Volatge Drop: Try to answer this.

I don't think there's any easy way of doing this, unless you know the full characteristics and the present charge state of your battery.

There is great variation of voltage drop with load between battery types, and also between different models of batteries of the same general type. Some batteries are better suited to high loads for a short time, while others are better for low drain over a long period, even though both may use the same technology (e.g. lead-acid), and have the same AHr rating.

Can it be done for both ways, currently I´m using a leisure batttery so its for low drain. All I need to know is what is the current load given the known drop in voltage. I am assuming I know the voltage with a permanent linked volt meter all the time.

The simple solution is to use a small resistor in series with your battery supply. By small, I mean less than or equal to 1 ohm (often called a shunt resistor). It will be important to know the exact value in order to be accurate.

Important: I would not depend on using an ohm meter for measuring this resistor because too many other factors will influence your reading. Instead, set up a simple circuit with the current monitoring resistor (the shunt resistor) included. Make sure that the test or calibration current is not enough to cause a significant heat build up. Perhaps it would be best to use a current close to what you expect to monitor.

Read or set the current level to some known or reasonable level, then measure the voltage across your shunt resistor. You may find the best results come from placing several 1 ohm resistors in parallel. Calculate your resistance R=V/I (it may vary with temperature). Now you can use this resistor in you circuit and monitor the voltage in order to determine the current drain from your battery.

Lastly, to obtain the best results you must solder your connections to your low resistance devices. Otherwise, contact resistance becomes a major source of error.

Try search results of "battery charge" or battery charge measurement" on CR4. You will find lot of useful information.

Un-Ub = r.I

where :

Un: Battery no load voltage

Ub: Battery output voltage

r : Battery internal resistance (refer to manufacturer )

I : Battery load current

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The voltage from 12.6 to 12.2V is as a result of the internal resistance of the battery.

It has a non-linear characteristic based on battery charged state and is only measured with specialist equipment.

Your easiest solution is to install a shunt in series with the battery line.

Shunts are available is many ratings. They are sold with or without matching ammeter .The ammeter is really a mV meter scaled in amps.

If you do buy the ammeter , remember you CANNOT cut the leads which come with the ammeter since the meter resistance is calculated with this length of wire.

You must use the complete length supplied.

Standard shunts is sized to develop a 50mV or 100mV drop at the rated current of the shunt. e.g a 100A 50mV shunt will debvelop 25mV at 50 A and 50mV at 100 amps.The voltdrop is linear with current. Shunts are available in ratings up to thousands of amps.

If you do not buy the ammeter option you can use a multimeter and measure the mV drop and calculate the current.

I have installed many on battery banks so I can monitor chrage and dischare currents. For that purpose , I bought the center zero ammeter so I can tell when the batter is charging and discharging.

http://www.allaboutcircuits.com/vol_1/chpt_7/2.html

Since batteries have a continuously changing drop in voltage and amperage while discharging, measurements change continuously also. isolating each voltage drop and measuring the amps for each drop can give you a rough idea of which drop is the most stable and then you will know where your largest fluctuations are coming from. If a consistent current flow is needed a charging source will help.