Which MOSFET to use for battery voltage measurement?

Community General Discussions
1

Hello all,

I'm currently building a buoy with a string of sensors that measures temperatures at different dephts. I'll be making a blog or Something Else later.

The thing is, I want to measure the voltage of the battery (Li-Po) to be able to predict when it needs to be charged since the system will operate "out-there-in-the-middle-of-nowhere".

The thing is, I'm looking for a suitable mosfet which, when the gate is driven by 3.3V, will open completely to allow current to flow through the voltage divider, thereby allowing the arduino to measure the voltage on its ADC port.

Can every mosfet with a Vgs threshold voltage of a one or a few Volts be used (n-type if I'm correct)?

 

Thanks in advance.

2

Relay?

Would a relay be out of the question here?  Low power ones can be energised with 10mA or so and if it’s only for a short time occasionally then power drain would not be excessive.  When energised it would give you a solid, low resistance connection with no worries about channel resistances or switching thresholds.

 

3

I like that. Sometimes
I like that. Sometimes simple solutions are the best.

The relay could feed into the voltage divider and the power could be checked every X minutes or so. I’d probably check every 10 to 15 minutes for Groucho (he has a battery that should be able to power him for 6-8 hours or so) whereas I might check every 5-10 minutes for mini-Groucho who has a 12v 10 amp-hr LiFePo4 battery.

At the moment I can check things manually, but eventually Groucho’s battery will be hidden inside where I’m afraid I might forget to check.

On the other hand, I don’t know how much power a properly constructed voltage divider would use.

And there may be chips that will deal with a lot of the details of battery maintenance.

4

I have used the AO3400

I have used the AO3400 N-channel MOSFET for this on my https://www.tindie.com/products/Dead_Bug_Prototypes/extreme-low-power-data-logging-shield-for-arduino/ 

the gate is connected to the Arduino 5V pin and Source is connected to the power supply. 

@Yahmez, Yes a 1.2mA current draw it quite a lot when you try to conserve power between logging data. My low power shield only uses 0.03uA while in sleep mode.

5

Ok, for large bots, a simple voltage divider.
For smaller bots that need to conserve power, I’ll worry about conserving power somehow.

6

Thank you for your replies

Thank you for your replies and considerations, I’ll try to elaborate on some of the issues raised;

I’m most interested in a low-power solution, the current setup is as follows:

- 1 x Arduino Pro Mini, power LED disabled, running 8Mhz, 3V3, Atmel 328.

- 1 x SD card holder without further electronics, directly wired to the arduino

- 1 x RTC, DS3231SN, specifically chosen for its extended enviromental range and precision

- 1 x string of DS18B20

- 1 x Texas Instruments LP2985, 3V3 regulator to run the SD-card, Arduino, RTC and sensor string. Nothing about the voltage regulator is set in stone so this may be a point of debate. I hear good things about the MCP range of buck/boost converters so may be looking into those instead of using this one. http://nl.farnell.com/jsp/search/productdetail.jsp?SKU=1053672

- 1 x battery, nothing decided yet, the LP2985 can go as high as 16V so there’s a lot of room to manoeuvrer. 

At present, with the current sketch putting the Arduino to sleep to be waken by the interrupt of the RTC, the quiescent current during that period i 0,4 - 0,5mA. I haven’t got a good idea how to improve that. The RTC takes as little as 4uA, the SD-card holder nothing, the string nothing when it’s not active so the main power consumer is the Arduino itself.

I haven’t looked into the Arduino Pro Mini stock regulator but according to Sparkfun its a MIC5205. I’ll go an compare the two to see which one is better. Also I’m very open to suggestions about a good voltage regulator! :).

So, in general, I’m looking for a voltage measurement solution that draws little to no current when not in use. I’ve looked at other solutions 

 

Perhaps this is a solution?

OENJz.png

 

7

That’s not a problem

The 5 milliseconds isn’t really a problem. Reading through stackexchange, one solution which is very similar to that posted, is this:

Vut6g.png

I'll be studying this one to see if it will fit my needs and wants :)