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Old Nov 4, 2003, 3:19 AM   #11
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I also tend to think that the timer in my charger is not the only thing that monitors the charging. I tried to unplug and plug it again, thus presumably reseting the timer, and the light went back from green (trickle charge) to red (full current charge) indeed, but when I measured the current, it showed something like 12mA instead of 210mA for the full current. So, hopefully, the timer really is only a precautionary measure and the charger has other means to realize that the batts are almost charged. My only concern is if that trickle current is enough to fully charge the batteries if I leave them in the charger for some long period? I assume that 7 hours of 210mA can't fill up the 2100mAh batts, so can 12mA do the rest? And is there a way to calculate the proper charging time in such a case? To be more specific: the charger is VANSON SPEEDY CHARGER, model V-3969, pri.: 230V, 8.5W, sec.: 1.4V, 210mA, charging time (timer) 7 hours; the batteries are GP 2100mAh. With the time I'll probably get one of the Maha chargers, but they are not easy to find in this corner of the world, besides I don't feel like spending extra money if this charger can do the job.
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Old Nov 4, 2003, 10:25 AM   #12
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FYI

http://www.maxim-ic.com/appnotes.cfm...mber/676/ln/en

With C the current rating of your individual battery:
Quote:
NiMH batteries also differ from NiCd batteries in the method required to fast charge them. Both types can be fast charged with a current equal to or greater than the capacity (C) in ampere hours. This technique allows you to charge a battery in about an hour or less. Because of internal losses, a battery charged at C for one hour cannot reach full capacity. For full capacity, you must either charge for an hour at more than C, or charge at C for more than an hour. Charging losses vary with the charging rate and from battery to battery...

...As a secondary or backup measure, NiCd and NiMH chargers often monitor the battery's temperature (in addition to its voltage) to ensure that fast charging is terminated before the battery is damaged. Fast charging should stop when a NiCd's DV/Dt becomes negative. For NiMH batteries, fast charging should stop when the terminal voltage peaks (when DV/Dt goes to zero).
As to trickle charge:
Quote:
Trickle charging is simple for NiCd and NiMH batteries. As an alternative to fast charging, the use of a small trickle current produces a relatively small rise in temperature that poses no threat of damage to the battery. There is no need to terminate the trickle charge or to monitor the battery voltage. The maximum trickle current allowed varies with battery type and ambient temperature, but C/15 is generally safe for typical conditions.
i.e. 2100mAh / 15 = 140mA ---> 12mA is plenty safe (and will take a while!) :lol: :lol: :lol:
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Old Nov 4, 2003, 2:53 PM   #13
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As you can see from NHL's post, If a charger is charging NiMh's quickly and using a simple timer, it is pretty dodgy. The early chargers, some used for portable drills, used time and temperature ( a sensor in the batt pack, 3rd wire). For small batts like AA's temperature tends to be the last resort for safe termination of charge. There is insufficient case area or often no direct contact with a thermal sensor to use temperature accurately. Most use delta V which is monitoring terminal voltage. In any case, you often find the charge controller chip has a temp shutdown option, but this is rarely used, or if it is only monitors air temperature somewhere in the charger unit.

Damage to batteries can simply be looked at as how hot they get and for how long. A trickle charge of 12mA as excess charge will produce so little heat that the battery's external case will get rid of it - so as NHL says, no problem.

As to why a charger appears to start full charge soon after a fresh charge, one explanation is this will happen for a short time until the delta V threshold is repeated. But a more common problem occurs when one batt in a set of 4 charged together, is actually not matched to the others. When removing batts from full charge, I now measure the terminal voltage, feeding a small load using a digital voltmeter. If I see more than a few % low, I pick a pair of low ones's and re-charge those.

Chargers are pretty agnostic to mAhours, it's the terminal voltage changes that drive most of the charge regulator circuits. There are a few examples of cameras reporting low batteries that aren't and this is down to how good terminal voltage is, as a measure of charge remaining, (which it isn't in most cases until the batt is at the crash point).

The worst cameras are the ones's using proprietary batteries and integral 'intelligent' charge monitoring which shuts the cam down, when terminal voltage has 'aged' or fallen outside the expected design level, and there is still plenty of charge remaining. Many people replace their 'faulty' phone batts because of this. VOX
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Old Nov 5, 2003, 4:21 AM   #14
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Thanks for the link to the review about battery chargers - very informative one! One thing I still don't understand - can I measure how close the batts are to the full charge by their voltage? In other words, if I take the batts out of charger, measure their voltage and see that it's say 1.3V (or whatever is maximum for this kind), can I be safe assuming that their capacity is all filled up? I mean, batteries can't reach their nominal voltage fast and then sort of back up energy filling up their capacity? Sorry if it's a stupid question. If I get the idea of terminal voltage right, the batteries voltage seems to be an adequate indicator of their charge, right?
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Old Nov 5, 2003, 5:46 AM   #15
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Quote:
Can I measure how close the batts are to the full charge by their voltage? In other words, if I take the batts out of charger, measure their voltage and see that it's say 1.3V (or whatever is maximum for this kind)...
Yes, however the key here is how do you determine if 1.3V is the maximum? This peak voltage might varies with manufacturers as well as how each battery ages within the same kind (this also explain why independent circuit chargers are better than ones that charged the batteries in pair!)

The best is to take several measurements:
Quote:
For NiMH batteries, fast charging should stop when the terminal voltage peaks (when DV/Dt goes to zero)
ie you need to measure the rate of change... and you probably need a meter with a plotter capability (such as the ones with an RS-232 interface), and measure this terminal voltage in time, but quite tedious still.


IMO it's more convenient to get a faster chargers knowing that you have given it plenty of time to charge. If you measured 12mA and assumed the charger has quick charged for 7hr @ 210mA:
o 7 x 210 mAh = 1470 mAh
o (2100mAh - 1470 mAh) / 12 mA = 52.5 hours (ie another 2 days!) and this has not taken any charging loss into account ops:

Your charger must stay on full charge for at least 10-11 hr to fully charge a 2100mAh... Anything less and the batteries are undercharged. As a reference a good 1hr charger would have to put out at least 2.1A/cell (ie x10 time your current charger) and that's why they have such a huge power supply!
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Old Nov 5, 2003, 6:19 AM   #16
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Yeah, that's what I'm afraid of - if the trickle current is so low, it will take forever to charge high capacity batts. :roll: The seller, however, told me that it would take about 14 hours to charge 2100 mAh batts with this charger, I'm only wondering how he estimated it. Hopefully, the charger automatically chooses the current depending on the charging state - I've got the 12mA figure after 12 hours of charging. I guess I need to try it with empty batteries and check it every hour or something to get the whole picture.
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Old Nov 5, 2003, 10:25 AM   #17
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Quote:
Yeah, that's what I'm afraid of - if the trickle current is so low, it will take forever to charge high capacity batts
You might be a bit confused. Manufacturers can refer to 'trickle charge' as some low level of charge which needs a long time to charge up your batts (It will be more than 12mA). We have also been describing the normal function of a fast charger which when finished stays at a 'trickle' level of 12mA.

All NiMh batts lose charge when left, so look at this part of the end of charge cycle as a way of neutralising the self discharge effect - providing the batts are kept in the charger. Buy a good name fast charger and don't lose sleep over how it's working. I don't explore what each of my washing machine cycles does unless it screws up!VOX
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Old Nov 6, 2003, 6:17 AM   #18
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It's not only about testing this particular charger, but also about finding out if I can trust this particular seller - he assured me that they were doing tests, etc. If it turns out that he sold me not what he was saying, next time I'll think twice before buying from him something else.
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Old Nov 6, 2003, 8:30 AM   #19
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I think the phrase is 'Caveat Emptor', which is why many come here! But I've always believed that there is probably a similar phrase which sellers use to describe buyers.

The internet is a wonderful place to do your product research, but human relationships, or rather delivery of service and satifaction are intangibles, often measured in seller surveys and customer feedback at the end of the value chain.

If this is a problem area, consider buying from dealers who can offer guarantees and exchanges supported by other customers - even if you pay a bit more. In my opinion, many sellers in stores don't really know what they are selling, but they'll agree with you every time to take your cash! Worse still are the mail order brown box companies who rarely offer inside technical advice - just a product manufacturer help line.
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Old Nov 25, 2003, 11:50 PM   #20
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Maha sent me a review pack of their 2200mAH AAs and a charger...I have to figure out how I want to test them, but I'm excited. Only batteries that have ever shocked me while I was holding them...not sure if that's impressive or terrifying.
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