jackrae:
Yes, that's it, but as I said, the LED circuit will place a burden load on your rather small 3AH battery. Using the designer's resistor values, this is about 30mA. This current must be added to your actual system circuit load to determine total load and hence probable battery life. You could of course add an ON/OFF switch to the metering circuit so that it only draws current when a reading is actually required.
ok so what you're saying is that the 30mA of the battery indicator circuit won't let the meter give an exact indication right ?
Yes, it will give an exact indication of battery voltage, but not battery capacity.
As we have all tried to get across, there is no simple way of relating battery voltage to battery capacity.
Without knowing how the battery chemistry performs you cannot relate voltage to capacity.
Voltage will only give you a general idea of battery state
jackrae:
Yes, it will give an exact indication of battery voltage, but not battery capacity.
As we have all tried to get across, there is no simple way of relating battery voltage to battery capacity.
Without knowing how the battery chemistry performs you cannot relate voltage to capacity.
Voltage will only give you a general idea of battery state
ok great one more question: how do i know from this voltage method that the battery is empty ? will the voltage be like less than 1V ? from 12V to less than 1V ?
ok great one more question: how do i know from this voltage method that the battery is empty ? will the voltage be like less than 1V ? from 12V to less than 1V ?
You should not discharge a lead acid battery below about 40%, as a very rough rule of thumb. Continued discharging to less than that will shorten the battery's lifespan. Without a load, that is roughly 2V per cell. A "12V" battery has 6 cells, so you should stop discharging when it reaches about 12V, and recharge it.
You seem resistant to reading things you don't like to hear, but here is a link you -should- read:
Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation. Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.
Please note that it says "open circuit voltage", and "rested for a few hours". So a circuit that draws 30mA from a 3Ahr battery is going to disturb this voltage.
Note again, that this only gives you a rough -percentage- of the power left. You still need to know what the actual capacity is, and the only way to know that is to measure it by the methods described.
Do NOT discharge the battery below 10.7 volts (others may well suggest a higher value) unless you wish to destroy the battery in its first use cycle. 10.7 is generally considered to be the point where a lead acid chemistry is considered to be TOTALLY flat (empty)
Good practice states that you should never discharge below 50% of capacity and even that figure will drastically reduce the life of such a battery type. Note that at 50% depth of discharge, your battery meter will still read at least 12 volts. Hence the statement that voltage is not directly proportional to battery state of charge/discharge.
and keep in mind that the actual capacity of your battery is not the same if current it sources is low or high. With a high current sourced, the capacity is lower.
Given that and all you've been told before, the only way to design a "remaining % " indicator is to know exactly the discharging curve of this battery at the current it will be used, and the actual capacity for this current .
ok thanks guys great info so what i understand as a resume is :
for an indicator i must work on the current using the graph
i must get a bigger battery if i want more given time
never discharge the battery below 50%, (about this i have a question: charging the battery for 50% won't destroy it too ? )
another small question : the battery of the car why won't it be destroyed ? is it because it is charging and discharging most of the time so the capacity will be almost the same ?