12V-Lead-Acid-Battery-Low-Voltage-Indicator comparison

[Lucky-Luka]

Hi all
I want to power my circuit using a 12V lead acid battery.
I want to indicate when it's time to recharge it.
I've found several schematics.
I've picked two of them.
Which one is better and why?
Do you have any other circuits in mind for this purpose?
Any suggestions are welcome.
What is the voltage to consider as the low battery? 12V?
Thanks

[Ed.Kloonk]

I posted this just a few days ago.

https://www.eevblog.com/forum/beginners/the-simplest-12v-low-battery-chirp-possible-which-off-the-shelf-components/

[ledtester]

What's the capacity of your battery?

You want the circuit to consume as little current as possible, and from that perspective it seems that the 555 solution might be better, but both will consume at least a couple of milliamps.

Instead of just an indicator, I would look for a "undervoltage protection controller" which disconnects the battery from your circuit when the voltage gets too low. You can also google "undervoltage switch controller".

[Lucky-Luka]

I have in mind, in this preliminary phase, small batteries like the the 7200mAh ones.

[ledtester]

Here's a falstad simulation of the 555 circuit:

https://tinyurl.com/y6ak8yf8

If you run it you'll see that it just below the threshold voltage it consumes a lot (like 10mA) of current whereas above the threshold voltage it consumes around 2 mA.

The other circuit works the other way around -- when the input voltage is low the LED is dim and it gets brighter as the input voltage increases. Consequently it's on most of the time consuming about 10 mA.

The PNP circuit also doesn't have a sharp distinction between being above and below the threshold voltage -- it just gets dimmer and dimmer as the voltage drops.

Falstad simulation:

https://tinyurl.com/y67jdlao

[ledtester]

btw, this modification of the PNP circuit works a lot better:

https://tinyurl.com/y3qwefhp

[Zero999]

I don't see how the one with the BJT, in the original post, can possibly work. The current through the LED will be fixed at just over 9mA, at voltages over 11V and gradually fall, as the voltage goes about 11V, but won't turn off completely, until it's below the LED's forward voltage. The modified BJT one is much better.

I think falstad  is being optimistic. According to the data sheet, the 555 timer uses around 8mA and would drain a 7.2Ah battery in just over 5 weeks.
https://www.ti.com/lit/ds/symlink/ne555.pdf

In the other thread, I recommended the TLV6703. If you want to use stripboard, then there are adaptors, but it's possible to use SOT23 on stripboard, although it's not the easiest thing to do. Cut two tracks, bend pins 2 & 5, on the device upwards, solder pins 1, 3, 4 & 5 to the board and connect pins 2 & 5 with 32 AWG wire and make it as short, as possible.

I'd just use the adaptor though.
https://www.sparkfun.com/products/717

https://www.eevblog.com/forum/beginners/the-simplest-12v-low-battery-chirp-possible-which-off-the-shelf-components/msg3401354/#msg3401354

EDIT:
Do you want the LED to turn on, or off, as the battery falls? The TLV6703 will turn it on, when the battery voltage is low. The other circuits posted here will turn it off. I you want it to turn off, then use the TLV6713.

Here's a demonstration of how to use the TLV6703/6713. For a threshold voltage of 10.5V use 10M for R1 and 390k for R2. R3 limits the current through the LED. Modern LEDs are fairly bright, so might find 10k is adequate.

[Lucky-Luka]

Isn't 10.5V too low? I think I've read that the battery isn't happy when it's so low.
Can't I use TLC3702IPE4 as an equivalent TH alternative?
Thanks

[Zero999]

Isn't 10.5V too low? I think I've read that the battery isn't happy when it's so low.

Read the data sheet/manual for your battery.

Can't I use TLC3702IPE4 as an equivalent TH alternative?
Thanks

You tell me. Please look at the data sheets, then ask again, if you're unclear. You'll learn more by looking stuff up for yourself, rather than asking questions.
https://www.ti.com/lit/ds/symlink/tlc3702.pdf
https://www.ti.com/lit/ds/symlink/tlv6703.pdf

[Lucky-Luka]

I have checked: 10.5V is the limit they suggest and that chip is equivalent for my purpose so I don't need an adapter.

[Zero999]

10.5V is too low

Change the values of R1 & R2. They form a potential divider. The site linked below has a calculator. V_IN is the voltage threshold you want to trigger at and V_OUT is the negative going threshold of the TLV6703, which is 0.3945V.
https://www.random-science-tools.com/electronics/divider.htm

and that chip is equivalent for my purpose so I don't need an adapter.

No, the TLC3702 is not equivalent to the TLV6703. The TLC3702 is a dual comparator IC, whilst the TLV6703, is a voltage reference and comparator, in the same package. There through hold, voltage referenace and comparator ICs, which work up to 12V and have a low enough power consumption, for this application.

If you want a through hole solution, then you could use a low power comparator IC, such as the TLC3702 and a separate voltage reference IC, but it's a bit more complicated. I recommend using the TLV6703, with a suitable adaptor board.

[Lucky-Luka]

I have edited my previous post maybe when you were replying to me. I thought I have read in another forum/chat that 10.5V is too low but then I've read in a acid lead battery datasheet that 10.5V is the lower limit for the discharge they use for their Ah ratings... So I think 10.5V is ok for the battery...

[ledtester]

This is an example of a low voltage cutout circuit (or over-discharge protection circuit):

https://www.eeweb.com/low-voltage-cutout-for-12v-sla-batteries/



I guesstimate the current consumption to be around 7mA when on and 2mA when off. (It's kinda a neat design - maybe I'll actually build it and test it.)

What will work for you will depend on:

- the capacity of your battery
- the current draw of your application circuit
- will you be nearby when the circuit is in operation or will you let the circuit run unattended, and
- how often will you check the indicator (couple times a day, only once a day, once a week,...)

[nfmax]

Take a look at https://www.eevblog.com/forum/projects/battery-undervoltage-lockout/msg1060148/#msg1060148 which is my design for a low-voltage lockout. It takes almost no current - very much less than a micro amp - when shut down. It uses rather old parts. The exact type of MOSFET is not critical, so long as the threshold voltage is low enough. You can still find NOS of the LM10 if you look, but I believe there is a more modern part of very similar functionality (can't remember the part number, unfortunately)

[Zero999]

I guesstimate the current consumption to be around 7mA when on and 2mA when off. (It's kinda a neat design - maybe I'll actually build it and test it.)

It's a good design, but will draw too much current for a 7Ah battery. The resistor values could be increased and lower power components used.

Take a look at https://www.eevblog.com/forum/projects/battery-undervoltage-lockout/msg1060148/#msg1060148 which is my design for a low-voltage lockout. It takes almost no current - very much less than a micro amp - when shut down. It uses rather old parts. The exact type of MOSFET is not critical, so long as the threshold voltage is low enough. You can still find NOS of the LM10 if you look, but I believe there is a more modern part of very similar functionality (can't remember the part number, unfortunately)

The LM10 is still made, but is expensive. It's low enough power consumption, for this application to just use the LM10 and a MOSFET, without the 40106B
https://www.mouser.co.uk/ProductDetail/Texas-Instruments/LM10CLN-NOPB?qs=X1J7HmVL2ZGvfXMxJ2OzgQ==&vip=1&gclid=CjwKCAiAudD_BRBXEiwAudakX7Zar1_AghWBrk9axOxFezjpCBkiJ3_kMnFz4CR99XMCxJhkG9QRNxoCpu8QAvD_BwE

How about the TLVH431 and 2N7000?