## Battery Status Indicator

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What is the best way to measure remaining battery capacity? Say I have a segmented display (like the old school Nokia cell phones) with a battery graphic that is divided into 4 segments. These segments can be used to show 25%, 50%, 75% and 100% remaining. Is there a simple circuit (or chip) that I can connect to my microcontroller to gauge battery power remaining?

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Simple question, complex response. The most common way is measure voltage, and depending on battery chemistry, temperature, consumption, astral alignement, and other paremeters, you can have an aproximated idea about its status of charge, but this could be as acurate as 50%.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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There are ICs that measure how much charge goes in and goes out. Some Li-Ion battery packs have these 'gas gauge' ICs on board and can be read via a SMBus connection (a version of the I2C protocol). Linear Technology, Maxim, BenQ and maybe some others make these ICs.

NiCd and NiMH have the habit to keep a fairly constant voltage throughout their charge, suddenly dropping when they are almost completely exhausted; unlike Alkalines for example, their voltage slowly decreases over their lifetime.

Also, measuring the voltage without any load isn't very accurate; a load must be present.

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As Guillem accurately said, it's a tricky thing to do, at best it's a calculated guess. One way could be to measure the voltage at the battery, and do a math calculation based on current draw. Probably the most accurate way would be to measure the current draw, and take the batteries amp/hour rate and do the math for remaing charge, then turn off or on the appropriate segments

Jim

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It's actually not that bad. I'm doing this in a battery powered application at my company.

Here are some helpful points.
1. Decide what you want to define as "full" and "empty". For a 12V lead-acid, I use 12V and 10V.
2. When current is very low, you can simply measure the voltage of the battery and interpolate between full and empty.
3. If current is significant (voltage drop across internal resistor of battery), you need to determine the R of the battery. Do this by applying various loads and measuring the voltage at the terminals. Plot V vs. I. That's R.
4. You can measure current by putting the load in series with a 0.02 ohm resistor and using a differential op-amp circuit to amplify the difference by 50. This will give you the current (1V output = 1A). An AVR can measure this using the A/D inputs and make the calculation.

If you need part numbers for LED bar graph driver or op-amp configuration, let me know and I can post something. Hope some of this helps.

Regards,
Paul

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Looks like a little black magic is involved... I don't plan on implementing anything soon, just toying with the idea. When/if I do try to get a battery status indicator, Ill let you guys know how it turns out.

Thanks!