone who wants a result that holds up during a sale, trade-in, or dispute needs the latter.
What types of SOH testers there are
Consumer dongles with an app. Cheap and widely available, fine for getting an impression yourself. They copy what the battery management system reports, do not cover every brand by far, and do not provide a document that is useful to a buyer.
Professional diagnostic kits for the workshop. Multi-brand, fast, and resulting in a report. This category includes the TBE Toolkit and equipment from brands such as Aviloo and Moba.
Brand-specific dealer equipment. Accurate for that single brand, but useless for a mixed fleet and not independent: the diagnosis comes from the same party that handles the warranty.
What to look for when choosing
Five things determine whether a tester is a good fit for you. They carry more weight the more cars you process with them.
Coverage. How many brands and models are supported, and is your fleet included? We test over 680 models from 60 brands, including plug-in hybrids.
Speed. A measurement that requires a twenty-minute drive first is not something you quickly do in between trade-ins. A three-minute readout, however, is.
Outcome. Do you get a number on a screen, or a document with the chassis number in it that you can attach to an advertisement?
Independence. A result from the party selling the car carries less weight than a result from a third party.
Cost per measurement. Besides the purchase, the cost of each test matters. With three cars per month, the math is very different than with thirty.
The TBE Toolkit
Our own kit for car dealerships: the dongle, the app, and access to the test platform. You perform the measurement yourself on your own premises, and a certificate linked to the chassis number is generated immediately. The cost of the kit and exactly what is included can be found on the order page.
Buying a tester yourself as a private individual?
Usually not worth the effort. A dongle that supports one brand well often costs more than a single measurement at an affiliated garage, and you don't get a certificate out of it. Anyone wanting to have a single car inspected before purchase is better off with a one-off test.
Frequently asked questions about SOH testers
Can I read the SOH of my car myself?
Yes, you can get quite far with an OBD dongle and an app. You then read the data from the battery management system, which is different from a controlled measurement with a certificate.
How much does a SOH tester cost?
That varies widely, from a consumer dongle costing a few tens of euros to professional sets for the workshop. The cost of the TBE Toolkit can be found on the order page.
Does one tester work on all brands?
Not a given. Brand-specific equipment only works for that brand. Our set covers over 680 models from 60 brands, including plug-in hybrids.
How long does a measurement take?
With us, it takes three minutes. There are also test methods where you first have to drive a bit; those take considerably longer.
Measure it yourself or have it measured?
Car companies perform measurements themselves using the TBE Toolkit. Private individuals have a single test conducted at an affiliated garage, including a certificate.