12V Battery monitor - current and voltage

I found an article in the read-only forum regarding 12 Battery current monitoring. The thread did not finish with good recommendations, so I post this.

I would like a non-invasive measure, so I would like to use “clamp-on” current sensors like the MT7191-ND

I have some questions:

  1. What is the difference between AC and AC/DC sensors? Is it possible to convert a AC sensor to also work on DC?
    By mistake I bought a few different sizes of sensors like SCT-013. Can you make this work on DC?
  2. I want the current monitor to be as accurate as possible, but the current range from the alternator can be from 0-140A.
    How can I make the sensor as accurate as possible? (I read someone had used a ADC converter to make the sensor more accurate).
  3. Does any one have experience with these kind of DC current sensors and can recommend some? (I need different sizes (5,10,30,150))

Note! this is for a boat and not a car. I have 3 different chargers (alternator (140A), dual 220V (2x30A) charger and a solarpanel (<35W12V) charger). I have 2 battery banks, 1 Start bank and 1 (2x115Ah) service/consumption bank that will deep cycle.

Some features I would like the monitoring to have:

  • Realtime current consumption (broken down in some categories like lights, refrigerator, etc)
  • Realtime current charging (alternator, charger, solar panel)
  • remaining capacity of battery banks
  • remaining time until empty battery based on “current” consumption
  • battery health calculated from how much battery drains on a given consumption compared to a new battery bank.

Hope someone can point me in a direction.


Hey there, I was looking to do something like this as well. I'm also a sailor. I have somewhat the same setup as you. I have 3 100ah batts for the house and one for the starter.

Here are some things I have found. Maybe we can share notes.




Now to figure out how much juice is still left in the batteries will be alittle more tricky.

Trancen: Yes we can share notes. No problem. I am just in the planning, try and fail state now. The Current sensing is only one part of it. I plan to also monitor temperatures, RPMsignal from engine, Gas detection, etc.

My plan for the battery health is basically to monitor/log voltage and current. When batteries are new they will give me readings of how a new battery bank performs. this can later be used to compare as battery gets weaker. Need to start somewhere I think :-)

KE7GKP: By ADC, I mean Analog to Digital converter like this one: http://focus.ti.com/lit/ds/symlink/ads1210.pdf It has much higher resolution.

I also found a post where they described using a smaller sensor when currents dipped blow a certain point. Not sure how that could be done because I do not know what will happen to a 30A hall effect current sensor when 140A is going through.

Even with a 140amp alternator I don't think you will ever see anything remotely that high.

I have a 120amp one and I don't think I have seen more then 20 amps going into my bank that was down 30% from about 300amp hrs. Throw 140amp into a bank of batteries and I'm sure you will boil them dry in minutes.

I'm also planning to put in gas monitors. What I'm currently working on researching how I'm going to monitor my water tanks and grey water tank too.

I'm a fulltime liveaboard on my Catalina 34.

IIRC as a rule of thumb you charge wet cells at 10% of their AH rating, so for a 140A charging current you would need one heck of a battery bank. You have a 230Ah bank, so I would say anything more than around 20A would be an issue.

That makes the problem really easy, get an ACS712 current monitor chip, or read across a shunt with something like an AD8210.

To determine the SOC (state of charge) is harder, you have to implement "Pukets Exponent" (sp?) which basically says you don't get out of a battery what you put in. This is a number that you multiply the current out by (or I guess divide the current in) to allow for the inefficiencies in the battery.

Once again IIRC for wet cells the number is often used is 1.2, although in theory you should ask the manufacturer.

And of course you can't just read voltage to do this, you have to know the recent current history of the system, like whether the batteries were held in absorption for long enough etc.

It's a bit of a black art to which I'm not privy, still I think you can do a reasonable job with simple tools and algorithms.


You forget something. It is not only the battery that is a consumer when engine is running. I have actually measured with a clampamperementer loads of up to 99% on the alternator (140A). My 290hp Yanmar engine has a airheater that is on when you start the engine. This seems to consume everything for 4-5min after start. It actually will consume from alternator and the batterybank.

I also have 2x5hp thrusters (Aft and forward). They also consume from alternator and batterybank for the short seconds they run.

That is why I need sensors that can deliver up to max, if not they will blow up.

I know that I need historic currents to be able to give a healt state of the battery bank. This is the point of my system, it will log data to SD card and then I have data to calculate health over time.

Hope this clearify my situation.

So the big loads are off your start bank are they? I hope so because that’s no way to treat deep cycle batteries :slight_smile:

The fore and aft thrusters will pull 600A+ in total (did I get the numbers right) so running both at once (does that ever happen?) will certainly draw a lot from the batteries.

Assuming you can measure that I’m not sure how you relate that to battery SOC as 1sec @ 600A is not the same as 600secs @ 1A because I’m sure the relationship is not linear.

I know that I need historic currents to be able to give a healt state of the battery bank.

Yep, I’m just talking about a SOC meter to tell you the current state of charge, that’s different to historic logging to get battery health.


Yes. this will not be an easy task.

Actually I have a Battery seperator (not a diode-divider). This is a relay that connects the banks as long at the voltage is above a given level. This means that all my batteries+alternator will be used when thrusters run (if the voltage do not go under 12.5V). If voltage is goes under 12.5V the seperator closes and only my start and alternator gives power to the thrusters.

I do not want to make my sensors. I like some almost ready to use. I do not want to use "shunt" or hall sensors that require me to cut cables, because the boat is so new and have warranty.

That is why I would like to use sensors that go "over" the cables like a clamp.


Here's a site that discusses hall effect sensors ("clamp" type, although the ones shown there are fixed rings) http://www.scienceshareware.com/how-to-measure-AC-DC-current-with-a-hall-effect-clamp-.htm

You can buy a sensor for as little as $7 (Digikey part numbers are given), but you have to do processing on the "raw" output voltage signal. That site also sells boards that give you a more complete sensor.

You can buy multimeters that have DC clamp sensors, but they a little expensive.

Hello again I had a look at digikey and found this sensor: http://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/L01ZXXXS05.pdf 1% accuracy, and around $20 pr sensor.

If I understand this correctly, these are 5V, meaning that then can be connected directly to an arduino? For a 50A sensor this means the accuracy of the sensor is +-0.5A on 50A. and +-0.1A at 10A, is this correct? Arduino will have a 10bit resolution, so 0-50A will get a resolution of 0.049A. Is this correct?

I looked briefly on the Honeywell sensors, but they require a inverted amplifier (or voltage divider) in order to reduce input to 5V.

Maybe I will order a few of the tamura sensors to test.

I think I will have to make a PCB for them aswell and some "rubber" casing to make them a bit more suited for the environment they are going to operate in.

Another question that rises is how long sensor cables can be and how will they be affected by noice. Are shielded calbles good enough or do I have to convert it to a serial protocol as close to sensor as possible? (I will ofcourse test this with my scope, but just incase someone has theories or facts around this).


MartOn: For a 50A sensor this means the accuracy of the sensor is +-0.5A on 50A. and +-0.1A at 10A, is this correct? Arduino will have a 10bit resolution, so 0-50A will get a resolution of 0.049A. Is this correct?

Scanning sensor options and trying to get some realistic expectations is probably a good start on a project like yours.

Accuracy for the hall sensor you linked to above is with reference to full scale, so +/-1% error for the 100A sensor is +/-1A. This is also just part of the picture as there is offset error, non linearity and temperature related errors adding to this.

Sensor output is ratio-metric to 5V for 1.25 times the rated current. A 100A sensor will cover a range of 1.25x200A (+/- 1.25*100) and for 10-bit ADC, resolution is then 0.24A. Account for ADC errors of +/- 2LSB and you’re adding +/-0.96A to the above sensor error.

I use a current shunt connected inline with house battery ground. The shunt drops +-50mV for a full scale of +/-100A. To read this, I use a delta sigma 24-bit ADC and after calibration (range and offset) this gives me a resolution of approximately 50mA across the full +/-100A range.

BenF: Thank you very much for your insights. It seems these kind of non-invasive sensors are not optimal for my project.

Are you willing to share which shunts and ADC you use? Have you developet PCB for the sensor or is it usable without making a PCB?

As long as the size of the shunt is ok, it should be possible for me to insert them in my boat, but they need to be big, the cables are quite large :-)


Sorry for reviving an old thread here, but I am looking at similar stuff, looking at monitoring some 12V batteries on a boat and then using the itead 3g shield to report if they get critically low on voltage or similar. I can probably deal with just having a voltage divider, but was curious to hear how you gents ended up with your monitoring scenarios.

Happy to share mine once it's built, only just ordered the 3g shield and expect to busy learning some AT commands for a month or two, but after that should be free :)

Bump....very interesting topic, another Australian here keen to understand and learn from your work....any updates / sharing / comments welcomed... :)