Go Down

Topic: 12V Battery monitor - current and voltage (Read 8 times) previous topic - next topic


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 :-)

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. 


May 28, 2011, 09:08 am Last Edit: May 28, 2011, 09:17 am by Graynomad Reason: 1
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.
Rob Gray aka the GRAYnomad www.robgray.com

Go Up