Voltage drop and mAh capacity of Sanyo Eneloops

[Ohmic]

Hello all, Long time lurker, first time poster. Pleased to be among you.

I have an experience and accompanying query:

I have recently purchased myself a pack of 4AA Eneloops.

I have used them for a while in my LED bicycle light, then out of curiosity decided to discharge and then recharge them with my Supernova 250s 'smart' charger.

For those that don't know it, it's basically just a 12V 1-25 cell NiHM and NiCd charger which has a variety of charge/discharge current settings and provides an LCD for which current mAh capacity is displayed as well as voltage of the battery (or pack) being charged.

I have arranged it so that I have discharged then charged the cells in groups of 2 (series, to produce a 2.4V pack).

Is it normal for the voltage to drop to only 2.09 Volts when 1.5 Amperes is being drawn (discharging) from the cells? To me this seems like a significant voltage drop per cell (1.05V per cell), for not very much current. I didn't think 1.5A was a huge amount, and I am somewhat disappointed that 1.2V is not attainable at this current draw, Bearing in mind that the cells are not at all flat, since this is when they are fully charged!

Another concern I have is that the charger only managed to fit 1150mAh into the 2 cells before it marked them as "full" and then stopped the charge. (Charge rate was set by myself as 800mA, which I assumed would be ok for these cells).

The charger is old and somewhat dated, but still charges my Sanyo 7.2V NiCd packs without a hitch, and constantly produces expected results. (I use to race RC cars).

Does anyone think that either the battery case and accompanying connections is throwing the charger off somehow? Or is it possible that simply charging these batteries in groups of 2 is not reliable? This is actually the first time I have tried to use NiMH with this unit. Before this it has only charged NiCd for me, faultlessly.

Thankyou very much all for your valued opinions:thumbsup:

 

[Russel]

Welcome to the Candle Power Forums Ohmic,


The first time that I load tested an Eneloop while monitoring the voltage I made the mistake of using thin wire and too many connections. My test jig looked like a bunch of spaghetti. After is was pointed out to me the voltages were too low considering the current draw of one amp I improved my rig and came up with an entirely different result.

Subsequent testing with good wire and connections:

Regarding your charger, what make and model is it?

 

[Mr Happy]

I agree with what Russel said.

If you happened to have high resistance connections between the battery pack and charger the charger would see a larger voltage drop at the charger terminals than the battery itself is experiencing. Also this would cause the charger to stop the discharge prematurely, before the battery was fully discharged. If this happened then it would take fewer mAh to recharge the battery since it would be starting out partially charged.

I recommend you to double check that you have short, thick, low resistance connections between the charger terminals and the Eneloops.

(For the make and model of the charger, I believe Ohmic said it was a SuperNova 250S.)

 

[Russel]

...(For the make and model of the charger, I believe Ohmic said it was a SuperNova 250S.)

Oops!

 

[45/70]

Welcome to CPF Ohmic! :thumbsup:

Yes, something in your setup isn't right. AA eneloops shouldn't have any problem maintaining 1.2V/cell with a 1.5A load. Now, somewhere around 2A, the voltage will come down a bit. Here is a graph that SilverFox did with the eneloops, and a bunch of other AA cells, as well as a few AAA and other NiMH cells, in this thread.

If the cells haven't been subjected to overcharge/over discharge, cell reversal during discharge, have been properly maintained, and are in good health, they should hold up fine under a 1.5A load. Keep in mind that NiMH cells are more fragile under the above conditions, than NiCd cells.

One question, do you periodically discharge the cells at a slow rate (0.2C/400mA) and then do a 14-16hr 0.1C/200mA forming charge? I ask this because voltage depression would cause your cells to drop under load, as they appear to be doing, and this would help correct the situation. This at least is a possibility, and may be worth looking into.

Dave

 

[clintb]

Is the battery case one of the cheapie Radio Shack models with thin wire spring? If so, that, and other wiring like others have stated, may be your problem.

 

[Ohmic]

Thankyou very much everyone for your replies:thumbsup:

The 2 cell AA battery holder is indeed a cheap model, with thin gauge wire and an inbuilt switch!

Having run the discharge function again at 2 Amps, I measured with my DMM the voltage across each cell terminal (thereby bypassing all contacts in the battery holder).

To my pleasant surprise, I found that even at 2 Amp, each cell holds a steady 1.22V. Measured at the charger terminals themselves, I got just over 2V. basically 400mV at 2 Amps drop due to poor connections and batt holder.

It's the pesky battery holder and its wiring that seem to be creating a rather large voltage drop, and in the process, confusing the charger into terminating too early.

A better way of connecting cells to this charger are definitely in order

 

[Ohmic]

...On another note, what have people found to be the best way to connect batteries to the charger for the least resistance?

The Supernova 250s has banana plug connections.

Thanks!:wave:

 

[VidPro]

Short Copper wire. like speaker wire is often fair copper, cheap, and good resistance values. just solder (as opposed to crimp) that to your bannana plugs and much less resistance will exist through your wiring.