I would like some comments or suggestions about this design. I am building a battery backup module for two units that run on 18 volts. They each have a power supply that powers the device when AC is present. When AC is lost (power failure) they require about 35 mA @ 18 volts to stay alive in "sleep" mode. I will be logging current usage using a Zetex ZXCT1081 (not included in this post.) I also want to be able to test the condition of the batteries periodically by measuring the voltage while the batteries have a small load (17 mA) on them. (I have read that measuring voltage on batteries with no load does not provide an accurate representation of the charge left in the battery.) Any comments, suggestions, or criticism would be greatly appreciated. Thanks.

 

A non-loaded battery does not show charge accurately.

You can switch in a power resistor to load the battery for a few mS, check the voltage then switch the power resistor back out.

 

Ahhh... I think that's already what he had in mind there, Captain Obvious.

Thanks for saving the day though.

I think what he wants to know is the loaded and unloaded voltage, to make a comparison between the two.

 

Ahhh... I think that's already what he had in mind there, Captain Obvious.

Thanks for saving the day though.

I think what he wants to know is the loaded and unloaded voltage, to make a comparison between the two.

That is what the 'O' in 'O-3' stands for....Obvious

Obviously I outrank you.

Ya know, either things are changing without me noticing, or I am going crazy.

I could have sworn the first post was different when I was responding to it. ..hmm..

So, don't make me pull rank.

 

Ahhh... I think that's already what he had in mind there, Captain Obvious.

Thanks for saving the day though.

I think what he wants to know is the loaded and unloaded voltage, to make a comparison between the two.

A better way is to have two loads. One at about 90% the other at 50% of battery capacity. The voltage drop between the two is a measure of battery internal resistance. (charge). The mosfet connected to a pwm channel is a good way to do it.

http://www.google.com/url?sa=t&sour...3JmoDA&usg=AFQjCNHc9aH7-bvOR1xRJqaTuZoaHiMXRg

 

Where the heck is my "smite" key when I need it?

 

A better way is to have two loads. One at about 90% the other at 50% of battery capacity. The voltage drop between the two is a measure of battery internal resistance. (charge). The mosfet connected to a pwm channel is a good way to do it.

http://www.google.com/url?sa=t&sour...3JmoDA&usg=AFQjCNHc9aH7-bvOR1xRJqaTuZoaHiMXRg

Thanks, that is a good idea. An internal resistance check using 5 mA for 1 second and 505 mA for 100 ms would only consume 55.5 mA seconds of battery life. http://data.energizer.com/PDFs/BatteryIR.pdf

A closed circuit voltage (CCV) test using a 10Ω resistor for 1.5 Volts = 150 mA (I would use 120Ω for 18 volts) for one to two seconds would consume 150 to 300 mA seconds of battery life. http://data.energizer.com/PDFs/alkaline_appman.pdf - Page 12.

 

What about comments on the FET? It's not a high-side switch. It's not exactly a low-side switch. It's in between. When the FET is OFF then Source should be very close to ground. (0 volts.) When the batteries are fully charged and the FET is turned ON then Source should jump up to about 850 mV above ground. I am thinking that is acceptable because the datasheet says: "Very low level gate drive requirements allowing direct operation in 3V circuits. VGS(th) < 1.5V”

Does anyone see any problems with the design?