Battery Testing Plan

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Lenp
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Battery Testing Plan

Post by Lenp »

I have a product repair project that may involve testing hundreds of 6 Volt 2200 Ma NiCad / NiMh battery packs over a period of time. They may come in batches so speed and relative accuracy is important.

The product is consumer oriented so the battery quality is really subjective but bad batteries do need to be weeded out. Yes, I make a few pence selling replacement batteries but the ire of a customer who gets a bad battery that I marked good is more important to avoid.

The manufacturer suggests a procedure that involves installing the battery into their product, charging it for 5 minutes with their charger that sources 200 ma. max, then measuring the battery voltage. Aside from being time consuming, although this may fret out the shorted, open or reversed polarity cell, it is probably not really reliable.

Perhaps a better test would be to charge the cells at a higher rate, maybe C/5 or higher, for a brief time, not allowing the temperature to rise or cells to vent, let the pack equalize for a short time then test with a moderate load.

With this method I could use a processor to set the charge, settle and test times for multiple packs to assure consistancy. The processor could also switch the packs to a metered load for the test, or using a simple threshold detector, latch a good/bad led for each pack.

Comments are welcomed!



Len
Len

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MrAl
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Re: Battery Testing Plan

Post by MrAl »

Hi Len,


The only method i have found that works perfectly is to charge the battery and then discharge it into a known load while monitoring the voltage with a computer interfaced meter, then using that data to calculate the ampere hour capacity. The bad batteries always show a low capacity, however sometimes they have to sit for a day or two before the test. I liked this method so much that i made a special chip that interfaces with the computer that reads 4 cells at one time and records the data and the software automatically calculates the ampere hour rating as the test progresses. This is an almost fool proof method. The drawback is obvious however, and that is first that the cell needs to be charged (could be done in 15 minutes however) but more time consuming is the discharge, unless maybe you could use a much lower load than i usually use and test some cells to establish a 'norm' and then compare future cell tests to those reference tests. After all is said and done, i'd say the test would take at least an hour per cell, but you could do a bunch of cells at the same time if you set up for it. Using two such chips you could test 8 cells at a time. You'd have to be able to charge 8 at a time too though if you wanted the speed, and that would take 8 charging circuits that are able to put out 10 amps each (15 min charge) or 5 amps each (30 min charge).
There are other methods i have read about but i have never actually tried nor asked any real life users about. These methods involve a commercial battery analyzer which you can buy on the web. As i said though i have never tried one so i can not say how reliable their results are. Supposedly, they can tell you the capacity with one test and i think the test is fast. Im sure a quick search for "battery analyzer" would turn up lots of stuff. Just in case you cant find any let me know and i'll ask around.

NiCd and NiMH are funny because you can charge them and they look really good, but then when you discharge some bad ones they discharge quite a bit faster than a good battery would. I've seen fairly new batteries discharge twice as fast as they should, and i've seen some that discharge in a sort of normal time unless they sit for a week first and then they show very little remaining capacity (high self discharge). Sometimes the bad ones show up even in the charge curve, which is not normal looking for the type of cell.
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Rodney
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Re: Battery Testing Plan

Post by Rodney »

In working primarily with NiCad but also NiMh, I have found there is no foolproof and FAST way to check out old batteries. The only reliable way I have found is to charge them at about 0.1C (that is at 1 tenth their capacity) for 15 to 16 hours. This insures that all cells are fully charged yet the rate is low enough to not harm those that reached full charge first. Let them set for 24 hours and check the voltage to make sure they do not have a high self discharge capacity. Now discharge at a known rate (usually at 1/3C) and if the capacity is 80% or more of their rated capacity, they are good to go. As I said, not a fast way, takes about two days to reliably check out a battery. Then there is one more test you need to make if the battery must handle high currents, Draw that maximum current from the battery while monitoring the voltage to make sure it does not sag below the desired limits during the high current draw.
ringo47stars
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Re: Battery Testing Plan

Post by ringo47stars »

Like as Mr. Al said you could get them charged up in 15 minutes but I dont think it works for ni-cads. The idea was to charge the batteries up without much damage or heat so the chargers used to have cooling fans. So if you charge up batteries slowly like Rodney said then you might heat them up for a longer period of time unless you added a cooling fan or something similiar. A solar charger works for both types but you are probably thinking you might need a big one to get the charging power to fast charge them. One idea I was thinking was to use the solar charger in lieu of a regular charger and add more batteries for the power. Like if you had some batteries that had a full charge put them on the solar charger with the ones that needed charging to get the power to charge them up fast. It works with a regular charger but I havn't tried it with a solar one yet. You don't want to be like the car salesman who lost a sale because he didn't charge up the batteries. I found an example of that on Yahoo answers with a Ford Fusion no less.
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