Any NiMH battery experts?

..and just simple thermal inertia?. The heat will come from the core, but the sense point will be further away, so you would expect the heat to climb after current it removed, especially on shorter charge cycles.

-jg

It does not behave like that-- early in the charge cycle the temperature rise stops immediately upon removal of the charge current, so something else appears to be going on.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com

I cross-posted this to sci.chem.electrochem.battery... maybe someone there can answer your questions.

--
DaveC
me@bogusdomain.net

The temperature rise early in the charging cycle is due to resistve heating, and stops as soon as you stop the charging current.

The additional temperature rise you see in a fully charged cell is due to the recombination of the hydrogen gas being generated at one electrode with the nickel superoxide being generated at the other - the hydrogen gas is generated in solution and has to diffuse through the electrolyte before it can get together with the static superoxide to produce water and heat. The electrolyte layer is pretty thin, but diffusion isn't quick, so a perceptible thermal decay time after the charging current has been removed isn't implausible.

They didn't teach me about this in undergraduate chemistry back in the early 1960's so I can't give you chapter and verse - sci.chem.electrochem.battery should do better.

-- Bill Sloman, Nijmegen

Thanks, Dave. I would appreciate any suggestions or pointers.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com

Without seeing the data my first thought is that it takes time for the heat to migrate out of the cells, so the surface temperature should continue to go up for a while after a fast charge is terminated.

best regards mark

This guy --

-- is a retired battery engineer from GE/Gates. He seems to know his stuff -- he claims to have written the "Rechargeable Batteries Applications Handbook" from Gates. Certainly everything he's said about rechargeable batteries has either been right, our it's been out of my league. If you could get him interested in your problem he may help you out.

There are folks who fly large models with that many cells to get high power at only moderately high currents, so it's a known problem in the high-end RC electric-powered field.

--
Tim Wescott
Wescott Design Services

I think that's going to be necessary in order to prevent substantial differences, especially at the "center".

Assuming a spherical pack ;-) the surface area increases with r^2 but the heat generation and capacity with r^3, so larger packs are bound to run into issues.

There are apparently also some known issues with the particular Sanyo cells which we are using.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com

I have Done 11ish in a tight pack. Early in the charge cycle most of the energy is charging the cells. Later in the charge more and more of the energy is converted to heat. In my case I have seen the temperature rise 2 - 3 degrees C after charging.

This would only be possible, if the power dissipation had increased rapidly just prior to shutdown, so that the surface temperature was still rising during shutdown.

On a steady state situation with constant dissipation and constant surface temperature, the temperature starts to drop immediately after shutdown.

The situation is different e.g. in a fan cooled projector lamp, in which the fan keeps the bulb and base temperature at a reasonable level. Removing the lamp and fan power simultaneously, will cause a surface temperature increase towards the hot filament temperatures. This temperature increase is finally limited by increased radiation and increased natural convection, but the bulb or the base might melt before that.

However, if you keep the fan running after the lamp was shut down, until the internal temperatures have cooled to something slightly above the maximum surface temperature, the fan can be safely turned off, since the surface temperature can no longer increase above the maximum surface temperature.

So if there is a temperature increase in the battery pack after the charge was disconnected, there must have been a rapid internal temperature increase just prior to charge is removed, so that a thermal equilibrium was not reached or there were some chemical reactions after the charging. Paul

C/10 or something? It's really not hard to get 50°C or destructively more with fractional C charge rates.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com

In my case more like C/2. You have to charge at a minimum rate to insure a negative delta V Termination or a Delta T Termination. It should be listed in the data sheet for the cells.

Have you looked at

True.

I have seen a lot of curves, but not a minimum charge rate. I think it would be difficult to come up with an absolute number since it's quite dependent on the heat sinking and arrangement of the batteries within the pack.

Also, unfortunately, I'm having trouble getting detailed engineering datasheet information on the specific Sanyo "Superlattice EVO" cells that are used-- less data seems to be available on cells aimed at consumer markets.

...yes I have, thanks.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com

It is not a minimum charge rate. It is a minimum Rapid charge rate. Under that rate the termination curve do not apply. It it would be low rate and could be charged indefinitely. The data sheet should have a note at what the rapid charge rate for the graph was. If you want a different rate you will have to charge and graph it at hot cold and room temperature.

This could be heat working it's way out, or it could be reactions in the cell recombining H and O2 that continue to happen...

Sounds similar to what I saw with chinese cells that would spontaneously self-destruct. I hope that's not what's happening in your case.

Can you post graphs?