Backup battery underdimensioned

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Backup battery underdimensioned

Rask Ingemann Lambertsen-2
   I looked at the specifications for the backup battery[1]. The charge
current is specified as 25 uA in the charge/discharge graph, but the
pcf50633 can only be configured for 50, 100, 200 or 300 uA (with 200 uA
being used in the GTA02). Similarly, the discharge current of 5 uA is much
lower than the maximum 50 uA consumption specified for the pcf50633 in SAVE
state. Even if this abuse doesn't damage the battery, with a nominal
capacity of about 300 uAh, it will only last for about 6 hours. At 100%
depth of discharge, battery life is only 100 cycles.

[1] http://speed.sii.co.jp/pub/compo/battery/productSpecBatEN.jsp?recordID=77
    http://www.sii.co.jp/compo/catalog/battery_en.pdf

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Re: Backup battery underdimensioned

Werner Almesberger
Rask Ingemann Lambertsen wrote:
>    I looked at the specifications for the backup battery[1]. The charge
> current is specified as 25 uA in the charge/discharge graph, but the
> pcf50633 can only be configured for 50, 100, 200 or 300 uA (with 200 uA
> being used in the GTA02).

The data sheet says:

| 4-6 Maximum Charging Current (mA)
|       The battery voltage is 3V or more   0.1
|                            Less than 3V   0.5
|                          Less than 2.8V   5.0

So I think we're okay. Not sure about the discharge current. The
battery's data sheet only specifies a "Standard Discharge Current"
(5 uA) but doesn't say what happens if we exceed it.

Also, the PMU's data sheet specifies only a maximum total supply
current, leaving it unclear on which path this goes, and if it
also applies if there's no main battery. Also, this doesn't say
much about the average current.

So I'm not sure we have a problem with the charge/discharge current.

However, the backup battery has been trouble. First of all, it
doesn't solder well. Second, it does seem prone to failure, but
nobody knows exactly why. I'm also not sure if this happens a lot
in the field or not. Third, if it works, it helps to increase the
number of PMU configurations we have to worry about after reset.

There's also the question of what it really does. We don't actually
want the PMU to remember all that much state, and we have to be
prepared to have a functional system in case the backup battery dies
anyway. We do want the time, but we also have at least three other
RTCs in the system (GPS, CPU, GE865), plus we have a number of
potential time sources (GPS, GSM, NTP if we have IP connectivity),
so resetting the clock after a battery change may be an option.

When planning GTA03, we considered just removing the backup battery
and replacing it with a supercap. This idea was eventually rejected
as being too risky.

The maximum backup battery charge voltage is 3.0V (typ), the backup
battery presence detection voltage is 1.3V (typ). If we assume that
the RTC does its worst and continuously draws 50 uA, a 100 uF cap
would only last about 3.5 seconds.

I'd love to get rid of the backup battery. I'm just not sure we can.

- Werner

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Re: Backup battery underdimensioned

Rask Ingemann Lambertsen-2
On Wed, Aug 19, 2009 at 02:35:58PM -0300, Werner Almesberger wrote:

> Also, the PMU's data sheet specifies only a maximum total supply
> current, leaving it unclear on which path this goes, and if it
> also applies if there's no main battery. Also, this doesn't say
> much about the average current.

   This bit on page 16 makes me worry (last six words):

        Save

        In the Save state, the power management section is supplied from
        either the system voltage (if VSYS > Vth(syspres)) or the backup
        battery (if VSYS < Vth(syspres) and VBUBAT > Vth(bubpres); see
        Figure 6). Only the internal supplies, the 32 kHz oscillator and the
        real-time clock will be active. The GPIOs will maintain their state.

   In particular, GPO has a push-pull output which we use for EN_USBHOST.
It's not clear from the PCF50633 manual how much that affects current drawn
from the backup battery or which state consumes the least power.

> plus we have a number of
> potential time sources (GPS, GSM, NTP if we have IP connectivity),
> so resetting the clock after a battery change may be an option.

   At least with the default ext2[1] settings, this causes two runs of
e2fsck: The first one because the last mount time is in the future and the
second one because the fs has gone 3000+ days without being checked.

[1] Yes, I know, people shouldn't be using it, but ...

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Re: Backup battery underdimensioned

AlvieBoy
In reply to this post by Werner Almesberger
Werner Almesberger wrote:
> The maximum backup battery charge voltage is 3.0V (typ), the backup
> battery presence detection voltage is 1.3V (typ). If we assume that
> the RTC does its worst and continuously draws 50 uA, a 100 uF cap
> would only last about 3.5 seconds.

Btw I just found these ones:

http://dkc1.digikey.com/us/en/ph/Panasonic/rf_rg.html

0.68F to 1F.

Álvaro


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Re: Backup battery underdimensioned

Werner Almesberger
?lvaro Lopes wrote:
> http://dkc1.digikey.com/us/en/ph/Panasonic/rf_rg.html

Hmm, 6.5 mm tall and ~20 mm diameter ? ;-)

We actually once did find a supercap that had the size of the
backup battery.

Naw, let's not go down that rathole. For gta02-core, the old backup
battery will do. I'm trying to find out if the can use the modem's
RTC to bridge any extended loss of power. If yes, I'd prefer to
remove the backup battery entirely, so that the PMU always resets
to NoPower state when the battery is removed.

- Werner

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Re: Backup battery underdimensioned

Werner Almesberger
In reply to this post by Rask Ingemann Lambertsen-2
Rask Ingemann Lambertsen wrote:
>    This bit on page 16 makes me worry (last six words):
[...]
> real-time clock will be active. The GPIOs will maintain their state.

Good point, yes. If we're in a controlled power-down state, EN_USBHOST
should be low and not draw any significant current. But if we dropped
dead with USB host enabled (which makes an excellent reason for running
out of power in the first place), then we'd leak VB_SYS/100kOhm or about
20 uA (with VB_SYS <= Vth(syspres).)

>    At least with the default ext2[1] settings, this causes two runs of
> e2fsck: The first one because the last mount time is in the future and the
> second one because the fs has gone 3000+ days without being checked.

Automatically running e2fsck is proabably a rather misguided idea in
the first place ;-)

"Why the heck does my phone freeze when I try to turn it on ?"
"It's just fsck'ing."

- Werner

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