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I'm trying to design a product that is powered from the 12V rail inside the car. In normal driving (engine running, electronics on etc.), what would be the minimum voltage of a low battery?

Reading online shows a lot of forums and mismatched information. The lowest appears to be 11.3V? Not sure how correct this is?

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    \$\begingroup\$ Do you want to work while the engine is being started, while the starter motor is "cranking"? The battery voltage can be very low during that short period of time. \$\endgroup\$ – Elliot Alderson May 3 at 13:18
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    \$\begingroup\$ No, just when the vehicle is running in normal operation. \$\endgroup\$ – Fat Diode May 3 at 13:21
  • \$\begingroup\$ Why don't use a 5V USB adapter (phone charger), designed to be plugged in the lighter? A quality one will be designed to meet all the nasty voltages that are encountered (see Neil_UK's answer ...) \$\endgroup\$ – anrieff May 3 at 13:27
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    \$\begingroup\$ I need 11V+ for my device to reliably work and I don't have the room for any switching devices. \$\endgroup\$ – Fat Diode May 3 at 13:40
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Inside a car is a particularly brutal environment.

It depends whether you're making a one-off for your own joy, or manufacturing something to sell.

There are two specifications, working, and not being damaged.

For a one off, working from 11 to 14v would be good enough. To sell, you would need to work down to 8v for ancillary functions, and to 5v for engine critical functions.

To avoid damage, it would be a nice idea (for a one-off) you must (if you're selling) be able to tolerate sustained reverse 12v (if somebody tries to jump start with a reversed battery), sustained 24v (if a garage jump starts with a lorry battery) and transient 160v (load dump when the last heavy load is removed from a charging system).

You might want to think about the highest temperatures you can meet on a sunny day in the cabin, or in the engine compartment.

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  • \$\begingroup\$ "For a one off, working from 11 to 14v would be good enough." - As far as I know, ~14.5V is common in a running car (to charge the battery), so I'd design for at least up to 15V. \$\endgroup\$ – marcelm May 4 at 12:42
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From practical experience (worked at an electronics supplier for a car company), during starting it's common to see 7V. This is of course intermittent. For cheap cars, it's still considered somewhat acceptable to have subsystems turn off in this case.

Now that the EU strongly favors start-stop systems which turn the engine off at traffic lights. These are basically restarts with still-warm engines, so less taxing on the starter engine. You won't see drops to 7V there, but I wouldn't bet on the battery staying over 11V either. But how far do they drop exactly? It's an optimization decision by the car manufacturer in the end. These start-stop systems do try to estimate if the engine is "warm enough" to do a rapid, clean start. That's not an absolute criterion, though. Being more aggressive in turning off the engine means less CO2, but rougher restarts. The CO2 norms are being tightened frequently, so it may still be wise to design for that 7V as the worst case.

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