Help me reduce the voltage at Vb in a coin battery for RTC

Hello,

I'm using rechargeable coin battery for RTC. At first this new battery comes with 4Volt but on top of battery they mark it as 3.6V. The RTC Vb requited is 3V/27uA but the new battery given extra 1V.

In between the battery and RTC Vb i tried put a higher 100K resister to reduce down the current flow. With this method i able to reduce down the voltage at Vb but still can't reduce down to 3V for RTC required. Finally i try to use a 3V zener diode to regulate down the voltage but the battery life can't be last longer. The circuit shown below:



Is there any better suggestion?

Thank you,..

Re: Coin Battery for RTC

You can use regulator ic for increase your battery life.
However usually long life nonchargeable battery used for rtc because power consumption of it is very low.

Re: Coin Battery for RTC

Hello Dv_sa559,

Thanks for your reply ya..

Normally the regulator draw more current compare with zener diode, am i rite?

I have another concern. Is it possible to increase the resistance to 150K at R2 to limit the current. Since the RTC requited 27uA only. Not sure whether the input voltage will damage the RTC?

Thank you,.

Re: Coin Battery for RTC

Can you clarify about the involved RTC chip? Industry standard RTCs have a current consumption fairly below 1 uA and can easily tolerate above 4V supply voltage. Using any kind of voltage regulator, either Z-diode or integrated regulator would considerably reduce battery lifetime and is an absolute No No. If a Z-diode overvoltage protection would be needed in special cases, the zener voltage must be high enough to draw effectively no current in normal operation, requiring zener voltages around 4.7V.

Re: Coin Battery for RTC

The usual solution is to connect a diode in series with the coil cell so it drops an additional 0.6V or so. It will also allow you to power the RTC from a different supply without current flowing into the battery if its only there as a backup supply and it gives some degree of reverse battery protection (believe me it happens !)

Brian.

Re: Coin Battery for RTC

Hello All, Sorry for late reply.


FvM,
Ya, i'm using mbed controller. I don't think the tolerate can go up to 4V supply because when i put the new battery 4V it just work fine within few hours. After the day i check it back the Date/Time again the time is reset again. Actually it is quite dangerous testing, mbed controller Vb only can accept 3v~3.3v max. Now i put back a normal 3V battery it working fine. But i wish to put a rechargeable coil battery for my RTC.

betwixt,
Ya, early i was thought that too..put a normal diode it will drops a 0.6V. So that in between 3.4V might still acceptable for RTC but now the diode that i add is just drop around 0.2V only. I guess may be the RTC is draw very least current so that the diode wouldn't drop until 0.6V. If this the case i need to add x4 diode then only can get 3.2V. :sad:

Is there any alternative way to solve the problem?

Thank you,.

Re: Coin Battery for RTC

The involved hardware (RTC chip, processor with built-in RTC?) still isn't clear. You should clarify for a serious discussion.

Re: Coin Battery for RTC

At risk of wasting a little charge from the cell, which is something any regulator would do anyway, try using the series diode but follow it with a bleed resistor to ground so a mnimum current is established. If the diode isn't dropping enough voltge it must be because it's leakage current is too high, adding a resistor will sink the leakage to ground. The value is best found by experimentation but I would suggest starting with 3.3M Ohms which will leak about 1uA.

Brian.

Re: Coin Battery for RTC

Hello FvM,

Sorry for that . I still new with that controller. Ya, i think should be built-in RTC. That controller is quite expensive for me. So, i quite worry will damage the RTC.

Thank you,.

---------- Post added at 08:46 ---------- Previous post was at 08:28 ----------

Hello betwixt,

May i know why we need to connect a bleed resistor to ground? What's purpose for the bleed resistor? If we connected in this way the battery need to draw the current for RTC plus the 3.3M Ohms resister, am i rite? If this the case the battery will dry faster when no power supply compare with normal battery (3V) which we don't need to add diode and resistor.

Can i have a try with 2M2 Ohm? because i don't have 3.3M Ohm.

Thanks for you all advising...

Re: Coin Battery for RTC

The battery will discharge faster, there is no doubt about that but if the current is extremely low it will not significantly shorten it's time between recharges, the internal leakage of rechargeable cells is already quite high anyway. The resistor is to overcome the leakage through the diode. Although a perfect diode will drop 0.6V (if it's silicon) it also has a tiny leakage current, you can think of it as having a high value resistor in parallel with a perfect PN junction. The reason you only see 0.2V drop is some of the current being drawn by the RTC is being supplied through the diodes leakage. Adding the 3.3M or 2.2M resistor will increase the current drawn through the diode and make its leakage current less significant. 2.2M should be OK to try the idea out, you really want to use the highest value possible that still makes the diode drop enough voltage.

Brian.

Re: Coin Battery for RTC

Instead of diode you may like trying a small LED instead. Please note that the red LED voltage drop is less than of the white one.

Re: Coin Battery for RTC

I would advise against using an LED for two reasons:

1. it isn't the amount of voltage drop thats important here, its the effects of leakage current. LEDs have significantly higher leakage than standard diodes so it may drop less voltage rather than more.
2. we are talking about nA or uA currents in this circuit, an LED will act as a voltage source if light falls on it and this may be sufficient to either boost the voltage to a dangerous level or cancel the supply entirely.

Brian.

Re: Coin Battery for RTC

betwixt said:

... its the effects of leakage current,...if light falls on it...

Click to expand...

You are right.
On my side... I usually test what I have on my hands (using all what I have in my mind :wink: ) then I choose the best result. If it happens that none of the results is acceptable practically (while being reliable), I surely redesign my circuit to get a more convenient solution to my application (in where I live) :grin:

Kerim

Re: Coin Battery for RTC

I still new with that controller. Ya, i think should be built-in RTC. That controller is quite expensive for me. So, i quite worry will damage the RTC.

Click to expand...

I'm sure, that there's a lot of reasonable options, that could be decided, if knowing about the exact hardware specification. Without it, it's just guessing.

Re: Coin Battery for RTC

Hello Dear All, very interesting discussion between Betwixt and Kerimf. Betwixt said is true, this circuit just need nA/uA currents. LED is will draw more current compare with diode. Perhaps we can use this solution where is no power-hungry concern in the circuit. We wish to keep the backup battery long life to run our data/time.

I did tried Betwixt advices connect a 2M2 resistor to ground and the voltage drop 0.42V which is additional drop another 0.22V. So, now is 3.58V but it still haven't reach the requirement 3.3V. Is it we still need to use lower than 2M2 resistor?

I have another new concern, new rechargeable battery come with 4V. Most of the coin rechargeable battery voltage is status 3.6V. I have another one rechargeable battery is drop until 3.29V when doing testing. Currently using ~3.8V to recharge the 3.29V battery with whole night but until the next morning the battery just get 3.333V. I wonder is it slow charging or it is the charging voltage just until 3.3V only or.. is it very hard for us to charge back to full voltage 3.6V or 4V?

If we can't get back to full voltage, let said now is 3.3V. My concern is if after added the diode and resistor the voltage will drop another 0.6V. So we just supply 2.7V to RTC. The minimum voltage for this RTC is ~1.9V. So, I wonder how long the RTC can operate in between 2.7V-to-1.9V voltage.

Thank you,.

Re: Coin Battery for RTC

The 2M2 resistor proves where the problem lies, clearly it is residual current leakage through the diode and although it is only one or two uA it is sufficient to cause the problem.
I would stay with the series diode idea but try a different diode to see if you can find one with lower leakage. There are special low leakage types but you should be able to use something like a 1N914/1N4148 small signal diode. Try to avoid power rectifiers (1n400x etc.) as these are optimized for high current or voltage rather than low leakage. Also avoid Shottky diodes but do use a silcon one rather than germanium or other semiconductor materials. Silicon normally gives best leakage characteristics.
If that fails, you can drop the resitor to a lower value. Bear in mind that rechargeable cells already have a high internal leakage so as a proportion of load the extra current though the resistor is quite small.

Brian.

Re: Coin Battery for RTC

Hello Betwixt,

Yeah, you can guess what I doing. Ya, i did try 1N4001 and Shottky diodes. Now then i realize. Ok, i will try to get 1N4148 and have a try. See what's the result and then get back to you ya..

Thanks a lot..

Re: Coin Battery for RTC

Do you have (or could you have) any regulated supply above 3V?
On my side, I would use and test the following:
(1) I use 2 AAA normal batteries which supply, when both are very fresh, about 3.3V then drop to 3V in most of their life time.
(2) Though the RTC trickle current is very small (75u), there is no need to take it from the battery when the mains power is on.
(3) If we assume that 5V regulated is available, I connect a resistor 27K between the battery and this supply; (5 - 3)/0.075

In this case, if the main’s is always off, the RTC would keep running for about 9 months!

Since in my products, the external RTC IC draws less than 1uA when standby on battery, I didn't see the need for any trickle current. In these applications, the life time of the normal AAA battery will depend on other factors than current.

In case you have no space (which is likely your case), I am not sure if there is an equivalent non-rechargeable coin battery to normal AAA, in respect to the voltage characteristic.