Measuring battery voltage

[cmassicotte]

I have a Radio Shack DMM that I use to measure the voltage on my batteries.

I think I have read somewhere here that to get a true measure of the voltage you have to measure against resistance. Is there a device that can do this? I don't have the skill sets to build one, so I would need something I can buy from Radio Shack or some other local electronics store - perhaps Frys.

Part two of this question - am I correct in assuming that if you measure the voltage you are also measuring how charged the battery is?

Tnanks for your input

Chuck

 

[WildRice]

Yes, the battery has to be tested under load. I am unsure how much load to use tho. Since alky batts have a discharge curve, a level can be determined. Kinda like the included testers on many 'AA' batteries.
Jeff

 

[elgarak]

Just to make sure: There is no such thing as "true" voltage. Only measurements that may or may not make sense for you purpose.

If you measure just the voltage of the battery with a high-ohmic multimeter (most DMMs have internal resistance of 10 MOhm, that's high enough), you measure the open-circuit voltage of the battery. Due to the high resistance of the DMM, no current is flowing. Power is the product of voltage of current. No current means no power draw from the battery.

In a circuit, say flashlight, you draw a current and power at a lower voltage. But you are not interested in this, since you want to know the charging status of you batteries.

For alkalines, the open-circuit voltage is a fairly reliable means to determine the charging status. Sorry, I don't know good threshold values of the top of my head.

For Lithiums or NiMH the open-circuit voltage doesn't tell you much about the charge status, since the batteries keep the same voltage over most of their discharge. The onyl thing you can tell is if the battery is empty, but you can't distinguish a quarter-full one from a three-quarter-full one. Here you have to measure the current you can draw from the batteries (you can do this also for Alkalines for better results). If you simply connect the DMM in a current mode, you measure the short-circuit current. This tells you the charging status, but it's dangerous, since the current might be high, resulting in hot batteries, cables, blown fuses, outgassing batteries, in worst case exploding batteries. So you have to put in a resistor in series with the battery. The cheap battery status meters simply use the same resistor for all batteries. Of course it's better to use a resistor value fitting the battery (which I don't know), and you can also measure both the voltage of the battery and the current simultaneously (with a second DMM).

 

[Lynx_Arc]

I have been trying to figure out battery condition for awhile and have come across these conclusions:

alkaline voltage after resting for half a day are a good measure of capacity when the voltage is 1.1v or more, less than that the reliability goes down and you need to do the flash amp or 10amp scale short test mentioned in elgarak's previous post.

Heavy duty batteries vary widely, but most tend to fall in line with alkalines to the 1.1v line, a few the voltage never sags at all so when in doubt do a flashamps test.

The highest flashamps I have measured on alkalines AAs is about 8amps on new ones, I don't have all brands so I suspect some may be higher than that, nimh AAs measure 13A and above fresh, I have tested them long enough to see the number but am cautious to recommend not doing this to nimh but instead just put cells measuring 1.25v or less on a charger if you think they are low.

I find AA/AAAs less than 0.5 flashamps and/or 1.2v tend to be too low to useful in incan based lights but are still useful in LED based lights, when they get to 0.1 flashamps they are only useful in a very low drain led like a <10ma nightlight.

They make battery testers that have resistors in them but I don't see paying much for them, I just use the 10amp DMM scale and measure flashamps if voltaga is less than 1.1v and automatically charge nimh if below 1.25v because I have a fast 1hour charger it is not worth the risk of flashamp testing to eek out one extra charge cycle out of them.

 

[mrme]

It isn't terribly scientific, but I compare the current the batteries put out in the device they are in.

For example, my Energizer Double Barrel I keep next to my bed pulls just over 500 ma on fresh batteries from 2 parrallel sets of 2 aa batteries. I just pull the battery cap off and measure the current between the batteries and the body. If it still reads about 500 ma, I leave them. Once they fall much below 500 ma, I replace them.

Somone please tell me if there is a better/easier way. I like this way because it tells me the suitibilty of the batteries for the device I am using them in.

Oh, how about a story. I ordered some CR 2016 lithium coin cells off ebay for my Inova LED keychain light. It pulls 50 ma on fresh cells and I replce them at about 10-15 ma. The cells I recieved were supposed to be "new and fresh" becuase they had been tested with a DMM.

Well, they could barely pump out 5 ma. Yet, their open circuit voltage was indeed 3.15 v, .1 v higher than a set I had theat really were "new and fresh" that could put out a full 50 ma.

I told the seller about it and explained how to test with a resistor. Well, the seller refunded my money, so it is all good, I guess. But I still need some good 2016's.