Re: Rechargeable 3v battery details for microcontroller (Can't lose data!)

From: Mac (foo_at_bar.net)
Date: 01/18/05 

Date: Tue, 18 Jan 2005 05:01:56 GMT

On Mon, 17 Jan 2005 09:03:26 -0400, Kevin wrote:

> On 17 Jan 2005 03:41:55 -0800, "David L. Jones" <altzone@gmail.com>
> wrote:
>
>>What is your total circuit current draw? (is it just the PIC?)
>
> Don't have a circuit draw yet... still doing preliminary design on
> paper (plus waiting for parts to come in). I know this is not ideal
> for calculating battery life ;-)
>
>>How often does your PIC wake up?
>
> Estimate once every 2 seconds via 32.768 KHz Timer1 wake-from-sleep
> interuppt. Probably execute code for about 8 or 12 (say 16mS worst
> case) milliseconds when awake on battery power. By my calculation the
> PIC will be asleep for 99.2% of the sleep time. I plan to power
> external circuitry from one of the PIC's output pins to further reduce
> external current draw to a minimum by switching off external circuitry
> when asleep.
>
>>Can you run your PIC slower from 32.768KHz instead of 4MHZ?
>
> Thought of this, but due to some signals that I have to monitor I have
> to run at 4 MHz. I am running at less than 5V (3v on battery), so
> power consumption should hopefully be OK.
>
>>How long does your data logger need to be installed for?
>>Can you tolerate changing the battery say once every year or two?
>
>>If you are talking only a mA or two total current then a set of
>>Alkaline D cells may last for several years. In that case you might be
>>able to forget about a rechargable solution.
>
> Well, was aiming for at least 5 years, maybe more (10 is even better).
> That's why I was thinking of rechargeable batteries as opposed to
> nonrechargeable ones.
>

I went to the Duracell website to see if I could find numbers on D cells.
Duracell batteries will last for at least 5 years on the shelf. I'm sure
Energizers will, too, but I happened to go to Duracell's website.

If you discharge a Duracell D battery down to 1.1V, with a constant 0.25W
power draw, it is more than a 10 amp-hour cell. So two D's, discharged
down to a combined series Voltage of 2.2 V at a power rate of 0.5W would
be more than 10 Amp-hours also. At very low discharge rates, such as you
are contemplating, the cell will last much longer. And if you allow it to
go down to 1V, you will get even more life out of it.

1000 discharges on your 17 mAh battery would be sort of like 17000 mAH, or
17 amp-hours. In reality, of course, this model of 1000 full charge
discharge cycles is totally bogus. The device probably can't support
anywhere near that many full discharges, and you won't be discharging it
very deeply anyway. Or so you say. But I don't know how to assess the life
of the battery at the shallow discharge rates you are talking about.

Do keep in mind that if you discharge it very deeply even once, it will
basically be toast.

It would help if you had some idea of total backup time you expect to need
over the 5 or more years you want this thing to work. Can you estimate a
percentage availability over the 5 years?

Anyway, it seems quite possible that two non-rechargeable D cells will
last just as long or longer than your 17 mAh cell, and will probably be
simpler to use. I don't know whether your design can tolerate the
weight/size of them, though.

>>If your PIC spends most of it's time in sleep mode then you might be
>>talking tens of microamps of total current, in which case the batteries
>>will work for near their shelf life. Lithiums may get you 5-10 years.
>>
>>The simplist and most robust solution is just some Alkaline or Lithium
>>non-rechargable batteries. If you can get away with it, this is your
>>best option.
>
> I thought (for low discharge rates) Alkalines weren't preferably
> because of their self-discharge rates? Not sure Alkalines will last 5
> years (what about leaking?).

Oh, yes. Duracell batteries will last 5 years on the shelf. I'm not sure
how storage temperature affects shelf life, but at room temperature, you
should get 5 years from your Duracell batteries. (and probably Energizers,
as well).

Incidentally, high temperatures greatly accelerate the aging of Lithium
primary cells. I wouldn't be surprised if it does the same thing to
rechargeable cells, too.

--Mac

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