LED voltage bar graph

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mrdarrett 18 years ago

I'd like to have a "bar graph" of LEDs tell me how far along my 6V SLA is on its charging.

I saw this, but I think it's overkill:

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5 LEDs would be plenty.

Any suggestions for a discrete version of the LED voltage bar graph? I'd like to tune it from 6.0V (first LED) to 6.8V (fifth LED) in increments of 0.2V.

It's not particularly important whether all preceding LEDs are lit, or if just the Nth LED is lit, if this simplifies the circuit at all. (For example, if voltage is 6.6V, corresponding to the 4th LED, it's ok if only the 4th LED is lit, and not LEDs 1 through 4, if this makes design easier.)

Thanks,

Michael

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Bob Monsen 18 years ago

If you have a voltage reference, you can easily scale voltages so that a couple of quad comparators, along with 100k trimmers can be used to do something like this.

Each stage is simple, it's an open collector comparator with the - input at the range, the + input at the battery voltage, and the output connected to a voltage source through a resistor and LED.

Vbatt 12V | |\\| | 6.8V .-------)------|-\\ ___ LED | | | | >--|___|-->|-----o .-. o------|+/ 1k | 10k | | | |/| | | | | | '-' | | | | |\\| | o-------)------|-\\ ___ LED | | | | >--|___|-->|-----o .-. o------|+/ 1k | 10k | | | |/| | | | | | '-' | | | | |\\| | o-------)------|-\\ ___ LED | | | | >--|___|-->|-----) .-. o------|+/ 1k | 10k | | | |/| | | | | | '-' | | | | |\\| | o-------)------|-\\ ___ LED | | | | >--|___|-->|-----o .-. o------|+/ 1k | 10k | | | |/| | | | | | '-' | | | | |\\| | 6.0V '-------)------|-\\ ___ LED | | | >--|___|-->|-----' '------|+/ 1k |/| (created by AACircuit v1.28.6 beta 04/19/05

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You'll need to figure out how to get 6V, 6.8V, and 12V. You can use LM324s instead of comparators, and use the extra three opamps to buffer voltage dividers. If you have a 12V reference, like an LM7812, that would make this easier.

Regards, Bob Monsen

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Phil Allison 18 years ago

** So YOU still refuse to use the proper charge method.

Constant voltage @ 13.8 volts.

Then the current drops over the charge cycle.

....... Phil

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mrdarrett 18 years ago

13.8 volts into a 6V SLA... ha ha ha. You're a hoot.

Actually I set my LM317T to 6.75V, and I just want to know how long it will take for the battery to float up to that.

Michael

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Eeyore 18 years ago

Obviously 6.9V for a 6V battery.

Phil is right that a simple voltmeter is not a good indicator of the level of charge, not least because the terminal voltage will rise whilst being charged by varying degrees.

Graham

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mrdarrett 18 years ago

Yeah I know, I was just giving him a hard time. ;-)

Good point. I did notice that the battery would go up from its original voltage to the charge voltage within a couple of minutes.

So if I've got a 10 A-h battery, and it charges at float at 500 mA, it should charge for 20 hours at float to receive a full charge?

Michael

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Eeyore 18 years ago

More like 30 hours actually since the process isn't 100% efficient. Also, the charge rate will taper off with time.

Graham

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John Fields 18 years ago

The proper way to charge a sealed lead-acid battery is to limit the initial charge current to that value specified by the manufacturer while monitoring the voltage across the battery. Once the battery voltage has risen to to the value specified by the manufacturer, then that voltage should be maintained until the charging current drops to the value specified by the manufacturer. Then, depending on whether the battery will be used in cyclic or float mode, one of two actions should be taken. Here, From: http://www.power-sonic.com/site/doc/prod/73.pdf For a 12AH SLA: Charging Cycle Applications: Limit initial current to 3.6A. Charge until battery voltage (under charge) reaches 7.20 to 7.35 volts at 68°F (20°C). Hold at 7.20 to 7.35 volts until current drops to under 120mA. Battery is fully charged under these conditions, and charger should be disconnected or switched to ?float? voltage. ?Float? or ?Stand-By? Service: Hold battery across constant voltage source of 6.75 to 6.90 volts continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.

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Eeyore 18 years ago

of

charged

I completely concur.

In this instance the OP could perhaps organise a suitable current limit around his regulator.

Graham

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mrdarrett 18 years ago

V SLA

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harged

Thanks a bunch.

6.75V not etched in stone... 6.75 to 6.90 seems more reasonable.

Michael

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LED voltage bar graph

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