Battery Extender Best Performance - Page 3

NZM · 02-17-05, 01:14 AM

The reason NiMH batteries are best suited to this task is not the voltage or the mAH ratings. It is that NiMH batteries have a much lower internal resistance than Alkaline or Carbon batteries.

The internal resistance of a quality Alkaline battery (such as Duracell, Procell, Energizer) will vary with load. The higher the load the higher the internal resistance, and will be in the range of 0.3 to 0.8 ohms depending on the load. NiMH batteries have an internal resistance of approx 0.03 ohms which is reasonably constant with load, or less then 10% that of Alkaline batteries.

We know from high school science that according to Ohm's Law, Voltage = Current x Resistance. So there will be a voltage drop caused by the current flowing out of the battery through its internal resistance. So for a high load, say 500mA, the drop for a NiMH will be approx 15mV, while for the Alkaline batts will be more like 400mV.

Now in the battery extenders described in this thread, the four batteries are in series, so the total internal drop for NiMH is 60mV (from a nominal 4.8V to 4.74V) while the Alkalines drop more like 1.6V (or from a nominal 6V to 4.4V). For Alkalines, it gets worse as the load increases.

This is why NiMH batteries are very good for high drain devices. They will maintain very close to their nominal voltage even under very high loads. Alkaline batteries are great for low to medium loads, but when the current drain is big, the loss due to internal resistance (which results in heat generated inside the battery) becomes a limiting factor.

So you can see that even though you may have 4 x 1.5V Alkaline batteries installed, the problem for a high drain drive may not over-voltage, but may under voltage as a result of the internal resistance loss.

As for the discussion on reverse charging (the Axim internal battery charging the battery extender), this is very simile to test. Connect the cable to the Axim and leave the USB conector free. Take a volt meter and test for the presence of 5V on the connector. If no voltage, then the Axim is not outputting battery voltage via the battery extender cable and therefor is not capable of reverse charging. Mine arrives in the next few days and I will test.

k1darkknight · 02-17-05, 12:34 PM

Originally Posted by frankenbike

I've pretty much got one of everything at this point.

So, what would you say would be the best charger/extender (in terms of reliability, then battery life) for extended use of an Axim, with WiFi, mp3 playing, video playing, or WiFi and mp3/video at the same time? (I don't have an Axim yet, but plan on getting one with my tax refund)
Also, would it be better to stick with the basic 1100 mAh battery for these extenders/chargers, or would the 2200 extended one be even better still?

Thanx in advance!
k1darkknight, aka Amp-hungry

NiwRoC · 02-17-05, 01:18 PM

I must say some very interesting info. I was thinking about purchasing one and will get one for sure! Thanks.

**ksong12** · 09-09-06, 05:41 PM

Originally Posted by gm646

I've done some work with these, and made one for myself....it's only good to use it to either leave your unit off and charge it, or wait till the unit is just about completely depleted in battery life and then use it. Because if you use a charged extender and your charged battery at the same time you hardly gain any life

Because it's like having a second HUGE battery. think about it... 4.8v at 8000mah or so. 2a x 4, at 1.2v each

I know its kinda old, but have you actually THOUGHT about it? If you want the "8000 mah or so", you would have to put the batteries in a paralell (i can't spell) circuit, giving you 1.2 v. If you want the 4.8 v, then you will have to get 2000 mah, or whatever your battery is.

So, you get one or the other, not both, and, obviously, you would have to choose the series circuit to make that little orange power light light up.