My HP67 Has Melted!!!

[Ignatz]

Noticed that my HP67 seemed to be losing charge, even when switched off... plugged it in for a recharge. A bit later, turned the calc over, the battery door had buckled at the top from heat. Managed to remove the battery (only a few months old). It was hot to the touch... unplugged, let everything cool down, put a fresh battery in. A few hours later, back into the office to the smell of burning plastic... the battery door had melted!!! This battery was too hot to even hold. This morning, the hot battery is dead. Plugged in the first battery (the one that was warm and caused the buckling) to monitor the situation: calculator still functions (phew!) and, even after a few hours charging, the first battery is still cool to the touch.







Any idea what went on there?

[Maximilian Hohmann]

Hello!

(03-20-2020 11:48 AM)Ignatz Wrote:  Any idea what went on there?

The only thing that can cause a battery to get that hot is a large current draw. A short circuit or an "almost short circuit". Or an excessive charging current. Or, as in your case, a combination of the two: First the calculator discharged it's battery rapidly and when you plugged in the charger, it rapidly charged the already hot battery thereby heating it up even more.

To start with I would look at the power supply circuitry in the calculator.

Regards
Max

[BillBee]

I've seen Lithium Ion battery packs melt - (mostly off brand replacement batteries for cordless tools). NiCd seem to be a less sensitive to the critical temperature point where melt down starts to happen. Li ones can take more charge the hotter they get and perpetuate the problem. An EE guy would know the internal structure and about what can shift internally causing it.

Makita chargers, for example, have a temp cut off for the NiCd batteries.

My rule is to always keep an eye on rechargeables.

Glad all is good except for the door.

-B

[Maximilian Hohmann]

Hello!

(03-20-2020 12:28 PM)BillBee Wrote:  I've seen Lithium Ion battery packs melt ...

This is due to an effect called "thermal runaway" (https://en.wikipedia.org/wiki/Thermal_runaway#Batteries). Low power density NiCd or NiMh cells as in the HP-67 should not be affected. I am 99% sure that this calculator melt down is not primarily caused by the battery but by the circuitry around it.

Anyway, this example shows that it is always wise not to charge historical electric devices unsupervised.

Regards
Max

[Ignatz]

(03-20-2020 12:55 PM)Maximilian Hohmann Wrote:  Anyway, this example shows that it is always wise not to charge historical electric devices unsupervised.

Regards
Max

Agreed. If I hadn't caught it when I did, I might have been looking at fire damage... Odd that the same battery (the first one) is as cool as a cucumber after five hours on charge this morning...

[aurelio]

At least the calcolator looks in a good shape and working condition...
I've seen that are for sale alkaline c. containers for the classics, you could use in futre one of that, but you have to find first a cover for the battery compartment.

[Ignatz]

(03-20-2020 01:16 PM)aurelio Wrote:  At least the calcolator looks in a good shape and working condition...
I've seen that are for sale alkaline c. containers for the classics, you could use in futre one of that, but you have to find first a cover for the battery compartment.

These batteries were new, after-market ones... Luckily, Twoweims has posted an .stl of the battery cover on here:

https://www.hpmuseum.org/forum/thread-14095.html

[Don Shepherd]

The exact same thing happened to my 65, the battery door started to melt while recharging. Fortunately, the 65 still worked, but I NEVER charge batts in the calculator anymore. Always use a third party battery recharger.

I too would have had a fire if I hadn't checked. I checked about an hour into recharging. I should note that I never had this type of problem while using the AC charger that came with the 65; I would charge overnight using that one and there was never a problem. The problem was using a USB-based recharger.

[aurelio]

(03-20-2020 01:43 PM)Don Shepherd Wrote:  The problem was using a USB-based recharger.

Which kind of problem, Don?

[Massimo Gnerucci]

(03-20-2020 01:19 PM)Ignatz Wrote:  

(03-20-2020 01:16 PM)aurelio Wrote:  At least the calcolator looks in a good shape and working condition...
I've seen that are for sale alkaline c. containers for the classics, you could use in futre one of that, but you have to find first a cover for the battery compartment.

These batteries were new, after-market ones...

As clearly stated on the door's label you should "USE ONLY H.P. BATTERY PACK" OTHER BATTERIES MAY DAMAGE CIRCUITS

:)

[Don Shepherd]

(03-20-2020 01:50 PM)aurelio Wrote:  

(03-20-2020 01:43 PM)Don Shepherd Wrote:  The problem was using a USB-based recharger.

Which kind of problem, Don?

The battery compartment door getting too hot and partially melting.

When my original HP power adapter was functional, I could charge rechargeable batts overnight in the calculator and not worry about anything. But then that HP adapter got a short in one of its wires and stopped working. I got a USB charger on Ebay and that seemed to work fine overnight, but then I got a different USB charger and that one caused the melting. So I learned I couldn't trust the USB chargers anymore and bought a third party batt recharger (recommended by Dave Britten) and now I always use that.

[burkhard]

(03-20-2020 02:39 PM)Don Shepherd Wrote:  So I learned I couldn't trust the USB chargers anymore and bought a third party batt recharger (recommended by Dave Britten) and now I always use that.

So you bought a third party EXTERNAL charger for Classic battery packs?
If so, can you furnish a link? Thanks!

[Ignatz]

(03-20-2020 01:43 PM)Don Shepherd Wrote:  <snip>
The problem was using a USB-based recharger.

Yes, this was a USB charger, too... I would definitely be interested in an external charger for classic batteries, too...

[BillBee]

(03-20-2020 03:58 PM)Ignatz Wrote:  

(03-20-2020 01:43 PM)Don Shepherd Wrote:  <snip>
The problem was using a USB-based recharger.

Yes, this was a USB charger, too... I would definitely be interested in an external charger for classic batteries, too...

The Radio Controlled hobbyists use USB chargers which are supposed to be really configurable - charge rate/discharge rate. I ran across the topic when looking at refreshing some "old" 9.6v NiCds (the theory was dentrite build up limited the charge retention). That could be a good first step.

-B

[Dave Britten]

NiMH batteries will overheat if they are overcharged by a NiCd charger. Don't charge NiMHs with anything designed for NiCd batteries (e.g. HP calculators/chargers).

[Hans-Peter]

Google this:
3 cell nimh charger module -lithium
Then you will find somme charger modules for nimh with the delta V method.
Im testing those for 3 cells with my new oszillioscope when i got a bit of time.
I have also solution in mind with a temp sensor like this:
Max712 Chip

[Paul Berger (Canada)]

(03-20-2020 02:39 PM)Don Shepherd Wrote:  

(03-20-2020 01:50 PM)aurelio Wrote:  Which kind of problem, Don?

The battery compartment door getting too hot and partially melting.

When my original HP power adapter was functional, I could charge rechargeable batts overnight in the calculator and not worry about anything. But then that HP adapter got a short in one of its wires and stopped working. I got a USB charger on Ebay and that seemed to work fine overnight, but then I got a different USB charger and that one caused the melting. So I learned I couldn't trust the USB chargers anymore and bought a third party batt recharger (recommended by Dave Britten) and now I always use that.

Do these USB chargers have any kind of current regulation in them? The charge connectors on a classic are connected directly the battery terminals and and the wall wart limits the charge current.
Paul.

[Don Shepherd]

(03-20-2020 03:27 PM)burkhard Wrote:  

(03-20-2020 02:39 PM)Don Shepherd Wrote:  So I learned I couldn't trust the USB chargers anymore and bought a third party batt recharger (recommended by Dave Britten) and now I always use that.

So you bought a third party EXTERNAL charger for Classic battery packs?
If so, can you furnish a link? Thanks!

No, I need to clarify. I'm not using a "classic battery pack." I'm using individual AAA rechargeable cells with a small plastic holder that holds the individual 3 cells. I got the holder on Ebay at a reasonable price. Here is the holder:
https://www.ebay.com/itm/Rechargeable-HP...SwVDpclCst

And I charge the 3 cells with this:

https://smile.amazon.com/gp/product/B07J...UTF8&psc=1

[[kby]]

Always the opportunist—do save the latches on the melted battery door. Doors are not impossible to gond, but the slide latches are usually broken.-kby