Measuring `State of Charge' of A Battery

Download data from the battery manufacturers web site (essential if you are to get the best from your battery...trust me I've ruined one )

If you are discharging into a nice steady load then the voltage should give a good indication, but it will be load dependant and may even need you to take measurements and graph it yourself if you want real accuracy, it may well also be temperature dependant.

The data from the manuf' should give you a good start point.

Make up a simple comparator with an adjustable voltage reference and some maybe a little positve feedback so it latched nicely, have the output drive a nice LED. There should be loads of circuits for this sort of thing on the web or in application notes for comparators (National Semiconductors usually have excellent app' notes)

Have fun

Del

Your 12V battery is made up of 6 cells. Cell voltage is the primary factor in determining when a battery is fully discharged. The manufacturer has specific minimum cell voltage values which must not be breached. Exceeding the min. voltage can lead to cell reversal, where the polarity of a cell reverses, and that cell begins to charge from the discharge of the remaining cells. This causes extreme gassing and flammable/explosive conditions.

If you can measure the voltage of each cell individually, a reasonable approximation can be made of the state of charge. Even though the voltage is load-dependent, it is still the best method of estimating remaining capacity for a given discharge rate. Connect a wire from each cell terminal to a 2-pole, 6 position switch. Connect an inexpensive digital voltmeter to the center terminals of the switch. Use the switch to select individual cells for display. If the voltage drops to minimum, you can get more energy out of the battery by reducing the load current (which will allow the voltage to rebound a bit).

Most commercial VRLA's don't provide access to inter-cell contact points to check the voltage of every cell. The next best indicator is specific gravity of the electrolyte in each cell. There are instant reading SG meters, but they are not cheap. Cole-Parmer advertises one as "low-cost" for US$1695.

check out www.batteryuniversity.com/parttwo-35.htm You may also want to have a look at the Rolls battery web page. Rolls is the top lead acid battery maker in the world. I have seen 30 year Rolls battery's still showing no signs of failure.

What gauge wire are you using and how long are your wire runs.

Del and Power2 just gave you very detailed directions. My 2cent is reffered to your questions of charging the battery and avoiding discharging it bellow 50% capacity.

I had some solar modules with a capability of 2.5A at noon, on a sunny day of jully, down here, in Houston. So, if you connected such modules, in series, you would need a current generator to provide your battery with a tenth of its capacity as a charging current. If you charge a battery that is some 25 Ah, you don't need the current regulator. Now, a car battery, when it is charging (still under the current from alternator) will read even a 14 something volts, depending of its internal resistance. Without charging it is considered charged at 13.5V, and in need of charge at 11 something (again, the manufacturer if the one who knows better). So, for your small battery (7Ah), if you do not have data from the manufacturer, just measure the voltage at the end of charging day. As for not discharging bellow the 50% capacity (others will allow you to 30 - 35%), just calculate half of the battery capacity (3.5Ah), and if you allow for a night use of 10 hours, you can "provide" 350mA at 12.5V for that time.

Thanks to Del, Pwr2, Indel and the Guest.

Unfortunately, the mfr. does not have discharge graph showing voltage drop vs. load and time. He has asked me to find out the depth of discharge (DOD) as suggested by Indel based on load x hours and then charging current x hours. Full charge is confirmed if max voltage is shown. This method is not relaible, when i use solar panels for charging, as on some days the battery nay not get fully charged at all due to poor sunshine and the charging also may vary during the day making it difficult to predict the state of charge (SOC). Secondly, the battery capacity also depletes over a period. But, I guess I have no other alternative but to use this method with a lot of built in safety factor on load and charging / discharging time.

There is no scope for meauring individual cell voltage. I wonder how these battery manufacturers measure the charge and draw the SOC / DOD graphs.

Why wouldn't you use a deep discharge battery?

I am using a Deep Discahrge battery, but I understand, frequent deep discharges reduce the battery life.

The simple test on dry cell simply puts a resistive load across the cell which the scale indicates- it is generally useless from the consumer viewpoint, but valuable from the maker sales figures!. There are circuits around that measure discharge/charge over a small time period to give an estimation of battery capacity remaining- but in your case, surely the light output is viable?. The simplest method of indicating battery capacity(given that volts fall as capacity diminishes) is a simple zener diode/ led bank.