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WOW, what a great looking forum! The breadth of issues covered makes me wonder where to start - as an Arduino newbee.

I have a reasonable background in hardware and software but would appreciate pointers as to where to look for ideas.  The arduino looks like a way to do just what I want to do. I don't need to reinvent the wheel.

I want to measure car battery voltage and log it on a continuous basis - maybe once every 15 minutes - preferably with a time/date stamp. If the voltage drops below a pres-set value I would want to activate an alarm of some sort (sound or light).  I have a second battery in the vehicle that I would use to power the arduino and associated circuitry.  I'd log for say two weeks before overwriting stored values.  (There are periods up to a couple of weeks when I would not use the vehicle.)

I had a serious issue with a new car (campervan) that the dealer can't/won't help with.  The main battery voltage dropped to 5.7V overnight with apparently nothing turned on!  I think it has happened twice. This is almost fatal in a diesel and I cannot afford to be stuck out of mobile phone range with this problem. I want to see if I can work out what causes the problem and when.

I'd appreciate suggestions about the arduino I might look at and interface boards that would be useful.  Any polite suggestions appreciated.

Thanks in advance,

transparu
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Left Coast, CA (USA)
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Well a good basis for your datalogging application would be a nice SD/RTC shield that I just happen to write a short review.
http://www.adafruit.com/products/1141
http://arduino.cc/forum/index.php?action=profile;u=7576;sa=showPosts

So welcome to the Arduino world, hope you have a lot of fun and keep us posted on your project progress.

Lefty

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Quote
I had a serious issue with a new car (campervan) that the dealer can't/won't help with.  The main battery voltage dropped to 5.7V overnight with apparently nothing turned on!
This doesn't sound good. If the dealer won't fix it, I'd take it somewheres else. There must be
many repair shops around.

Alternatively, for a 100 Amp-hr battery to discharge in 12 hours would mean a drain of 8 Amps
continuous, which is quite a bit. For starters, with everything turned off, I might try pulling each
of the fuses in the fuse block, and measure the current through the contacts by connecting an
ammeter across the fuse terminals. Most DMMs will have a high Amperage range.
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I want to measure car battery voltage and log it on a continuous basis - maybe once every 15 minutes - preferably with a time/date stamp. If the voltage drops below a pres-set value I would want to activate an alarm of some sort (sound or light).  I have a second battery in the vehicle that I would use to power the arduino and associated circuitry.  I'd log for say two weeks before overwriting stored values.  (There are periods up to a couple of weeks when I would not use the vehicle.)

I had a serious issue with a new car (campervan) that the dealer can't/won't help with.  The main battery voltage dropped to 5.7V overnight with apparently nothing turned on!  I think it has happened twice. This is almost fatal in a diesel and I cannot afford to be stuck out of mobile phone range with this problem. I want to see if I can work out what causes the problem and when.

Not really helping on the Arduino side, but there are well known and straight forward ways to track down current drains on a vehicle - all it takes is a little time. You could do this yourself far more easily than you could implement a voltage logger. Also consider getting a smart battery isolator which will cut off the battery before the voltage drops to the point that the battery itself is at risk. The further the battery is discharged below its nominal 12V the greater the risk of damage. By the time it has been discharged to 6V I think it is probably ruined. The isolator won't cure the problem, but will prevent it from damaging the battery and also in most cases leave you with enough charge to start the engine. This also comes in handy if you ever find you've left the lights on etc.
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Hello
When you say new car(camper van) is this a brand new vehicle or  just new to you? What I am trying to determine if this is a new battery being drawn down by either not being charged properly or drawn down externally by some load current. If this a older battery, it may have bad cells which can be tested with an inexpensive battery specific gravity tester.If there is a external drain then that should be fairly easy to find..It was suggested to try another repair shop and  that would be a good place to start as most likely they would have the proper equipment and a good tech.
goodluck
jolphil
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Folks,
Thanks so much for comments and suggestions. smiley Lefty, your links are most interesting and useful.  Much appreciated and I'll get into the Arduino with these as a basis. I do realised this is an Arduino board and NOT a campervan one, hence the nature of my post.

To answer other comments/suggestions.

I bought the (German) vehicle new in Jan 2012, factory configured to my specs, and had it professionally converted in Australia before I saw it.  The intelligent charger for the house batteries can only operate when the vehicle alternator is actually charging and in any case it will only pass current to house batteries (two, high capacity) when voltage is above about 11 volts.  (ALL circuits in the camper side of the vehicle had been switched off for a week and the house batteries were at 100% charged state at the time I found the problem.) The local service agent checked the vehicle electronics/electrics and could find no fault. The vehicle has the Canbus system. Battery was tested too and found to be fine (fortunately). They were persuaded this was a reasonably warranty issue and didn’t charge the $175 fee but they weren’t prepared to look further. Given that the issue has occurred only once (or possibly twice) in the 10 months I’ve been driving the vehicle I am probably not surprised they didn’t find anything.

I said in my post the voltage drop was overnight.  It might have been a couple of nights/day. I am capable of measuring voltages and current draw etc, but given that this has occurred so infrequently and that the car often sits a week or two unused with no issue, individual measurements seem unlikely to help much.

I suspect I would void my 3 year warranty if I put an isolator between the main battery and the vehicle. I’ll ask.

 I have driven my other camper for 28 years and never once had such an issue!

Folks, thanks for your interest.

transparu
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I don't see that adding an isolator would have any bearing on warranty. It does not require any modifications to the vehicle.

It would be worth checking for interior light switches left in unusual positions, door switches not reliably making contact and trying to switch the interior light on, door mirror adjuster switches being left in unusual positions.

None of these *should* cause a current leak but I've had cars with some distinctly unusual wiring designs, including circuits switched through completely unnecessary relays that then consume current for no obvious reason. For example, one car had a door switch that operated a relay to turn the courtesy light on, and an additional switch on the courtesy light that could turn it on or off. So if the door was left ajar with the light switched off, the relay would still draw a significant current and flatten the battery over a day or so. It sounds to me as if your vehicle has some highly non-standard electrics and I wouldn't assume that these have all been designed sensibly.
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None of these *should* cause a current leak but I've had cars with some distinctly unusual wiring designs, including circuits switched through completely unnecessary relays that then consume current for no obvious reason.

That reminds me of a strange wiring 'feature' I had in a yellow 1966 Ford Mustang GT fastback when I was young. With no key in the ignition at all, if one turned on the emergency flasher, switched the turn blinker on and stepped on the brake pedal the radio (if left turned on) would pulsate on and off in time with the blinker. Don't ask me how I learned that sequence but it was kind of a cool 'feature' to show off to people.  smiley-grin

I loved that car.

Lefty
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Hi LEfty, I am pretty new here also, but I am trying to do something similar.

WHat I have learned is:
You need a Arduino
You need a data logger - https://www.sparkfun.com/products/9530
You need to then apply a Voltage Divider to your source, as its 12v, and you need it at/under 5v. - http://www.sparkfun.com/tutorials/207

Then you can plug VOut into your A0 pin, and it should give you a pretty accurate reading.

(I am in the middle of setting this up, but this is what I am moving towards).
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