HP Forums
OT-N cells hide & seek - Printable Version

+- HP Forums (https://archived.hpcalc.org/museumforum)
+-- Forum: HP Museum Forums (https://archived.hpcalc.org/museumforum/forum-1.html)
+--- Forum: Old HP Forum Archives (https://archived.hpcalc.org/museumforum/forum-2.html)
+--- Thread: OT-N cells hide & seek (/thread-223083.html)

OT-N cells hide & seek - Matt Agajanian - 05-25-2012

Hello all,

To avoid the fried 28S fiasco I described earlier, outside of the 'Shack' store, where might I easily find N cells?


Edited: 25 May 2012, 11:16 p.m.

Re: OT-N cells hide & seek - David Ramsey - 05-25-2012

To avoid the fried 28S fiasco I described earlier, outside of the 'Shack' store, where might I easily find N cells?

Almost anywhere, in my experience. Most drug stores have them here in Reno. If not, see if you have a local Batteries Plus.

Re: OT-N cells hide & seek - Matt Agajanian - 05-25-2012

Okay. I just figured that sizes which are outside of the everyday AA, C, D, hearing aid batteries would be more common in hobby, electronics, camera shops. But, if N cells aren't that specialised, finding them in CVS, RiteAid, etc. makes the accessibility if and when I need these batteries all the easier.


Re: OT-N cells hide & seek - Luiz C. Vieira (Brazil) - 05-25-2012


I found that the 2/3AAA are fair replacements. They almost match the length - they're a bit lengthier, which helps with the contacts - and have a smaller diameter. I use them with the HP41 series for some time now, no problems. I guess they will fit fine with the HP28 as well. See the picture below:

Click to enlarge (approx. 900X800)

Hope it helps somehow.


Luiz (Brazil)

Edited: 25 May 2012, 11:59 p.m.

Re: OT-N cells hide & seek - Bart (UK) - 05-26-2012

I would be careful using 2/3 AAA in a 28C/S as the extra length (as slight as it may be) could put extra strain on the battery door, particularly as 2 of them are in series. That battery door is a known weak design.


Re: OT-N cells hide & seek - Matt Agajanian - 05-26-2012

I must agree. As I experienced, my recent activities helped me learn NOT to try to squeeze even so slightly different batteries into a calculator made for a certain, specific type.

Besides, the 28 battery door is tricky enough to try to put on and secure while trying to 1--fit it into the compartment grooves while 2--pressing down to push the batteries down in the chamber against an amazingly tense spring.

Edited: 26 May 2012, 12:16 a.m.

Re: OT-N cells hide & seek - Garth Wilson - 05-26-2012

Mouser (www.mouser.com) has Energizer alkaline N cells, cat. no. 525-E90, for $1.42 in singles, $1.39 each for five, and $1.34 each for 10. Their Gold Peak alkaline N cells cost less than half that much, but someone here said they had a bad experience with Gold Peak.

Re: OT-N cells hide & seek - Raymond Del Tondo - 05-26-2012

where might I easily find N cells?
eBay. A pack of 20 Duracell Plus costs about 11.50 Euro incl. shipping within Germany. Pricing should be similar in the USA.

N-size & 2/3AAA: actual size compare (must see) - Luiz C. Vieira (Brazil) - 05-26-2012


Actually, regular N-size cells are a bit lengthier than the 2/3AAA. The picture bellow compares both.

Click to enlarge (1070 X 752)

The alkaline N-size in the picture is the one that originally came with the HP28S. Notice that the tip in the positive terminal exceeds the original 2/3AAA size.


Luiz (Brazil)

Added after editing: I've been using N-size rechargeable batteries for more than 4 years; have not thrown away any battery since then. Eco-friendly, I'd say, these rechargeables... 8^)

Edited: 26 May 2012, 12:26 p.m.

Re: OT-N cells hide & seek - Les Wright - 05-26-2012

Luis, do you have a sheath or extend to recharge them in a standard charger, or do you have a specialize charger?

Re: OT-N cells hide & seek - Luiz C. Vieira (Brazil) - 05-26-2012

Hi, Les.

I have a standard charger I want to modify and use it for charging specific batteries. I just want to re purpose the case, line transformer and batteries compartment. This standard charger has just rectifying diodes, some current-limiting resistors and one small current meter, and I want to build something more reliable and precise inside of it. It's been hard to find some time to get my classes ready to carry on, what to say about time to a single project like this one...

What I use to recharge the NiCads and NiMH is single pieces of double-sided, fiberglass PCB's with 1/4 resistor soldered on them (picture). These little PCB's are placed between the batteries and the charger contacts, like this, so the current is limited to approximately 1/10 nominal battery current. I use the 47-ohm resistor for the ones below 200mAh (I have 12 of 190mAh and 4 of 150mAh) and the 27-ohm resistor for the 400mAh and 500mAh NiMH batteries. I let them charging for no more then 10 hours, and from time to time I measure the voltage across the resistor to make sure it has the average 1/10 charging current. As you can see in the picture, I use two N-size batteries at once for each AA-size battery holder. I do this mostly because I measured the voltage in the charger contacts while unload and I found about 2.35Vcc. When I inserted one single 1800mAh rechargeable NiCad AA-size battery I measured about 300mA, and that freaked me out! So I decided to use two N-size batteries (about 2.3Vcc) with no series resistor and I measured almost 60mA. After a few tests and some computing (Ohms rule, nothing fancy) I got to the resistor values.

I do not care for charging batteries this way. It is a once-in-three-months operation, give it or take, so the full extent of the batteries lifetime expectancy will be probably more than my own (I'm 50 Y.O., now). I have some almost 5 Y.O. batteries still running smooth after as many charges as I have submitted them to. In fact, I believe they will live long because I am not exceeding their specs.

Hope this helps.

Luiz (Brazil)

Edited: 26 May 2012, 9:41 p.m.

Re: N-size & 2/3AAA: actual size compare (must see) - Bart (UK) - 05-27-2012

Hi Luiz,

That is interesting. So the rechargeable N cells are shorter than the standard alkaline N cells. Could you post a picture of the rechargeable N cell next to the standard N cell? I already use rechargeable AA and AAA where possible, but I should find some N cells too, I'll be more eco friendly and possibly reduce the pressure on the 28s battery door too :-)


The four of them: size compare - Luiz C. Vieira (Brazil) - 05-27-2012

Hi, Bart.

Here we go:

Click to enlarge (approx. 880 X 1340)

The small ruler is in mm. and it is a regular, low-budget school ruler, so it is there just as an acceptable reference (have not found a better one...).

I added another N-size to the picture. I guess the inscriptions in each of them are enough to identify. Both batteries in the middle are N-size as written, but the NiMH from China has no tip at the positive pole, and its body is slightly shorter than the Sanyo Cadnica. And as you can see, all rechargeables are shorter than the alkaline.


Luiz (Brazil)

Edited: 27 May 2012, 1:40 p.m.

Re: OT-N cells hide & seek - Les Wright - 05-27-2012

When I inserted one single 1800mAh rechargeable NiCad AA-size battery I measured about 300mA, and that freaked me out! So I decided to use two N-size batteries (about 2.3Vcc) with no series resistor and I measured almost 60mA.

My AA/AAA charger is evidently designed for higher capacity batteries, as the LOWEST current offered is 200 mA. The design is such that 2 2/3 AAA's or N's would not fit end-to-end as in your pic, so I am looking at some sort of spacer--either packing in foil or metal rods cut to the correct length.

I take it it is unwise to charge smaller capacity batteries at 200mA? For my regular battery needs Eneloops and pre-charged Duracells, which hold a charge much longer, serve my purposes superbly, and I recharge infrequently, so I will likely expire sooner than the batteries. Looks like standard NiMH and NiCd need more attention to avoid overcharging--something I don't worry about with the Eneloops since I recharge them so rarely anyway.

Looks like I am looking for some resistors and schooling myself in fine soldering...

Thanks for the reply.


Re: OT-N cells hide & seek - Luiz C. Vieira (Brazil) - 05-27-2012

Hi, Les.

The reference for 1/10 of the nominal battery current as for charging current is kinda 'rule of thumb', mostly because the batteries should not heat it to the damage level. And it also matches the 10 hour standard charging time: 1/10 nominal current X 10 hours = 1 X nominal charge (or close to).

But if we consider that some AA batteries claim to reach 2800 mAh (2,8Ah) and some AAA batteries would go for 1200mAh, the same rule would give us either 280mA or 120mA charging current. If the battery charges at about 1.5Vcc, this is something close to 420mW power consumption, and part of it will surely be converted into heat. I do not like it at all, so I prefer reducing the current to no more than 80mA to 100mA and wait for 20, 24 hours, if applicable. Getting the batteries a bit warm, like a little below the finger temperature when touched, is my relative reference.

I know that some modern technology allows batteries to be more resilient, but I still use some NiCads and NiMH. LiPO and any others are relatively new and they come with some different, custom package. I have not use these yet, although I know they need much more care when charging, demanding specialized chargers.


Luiz (Brazil)

Re: OT-N cells hide & seek - Dieter - 05-28-2012

I take it it is unwise to charge smaller capacity batteries at 200mA?

It depends. The relevant figure is the relative (!) charging current C, i.e. current divided by nominal capacity. So charging a 500-mAh-battery at 100 mA is equivalent to 0,2 C. The respective charging time then is about 1,2 / C hours (which accounts for the typical 20% inefficiency of the charging process).

The usual very moderate low-speed charging current is 0,1 C, leading to a 12-hour charge for a completely empty battery. Most N-size NiMH batteries can be charged with 0,2 or 0,3 C as well, maybe also 0,5 C (cf. manufacturer's data sheet). FTR: The GP50NH data sheet allows 0,5 and even 1C (250...500 mAh) if a suitable charger is used.

So, if your charger can be set to 200 mA and the battery is rated 500 mAh, that's 0.4 C (resp. a 3 hour charge) which should be fine if you have a decent charger at hand.

Looks like standard NiMH and NiCd need more attention to avoid overcharging

Every battery needs attention to avoid overcharging. The faster you charge, the more important this point gets. But this is nothing you should have to care about - any decent (i.e. processor controlled) charger will handle this automatically and stop the charging process as soon as the battery is fully charged. This kind of charger is available for about 30 EUR/USD and less, so using a simple low-tech charger does not make any sense at all.

All this has been discussed here before, including different ways to use N-size batteries in suitable AA/AAA-chargers. Please take a look at this thread.