Are Bluetooth car battery monitors any good?

A Bluetooth battery monitor is a small box that fastens to your battery's terminals, measures its voltage around the clock and sends the readings to an app on your phone. They're cheap and popular, especially with people whose car sits for days at a time. They do work, as long as you know what a voltage reading can and can't tell you.

How does a Bluetooth battery monitor work?

Almost all of them measure one thing: voltage at the battery terminals, sampled over time. From that history the app draws graphs and gives you three useful readings:

Our battery voltage chart explains each of these in detail.

What a battery monitor is good for

Catching a drain. This is the strongest case for a monitor. A healthy parked car's resting voltage settles and then falls slowly. A drain shows up as a steady slide overnight, or a sudden drop at the same time every night. A monitor that logs while you sleep shows you that a drain exists and when it happens, which a one-off meter reading can easily miss. Finding which circuit it's on still takes a meter and the fuse box; see finding a parasitic drain.

Showing whether your driving keeps the battery charged. If the resting voltage the morning after each drive keeps creeping down over a week of short trips, the battery isn't getting enough charge back. That's one of the four usual reasons a battery keeps dying.

Flagging a charging problem. If the voltage with the engine running stays near resting voltage, the battery isn't being charged. On newer cars with fuel-saving charging control, a lower reading at times is normal, so look for a pattern rather than one reading.

Watching a stored car. For a car that sits for weeks, a monitor tells you when the battery has fallen far enough to need a charge, before it's flat.

What a battery monitor can't tell you

Measure battery health. Voltage shows charge, not the battery's ability to deliver current. A worn battery can recover to 12.6 V overnight and still collapse under the starter's load. East Penn calls a load test "the best way to determine if the battery is delivering adequate electrical performance" (East Penn); a shop's conductance tester is the quick alternative. Some apps display a "health" percentage. Treat it as an estimate from voltage, not a test.

Give a true resting reading right after you park. A battery that was just driven or charged reads high for a while. The Battery Council International defines open-circuit voltage as one taken "after a specified stabilizing rest period" (BCI), and MINI's dealer instructions only trust a reading after at least 4 hours of rest (MINI SI M61 01 02). Read the overnight value, not the one at the moment you park.

Reach you when you're away. Bluetooth only works within a short range of your phone. A monitor can store readings and sync when you come back, but it can't alert you at the office that the car at home is draining (our inference, for monitors without Wi-Fi or cellular).

Do battery monitors drain the battery?

A little, all the time. A monitor is always connected, so it is always on. Look for the maker's stated current draw and add it to your car's parked draw; a typical car draws a few tens of milliamps once asleep, which is the number to compare against (see parasitic battery drain). Subaru warns its technicians that "even a small, acceptable draw from a device can add up to an excessive overall draw" combined with the car's own (Subaru 07-236-25). If your car sits for weeks, a monitor's draw is part of the reason to check on it.

How to install a battery monitor safely

• Safety. East Penn's rules apply: eye protection, no sparks or flames, and don't lean over the battery, which gives off explosive hydrogen gas.
• Check your car first. Some cars shouldn't have their battery terminals loosened. Toyota Canada says that on models with a battery state sensor on the negative terminal it is "highly recommended not to disconnect the battery terminals under any circumstances" (Toyota Canada). If your manual says the same, ask a shop to fit the monitor, or use a monitor that connects somewhere else.
• Tight connections. A ring terminal under the clamp nut must not stop the clamp from seating firmly. A loose clamp causes the very starting problems you're trying to catch.

Plug-in monitors: OBD-II port or 12 V socket

Some devices read battery voltage through the OBD-II port or a 12 V power socket instead of the terminals. They are quicker to fit and don't touch the battery. The trade-off is that they measure through the car's wiring and a fuse, so their readings can differ a little from a meter on the battery posts, especially when current is flowing (our inference). For trends, such as an overnight slide or a deepening cranking dip, that difference matters less than having the readings at all.