Advice replacing coin battery on trapdoor RAM.

[Ral-Clan]

Hi, I've got an Amiga 500 with a SupraRAM 512K expansion in the trapdoor.  It has a real-time-clock on board that is powered by a coin battery (now dead).

Here's a great picture of the exact board I have and battery:
http://www.amiga-hardware.com/download_photos/supraram500rev1_top_big.jpg

I need to replace the coin battery.  The current battery is electrically bonded to the contacts on the board, so I will have to de-solder those.

I want to put a coin battery holder in there, but I need to know if I should put a diode so that the Amiga doesn't try to charge the battery.

I'm guessing that because the CURRENT dead battery is ALREADY a coin type (2032), then it probably wasn't rechargeable to begin with, and therefore I don't need to add a diode.

But I just want some confirmation of my theory before I go ahead with the modification.

Thanks.

Here's the battery that's already installed on the expansion board:
http://en.wikipedia.org/wiki/CR2032_battery

[djos]

2 things:

1/ that battery is not rechargable, it's just a normal CR2032 Lithium cell so you dont need a diode.

2/ just remove the dead battery and take it to your local battery specialist store and ask for a coin holder to fit the holes on the PCB, then you can replace the battery whenever you want.

[mrknight]

I would replace it with an identical one just to make sure everything will work as intended.

Tried to search for a circuit diagram but I didn't have any luck.
http://www.amiga-hardware.com/download_photos/supraram500rev1_back_big.jpg
http://www.amiga-hardware.com/download_photos/supraram500rev1_top_big.jpg

Have a look at the photo. These is a diode, D1 connected to the plus side of the battery. The other side of the diode is connected to what seems to be the supply voltage of the RT-clock. And this pin should also be connected to the voltage pin from the bus. This is all I can do now without having the proper data sheets and/or circuit diagrams close at hands. Since you know what a diode is, I guess you can easily verify this yourself;)

Good luck!

[djos]

Ah I didnt notice the diode - that battery must be an aftermarket fitment.

Just grab one of these from AmigaKit (has the diode already installed):

http://amigakit.leamancomputing.com/catalog/product_info.php?cPath=39&products_id=1093

[mrknight]

Ah I didnt notice the diode - that battery must be an aftermarket fitment.

I'm sure the coin battery is part of the original design. The battery is powering the RT-clock while the computer is off, and the bus voltage is powering the clock when the computer is on. Since the battery and the bus voltage is connected to the same pin (Vcc on the RT-clock), the supplies need to be protected from each other. That's why you have D1 at the battery side and D2 at the bus voltage side.

Just grab one of these from AmigaKit (has the diode already installed)

The circuit on AmiKit is just a coin battery replacement for rechargeable batteries and is redundant for this RAM expansion since it already got a protection diode.

[djos]

Hmmm, I must be tired today, I keep missing the blindingly obvious! :p

[Ral-Clan]

Good eye both of you....especially catching that diode at D1.

I already have a coin battery holder...and CR2032's are really common, so I'm going to go ahead with this mod.

Thanks everyone.

[danbeaver]

The holder, battery and diode is about a four buck project that takes five minutes to assemble. A dap of hot glue and you are done.

[mindprober]

Your price quote is spot on, even in 2014. The parts, including tax came to $3.97.

I just completed the modification yesterday. The time seems to be holding now.  

Anyone else think the battery is slightly loose in the holder though? Assuming you have the same one I do. I'm guessing this is a very common battery socket.

Also, I'm a little confused by the diode. Clearly, this card already has one, thus preventing any charging of the battery. If I simply replace the holder and battery, there isn't anything more that is needed, correct?

[danbeaver]

Nope, but if the holder is loose then two small dabs of hot glue on the edges will secure it and be easily removable.  I've found the holders are of three types akin to Goldy Lock's three bears: too loose, too tight, and the rarest, just right.  Most of mine have been too tight.