Dissection Minolta NP-90M battery pack for Minolta (Maxxum) 9000; MD-90, BP-90M

Andreas Thaler · Aug 19, 2024

The Minolta (Maxxum) 9000 comes with the Motor Drive MD-90 as an accessory, which can optionally be operated with the rechargeable NP-90M battery pack.

This is a high-quality part that is electronically connected to the motor drive and provides a portrait format shutter release.

The NP-90M contains nickel-cadmium batteries that have leaked over the decades.

I wanted to take a look at the inner workings of the battery pack with a view to replacing the batteries.

Loosening the screw connections.

The battery cells have leaked.

There are no nominal values applied to the battery cells below the plastic cover which I have sliced.

13 battery cells of 1.2 volts each are connected in series and soldered together.

Overview

The battery cells are only electrically connected to the NP-90M circuit via the red plus and black minus connections.

Switch, a small circuit board with a red LED, resistors and probably a transistor.

The corresponding charger NC-90M.

The question that arises with all old battery packs is which batteries can be used as replacements.

As far as I know, nickel-cadmium batteries are only available in the EU for certain applications and while stocks last.

Andreas Thaler · Aug 19, 2024

Optionally, the MD-90 can be operated with the BP-90M battery holder for 12 x AA.

This means that the motor can still be used today.

The MD-90 is one of the most valuable accessories I have seen for SLRs.

+++

All information provided without guarantee and use at your own risk.

Paul Howell · Aug 19, 2024

The motor drive is fairly fast, I think 5.5 FPS, but at 12 AA, it can heavy. I normally use the motor winder, it is a lot lighter at 4AA, but 2 FPS. There are few companies here in the U.S that will rebuilt NC batteries, but the cost is high.

ogtronix · Jan 18, 2025

I've just ordered some replacement N sized NiMH cells from Aliexpress for one of these. Kinda pricey and low availability, plus the usual risks of shopping on Aliexpress, but the only other places selling them are alarm battery places at like £5 a cell. As far as i'm aware NiMH cells are almost a 1:1 replacement for NiCd cells with the difference being how charging circuits detect the end of charge, but it doesn't appear any of the charging circuit was contained in the battery pack itself so that's easy to work with.

At least, i'm assuming that. I've not been able to get that metal plate off to take a closer look since a few of the screws are extremely stuck and now rounded off. Fortunately they go into round pillars so I can remake them, but i'll have to drill the old screws out at this point maybe. I wonder if the problem is leaked electrolyte getting into the threads.

Andreas Thaler · Jan 19, 2025

ogtronix said:
I've just ordered some replacement N sized NiMH cells from Aliexpress for one of these. Kinda pricey and low availability, plus the usual risks of shopping on Aliexpress, but the only other places selling them are alarm battery places at like £5 a cell. As far as i'm aware NiMH cells are almost a 1:1 replacement for NiCd cells with the difference being how charging circuits detect the end of charge, but it doesn't appear any of the charging circuit was contained in the battery pack itself so that's easy to work with.

At least, i'm assuming that. I've not been able to get that metal plate off to take a closer look since a few of the screws are extremely stuck and now rounded off. Fortunately they go into round pillars so I can remake them, but i'll have to drill the old screws out at this point maybe. I wonder if the problem is leaked electrolyte getting into the threads.

I have an NP-90M battery pack here, unopened and in very good condition. I can send it to you.

I am also happy to include the parts of the battery pack that I disassembled, including all the screws.

Of course free of charge

ogtronix · Jan 19, 2025

Very kind offer, thanks. I'll see if I can get this one apart first but I might take you up on it if I manage to completely mangle it.

ogtronix · Jan 24, 2025

Well the aliexpress order says the delivery date will be the 25th of March, and I can't try cancel it since it's already supposedly been shipped lol. So I took another look at the Landscape of Batteries and found they make 2/3 AAA sized cells that're a bit skinnier but the same length as N sized ones, and they're popular in those garden solar lanterns thus fairly cheap and available. I've ordered a set that should arrive alot sooner.

It's a shame no literature seems to state the expected mAh capacity of the NP-90M itself, but looking at search engine results for an old catalogue...

"N" at 150mAh when the 2/3AAA NiMH cells I just ordered claim to be 400mAh. Which I doubt is true but even if it's exaggerated by as much as half, it'd still be more capacity than a nominal NiCd cell of the era.

ogtronix · Jan 29, 2025

Alright, I got it working!

First step was cleaning the old stuff out. I removed everything including those contact posts that connect it to the camera, which were just press fit into plastic holes and could be twisted a bit to loosen them and then pulled out.

Cracked the case unfortunately trying to get the metal insert out. I thought it was held on with double sided tape but it was just the foam tape under the cells overlapping, but I flexed it a little to try peel it off the imaginary adhesive.

Half assed attempt to hide the glue repair. I might come back at this.

Spot welding the cells. I bought the battery spot welder a while back and the thing makes me nervous. Just a big old lithium battery you directly short out. They're vape batteries and designed for it I suppose, but I still don't like it.

Once everything was soldered back in place and tested working, I applied a few dollops of hot glue to hopefully prevent it sliding around. The new cells are a bit narrower and looser, and while the foam holds them in place, some sharp bumps or dropping it could have the packs slide and possibly short something. Also the removed and reinserted contact posts were glued in to hopefully prevent them rotating since their tabs could touch and short.

If you can get the top metal cover off there's little reason to dissemble it as much as I did.

The charging setup is quite crude, using an old RC hobby charger and the connector from a particularly busted motor drive. I'd hoped i'd be able to charge it all neat and properly through the DC jack but the hobby charger didn't like that, complained about over-voltage. Probably because there's a diode in the way or something... I think I forgot to take a photo of the top side of the pack's PCB, or i'd try trace it out. But yeah the two power pins are just directly connected to both end of the battery. I'll probably try make something a bit neater at another time.

But yeah, glad that's working now. The rechargable battery looks alot less silly than the huge tall AA one, although I think the AA one looks good with the control grip. Since they're quick detachable from the motor drive it might make sense to just keep the control grip always attached to the AA pack. You could use battery packs as a weird tripod quick release.

Something i've read occasionally is that the motor drive can only hit its top speed with the NP-90M, which didn't make alot of sense to me. The manual has the line "for maximum film advance speed of 5fps, fully charged NP-90M must be used" which could be read as saying the BP-90M can't hit 5fps, but it could also just mean you've gotta use a freshly charged NP-90M to hit the advertised top speed. But maybe mid 80s alkalines couldn't provide as much current as contemporary NiCd?

Either way it's hard to compare them now since 12 fresh alkalines is about the same voltage as 13 charged NiMH cells, then there's different voltage fall-offs and stuff. But i'd say there's no significant difference in performance speed-wise between the two battery packs. I also suspect the motor drive is regulating the framerate electronically anyways.

Andreas Thaler · Jan 29, 2025

Congrats!

Bushcat · Jan 30, 2025

The current you can pull from a cell is a function of its internal resistance. Ni-cd and Ni-MH have way lower internal resistance than alkalines (tens of milliohms compared with hundreds of milliohms), ergo you can get way more current from them, with less voltage drop, all things being equal.

Alexander K · Apr 9, 2025

ogtronix said:
Alright, I got it working!

View attachment 389293 View attachment 389294
First step was cleaning the old stuff out. I removed everything including those contact posts that connect it to the camera, which were just press fit into plastic holes and could be twisted a bit to loosen them and then pulled out.

Cracked the case unfortunately trying to get the metal insert out. I thought it was held on with double sided tape but it was just the foam tape under the cells overlapping, but I flexed it a little to try peel it off the imaginary adhesive.

View attachment 389295
Half assed attempt to hide the glue repair. I might come back at this.

View attachment 389296 View attachment 389297 View attachment 389298
Spot welding the cells. I bought the battery spot welder a while back and the thing makes me nervous. Just a big old lithium battery you directly short out. They're vape batteries and designed for it I suppose, but I still don't like it.

Once everything was soldered back in place and tested working, I applied a few dollops of hot glue to hopefully prevent it sliding around. The new cells are a bit narrower and looser, and while the foam holds them in place, some sharp bumps or dropping it could have the packs slide and possibly short something. Also the removed and reinserted contact posts were glued in to hopefully prevent them rotating since their tabs could touch and short.

If you can get the top metal cover off there's little reason to dissemble it as much as I did.

View attachment 389300 View attachment 389301 View attachment 389302
The charging setup is quite crude, using an old RC hobby charger and the connector from a particularly busted motor drive. I'd hoped i'd be able to charge it all neat and properly through the DC jack but the hobby charger didn't like that, complained about over-voltage. Probably because there's a diode in the way or something... I think I forgot to take a photo of the top side of the pack's PCB, or i'd try trace it out. But yeah the two power pins are just directly connected to both end of the battery. I'll probably try make something a bit neater at another time.

View attachment 389303
But yeah, glad that's working now. The rechargable battery looks alot less silly than the huge tall AA one, although I think the AA one looks good with the control grip. Since they're quick detachable from the motor drive it might make sense to just keep the control grip always attached to the AA pack. You could use battery packs as a weird tripod quick release.

Something i've read occasionally is that the motor drive can only hit its top speed with the NP-90M, which didn't make alot of sense to me. The manual has the line "for maximum film advance speed of 5fps, fully charged NP-90M must be used" which could be read as saying the BP-90M can't hit 5fps, but it could also just mean you've gotta use a freshly charged NP-90M to hit the advertised top speed. But maybe mid 80s alkalines couldn't provide as much current as contemporary NiCd?

Either way it's hard to compare them now since 12 fresh alkalines is about the same voltage as 13 charged NiMH cells, then there's different voltage fall-offs and stuff. But i'd say there's no significant difference in performance speed-wise between the two battery packs. I also suspect the motor drive is regulating the framerate electronically anyways.

Congratulations mate! I just opened the parcel with MD - 90 and NP-90M yesterday and found that the motor does not work, I decided to disassemble NP-90M, realizing that the old batteries could have leaked. And so it turned out, everything is covered with traces of old batteries. But I hope there is a chance to restore all this. I have a native charger, maybe it will help me. Please tell me what kind of batteries you ordered, I really want to do the same. And also, I don't have a battery spot welder, will this be a problem for me? Maybe I can use an ordinary microcircuit iron?

ogtronix · 2025-04-15T06:50:25-0400

Alexander K said:
Congratulations mate! I just opened the parcel with MD - 90 and NP-90M yesterday and found that the motor does not work, I decided to disassemble NP-90M, realizing that the old batteries could have leaked. And so it turned out, everything is covered with traces of old batteries. But I hope there is a chance to restore all this. I have a native charger, maybe it will help me. Please tell me what kind of batteries you ordered, I really want to do the same. And also, I don't have a battery spot welder, will this be a problem for me? Maybe I can use an ordinary microcircuit iron?

Thanks!

The cells I got are Sanik branded 400mAh 2/3AAA NiMH ones. They're slightly narrower than the original N sized cells, but higher capacity due to improved battery technology after 40 years - even if they're only half of the claimed capacity. There might be other sized cells that'll fit in the space available, but 2/3AAA is a reasonably common one for use in those solar lanterns you spike into soil.

Soldering directly to the battery ends can heat damage them, but you may be able to find them for sale with solder tabs already welded on. Here's an aliexpress listing for exactly that. The 800mAh claim is silly, but the one review says they measured out to 150mAh. If I remember right 150mAh is still slightly higher then the capacity of the original cells. But yeah you might be able to find a better or more convenient deal on a website local to you.

Also NiMH batteries as far as i'm aware aren't fully compatible with chargers made for NiCd batteries, something about how they detect the end of charging.

Chan Tran · 2025-04-15T07:45:20-0400

NiMH is fine. What's the dimensions of the orginal cells? Are there 13 of them or 14?

ogtronix · 2025-04-15T08:32:23-0400

There's 13 "N" sized cells, which are officially 30.2mm long and 12mm diameter, although there's probably a tolerance along with a difference in length between flat top and button top ones.

Again the 2/3AAA cells are much cheaper and easier to find and their capacity exceeds the original 40 years ago style NiCd cells in the pack. The N cells for sale tend to have the button tops too which, along with the possibility of imprecise spot welds making the battery pack slightly oddly shaped, might make a pack using them difficult or impossible to fit back into the shell.

Alexander K · 2025-04-16T04:28:12-0400

ogtronix said:
Thanks!

The cells I got are Sanik branded 400mAh 2/3AAA NiMH ones. They're slightly narrower than the original N sized cells, but higher capacity due to improved battery technology after 40 years - even if they're only half of the claimed capacity. There might be other sized cells that'll fit in the space available, but 2/3AAA is a reasonably common one for use in those solar lanterns you spike into soil.

Soldering directly to the battery ends can heat damage them, but you may be able to find them for sale with solder tabs already welded on. Here's an aliexpress listing for exactly that. The 800mAh claim is silly, but the one review says they measured out to 150mAh. If I remember right 150mAh is still slightly higher then the capacity of the original cells. But yeah you might be able to find a better or more convenient deal on a website local to you.

Also NiMH batteries as far as i'm aware aren't fully compatible with chargers made for NiCd batteries, something about how they detect the end of charging.

Thank you ogtronix !
I will definitely try to find these batteries and restore the device. I will let you know about the results here

Dissection Minolta NP-90M battery pack for Minolta (Maxxum) 9000; MD-90, BP-90M

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