Replacing Master RTC battery pack by a supercap

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eelco
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Replacing Master RTC battery pack by a supercap

Postby eelco » Wed Jul 05, 2017 4:51 pm

Hi,

Anyone got experience with replacing the RTC battery pack in a Master with a supercap*?

Cheers
Eelco


(*) a supercap is a capacitor with a relative large capacity, e.g. 0.47 or 1F.
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sydney
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Re: Replacing Master RTC battery pack by a supercap

Postby sydney » Wed Jul 05, 2017 5:23 pm

Me!
I think mine is 2.2F and has kept the settings for over 6 months of not being powered on.

Have a look here for my thread on the subject. viewtopic.php?f=3&t=10686&hilit=Super+capacitor

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fordp
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Re: Replacing Master RTC battery pack by a supercap

Postby fordp » Wed Jul 05, 2017 5:51 pm

That is strange as most supercaps will self discharge in a about 7 days.
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sydney
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Re: Replacing Master RTC battery pack by a supercap

Postby sydney » Wed Jul 05, 2017 6:51 pm

fordp wrote:That is strange as most supercaps will self discharge in a about 7 days.


I've been told by several people that what I've described is impossible. One person even 'did the maths' to prove it. Nevertheless I know that my master had not been powered up for over 6 months and the settings were still there.

I read a document which stated supercaps would discharge from 100% to 50% in 30 to 40 days - still much shorter than my experience.

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Elminster
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Re: Replacing Master RTC battery pack by a supercap

Postby Elminster » Wed Jul 05, 2017 7:35 pm

Schroeder's cap [sic]

If you dont turn it on are the setting there or not? They would be there for an infitinte time if you never turn it on. Or you could turn it on and find you have kittens. And if you have done 2 impossible thigns before breakfast ....

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1024MAK
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Re: Replacing Master RTC battery pack by a supercap

Postby 1024MAK » Wed Jul 05, 2017 9:12 pm

The amount of time that the CMOS RTC & SRAM chip can operate from a supercap depends on the capacity of the cap, the internal leakage of the cap, the current drawn by the chip, the voltage the cap was charged up to (and we assume that it got to 99.9% of charged, as capacitors never in theory ever get to 100%) and the minimum voltage that the chip will work down to... <breathes...!> :mrgreen:

In practice, I would expect a 2.2F cap to last at least a couple of months.

To put this into contex, a pack of three good quality, new AA cells will keep the chip happy for over two years. Or even longer.

Mark
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dominicbeesley
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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Thu Jul 06, 2017 11:38 am

I've had supercaps hold settings for a few months on a TRS80 model 100 laptop, I can't remember what size cap I used.

I'm planning on a supercap for my 6809 and was going to fit one to my master too. It might not hold settings as long but I distrust potentially leaky batteries

I'm not sure how close to the voltage rating to go, I usually leave a lot of headroom for transients etc but here where size and cost are a real consideration can a 5V supercap safely be used with a 5V rail?

D

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Elminster
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Re: Replacing Master RTC battery pack by a supercap

Postby Elminster » Thu Jul 06, 2017 12:00 pm

This seems like a great idea that I hadnt come across before. Perhaps when my batteries need replacing on the Master I will supercap it. It is on most weeks and doenst take long to *exec a *spool config backup if it does go flat.

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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Thu Jul 06, 2017 12:53 pm

It should be simple enough to connect one up a simple series resistor should do the trick (100 ohm or so to limit charging current) a parallel diode with the resistor might help extract the last drop out...

D

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1024MAK
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Re: Replacing Master RTC battery pack by a supercap

Postby 1024MAK » Thu Jul 06, 2017 4:05 pm

dominicbeesley wrote:It should be simple enough to connect one up a simple series resistor should do the trick (100 ohm or so to limit charging current) a parallel diode with the resistor might help extract the last drop out...

D

Yes to the resistor. But a parallel connected diode is not needed. At the very small current drawn by the CMOS RTC & SRAM chip in standby power mode, the voltage drop across the resistor is very close to zero, so the diode would never conduct.

As to the value of the resistor, 100 ohms is a good value. It will limit the inrush current when you switch your machine on (large capacitors act like short circuits when a DC circuit is powered up), and protect the supercap.

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Elminster
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Re: Replacing Master RTC battery pack by a supercap

Postby Elminster » Thu Jul 06, 2017 4:37 pm

Think I will add to the diy part of hardware list, which only has battery pack.

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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Fri Jul 07, 2017 10:25 am

Yes, that's what I thought with the diode...unlikely to do much.

It's hard to work out what the best supercap would be: There are some big 10F, 5F ones available but they have quite high leakage currents (>30uA compared to the small draw of the 146818 which is around 50uA if I'm reading the datasheet correctly), a lot don't publish the leakage current so no idea on how long they would last.

D

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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Fri Jul 07, 2017 10:38 am

Here's what I'm looking at for my next 6809 motherboard. The 146818 will be used pretty much like on the beeb but I've added a MAX807 to control the battery switchover. I'm not sure, I might add a battery connector too if the supercap proves to be too weedy I'm also looking to backup some of the ram(disk) the 512k static ram chip reckons to only need 4uA on standby!

I remember when the first Masters coming out the furore over the exploding batteries. I was looking at the charge circuit yesterday and I'm still not sure what they were trying to achieve!
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Re: Replacing Master RTC battery pack by a supercap

Postby johnkenyon » Fri Jul 07, 2017 12:39 pm

dominicbeesley wrote:Here's what I'm looking at for my next 6809 motherboard. The 146818 will be used pretty much like on the beeb but I've added a MAX807 to control the battery switchover. I'm not sure, I might add a battery connector too if the supercap proves to be too weedy I'm also looking to backup some of the ram(disk) the 512k static ram chip reckons to only need 4uA on standby!

I remember when the first Masters coming out the furore over the exploding batteries. I was looking at the charge circuit yesterday and I'm still not sure what they were trying to achieve!


Why not replace the 146818 with DS1307 (500nA in battery mode)?

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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Fri Jul 07, 2017 2:08 pm

It's a thought...mainly for ease of porting the software and the fact that I have one in my parts box....I'll give it a look though.

D

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Re: Replacing Master RTC battery pack by a supercap

Postby pau1ie » Tue Jul 11, 2017 1:07 pm

This is an interesting idea. My master currently has no battery, and it keeps it's settings OK, except for when I took it to Cambridge Abug. I realised that this is probably because the datacentre is plugged in, and somehow powering the micro. Maybe this is why Sydney's supercap lasts longer than expected?
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1024MAK
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Re: Replacing Master RTC battery pack by a supercap

Postby 1024MAK » Tue Jul 11, 2017 11:45 pm

I don't see how something on the 1MHz bus can power the CMOS RTC & SRAM chip without powering up the rest of the Beeb.

It is possible to load the configuration in from disk (apart from the correct time).

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Re: Replacing Master RTC battery pack by a supercap

Postby dominicbeesley » Thu Sep 21, 2017 12:53 pm

I finally got round to prototyping this this week. I've only used a 0.33F supercap (robbed from an old Sky box) as RS managed to deliver a potentiometer by mistake!

I'm finding results rather disappointing, it's dropping about 0.75 in the first hour and struggling to retain the clock over 24hrs...which seems about right for the stated 50uA drain, I was hoping it was a conservative datasheet!

Did anyone try a supercap and what was your experience?

D


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