L.E.D battery capacity & charging indicator

Discussion in 'Power Electronics' started by NZVengeance, Jan 30, 2014.

1. NZVengeance

2
0
Jan 30, 2014
Hi Everyone,

First time poster on these forums.

I am trying to build a circuit which will do two things

1. When a button is pressed the capacity of the battery is indicated. This will be via 5 LEDs (100-81%, 80-61%, 60-41%, 40-21%, 20-1%. I would also like the last two LEDs to be Bi-Colour (Green Red). When the capacity if above 40% the first two LED's are green but when the capacity drops below 40% they are red. this way when the battery is fully charged all the LEDs are the same colour.
2. When the battery is being charged it indicates its charge level. for example if you begin charging at 20% capacity the first LED is on solid whilst the remaining four are cascading in sequence 2-3-4-5 until the battery reaches 40% then first and second LEDs are lit and an the reaming 3 are cascading, this process continues until the battery is fully charged and all LEDs are on solid. As per point one I would like the first two LEDs to be red until the 40% threshold has be met and then they can turn green.

I would also like to build into this some circuitry to detect when then battery is fully charged and to stop charging. it could then perhaps check periodically to see if the battery needs charging or not.

I realise this is quite an ambitious project for my first but might as well start big

Last edited: Jan 30, 2014
2. GreenGiant

842
6
Feb 9, 2012
So you want to build 2 circuits, a charger and one for on the battery?

The one at the battery is going to be a function of voltage, you are going to need to know what voltages each of those percentages are. (For example a 3.6 volt litium battery would be 3.6-3.3V/3.3-3V/3-2.6V/2.6-2.3V/2.3-2V) Then you will have to either set up some circuitry with LED's so that they shut off after cirtain voltages (playing with voltage drop woo)

The charger circuit you will probably need a microcontroller for, in order to turn on and off like you were saying, not too hard but again you need to know voltages, and you have to have tolerances/timeouts setup so you dont burn anything.

3. BobK

7,682
1,688
Jan 5, 2010
Charger circuits are very specific to the battery chemistry, and the terminal voltage is really not a good measure of charge state.

To get accurate charge state info you have to integrate the current in (during charging) and out (during discharge) with high accuracy.

Bob

4. NZVengeance

2
0
Jan 30, 2014
I haven't settled on a battery for this project yet.

However, if i gave you the specifics of a battery could the circuit be designed then tweeked later on?

5,164
1,087
Dec 18, 2013
For batteries like Lithium it's very difficult during discharging because they keep a higher voltage right to the end and then just die. You would need a coulomb counter to get an accurate reading, but you would need to start off with a fully charged battery as Bob mentioned.
http://cds.linear.com/docs/en/datasheet/4150fc.pdf

Batteries like lead acid and NiMh are a bit more predictable. But during charging you would have to disconnect the charger, wait a while then load test it or measure the resistance of the battery or voltage to try and work out it's state of charge.

6. BobK

7,682
1,688
Jan 5, 2010
Usually, you would use a chip designed for the specific battery chemistry, so, no, you would be best to decide on that first, also the number of cells matters.

You could build a microcontroller based charger then program it only after you know the specifics of the battery.

Bob

7. KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
As BobK says, you need to tie your battery energy indicator to the battery chemistry, and monitor the current flowing in to (during charging) and out of (during use) the battery. You may also need to monitor battery temperature, because this affects leakage and other characteristics (and is a good way to detect end of charge, for some types at least), and age and number of recharges, since both cause capacity to drop.

I would use a microcontroller to keep track of everything. I designed a battery board (as part of a commercial product) that does these things, among others, for an NiMH battery pack. I have no experience with Li-Ion nor Li-Polymer batteries.

You need to first clarify your requirements. Decide on the battery, and the features you need. Do you want to include the charger? What is the input voltage? What accuracy do you want for your battery energy meter? If batteries are interchangeable, how will you track them? (We used a DS1822 which monitors temperature, provides a unique serial number, and has 16 bits of non-volatile storage, for this).