Maybe I'm going overboard here, but I found this post on bf.c and it is full of information so I thought I'd paste it here:
djb2
09-18-2006, 07:19 PM
The Service Indicator (SI) board is a circuit board in the instrument
cluster. Its most obvious function is to monitor the engine usage in
order to count down the time until the next recommended oil change or
inspection, reporting using the bar graph on the cluster. A more subtle
function is to read the "code plug" at start up and use this information
to translate fuel injector and ignition pulses to drive the tachometer
and fuel economy meter.
The SI board batteries provide back-up power to retain the service
counter when the main battery is dead or missing. Much like a UPS for
a desktop computer, they are intended to prevent data loss during a
brief power outage rather than to be a long-term power source. Since SI
board batteries are a pain to replace, you should not remove the battery
for storage and avoid leaving the car sit with a dead battery.
There are two major variations of the Service Indicator (SI) board in
the instrument cluster, the original type with NiCad batteries and the
redesigned retrofit board with long-life Lithium batteries.
NiCad boards have two 1.2V 'AA' cells in series. Fresh batteries will
power the SI board for about a month if the main battery is
disconnected, and will slowly recharge when main battery voltage is
restored. The NiCads typically to last for 5 to 15 years.
Lithium boards have two 3V lithium cells in parallel, which will last for
well over a decade of stand-by use. They should power the SI board for
several months if you remove the main battery, but this will permanently
consume part of their life.
The problem with NiCad board, and the reason for the SI board redesign,
is that NiCad cells don't fail gracefully. If you discharge an old cell
to zero volts (really below 0.6V), metal whiskers start growing
internally. These will short out the cell, preventing it from
recharging at all. The original SI board had two design flaws based on
connecting the microprocessor directly to the batteries: it would
discharge the batteries completely trying to preserve the counter state,
and shorted batteries will prevent the microprocessor from working at
all when the power is restored. Even if you don't care about the oil
change suggestion, other functions that depends on the coding plug
(e.g. the tach) will not work.
The Lithium battery board fixed the design problem. It has two
non-rechargeable 3V lithium cells that are diode isolated from each other
and main battery power. Either cell can provide back-up power, with the
load normally shared to provide twice the life. Even if both cells
fail, the main battery can still power the circuitry.
So now that you know how the SI board batteries work and fail, how do we
fix the problem? I'll start with the NiCad board, since people with bad
lithium batteries might not even know that they have have dead batteries.
There are several web pages with excellent instructions and pictures on
removing the SI board. I won't repeat them here, instead only covering
the battery information that isn't obvious from physical disassembly.
The original NiCad batteries are standard size AA cells (Varta 500 RST)
with board-mount tabs welded on the ends. Your options are to buy
replacements with the same board-tab connections from a specialty
battery supplier (hobby shop or on-line), buy cells with solder tabs or
wires attached, or use wire a remote battery holder to the circuit
board.
Whatever mounting option you choose, use *only* standard capacity,
high-temp NiCad cells. "Better" battery chemistry types (NiM-H,
Lithium) or specialized cells (rapid charge, high capacity) cells are
not better for this application. Non-NiCad chemistries will have
different voltages, and high capacity or rapid recharge NiCad cells have
design compromises that reduce longevity. NiM-H batteries are reported to
work, but the continuous trickle charging will destroy them.
A good choice of battery is the Sanyo high-temp standard capacity cells.
If you can't find cells with solder tabs or want to remote-mount the
cells for easier replacement, put them in a holder. Radio Shack has a
nice dual AA cell battery holder, part #270-408. It's fully enclosed
with wire leads already attached, and typically stocked in-store for
about $1.49.
I don't recommend Radio Shack as a NiCad battery source. They will do
in a pinch, but the cells are overpriced and not the best type for this
application.
Now on to the lithium battery SI board. This is much more academic,
since these board generally don't have problems. However our cars are
getting old, so some of these boards may start needing battery
replacements at some point.
Lithium Battery info
The newer style SI board uses a MnhO2 Lithium battery by Sanyo (CR1425OSE).
The board on my 1987 325is is dated 13.94, and numbered 992 647 196.
It clearly is a replacement, likely installed after only seven years.
The batteries on my board are dated 93-08, and measure 3.17V in 2004.
Nominal end-of-life voltage is 2.0V, but at 2.5V little capacity remains.
The two 3V cells are connected in parallel but isolated by diodes.
Either cell can fail individually and the board will still work.
The cells have an extra isolation film under the conformal coating,
presumably to protect the board from overheating or leakage.
More details about the battery:
Sanyo CR14250SE 3V 850mAh 14.5x25mm (1/2 AA size) lithium cell.
The 14250 is a standard primary (non-rechargeable) Lithium type, available
from multiple sources.
sanyo.wslogic.com/pdf/pdfs/CR14250SE.pdf
The 'SE' variant was optimized for high capacity and long shelf life.
Other variants are optimized for high discharge rate or light weight.
The standard 'SE' discharge rate is 0.5 mA.
This is ten times less than the power needed to light a small LED.
At the standard rate the cells will discharge in 72 days.
At a 10uA rate the cells will last 10 years.
The temp range is -40C to +85C, rather than -20C+60C of other brands.
The cell may weigh up to 11g, rather than as little as 7g.
The Sanyo 'SET-FT' suffix indicates welded tabs for board mounting.
Modern cells are available in 950mAh versions, but long shelf life and
seal life are more important than capacity.
Now I'm curious about a few things:
1) My cluster is a VDO but has a motometer SI board, is that normal?
2) Since we have the newer boards with Lithium batteries, shouldn't the service lights still work by running of the cars main battery after the SI batteries fail?
I've decided that since these batteries last much longer than the rechargeable ones, its not really necessary to install a remote battery pack. Plus I had no idea how to make one (do I have to use the same style Lithium batteries? do they have to be wired in parallel and isolated by a diode?) I would be interested in know how it is done though.
Topic: Service Interval light batteries (Read 29765 times)