Question about Battery Chargers (NiMH)

[poopeater]

How does the battery charger know the capacity of the cells that it is charging? For instance, say you have two sets of batteries. One set is 4 x AA cells, each having a capacity of 800mAh. The second set of batteries is 4 x AA cells, each having a capacity of 2500mAh. How does the charger know if you plug in the 800mAh set or the 2500mAh set? The amount of current to supply each set of batteries would be HUGELY different (on the order of 3x).

 

[Roff]

A quick Google search seems to indicate that 800maH is only available in AAA. Can you point to a source of 800maH AA's?

 

[poopeater]

A quick Google search seems to indicate that 800maH is only available in AAA. Can you point to a source of 800maH AA's?

I was just pulling numbers out of the air for my example. There may not be 800mAh AA batteries, but there are 1100mAh ones.

https://www.allelectronics.com/cgi-bin/item/NMH-110/160/NIMH_AA_CELL,_FLAT-TOP_.html

There is still a large enough discrepancy between 1100mAh and 2500mAh batteries that my question still stands though...how does the charger know the capacity of the batteries, and thus the charge current that it should use?

 

[audioguru]

A battery charger is supposed to be smart enough to determine when a rechargable battery is fully charged. Then the battery doesn't explode if it was already charged or if it has a low capacity.
A "smart charger" detects the voltage peak then small drop, the temperature increase, the pressure increase or a redundant timer is used to terminate charging when a battery is fully charged.

 

[poopeater]

audioguru,

thanks for the reply, but the full charge status has nothing to do with the proper charging current. Most manufacturers suggest a maximum fast charge current of 1C. If the charger is charging 2500mAh batteries, this charge would be 2.5A. If the charger is charging 1100mAh batteries, this charge would be 1.1A. Needless to say, charging 1100mAh batteries with 2.5A is definitely NOT the correct way to charge.

Again, I'm not looking for details on how they know that the battery is charged. I already understand that. I'm wondering how it determines the correct current to use for the charging process. These are two completely different things.

 

[audioguru]

Some 2500mA/hr NI-MH AA cells are fully charged in only 15 minutes today. Thats a blast of 10A (or more)!
I think an ordinary cell wouldn't last very long with such abuse.
The ordinary cell would have a higher internal resistance which the charger can probably detect, then reduce the current.

 

[poopeater]

Yeah, I was thinking that it could maybe detect the internal resistance of the battery, but the difference between two AA batteries of different capacities is so small! It's like 5mohm difference!

I had no idea that some chargers blast the things with like 10A. That's just nuts.

 

[Roff]

I'm guessing that "generic" chargers are designed to charge the lowest-maH batteries at 0.05C to 0.2C. Higher-capacity batteries will just take longer to charge.
If you buy batteries and a charger in a package deal, the charger may be roughly optimized to the capacity of the batteries.
I seriously doubt that they sense internal resistance, but I could be wrong.
These are just guesses.
And then there are the insanely fast chargers that RC hobbyists use, which Audioguru has already mentioned.

 

[bananasiong]

can i know how long should the battery be charged?

i've two chargers, one energizer, the other one MWalker
for a 750mAh AAA, the table on the MWalker states that 5 to 6 hours, while the Energizer states that around 12 hours..

that is so much different... does it depend on the charger?

 

[poopeater]

Hi Ron,

Thanks for all your help and inputs. What you say about the chargers makes a lot of sense. The bigger batteries simply take longer to charge because the charger is built for lower capacity cells. Duh, I'm stupid *smacks self in head*. That makes perfect sense.

Anyway, I was looking at some simple trickle charge circuits, and had a few questions I was hoping that you could help me with.

I see a circuit that is very simple. It's a DC source feeding an LM317 regulator that is configured to provide a constant current. This is ideal as a trickle charger. However, you need a DC source that is at least 4V above the charged value of your battery in order for the circuit to work.

My question is: Why not just take a DC source and add a series resistor to the circuit to provide the 'constant current' to the battery? It seems that you could get rid of the need for 4V of overhead, and that the end result would be even more ideal than the fixed value trickle charger.

For example, say I had some 5000mAh batteries (4 in series) and configured an LM317 to provide them with 500mA. This circuit would need ~10V to operate properly. Why couldn't I simply take a 6.5V DC source, add a 2ohm resistor in series, and connect that to the battery? Unless I'm missing something, that would provide the following:

discharged: 6.5V through 2ohm into ~4V battery = 1.25A (0.25C) charge current
charged: 6.5V through 2ohm into ~5.6V battery = 0.45A (0.1C) charge current

This circuit would be simpler, would charge the battery faster, and would require less DC overhead from the input supply. Am I missing something?

I'm building a charger out of solar panels, so the need for 4V of overhead is going to cost me a LOT of real estate in building the panel.

 

[poopeater]

can i know how long should the battery be charged?

i've two chargers, one energizer, the other one MWalker
for a 750mAh AAA, the table on the MWalker states that 5 to 6 hours, while the Energizer states that around 12 hours..

that is so much different... does it depend on the charger?

It depends on the charger. The energizer charger probably charges much more slowly than the mwalker. If it's taking 12 hours to charge, then the energizer version probably is just providing a trickle charge to the cells. The reason this is done is because it makes the charger way less complicated and way cheaper to produce. Modern NiMH cells can sustain a trickle charge of 0.1C for many hours at a time with no real harm to the batteries.

My guess is that the energizer is not a 'smart' charger, which makes it cheaper and more appealing to the masses.

 

[audioguru]

Modern NiMH cells can sustain a trickle charge of 0.1C for many hours at a time with no real harm to the batteries.

In their Ni-MH Applications Manual, Energizer recommends a trickle-charge rate of only C/40 for a cell's longest life.

 

[poopeater]

bump...hoping that Ron or someone else can help me with some of my questions.

 

[Roff]

I don't have any experience with NiMH battery chargers, but from what I've read, I think your idea will shorten the life of the battery. There are lots of articles on the Internet about how NiMH's should be charged, and I think there are lots of construction projects that don't follow those guidelines. You might look at chips that are designed just for that purpose.

 

[dknguyen]

Trickle charging NiMH is kind of pointless. They charge fast and have a high self-discharge rate so you charge them before use. Or you can charge them, then just leave them out and then charge them to top them off before use. You aren't supposed to leave them on trickle charge so you can use the battery at will at a moment's notice. That's what NiCds are for.

The OP is misunderstanding something. You can charge a 800mAh or 2500mAh battery with the SAME current The only difference is that the 2500mAh battery will take longer to charge. So the charger doesnt know. It just charges using the same current. Bigger batteries take longer to charge.

You don't need to know the size of a fish tank to fill it...you just keep filling it until it overflows. In the battery's case, you keep filling the battery until the voltage stops rising and dips a bit.

On RC chargers and other computerized chargers, you set the current you want to use.

 

[Oznog]

First off you need to read up a bit on battery charging algorithms. Know that sealed lead acid, NiMH, and Li-Ion are charged in completely different ways.
All of these are damaged by overcharging. "Trickle charging" does not get around the problem, it just damages it slower but it'll still be damaged and usually the "trickle" part means people intent to leave them on the battery for days/weeks/months past the time it took to fully charge them.

NiMH is often charged by applying a constant current and then looking for a temporary drop in the voltage that occurs around a full charge. It's easy to miss so there are other detection methods involved, like for one if it's an AA cell it should definitely just shut off if it's already given it 2500mah.

There's a LOT out there on charging the tricky batteries (NiMH/Li-Ion/LiPo). Some of it is proprietary or sort of an "industry secret" though.

And the short answer here is, "can I pick up an unknown NiMH battery and check its charge state?" the answer is there's no practical, reliable way. Unless it's empty of course, that's easy to see. If you want to determine how far along it is in the charging process at any point, also not very reliable. You can only figure out when it's through, and then only with some particular agorithms.

 

[poopeater]

Trickle charging NiMH is kind of pointless. They charge fast and have a high self-discharge rate so you charge them before use. Or you can charge them, then just leave them out and then charge them to top them off before use. You aren't supposed to leave them on trickle charge so you can use the battery at will at a moment's notice. That's what NiCds are for.

The OP is misunderstanding something. You can charge a 800mAh or 2500mAh battery with the SAME current The only difference is that the 2500mAh battery will take longer to charge. So the charger doesnt know. It just charges using the same current. Bigger batteries take longer to charge.

You don't need to know the size of a fish tank to fill it...you just keep filling it until it overflows. In the battery's case, you keep filling the battery until the voltage stops rising and dips a bit.

On RC chargers and other computerized chargers, you set the current you want to use.

Thanks for your inputs. I need to trickle charge these NiMH cells because I am building a solar charger for use in sub-sarahan Africa, where there is no electricity. They will be using these cells to power flashlights, radios, etc.

They won't be left trickle charging. I would say that they'd probably be left outside charging all day long, and then used until they needed to be charged again. They will only be charged when needed, basically.

Ron, why would using a simple resistor shorten the life of the battery? It would basically allow for 0.2C charging until the cells reach near the capacity, and then the resistor will automatically reduce the current to 0.1C or less. Why would charging at 0.1-0.2C shorten the life?

 

[poopeater]

First off you need to read up a bit on battery charging algorithms. Know that sealed lead acid, NiMH, and Li-Ion are charged in completely different ways.
All of these are damaged by overcharging. "Trickle charging" does not get around the problem, it just damages it slower but it'll still be damaged and usually the "trickle" part means people intent to leave them on the battery for days/weeks/months past the time it took to fully charge them.

NiMH is often charged by applying a constant current and then looking for a temporary drop in the voltage that occurs around a full charge. It's easy to miss so there are other detection methods involved, like for one if it's an AA cell it should definitely just shut off if it's already given it 2500mah.

There's a LOT out there on charging the tricky batteries (NiMH/Li-Ion/LiPo). Some of it is proprietary or sort of an "industry secret" though.

And the short answer here is, "can I pick up an unknown NiMH battery and check its charge state?" the answer is there's no practical, reliable way. Unless it's empty of course, that's easy to see. If you want to determine how far along it is in the charging process at any point, also not very reliable. You can only figure out when it's through, and then only with some particular agorithms.

These batteries will not be continuously trickle charged. At most, they will be trickle charged for up to maybe 20 hours prior to use. According to all the specs I see, modern NiMH cells can handle a 0.1C trickle charge without damaging the cell. In the case here, the batteries will likely only be 'overcharged' by 8 hours or less. Damaging the cells is not my issue.

My question now is, why can't I simply charge these batteries from a DC source with a resistor in series? In this configuration, you can select a resistor such that when the cells are depleted, they are fed ~0.2C charge current, and when they are near to being topped off, they are fed a 0.1C charge current. Why won't my idea work? Why would I need an LM317 or a separate 'smart charge' chip to do this?

 

[ThermalRunaway]

I can't offer you much help on your battery charging questions, but I do remember reading all about this in an issue of Elektor Electronics. They were doing a feature on solar panels and the best way to make use of the energy from them. If I remember, they also looked at the best way to charge batteries from a solar panel, and offered a project solution for it.

I can't remember which issue of Elektor this was but it would most likely have been within the last two years. You could try searching their back issues, or perhaps emailing them to enquire about the article. Quite often you can download single articles from their website for a small charge which saves you buying the whole magazine.

Hope this helps.

Brian