Hi All. I suspect I have a bad battery although it's only about 7 months old. Overnight voltage can go down to about 11.25V. I had it checked at NAPA where I purchased it and they say it's good after testing. I checked the drain while parked and it was about 325ma. Because of that, I don't think there's anything significant draining the battery overnight. I would like to monitor and document the current usage. After a lot of poking around on the internet I sense the way to go is a shunt resistor (150A in my case) and a sense amplifier. I'm looking for some advice in what is currently available in a sense amplifier that would be easy to use. Beyond that reading the output and gathering the data with an Arduino should be pretty simple. Any advice? - Scotty
Don't know what modern cars with electronic circuits use in standby, but 325mA sound a lot.
That will take an average 40Ah car battery down in a few days.
Better check what's draining the battery, maybe by pulling some fuses.
Leo..
That seems high. Some quoted specs are 50 or so mA. Some modules/systems may run for several minutes after turning off the ignition before completely shutting off or going into low power state.
I build a voltage and temperature logging system for my solar panels. Adafruit.com has a very nice logger shield that has a real time clock and writes to an SD card. This was my very first Arduino project and it was up and running in 15 minutes. They have great tutorials and example code. I used a voltage divider (just 2 resistors) to sample the voltage at whatever interval I want. The shield is $14 on their site and $19 at MicroCenter. I'm not sure about building a current sensing device, I do use shunts in my solar system. I suppose you could feed the taps into an Arduino Analog pin (after safe leveling) and log it. Logging the Voltage would be easy as well.
I agree with Leo, maybe time to start pulling fuses. It would be more fun however, to build the Logger first! 7 months is way too quick for a battery to crap out.
325ma is pretty high. I would question a bad battery since the normal failure mode for 12v lead acid batteries is for one cell to go bad (what happens is the sludge builds up, shorts the plates out and the cell goes dead which results in very high internal resistance in that cell so you are not able to pull any power to start the engine etc. With the old style batteries with the exposed bus bars, you could jumper the bad cell and start the engine from 10v instead of 12. With the newer batteries, you can't get to the bus bars. "Progress". Simple tests - check the specific gravity (with a hydrometer) of the cells. They should all be about the same. Check the voltage when the engine is running - it should be around 13.7-14.3v. You can also use a high rate discharge tester (Harbor Freight even carries them) to see what the battery does under load. I have yet to have a battery problem that did not show up with either the charging voltage being off (indicating a bad charging system) or goofy readings on one cell with a hydrometer. I did have one "Diehard" battery from Sears years ago that would only work if you tipped it slightly ... turns out they had not welded the internal bus bars for one cell and moving the battery made the connection. Great quality control (NOT).
gpsmikey:
With the newer batteries, you can't get to the bus bars. "Progress". Simple tests - check the specific gravity (with a hydrometer) of the cells.
Most of the new batteries are sealed, so no hydrometer tests either.
Maybe check the glovebox light.....
At that rate of discharge the battery may not be getting up to 100% charge during trips, in which case
it may be degrading rapidly (lead acid needs to be stored fully charged, not partially). Find the leak and fix it, and give it a proper top-up charge to make sure its at 100% before overnighting it again.
Faulty diodes in the alternator can give this effect
regards
Allan