What is the recommended battery mAh rating for Williams 80' MPUs ?

andykmv

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What is the recommended battery mAh rating for Williams 80' MPUs ?

i was looking at some lithium cell options for a better nvram/cmos backup battery for my Williams Defender etc using parts i have available. i have bunch of ds1225 nvrams so i might as well hack one of these for use.

There has been some discussion on the board about how long some of the button cells last as cmos backup batteries (or don't last according to some)

For 5101 CMOS Ram battery backup, what is the recommended milli-amp-hour rating for the backup battery to achieve at least 12 months backup reliably ?
 
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Recommended procedure is to throw away the batteries and use a non-volatile RAM :)

yeah - i have been looking around for suitable parts that dont have embedded batteries. iirc you mentioned a particular part (EDIT: Simtek 25c48) a little while ago, but some of those similar that i have looked at recently were rather pricey.

i have a bunch of ds1225 devices at hand (20) that would have been pulls, and therefore may or may not have good batteries.

i can bypass the internal battery quite easily with this device as the controller ic is visible underneath so access to the VBATT & gnd pins is easy (either a DS1210 or DS1218 nvram controller depending on the nvram module model) . i'll try acetone first but i am not sure if that will work to dissolve the potting around that ic, so may have to resort to a gentle gouging with a dremel tool.

hence the request for some advise on a suitable Ahr capacity lithium cell that i can readily attach to the ds1225 to give trouble free >12month operation (ideally another 10 years capacity so i could go over the top and hook up a 2.1Ah capacity 3.6v lithium cell) but would rather a lower cost option.

i should stop being lazy and do some calculations myself based on the 5101 cmos ram current draw etc.

so the question remains: what is a suitable Ahr rating for approx 12 months trouble free operation ? maybe those of you that have installed the button cell replacement kit for the cmos battery could pipe up with what make and model of battery cell that you used that lasts or lasted for 12 months or so ? :)
 
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When I bought my stash of STK25c48, it was an EOL buy... with the right adapter board, any of the simteks with "autostore" should work fine... the ones with triggered store/restore, you'd need to wrap addttional circuitry around to make them store at power down and restore at power up...
 
When I bought my stash of STK25c48, it was an EOL buy... with the right adapter board, any of the simteks with "autostore" should work fine... the ones with triggered store/restore, you'd need to wrap addttional circuitry around to make them store at power down and restore at power up...

aha! autostore. cool. i'll see if those ones for sale were autostore. thank you!

i'll do a hardwired hack for the ds1225 chip first, followed by an autostore type chip when i can get a pair, then tackle the design/pcb for a triggered store chip later if necessary.
 
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got off my butt today and started doin some calcs to find out what the minimum AH rating should be to support the Defender battery backed 5101 CMOS RAM circuit and related components for a minimum period of 12 months.

started with identiying the components including:
1. IC 1E - 5101 CMOS RAM
2. IC 6H - 4071 CMOS OR gate
3. R51 - 330k ohm resistor
4. C3 - 0.1uF - disk ceramic cap ?
5. D13 - IN5817
6. D14 - IN4148

and collecting the specification sheets for each key semiconductor component (1,2,5,6).

spending some time working out the current draw during the scenarios
a) "normal operation", and
b) "battery backup"

findings to follow.
 
Calculating the Ah rating for the Defender 5101 battery backed CMOS RAM Device
===============================================================================

1. 5101 CMOS RAM Specs:
---------------------------------------------------------
Power Supply Range | 4V to 6.5V |
---------------------------------------------------------
Operating current | 8mA |
---------------------------------------------------------
Min voltage for battery | 2V |
for memory retention | |
on stand by | |
---------------------------------------------------------
Quiscient current | 25-200uA |
---------------------------------------------------------
Quiscient mode 0.95 of Vdd, presuming
this means 95% of 4v,
then 3.8v is probably
the voltage at which
quiscient mode begins
---------------------------------------------------------

when the MPU is running normally, normal operating current is 8mA. also, when the MPU is being shut down, either normally

or under a power failure scenario, there will be a very small period of time where data may be being written. normal

operating current during that period is 8mA

when the 5V supply rail drops to =<4.75v, the reset circuit inhibits further writes.

with a traditional 3xAA battery pack the terminal voltage is 4.5v with fresh batteries. there should be no current draw on

the Battery Pack until the main psu voltage drops below 3.8v (ie the terminal voltage of 4.5 minus the in4148 diode forward

voltage of 0.7v). As that is about the point at which the 5101 goes into quiscient mode, then the current drawn from the

battery pack is in the 25-200uA range.

with a fresh 3.6v lithium cell, there should be no current draw on the Battery Pack until the main psu voltage drops below

2.9v (ie the terminal voltage of 3.6v minus the in4148 diode forward voltage of 0.7v) As that is about the point at which

the 5101 goes into quiscient mode, then the current drawn from the battery pack is in the 25-200uA range.

so, if a 3.6v button cell is used as the CMOS RAM battery backup, less the 0.7v forward voltage of the in4148 diode, then

the chosen cell should retain its terminal voltage above (2v+0.7v) or >2.7v to ensure the supply voltage to the 5101 is

above the minimum voltage of 1.5-2v for memory retention. therefore, the cell chosen must maintain the cell terminal

voltage above 2.7v for the desired period (say a min of 12 months) at the max est current draw of 200uA. If you choose a 3v

cell, then the voltage to the 5101 is 3.0-0.7=2.3v when the battery is fresh.

if you choose a typical lower AH rating cell, say 90mAh, there is approx 1/20th the capacity and therefore may only last 3-4

weeks!

As a back of the envelope calculation, If the current drawn is x amps, the time is T hours then the capacity C in amp-hours

is C = xT. In our example, the circuit draws max of 200uA in quiscient mode, for 10,000 hours the Amp Hours (C) = 0.0002

Amps * 10,000 hours (approx 1.2 years) = 2Ah depending on the cell type, the terminal voltage will drop rapidly as it nears

the end of its capacity, so a 2Ah battery may be only good for 8000-9000 hours at 200-300uA

2. Now we need to add the extra current drawn by IC 6H and the associated 330k resistor.

4071 CMOS OR gate 14 pin package. Typical operating specifications for this:
(TBA) - 4584 uses between 30 and 120uA
and the associated 330k resistor:
i=3.8v/330,000 ohm=1.15*10exp-5=0.0000115 or 11.5uA

therefore:

if we use max values, Amp Hours (C) = 0.0003315 Amps * 10,000 hours (approx 1.2 years) = 3.3315Ah
if we use typical values, Amp Hours (C) = (0.000050+0.0000115+0.000030 Amps * 10,000 hours (approx 1.2 years) = 0.915Ah

so i estimate a 4.5v or 3.6v 2Ah battery would do the trick then.

now the ds1225 is a little different with a quoted standby current of 5-10mA (when chip not selected but presumeably in an

active circuit. therefore if we use max values as quoted,
Amp Hours (C) = 0.01 Amps * 10,000 hours (approx 1.2 years) = 100Ah. now that just doesnt look right. given the size of

the packaging, lets say it is a 2Ah battery, divided by 10000x10 (10 yrs) would yield a mem retention mode current of 20 uA.

so lets now say 20uA * 10000 hrs = 0.2Ah. now thats more like it! this est is borne out by the ds1210 spec sheet which

quotes 0.1uA for the ds1210 and 10uA for the ram, which would mean a 1Ah battery would suffice for 10 years! that is without

the 4071. maybe 40ua *10000hrs=0.4Ah. still a tiny cell, and 10yrs for 4Ah.


any flaws in the logic or methods of approximation ?
 
Looks like from the way this thread "dropped off the face of the earth" makes me think this was a little "over" most people's heads! (it was definitely more than i could get my head around)... which is a little unfortunate- would have liked to hear more.
 
Oh I don't know, I just have a defender and I just installed a kit from BOB Roberts. Dont think it's a lithium though:)
 
aha! the old i dont know what i dont know until i know i dont know it trick! :D

i am guessing you want an idea how long the battery in the kit should last.
should be able to work that out from the battery make & model info on the battery.
 
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