Minimum 'charge rate' not scaling with battery size/capacity

While a number of manufacturers have stated that the minimum charge rate be greater than 0.5C to generate a strong enough signal for chargers to detect and stop charge, there was a point raised that it should be a "fixed" number or fixed current level...especially for smaller capacities, see below:
If you look at the numbers, the AAA cell terminates well and is charging at 0.4/2000 or 0.2C rate. The AAA cells is charging at 175/800 or 0.218C or slightly higher but does not terminate.

I have another charger that shows exactly the same pattern with vibrant AAA eneloop batteries where the rate is 0.14C (175mA) and that batteries do not terminate, yet, the AA batteries at the same 0.14C rate do terminate ???

When I alter the AAA charge to about 280mA (increase of 170 mA), the termination is very reliable.


Based upon these empirical tests on a few chargers, I would suggest that a minimum current of 280mA be used for both AA and AAA batteries if smart termination is to be used. Out of curiosity, I want how low it can go ?
All things being equal, the smaller AAA battery should be able to terminate more reliably at a lower current level but I believe it does scale (with capacity) from the AA number.


Last, as the charge current drops, I believe the termination algorithm has be that much better (than at higher currents). The units that terminate with the lowest currents should be the most tolerant of less than vibrant batteries.

Interesting observation, PeAK. Perhaps there's a minimum threshold rate needed regardless of cell size. Mr. Happy's observation seems to have merit too. Whacking a D cell with 5 - 10 amps of charge current is a lot, regardless of size.

jayflash said:

Interesting observation, PeAK. Perhaps there's a minimum threshold rate needed regardless of cell size. Mr. Happy's observation seems to have merit too. Whacking a D cell with 5 - 10 amps of charge current is a lot, regardless of size.

Click to expand...

One of the things about charging a low currents with a smart termination is that the backup overtemp design is made more difficult. The heat generates a smaller temperature rise and this requires a more difficult time for the engineers to get it right in the design.

As you said, the other end of the spectrum requiring larger currents for large capacity batteries seems also to be a point where things break down. Again as we deviate from 0.5C for D size batteries, I think the batteries will generate less heat and lessen not only the negative delta-V signal...it will also make the backup overtemp circuits have less temp to work with.

The temperature rise of an object depends both on the heat input and the heat losses to the surroundings. Now the heat losses depend a lot on the surface area to volume ratio. An AAA cell has a very large surface area relative to its size, so it cools quickly. Larger cells, not so much.

If I take a hot AAA cell of the charger it cools down in seconds. If a take a hot AA cell off the charger it takes at least a few minutes to cool to the same extent. I bet a D cell would take half an hour to cool down.

So a relatively smaller current charging a D cell could still produce a decent temperature rise.

Mr Happy said:

The temperature rise of an object depends both on the heat input and the heat losses to the surroundings. Now the heat losses depend a lot on the surface area to volume ratio. An AAA cell has a very large surface area relative to its size, so it cools quickly. Larger cells, not so much....So a relatively smaller current charging a D cell could still produce a decent temperature rise.

Click to expand...

Hmmm...this may be why we do not see many C/D sized batteries:

1. The C-rate needs to be greater than 0.5
2. For the same C-rate, larger diameter/volume batteries will heat up more.

Perhaps the size of AA batteries are at a point whereby the temperature is at the edge of the temperature envelope extreme. To stay below "this temperature" the only solution would be to lower the C-rate and accept the weaker negative delta-V signal.

There is also an unproven hypothesis that the −∆V signal is directly attributable to the temperature rise at the end of charge. The reasoning would be that the increased temperature reduces the internal resistance of the cell (higher temperatures increases the mobility of ions in the electrolyte), and the lower internal resistance reduces the resistive component of the voltage difference and thus causes a small drop in voltage. This is one reason why higher charging currents would lead to a more pronounced signal—they produce a larger resistive component to the overall voltage difference during the charge.

Evidence for this assertion includes the common observation that AAA cells give a less detectable −∆V (they don't warm up as much), and the observation that putting a fan on the charging cells can also cause missed or delayed terminations.

Therefore C and D cells may still exhibit a detectable end of charge signal at lower charging rates due to the more pronounced temperature rise inherent in their larger size.

This was documented in no longer publicly available Sanyo documents. There's a "break through" threshold where no charging will take place at all below this point.

For example, a 3,000mAh cell can be charged at 300mA for 12-16 hours, but it may not accept ANY charge at 15mA even if it was charged for a week.