Moe wrote:The reason car manufacturers haven't already done this is because it's not really practical. The main problem is that battery voltage drops under load, so voltage is not a reliable measure of battery condition unless everything is switched off.
If you were to simply break the circuit when the voltage dropped below X, you would never be able to start the car because the starter motor would trip it every time.
When the engine is running you'd be measuring alternator voltage, not battery voltage, so the measurements would be meaningless. As others have hinted, cutting the battery when driving would be very unwise anyway - under conditions of very high load and low alternator output (low revs and all consumers on) the car could be reliant on the battery and your system would kill it. Also, if you have a fault with the charging circuit then sometimes you can get home on battery alone (this has happened to me twice!).
So, you'd have to link it to the ignition somehow so that your cutout was disabled when the ignition was on.
This could work with the engine off, but even then it's not simple. Choices are a) measure voltage, current, ambient temperature etc, try to predict what the expected battery performance should be and compare it with actuals; b) wait for periods of no load to do your measurements; c) rely on voltage alone and prevent the car owner from using his/her lights and/or stereo on battery power.
Finally, the practicalities. This would be a big relay - starter motors can take 200A - not impossible, but expensive. And the system would have to fail-safe so a software glitch (or dead battery that kills the Pi itself) doesn't kill the car altogether.
You raised some good points, I thought myself that a relay will probably not work, as the starter pulls a huge amount of amps in a short amount of time (that is why we have these large batteries in the first place).
I was thinking about a screw design, that the pi could actuate a little motor that would turn a screw and disconnect it that way.
And of course it would have to be fail save.
I would probably make something that the system will not even work when the engine is running, as there can only be two situations when the engine is on:
1. Battery is being charged or alternator is providing enough juice for the whole car to run.
2. Alternator is dead, battery must be available to get to a repair shop, time is ticking down till the car will not have more energy for powering the ignition.
In both of these cases, the battery does not get discharged due to a mistake and the cut off circuit would not serve any purpose.
I would make it time delayed, so the disconnect would only kick in if battery voltage drops to less than 10 volts or so and STAYS there for a long time. Starting would not trigger it, as it would be too slow set.
An interesting fact: If you connect a battery monitor to your cigarette lighter, you will notice that battery voltage (and system voltage) does never ever drop even below 12 volts when the engine is on and the alternator is running. The reason for this is that you can't charge a car battery at 12 v or below, you need about 13 or even 14 volts and the alternator provides that, even at idle, it never drops that low. If it does, then the electrical system is damaged and the battery will start getting discharged (and the battery warning light will come on).
I have never seen a situation with my car that would ever need the battery to support the electrical system, the alternator will provide enough power (again, more than 12 v even, since it must charge the battery, even at idle (which the voltage measurement confirms). If the car uses more power, the car will automatically raise the RPMs to produce more power and this is with a 1995 Toyota, so this mechanism seems to be really robust and old.
Now the only time you could collapse the voltage enough when just running on the alternator alone is when you have a sudden spike in power consumption that coincides with extreme low rpm, like when slowing down suddenly, turning on the high beams, brake lights, seat heatings and window defroster, but this is only with a totally busted battery. If your battery is connected, it will always act as a safety buffer.
It is a pretty robust system, but again: The cut off circuit would only be active when the engine is off or more precise: if the key is not set to ignition.
The cut off voltage should be easy to determine, I have noticed that battery voltage will not drop fast in a healthy battery, even if it is not being charged.
But there is a point of no return, which might be different for every car. It is the point when the battery reaches a certain low voltage where it can no longer provide enough power for the starter and the ignition at the same time (it can happen that you can crank the starter still, but the ignition voltage is too low and the car does not start). Battery health is not such a big factor really, as in a weak or damaged or old battery, the voltage will just drop faster and reach the cut off voltage.
BUT and here is the kicker: Batteries often die because there is no protection circuit like the one I propose:
Normal car batteries do not tolerate deep discharging, they get irreparably damaged. Every time they run down, they take a little damage, till they are all broken. There are of course marine batteries, that allow deep discharge.
So I wonder how long my batteries will last in the future once they cannot possibly ever run down.