A question for the techies out there...

So my CIB NES set is almost done, and I was all set to go replacing all my CR2032 batteries in one fell swoop, when it occured to me that there might be a better option.  So my question is thus: could I replace the RAM chip in games with battery backed RAM with a compatible NVRAM chip in order to keep saving games on said carts well into the future?  If not, hey, no biggie; but if so, these would be my follow-up questions:

A - Is there a standard RAM chip used in all battery backed US NES carts or do they all use different chips?
B - Which presently available NVRAM chip(s) would be compatible?
C - Would program code have to be altered at all or would it be as simple as swapping in the NVRAM chip with a soldering iron and cutting out the battery?

The reason I ask is because I use Dallas NVRAM chips in a lot of my arcade games to save high scores on my cabs even after power-down, so I figure the NES games with battery back-up just may work on the same principle.  Ideally, I'd want to replace any game with a high score table with NVRAM, but I do understand that most games not designed to save scores out of the box will almost certainly have a subroutine in their programming to wipe and initialize the high scores with a preset default on startup.  But as far as the battery-backed games are concerned, this obviously wouldn't be the case.

So anyway, has anyone looked into this?  If so, please enlighten this aging 8-bit gamer  

Isn't NVRAM just SRAM with an integrated battery? If that's the case, swapping out the batteries would be the better option.

If you don't want to deal with batteries at all, FeRAM is what you're looking for. I haven't done any research on NVRAM, but if you went with FeRAM you'd probably be looking at around $13 per chip + an adapter to mount it. You'd also need some logic on the adapter to make the chip work properly.

Yeah, Dallas NVRAM chips are an SRAM with a battery inside. That's why they're so big, they wouldn't fit inside an NES cart.

Ramtron makes (or made, not sure if it's still in production) an FRAM chip that is a drop-in replacement for SRAM. It was in .6" wide DIP package, I tested one on NES and it worked fine. You probably wouldn't want to use it though. I believe with all 5V FRAMs, reads are destructive. The number of reads allowed is pretty high, but it is finite. Plus I'm sure they're over $10 each. edit: it was FM1608, and it is out of production (in DIP package, anyways).  If you wanted to use it for the fun of it, this should work if you can find them on ebay or something (be careful about fake chips on ebay though).

There was another type of chip, sorry I don't remember the part number or manufacturer, but I believe they were drop-in replacements as well. They were SRAM + Flash chips, and when powered on/off they would backup/restore the SRAM to/from Flash. I'm sure this was a really exotic boutique type of item that wouldn't be cheap or easy to get.  I'm not certain it would work with NES games (probably), but it is almost surely obsolete as well.

The RAM chips in NES carts are probably not exactly the same part number, but certainly are standard pinout and function. Judging by how many of my NES carts still have saves, I'd say they did a good job on selecting the right RAM chip and backup circuit. At my work I repair a lot of units of a product from the late 90's that used battery-backed SRAM, some of RAM chips (even of the same part number and manufacturer) just drain them faster than others. And CR2032s, the voltage doesn't drop at a linear rate over time, it'll stay near 3.2V and at some point (not sure what point exactly) it'll just drop off a cliff and be effectively dead. The rule I go by when repairing, is if it's below 3.0V, I replace it. They're only like 20 cents if you buy in bulk, and with NES carts they should be good for 10~20 years.

Thanks for the detailed input. It sounds like replacing the batteries with new CR2032's is my best bet, then I don't have to worry for another couple of decades. I was just looking for a permanent solution, but twenties years of save data is going to have to do...

"NVRAM" is more a configuration than a type of memory. The SRAM in the carts is already configured as NVRAM by way of a battery. A lot of PCs with battery-backed CMOS settings have BIOSes that refer to it as "NVRAM" (usually CMOS Setting, BIOS settings, etc, but sometimes the POST will test and say "NVRAM good" or "CMOS Checksum Error - NVRAM cleared" or something. If they sell a chip called "NVRAM" I imagine that it's just going to be the same thing the carts already have but in a single component package or something like a SRAM with a capacitor that will loose it's data if it isn't powered every now and then.

I wonder if you can get a stacked battery holder that connects them in parallel for even longer battery life.

If you're going to replace the batteries, you might want to go with BR2032 batteries instead of CR2032. From what I've read, BR type batteries are more appropriate for this application.

I would just buy good name brand CR2032 batteries & either wired holders (or if you want to spend a bit more time board mount holders) I've recently went through about 20 copies of Zelda's some that date back to 87 & all but 2 had saved data and the 2 that didn't I think were just empty to begin with. I don't recommend buying cheap ones because I can say that the sunbeam ones from dollar tree don't last near as long in my wii fit meter & their expiration date is like 5 years less than the energizers I bought at walmart. 2 pack of energizers were like $4.50 ish. I'd prefer panasonic, maxell, sony, or duracell but all wally world had was energizers & I was switching out batts in zelda 1 and 2 so I wanted to go ahead & get a couple. You probably saw my post of changing them using 2 different methods & even retaining my save. Really simple. I wish someone would make something that would go between the NES & cartridge (think Action Replay which I have one but never tried since it won't fit nes without modifying) that would have save points using modern components that are reliable & based off flash memory. I can't believe it's not been done since I know the tech to do so was around all the way back in the early 90's with action replay.

There are options on several platforms:
Retrode
Hyperkin Retron5
INL Kazoo (well, I know it can dump the ROMs at least)
Far Front East Super Wildcard DX2
Bung Game Doctor SF7 (and earlier)
Naki Game Saver+ (tricky but possible)
Bung PCjr (use with NES to Famicom adapter?)
Mr. Backup Z64
Bung V64
Bung V64jr
Super UFO CD64
GameShark/Action Replay + ProCom or whatever PC cable/card they used
GameShark Pro/Action Replay Pro + Dex Drive and/or parallel port cable
Bung GB Xchanger
GB Transferrer
...about a million GBA flash linkers

Definitely have a few options for different platforms with battery saves.

BR2032 has a lower voltage and lower mAH. Only advantage I can see is that it can tolerate more extreme temperatures, but all of our game paks have a warning about exposing to extreme temps anyway.  

If there's some other advantage like they expire more slowly when not used or when used in extremely low current applications then I'm not reading it in any comparisons. It wouldn't surprise me if it did have a superior shelf life, I just don't see anything to indicate it. My understanding is that Super Heavy Duty batteries are inferior in every way to alkaline except shelf life, which is why they are still included with remote controls, smoke alarms, and similar things.

There's a "Pro Action Replay" (cheating device, Europe only I think?) and "Game Action Replay" (save state device).  Yeah those kind of saving devices have been around for a while, but if you look at what's inside a Game Action Replay it's pretty intense, it's a huge custom chip which must have been really expensive to make.  Something like that can be made today with an FPGA, but it is similar in complexity to something like the PowerPAK.  And the PowerPAK of course does support save states, with thefox's mappers.  Not as cool as doing it with original cartridges, but there is that.

About BR2032s, those can be ordered with pins on them like the batteries in Nintendo's cartridges, I think that's what DxS was referring to.  They are the same 3V type as CR2032s, does look like they have less capacity but not by a huge amount (Panasonic for example says 225mA/h for CR2032 and 190mA/h for BR2032.  They come in different pin configurations though, I can't say for sure if it's the same pin style as Nintendo used (one of them looks like it, but could be spaced differently, you could measure on in an NES cart then compare it to the pin spacing in a datasheet if it's given there (check Digikey).  If it fits it would be easier to install, probably still costs more than a CR2032 and a holder though.  I could be wrong, but I'm pretty sure there isn't a board-mount battery holder that matches the hole size and spacing on the NES board, so probably has to be wired up to it.

It is incorrect to say that all NVRAM has internal batteries. Some NVRAM contains F-RAM, which has a 150+ year life span with 100 trillion reads/writes. This is commonly used in older pinball games.

https://www.pinitech.com/products...

Indeed, there is this kind of NVRAM already used in NES cartridges and such.

https://www.instructables.com/id/...

So, replacing the battery with NVRAM is a perfectly legit upgrade to do to cartridges.

Lots of misinformation in this thread.