Inconsistent results when charging batteries?

Hi guys, I hope someone can explain this to me. I’m pretty much a noob when it comes to understanding recharging batteries.

I bought an Xtar X4 extended version charger, which displays results during and upon completion of battery charging. When charging more than one battery of the same brand and model, all are finally displayed as “full” but the mah readout can be dramatically different. This can occur with even new Li-ion and Nimh cells of good brands bought from our reliable dealers. So what’s up with that? Thanks. Peter

It shows the capacity that was put into the batteries, but doesn’t include the capacity that was remaining before charging.

Oh great, thanks for explaining that! I had no idea!

Ditto on what “Sammy” said.
You can get a……very……rough idea by looking at the voltage immediately when starting. This will be a little more consistent with Li-on than NiMh.

NiMh voltage does not track well with capacity. It is also a more difficult and inexact charging methodology, which can get even more erratic as NiMh ages, leading to possibly early termination, or delayed/non-termination. The only way to get an accurate capacity on NiMh is to charge it, then do a dedicated discharge to a known voltage, like 1.0v.

Without buying a more expensive analyzing charger with discharge functions, if you ever want to figure out the “whole” capacity of a cell, you can try to emulate the factory methods. To come the closest you need to look up the factory data sheets for the model of cell, but generally they will discharge at a pretty low rate down to about 2.7 volts (give or take, depending), and then charge it up at a very low rate until it’s full. The low charge/discharge rates are important, and having things at normal room temperature is also important…in the name of science. :slight_smile: The charge rate is usually something like 0.2C or 0.25C, where C is the capacity in mA/hr. So for a 3000mAh cell (call it 3 amps), the 0.2C charging rate would be .6 amps or 600mA. This will get you a pretty good approximation on cells that are new or haven’t been used a ton - for factory comparison - and can be useful to see how that cell might be changing with extended use/cycles. In real life use, however, you’ll rarely get the full capacity in run time because the discharge rates are often higher than that factory testing method.

Thanks everyone! This forum is such a great resource. -Peter