Mark wrote:There are other termination methods:
dT/dt - in this method, the charger is looking for the rapid temperature rise at the end of the charge. The problem with this method is that high impedance cells may get quite warm during the main part of the charge and then not show too much of a slope at the end - this leads to missed terminations and overcharging the battery.
I just wanted to add to the discussion that the dT/dt is probably the least fortunate method to implement in a powerful multi-bay universal charger. Powerful chargers work with high charge and high discharge rates, generating heat both in the batteries and in the inside of the charger. To keep the temperatures of the electronic parts and components at an acceptable level the device would come with a heat sink and also a ventilation fan. All powerful hobby chargers and also some notable multi-bay universal chargers (NC2500, BT-C3100, MC3000, a.o.) have an internal fan. I've been recording the temperature graphs of the batteries in such a fan-operated device and one can easily see the influence of the fan on the temperatures. Battery temperature depends on the environmental air temperature, the cell internal heat generation, the two cold/warm/hot anode|cathode metal contacts, and on the radiative heat transfer from hot emitting sources, and finally the hopefully unhindered heat conduction to the temperature sensor. A battery surrounded by hot batteries will pick up some of their emitting heat, or fan-cooled internals will also indirectly reduce the temperature of the batteries in the bay through the cooled off metal contacts. And in general, to maintain the cell health, a charger should always try to keep the temperatures low by removing the cell heat as effectively as possible. Therefore in practice the dT/dt method is impossible to model, if the programmer tried to take all physical effects, side effects and potential influences into account. For a single-channel NiMH charger without bay and without automatic fan, like a typical hobby charger in a dark lab with constant room temperature, the dT/dt method would have a place. But with an integrated bay and 2+ channels the whole method becomes a complete mess and not reliable in the end.
See for example the below 4-set of graphs representing the four battery slots of MC3000. Slot#1 was occupied with a 1.2V-1s3p round 3AA-to-D-size parallel adapter containing three Eneloop AA's, slot#3 the same, whereas slot#2 and slot#4 were loaded with 1.2V-1s4p round 4AAA-to-C-size parallel adapters both containing four Eneloop AAA's. Yes, basically i was charging 14 Eneloops at the same time, with a massive 3.0A per slot! Anyhow, the effect of 'neighboring influences' on the external temperature sensors can be seen very well: slot#3 gets hotter than slot#1 because it is surrounded by slot#2 and
slot#4, and as soon as the 8 AAA's have finished charging and begin to cool off, slot#1 and slot#3 quickly drop in temperature too. Obviously, most of the temperature (sensor) rise in slot#1 and #3 was attributed to the emitting heat of the neighboring compact/packed 4xAAA's. One can see how the temperatures rise after ~1h57min, near the end of the charge termination, marking a local minimum in the curves.
When i did this test run, i didn't have the temperatures in mind, actually. The original purpose of the test run was to demonstrate that it is possible to charge (equally conditioned) Eneloops in a parallel configuration without problems and that the MC3000 can do so with up to 1C
per cell when charging 16 AAA's
simultaneously, or up to 0.5C/AA
cells, or as in this example 8 AAA's and 6 AA's. The use of the dimensionally different sized adapters, a densely packed C-size adapter vs. a spaciously packed D-size adapter, only intensified the adverse effects of heating, heat accumulation, heat emission vs. fast cooling: an extra variable which entered the physics in our example are the plastic adapters themselves! The D-size adapter has thick plastic walls at some spots and they definitely reduce the amount of cell heat reaching the temperature sensors.
This example also illustrates that it would be negligent and imho wrong (for a post writer or a charger reviewer) to have an isolated look at a single slot and its performance, or even worse, to run a test in just 1 slot and leave all other three slots empty. Well, one could do the latter. But then you would never know whether and to which extent running charge/discharge programs in the other slots at the same time affected your original temperature measurements. There you have it, battery temperature is influenced by too many variables, it is too sensitive a physical quantity to build one's NiMH termination method upon.