HP-28S battery door

[robve]

(09-06-2018 03:02 AM)burkhard Wrote:  

(09-04-2018 09:17 PM)burkhard Wrote:  I had considered (but haven't tried it yet) trying either a flat top N or possibly even a flat top 2/3 AAA instead of the normal N. The normal "N" battery should be slightly shorter (better) than a normal "2/3 AAA", but a flatop "2/3 AAA" might be slightly shorter (better) than the normal "N", helping to reduce the static loading of the door:body interface. I haven't confirmed the measurements yet, though.

OK, I just did confirm it. The "regular" alkaline Energizer N batteries HP expected you to use in the calculator I have measure 29.45 mm long. The "flat top" Chinese generic 2/3 AAA batteries I have measure 28.35 mm. So, each of the "flat top" 2/3 AAA is more than a millimeter shorter than the specified N batteries.

I tried out some of these "flat top" 2/3 AAA in a 28C just now and indeed there is noticably less pressure on the battery springs. Remember, the clamshells use 3 batteries, a stack of two and a single in parallel. So the stack of two side is ≈ 2.2mm shorter than a stack of two regular N batteries. That's noticeably less preload on the spring. I'm not sure if that substantially reduces the risk of side door area breakage, but it couldn't hurt.

They are a little smaller in diameter than N batteries as well, so no risk of getting stuck.

Oh yeah... as a bonus they seem to work OK as well. Everything seems to make contact... the calculator turns on and looks like it operates properly.

These are the exact batteries I have, purchased off eBay from a Canadian battery seller. They are *cheap*, only about $1US each *including* Canada → US shipping. They are NiMH as well, so rechargeable with the right adapters rigged up to get them to AAA length and preferably the charging current reduced somewhat.

Fantastic recommendation!

I recently acquired a new CIB HP-28C, which is my first of the folding models. I am very impressed with it. I worried about the battery door cracking. The seller had put new LR1 size N batteries in the HP-28C to test it and ship it, which I nervously took out by pressing on the door while sliding. The eBay seller also included a used HP-28C with manuals as a surprise bonus! Perhaps he included it in case I cracked the door of the new one, so I have a second one to play with.

The flat top NiMH 2/3AAA batteries work perfectly and do not strain the door. They do charge up fine in a charger of high quality (which appear harder to find these days?). I've used a bolt to fit them in the slot as pictured.



This LR1 size N battery is actually over 30mm, so definitely oversized. The flat-top 2/3AAA NiMH battery is about 28.3mm and fits perfectly with only minor pressure on the door as should be. They are a bit thinner, but do not rattle inside the battery compartment.

Don't forget: you will need two sets of three 2/3AAA NiMH batteries to swap depleted ones with charged ones, to avoid "memory lost".

- Rob