Safe Limit To Prevent overdischarge On NiMh Cell?

Just how bad it is to overdischarge a cell. Also where is the safe limit should you stop to avoid the cells from becoming overdischarged?.

I ask this because I am doing investigation to what mr happy said about overdischarge which probably did not cross my mind.

I noticed when all the 3 led lights goes out on my usb charger. I take out the cells and measure the open circuit voltage with a meter. All cells reads 0.95v one cell read 0.89V. That's just with no load. did I overdischarged my duraloops??

I am telling ya, this USB portable charger is not gentle on the AA cells at all. I immediately started charging the cells at 700mA just in case.

Just hope the duraloops can handle it. On the plus side though my phone was charged up to 85% from 8%.

I'd say you're over discharging the cells, Mario.

Ideally, you want NiMH cells to recover to 1.20 Volt, or over after they've rested 5-10 minutes. Technically they're still OK if they recover to 1.00 Volt, but the cells will last a lot longer if you go by the 1.20 Volt rule.

What happens to a NiMH cell when it is over discharged, is that the separator is weakened. Repeatedly over discharging a cell will lead to an early demise, as the separator will begin to "leak" and the cell won't hold a charge well.

Dave

So that means that the 3 Led battery status is to sensitive on the usb charger. I believe that there is no cutoff point built in. I am getting tired of having good cells developing premature failures.

Hopefully no damage has been done. Just got to keep an eye to avoid over discharging next time. Nothing more annoying is when your batteries goes flat, especially in a high drain device

It could be too, that those types of chargers were designed primarily around the use of alkaline cells. In this case, they would want the charger to drain all available capacity from the cells. An undesireable aspect of our "use and pitch" society.

I'm not at all familiar with using such chargers, but what I'd try, is recharging/replacing the cells when just one LED is left. Possibly, this would prevent over discharging your NiMH cells. Might be worth a try.

Dave

what about the duraloops? these cells should be more resistant to this kind of abuse. Not saying I should keep doing this. Just that this mobile charger drains the cells pretty quick. 1 hour tops i think. Definitely does not last 2 hours. How is it a over discharged cell causes damage inside of NiMh cell? I do not get it??

I thought it was the overcharging you have to worry about. Now your saying I have to worry about over discharging??. You know why can't batteries have a simple discharge characteristics. Like a water in a sealed tank analogy. Too much water in the tank it will explode unless it "vents" But over draining is where I do not get of how a cell can be damaged that way.

MarioJP said:

I thought it was the overcharging you have to worry about. Now your saying I have to worry about over discharging??. You know why can't batteries have a simple discharge characteristics. Like a water in a sealed tank analogy. Too much water in the tank it will explode unless it "vents" But over draining is where I do not get of how a cell can be damaged that way.

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it's more like a pressure stabilized structure. overpressure bursts, under-pressure buckles.

well that is interesting. The question is did the duraloops got damaged?. It was only over discharged in a short period of time.

The way I understand it is that if you overdischarge them, it doesn't matter for how short a time - it's like if you crush a coke can - crushing it slowly doesn't mean it will be any less damaged.


I'm not a great electronics expert by any means, but I've got over 15 years experience in building and maintaining computer systems on both hardware and software sides, so I know a fair bit about USB ports etc. I think you may want to check your USB charger. A single USB port only supplies 500mA 'at specification' so if this were a single-cell charger and you're saying it charges at 700mA it would already be over specification. My laptop just refuses to let that happen - some systems are more forgiving (usually desktops) and will supply a lot more - some will limit the output to 500mA. A few (but not many) let you use the data pins as power sources as well and you can get up to 1.4A that way, but it would be really risky to rely on that as a power source.

It might be that your cells are not charging properly in addition to being over discharged. Get your multimeter out and check charge rates etc. to see if they are genuinely maintaining what you expect, and do this will ALL the charging bays full. It's not unusual to see USB devices 'split' the current (ie it might genuinely charge a cell at 700mA but only 350mA if you put two in there).

WellI actually have. 5 LED tester for testing single double AA and AAA cells. And its quite sensitive too. There is a switch to set it for 1.5v for primaries 1.2v for secondary.

And I also get a voltage reading using a multimeter.

On the batery tester. I get all 5 green led lit on each cell. (4 cells)

On the multimeter. The voltage readings is 1.42v each cell (4 cells)

4 cells would be somewhere approaching 1amp at 5V by the time you've allowed a bit for circuit efficiency, driving LEDs etc, so would be double the 'spec' of the port. Perhaps somebody more experienced than me can assist here, but I think you need to check the charge current rather than the voltage across the cells. I do this using the "bits of tinfoil and cardboard" method - picture in this post...

https://www.candlepowerforums.com/posts/3241787&postcount=9

...the DMM should be connected to one of each bit of the foil, and you measure the current during charge. On one very bad charger I had, it charged at 1amp with a single cell, but with 4 cells loaded it charges at 250mA... yet is supposed to be a 1.5 hour charger. It may not have always operated that way, it was pretty old.


However, in terms of discharging, I'd like to know how you're supposed to tell they are hitting the point of over-discharge if your circuit is regulated...

I've seen a number of USB based products that can move 700mA - I too had thought the spec was 500mA. Not sure if the spec changed or if there is built in overhead or what....

The spec hasn't changed, it's just become commonplace to abuse it. It's down to the individual computer to decide how to deal with that abuse, and that can even vary between different BIOS implimentations on the same computer model.

However, remember that is 500mA at 5V = 2.5 watts. If you're using a 1.5V charging circuit, you could in theory draw (2.5w / 1.5v) 1500mA if the circuit was nice and efficient... however very few are.

Magic Matt said:

However, in terms of discharging, I'd like to know how you're supposed to tell they are hitting the point of over-discharge if your circuit is regulated...

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Hey Matt. With most single cell regulated lights I have, and most of them are, you can tell when the cell is low, because the light drops out of regulation. In all cases, with my lights anyway, of which none are "DX, xxxxxFire Specials" etc. (not sure that matters, but probably does), the voltage of the cells, after a short rest, is 1.20 Volts, or above at this point. In addition, most of my lights either have a voltage readout, and/or a warning, then shutdown, before the cell is drained to a low enough voltage to over discharge them.

With Mario's AA charger, there really isn't anyway to tell, except for the 3 LED indicators. As I mentioned, it was most likely designed for the use of disposable cells, so they probably didn't really think too much about it when they designed the charger.

Mario, I'm sure the Duraloops will fare better, but it's still not a good idea to over discharge them.

Dave

Matt - No, I mean I saw a number of 5V 700mah (3.5W) products at CES this year. I should have been more clear in my first post.

MattK said:

Matt - No, I mean I saw a number of 5V 700mah (3.5W) products at CES this year. I should have been more clear in my first post.

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Depending on what they were, they might be drawing extra off of the data lines to get a little extra juice. You'd probably get away with that on a lot of systems. Not guaranteed though. Some good quality portable hard drives that require >500mA at 5V use a double-USB plug with the data pins only connected on one plug; the limit of 500mA per socket can be got around that way.

These were single plug devices....

I think the standard presumably says you can rely on at least 500 mA being available on a USB socket? Which means that more might be available, but you should not rely on it. And if you build a device that tries to draw more than 500 mA you are exceeding the standard and it might not always work.

Well when it gets to the one led left to almost going out. The open voltage on all 4 cells reads 1.14V

whats interesting about this usb charger though is that the 4 cells is split into pairs. Like if having 2 independent circuit per each pair.

the bay design is like this if the usb port is facing away or bottom of charger facing you

first slot - top + Bottom
Second slot + top -Bottom
Third slot +top - Bottom
Fourth slot - top + Bottom

slot 2 and 3 are what I like to call "the junction slots". If the charger is on and wanted to remove the cells. You would have to pull the cells from slot 2 and 3 at the sametime otherwise you disable one pair while the other pair is draining and also would be putting more stress on that pair if the charger is charging a usb device. Luckily there is a switch to turn it off.

So this type of config is not in series of 4 like most devices are. Pretty cool now that i look at it as this type of setup minimize polarity reversal on the AA cells.

MarioJP said:

Well when it gets to the one led left to almost going out. The open voltage on all 4 cells reads 1.14V

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That sounds like a much better point to stop and replace the cells, Mario. If I were you, I'd just go with that plan. Your cells, whatever brand you use, will be a lot happier.

Dave

I use the duraloops