Charging NiMH AAA and AA Cells, Technique?

[+Ratsneve]

After reading a number of threads trying to cover this subject I thought I would make a fresh post.

 

I believe automatic chargers use over voltage circuitry to detect when a cell is fully charged. But I read that if you charge at a low current (such as 200 mA for AAA cells?) the overvoltage detection might not work.

 

On the other hand if you use a higher charge current the cells can get quite hot as the charge shuts down to a trickle and this does not prolong cell life.

 

I have the BC-900 charger and it seems to be working. But charging at 200 mA seems to take a long time and I'm not certain that the circuitry is detecting overvoltage very fast or properly. I'm testing a set of AAA cells now and the mAh seems to be stuck and the test cycle won't end.

 

Do you think charging AAA and AA at 500 mA instead of 200 mA might produce better quicker results and not really be detrimental to the cell life?

 

Any other thoughts on this that I've missed? Such as, if 500 mA would work well for AAA cells, might 700 mA be a better charge current for AA cells? These changes would really cut down on the normal charging times. I know I could charge faster still if I was in a hurry--to the possible detriment of battery life.

 

Anything else?

 

Thank you.

Edited July 6, 2008 by Ratsneve

[+fourbeer]

After reading a number of threads trying to cover this subject I thought I would make a fresh post.

 

I believe automatic chargers use over voltage circuitry to detect when a cell is fully charged. But I read that if you charge at a low current (such as 200 mA for AAA cells?) the overvoltage detection might not work.

 

On the other hand if you use a higher charge current the cells can get quite hot as the charge shuts down to a trickle and this does not prolong cell life.

 

I have the BC-900 charger and it seems to be working. But charging at 200 mA seems to take a long time and I'm not certain that the circuitry is detecting overvoltage very fast or properly. I'm testing a set of AAA cells now and the mAh seems to be stuck and the test cycle won't end.

 

Do you think charging AAA and AA at 500 mA instead of 200 mA might produce better quicker results and not really be detrimental to the cell life?

 

Any other thoughts on this that I've missed? Such as, if 500 mA would work well for AAA cells, might 700 mA be a better charge current for AA cells? These changes would really cut down on the normal charging times. I know I could charge faster still if I was in a hurry--to the possible detriment of battery life.

 

Anything else?

 

Thank you.

 

Blindly using one charge current for all batteries (of a specific physical size) is the wrong approach. Charging current should be about 0.5*total capacity.

[+Ratsneve]

It doesn't look like this is rocket science. The four AAA cells are 800 mAh capacity. Position 1 and 4 just finished. 1 is at 800 mAh and 4 is at 795 mAh. The problem is that they have been at this capacity for over a couple hours. The cells are barely warm. I'm thinking "overvoltage" was totally the wrong word but "temperature" works.

 

Cell #3 just finished showing 842 mAh capacity.

 

Are cells low or high in their capacity indicative of anything? What is the nominal capacity tolerance? I guess this discussion should be on a different forum. Oh well. I'll do the next set at a higher charge current.

 

BTW, the BC-900 trickle charge is around 15 mA and the discharge current is fixed at around 1/2 the charge current that is set.

 

What is the indication to use when a set of cells needs conditioning?

[+Ratsneve]

Blindly using one charge current for all batteries (of a specific physical size) is the wrong approach. Charging current should be about 0.5*total capacity.

Okay, so if the AAA cells are 800 mAh then 200 mA charge current is on the low side. 400 would be about right. And 500, which is the next charge current increment for the BC-900, is on the high side. But is 500 too high to use or would it likely be close enough to work fine? I suppose how hot the cells got after the charge is finished would be an indication.

 

Thanks

Edited July 6, 2008 by Ratsneve

[WR8Y]

I believe automatic chargers use over voltage circuitry to detect when a cell is fully charged.

I wouldn't say "overvoltage". Sure, you COULD use a strictly high voltage indication to assume a full charge, but the more commonly accepted indication is that a charging cell, (given a reasonably steady charge current) will show a slight DIP in voltage when fully charged.

 

This is more pronounced in Ni-Cads than Ni-MHs, therefore, heat is often used to determine full charge when the NiMH is being fast charged (charge rates over 1/3 of capacity.)

 

The most reliable way to charge a NiMH is to apply 1/10 of the rated current for 15 hours. (Say you have a 100 maH battery. Charge it as 10 ma for 15 hours - NiMH's have a charge efficiency of about 66%, so you've gotta put more INTO it than you get OUT of it.)

 

If you insist on fast charging (and we all do!) then sensing of the cell's heat is supposedly the way to go.

 

But I read that if you charge at a low current (such as 200 mA for AAA cells?) the overvoltage detection might not work.

Well, the 'delta V' process I describe only work reliably with a low rate.

 

Do you think charging AAA and AA at 500 mA instead of 200 mA might produce better quicker results and not really be detrimental to the cell life?

Heating a battery up like that is NOT good for it, period. If the surface of the battery is getting warm, think how hot the INSIDE is getting.

 

I tried a compromise when on vacation. I used a 2 1/2 hour charger in the truck that charges at 1000 mA. It uses heat to determine charge in addition to a timer - it cost $11 at WalMart. Since I was driving 14 hour days, and since I could time the charge (and had already measured the charge current), I let it charge my batteries (AA's at 2100 maH) for almost 3 hours - I had it plugged into my acc outlet in the direct flow of an A/C vent - the batteries never got warm! At nearly 3 hours, I pulled them from the charger.

 

I could get 12 hours use from them with the backlight on and external antenna plugged in. Usually, I get 10 hours maximum.

 

BTW, I use Kodak 2100 maH AA's from WalMart - the cheapest NiMHs I can find at $7 for four of them.

[+Ratsneve]

Cell #4 has finished at 809 mAh. The highest 800 mAh cell so far came up to 842 mAh--that's a little over +5%. Does it mean anything if a cell charges that high? What if it was -5% low or in this case an 800 mAh cell charges to only 760 mAh?

 

The BC-900 manual says the charging unit uses both "overheat detection to protect rechargeable batteries from over-charging" AND "Minus delta voltage (-dV) detection for charge termination." Good.

 

I've started a second set of the same type AAA at a 500 mA charge.

Edited July 7, 2008 by Ratsneve

[WR8Y]

Cell #4 has finished at 809 mAh. The highest 800 mAh cell so far came up to 842 mAh--that's a little over +5%. Does it mean anything if a cell charges that high? What if it was -5% low or in this case an 800 mAh cell charges to only 760 mAh?

 

105% is fine, at work, we often see new batteries, well - sometimes 6 month old batteries that go 105 to 110%. And 95% is certainly within tolerance. (We condemn a battery when it gets down to 80%, mostly because Motorola will replace a battery under warranty if it's under 52 weeks old if it tests under 80%.)

 

The BC-900 manual says the charging unit uses both "overheat detection to protect rechargeable batteries from over-charging" AND "Minus delta voltage (-dV) detection for charge termination." Good.

I am gonna go get one of those chargers.

 

No product is perfect,but I still want one.

 

Impressive Charger!

[+Ratsneve]

105% is fine, at work, we often see new batteries, well - sometimes 6 month old batteries that go 105 to 110%. And 95% is certainly within tolerance. (We condemn a battery when it gets down to 80%, mostly because Motorola will replace a battery under warranty if it's under 52 weeks old if it tests under 80%.)

So, before you condemn a cell because its capacity has fallen to 80% do you refresh it to try to restore capacity above 90%? How well does doing that technique on a NiMH cell actually work?

 

So far so good with the La Crosse BC-900. I'm now doing a discharge & charge on one AAA 800 mAh cell at a 250 mA discharge & 500 mA charge current _while_ also doing a test (charge-discharge-charge) on one AAA 1000 mAh cell at a 250 mA discharge & 500 mA charge current _while_ also doing a test on two AA 2000 mAh cells at a 350 mA discharge & 700 mA charge current.

 

The big tricks to learn with this charger are that you can only change the charge/discharge current right after inserting the battery (8 seconds). If you mess this up you must remove and reinstall the battery. If you don't want to disturb other batteries in the charger you absolutely MUST press the slot location button corresponding to the cell just installed before anything else or you will upset the mode of everything else going on.

 

Once you have things set there is a brief delay and then the display flashes off then on again and your settings are locked until you take specific action to chage something.

[WR8Y]

 

So, before you condemn a cell because its capacity has fallen to 80% do you refresh it to try to restore capacity above 90%? How well does doing that technique on a NiMH cell actually work?

If the battery is over 4 years old, I don't bother, by that time the cells are old enough (and since most of our customers are public safety and use their batteries almost every day) that we replace the battery.

 

Restoring/reconditioning such an old battery has just a temporary effect.

 

Now, a battery of just 1-2 years age is a different story! I WILL say, that NiMH seem to not be all that prone to capacity loss as the NiCADs - NiCADs were awful about 'memory effect'.

 

I can't say much about how well reconditioning works on NiMHs - I have not seen them suffer capacity loss very often. The few that HAVE done so, were under Motorola's 1 year warranty, so we just replace them and send the bad ones in.

 

It sounds like the BC900 acts very much like out commercial charger/conditioners, and does all that they do - but for 10% of the cost!

Edited July 7, 2008 by WR8Y

[+benjamin921]

I use the MAHA MH-C9000 CHARGER and it acts pretty much like the 900. I will not use a high charge but also nothing too low. For the AAA 800's I charge either at 400 or 500 and it never gets hot.

[gallet]

The MaHa manual recommends at least .3C, before I bought it I had planned on charging at 200mAh but the manual convinced me this was not good with regards to detecting the voltage drop. So I've been charging my 2700 sanyos at 1000mAh it switches to trickle after about 150 minutes and the batteries only get ever so slightly warm, not much above ambient temperature.

 

My old so called 'smart' charger, destroyed my previous batteries in 6 months and got quite hot charging at a lower current. Go figger.

 

I like the way the MaHa gives a continual voltage readout, there's enough info for even the most anal of battery afficiondos.

[+benjamin921]

I have been charging my 2700's @700 with no issues.

[WR8Y]

I have been charging my 2700's @700 with no issues.

A 26% rate!

[+fourbeer]

The MaHa manual recommends at least .3C, before I bought it I had planned on charging at 200mAh but the manual convinced me this was not good with regards to detecting the voltage drop. So I've been charging my 2700 sanyos at 1000mAh it switches to trickle after about 150 minutes and the batteries only get ever so slightly warm, not much above ambient temperature.

 

 

Interesting. My Maha manual recommends 0.5C I wonder why they are different.

[+benjamin921]

They changed that a year or two ago. I forget the reason but if you search over at the Candlepower forums, you might find your answer.

[WR8Y]

Some Motorola chargers are using rates of 1.0C - yikes! Not sure how that's gonna work out, unless there is something about NiMH chemistry that prefers high charge rates - but that goes against decades of conventional battery care teachings.

 

For NiCads, the old "Rapid" SP50 chargers used a rate around .3C, took 3-4 hours to charge on high rate. THOSE batteries lasted a LONG time. The MT500s and others that used a .6C to 1C charge rate didnt' last as long.

 

NiMH has been touted in my industry as being "A great improvement" over NiCAD batteries. Sounds like it IS indeed an improvement: faster charging is possible and maybe even GOOD, where with NiCADs fast rates were obviously not good for the battery.

 

And the "Memory effect" with NiMHs is certainly less of a problem, we'v seen that here in the shop.

Edited July 7, 2008 by WR8Y

[WR8Y]

I like the way the MaHa gives a continual voltage readout, there's enough info for even the most anal of battery afficiondos.

And there's quite of few of us here - aren't there!

I see that I'm among my own kind here.

[+Ratsneve]

The MaHa manual recommends at least .3C, before I bought it I had planned on charging at 200mAh but the manual convinced me this was not good with regards to detecting the voltage drop. So I've been charging my 2700 sanyos at 1000mAh it switches to trickle after about 150 minutes and the batteries only get ever so slightly warm, not much above ambient temperature.

 

My old so called 'smart' charger, destroyed my previous batteries in 6 months and got quite hot charging at a lower current. Go figger.

 

I like the way the MaHa gives a continual voltage readout, there's enough info for even the most anal of battery afficiondos.

I'm pretty sure it wasn't a low current charge that killed your batteries with your old "smart" charger but that that charger didn't detect and shut the charge down to a trickle when the cells were full.

 

The BC-900 gives a voltage readout too but I'm not going to get that anal about it either.

 

What I don't have enough (anal) information on and to my surprise the BC-900 La Crosse tech support number couldn't help at all on was when one might consider wanting to refresh a battery and how important or how well that might work. I'm sort of thinking that if cells charge up to at least 90% of their capacity I'll continue to use them but if they fall below 90% I'll run them in the Refresh cycle and see what happens. If they come back to within 90% full charge then I'll use them and see how long that lasts for. And from what I've read here I may not really see many of these cells until they get 3 or 4 years old. I have some that old too I think but they are a much lower capacity too so best to just dispose of them and get higher capacity ones.

Edited July 7, 2008 by Ratsneve

[+Ratsneve]

The MaHa manual recommends at least .3C, before I bought it I had planned on charging at 200mAh but the manual convinced me this was not good with regards to detecting the voltage drop. So I've been charging my 2700 sanyos at 1000mAh it switches to trickle after about 150 minutes and the batteries only get ever so slightly warm, not much above ambient temperature.

 

 

Interesting. My Maha manual recommends 0.5C I wonder why they are different.

I'll bet users and the industry discovered that NiMH can take a little more charge then 0.3C even it the cells get a little hotter. It may not be that much to their detriment and that age (3 or 4 years) may be the bigger problem which there is no escaping from.

 

I couldn't read anywhere in the BC-900 manual what an optimum charge current forumula was. They do say to just use the lowest charge or 200 mA. That might be okay for small capacity AAA cells but large capacity AA's would take forever. And at the end of the day would they last longer then 3 or 4 years anyway?

[gallet]

@ 'Ratsneve

 

I'm pretty sure it wasn't a low current charge that killed your batteries with your old "smart" charger but that that charger didn't detect and shut the charge down to a trickle when the cells were full.

 

I'm certain that's the case.

 

What I don't have enough (anal) information on and to my surprise the BC-900 La Crosse tech support number couldn't help at all on was when one might consider wanting to refresh a battery and how important or how well that might work. I'm sort of thinking that if cells charge up to at least 90% of their capacity I'll continue to use them but if they fall below 90% I'll run them in the Refresh cycle and see what happens. If they come back to within 90% full charge then I'll use them and see how long that lasts for. And from what I've read here I may not really see many of these cells until they get 3 or 4 years old. I have some that old too I think but they are a much lower capacity too so best to just dispose of them and get higher capacity ones.

 

After a normal charge the MaHa, and I presume the La Crosse would tell you how much current went in. Problem is that on a normal charge that would only be a consistent indicator if the batteries were left to drain completely before charging. MaHa recommend to run there charge/discharge/charge cycle, (which gives the ammount of current discharge) every 10 runs, then to match the batteries. They recommend a forming or 45 hour break in cycle after 100 runs.

[+floydgg]

On the MAHA website for 2700 mA batteries it recommends charging at 1300 mA and discharging at 700 mA for the regular charge (with the refresh every 10 charges and the "break-in" every 30 cycles). (At least for their MH-C9000 charger).

 

http://www.mahaenergy.com/store/mhc9000faq.asp

Edited July 8, 2008 by floydgg

[+Ratsneve]

On the MAHA website for 2700 mA batteries it recommends charging at 1300 mA and discharging at 700 mA for the regular charge (with the refresh every 10 charges and the "break-in" every 30 cycles). (At least for their MH-C9000 charger).

 

http://www.mahaenergy.com/store/mhc9000faq.asp

Thanks for this information. I wonder if I'm really going to track things this closely--no, I don't think so?

 

A charge of 1300 mA is about .5C of 2700 mAh batteries. The BC-900 discharge current is half the charge current.

 

I ran across something strange with my BC-900... [Edited out... my error. The BC-900 has been working fine.]

Edited July 8, 2008 by Ratsneve