Hello everyone, I have a Li-on battery in my project which means a supply voltage arround 4V at it's peak.If I use a relay which works at 3.3V would be any problem?
It's a project that will work 6-7 months per year so I want a safe solution.Also I can't change the battery to lower or higher voltage.
.If I use a relay which works at 3.3V would be any problem?
That should be no problem
Can use a suitable resistor in series with the relay coil.
That also reduces battery drain.
Post a link to the relay if you want help with that.
Leo..
The datasheet may show the max and min voltages.
A simple trick for power saving for a relay is as follows.
Determine the voltage at which it releases.
Calculate a resistor* to put in series with the relay coil that will give it a voltage slightly higher (maybe 0.5V higher) voltage than this.
If you use the relay with this resistor value in series with relay coil it won't operate, but it will stay operated once it has operated. To make it operate with the resistor in series with the coil put a capacitor across the resistor. The capacitor will give a brief burst of current while it charges, which will operate the relay, the resistor will keep it operated with the minimum current needed.
I know of no way to calculate the capacitor needed, you have to experiment with that. Probably a few 10s of microfarads. Too big won't matter, too small and the relay won't operate reliably.
*Or use trial and error to find the highest value resistor that allows the relay to stay operated.
You can calculate the capacitor from the operating time times the current = charge required to operate
reliably. Arrange the capacitor keeps the voltage across the relay winding above the minimum operating
voltage with this charge flow.
For now ignore that and just ensure the max and min winding voltages surround the actual range of
voltages, there's usually a wide range.
You're probably OK, but without the relay specifications or your schematic, we're just guessing.
Li-ion cells are 4.2V when fully charged, and that decreases over time with charge/discharge cycles.
Here is the typical discharge profile that shows that the battery quickly drops below 4V then stays at 3.75V for most of the remainder of it's useful charge.
This is good idea might be good way to control sensitive solenoids with super low resistance.
PerryBebbington:
A simple trick for power saving for a relay is as follows.Determine the voltage at which it releases.
Calculate a resistor* to put in series with the relay coil that will give it a voltage slightly higher (maybe 0.5V higher) voltage than this.
If you use the relay with this resistor value in series with relay coil it won't operate, but it will stay operated once it has operated. To make it operate with the resistor in series with the coil put a capacitor across the resistor. The capacitor will give a brief burst of current while it charges, which will operate the relay, the resistor will keep it operated with the minimum current needed.I know of no way to calculate the capacitor needed, you have to experiment with that. Probably a few 10s of microfarads. Too big won't matter, too small and the relay won't operate reliably.
*Or use trial and error to find the highest value resistor that allows the relay to stay operated.
Thanks everyone for the replies!The relay is this https://www.cableworks.gr/images/thumbnails/1143/1000/detailed/259/3_3v_relay_module.jpg
I found a datasheet which includes also other series with 5V,9V etc but without any information.The idea with the resistor and the capacitor is good if I don't find the max voltage I will try it.
share the link... in the spirit of open source sharing.. it may help someone else, that's how this forum rolls. 
Ιt says "max continuous voltage : 110%UN".So the maximum voltage is 3*1,1=3,3?
Coil resistance according to several sources could be 25ohm,
so you need a ~6.8ohm resistor in series with the coil.
Tell us more about your project. Maybe you don't even need that power-hungry (~125mA) relay.
Leo..
My project is a beehive scale.I have already built one that works fine and now I'm building 2 more.I use 2 arduinos one nano which is in sleep mode and controls the relay and one RTC to wake up 2 times per day for 1 minute.And I use one Uno with a GSM shield, load cell and a temp sensor.Through the relay I give power to Uno.
I had tried before a year maybe to use only nano with the GSM shield connecting the pins but I didn't made it.I think that the shield even if turned off she kept drawing some current but I'm not sure.I didn't use relay for that just connected the pins.
Here is the schematic:
About the relay I found out that with an srd 5vdc sl c relay and an almost discharged Li-on battery at 3.5V it works fine.In the datasheet says that the pick up voltage is 70% which is 3.5V.I'll continue testing it and maybe I'll use that.The other relay the jqc at 5V didn't work with a charged battery at 4V even if the datasheet said 75% pick up voltage.I don't know why.
