Measuring charge of NiMh batteries with PIC (adc)

Christophe · Joined: 10 May 2005 Posts: 323 Location: Belgium

Hello,

We are charing 4X niMh 1600mAh battery pack at 600 mA. The nominal voltage is 5V. They seem to be charged at 6.1V

We are using the LTC4060. Review the schematic:

http://users.[spam].be/chs/Werk/Laadcircuit.jpg

Battery voltage is measured by a PÏC processor ADC via a voltage divider

Now I am searching for a way to know how much % the batteries are charged? I can detect the start and the end of a charge cycle.

Is the max. voltage always the same when batteries are full?

If the max battery voltage gets higher, does the charge curve gets higher linearly?

Humberto · Posted: Fri Jun 23, 2006 7:20 am

Christophe,

Microchip has a very good AN: Intelligent Battery Charger Reference Design that you would read it before to start, it has some algorithms and reference values that support NiMh technology.

http://ww1.microchip.com/downloads/en/AppNotes/30451c.pdf

Humberto

Christophe · Joined: 10 May 2005 Posts: 323 Location: Belgium

Tks for the input but all that is done by the LTC4060. I only want to know the %charge when charging.

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

Read the application notes anyway. One thing that is not easy to estimate is % charge based only on the pack voltage. You really need to be counting coulombs. Count them going in and count them coming back out again.

Look around on the TI web site, they have some ICs that do this and probably enough information to figure out how to make a PIC plus a few other parts emulate the same function.
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Ttelmah · Guest

The normal key, is Dv/Dt.
You cannot really rely on voltage with almost any battery. However when they become fully charged, the cell voltage will stabilise, then drop, and this is the indicator of fully charged, normally used for NiCd batteries.
Unfortunately, NiMh, batteries only show a tiny drop, so this is not a common method on better chargers. The rate of change of temperature, rises sharply at the full charge, so heat is a good indicator of NiMh charge completion. However some chargers do just look for the levelling of the voltage characteristic. 'Belt and braces', with a limit on heat, combined with voltage change detection, is also effective.
You can use the cell voltage as a _very_ approximate indicator of charge state, on the way to this point.

Best Wishes

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Christophe · Joined: 10 May 2005 Posts: 323 Location: Belgium

Thks 4 the input. So I've learned that the only accurate way to measure battery charge is a fuel gauge IC counting coulombs in and out. Now as hardware is allready finished I cannot implement this.

With the LTC4060 I've got -DV/Dt; DT/Dt and a time limit. So that should be safe.

As for battery charge I've only got the battery voltage.

So
6.1V = 100%
5.9 = 90%
..
5.5 = 10%

using the curve and a lookup table.

Q: Is the maximum battery voltage always _approximately_ the same at full charge?

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

No. It depends on the temperature of the battery pack.
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JFK · Guest