measuring actual battery capacity mah

[+ras_oscar]

I seem to be getting more operating hours out of my 2000 mah nimh batteries than my 2675 mah sets. Before I cry foul is there a simple way to measure the capacity? Is it a linear relationship to voltage? I have a multimeter and can do simple electronics

[+GrnXnham]

I seem to be getting more operating hours out of my 2000 mah nimh batteries than my 2675 mah sets. Before I cry foul is there a simple way to measure the capacity? Is it a linear relationship to voltage? I have a multimeter and can do simple electronics

 

My charger tells me the capacity--La Crosse BC-9009

 

I don't think there is a linear relationship to voltage. As I use my rechargeable batteries more and more, they eventually diminish in capacity but the voltage is always maxed out when fully charged.

 

Are your 2675 cells older than your 2000 cells or have they gone through more charge cycles? Are your 2000 cells "low-discharge" or Eneloop batteries?

Edited December 10, 2009 by GrnXnham

[+g-o-cashers]

You need to have some sort of load circuit where you can measure the current being drawn from the device and measure it over time. Most good NiMH (Maha, LaCrosse) chargers have a measurement mode which will do this for you and report the battery capacity based on this type of test.

[+twolpert]

I seem to be getting more operating hours out of my 2000 mah nimh batteries than my 2675 mah sets. Before I cry foul is there a simple way to measure the capacity? Is it a linear relationship to voltage? I have a multimeter and can do simple electronics

It's not that simple. Each battery type has a discharge curve (not entirely linear) which relates voltage to remaining capacity. IIRC, the curves for NiMH have an abrupt drop at the low end. More to the point, you are at the mercy of the generic curves built into your GPSr. In most cases, the unit will give a low battery warning (or shut down) when the batteries still have some capacity left. This is because the unit compares the supplied voltage to its internal curve and warns/dies when it judges that the batteries are nearly dead.

 

My guess would be that your 2000 mAh batteries are low-self-discharge (like Sanyo Eneloops) -- and that your 2675 mAh batteries are ordinary NiMH. The low-self-discharge types have a longer "shelf life" after charging than ordinary NiMH. Ordinary NiMH lose a substantial amount of charge in just a few days. So if you let them sit in your kit for weeks without using them, they will not perform as well as lower capacity low-self-discharge that have been sitting for the same amount of time.

 

Other possibilities include failing to properly "condition" regular NiMH (low-self-discharge should not require it) before first use, worn out batteries, mismatched pairs of cells, and so forth.

 

The only reliable way to measure the actual capacity used is to fully charge the batteries, discharge them in your unit and recharge them in a smart charger that will show you how many mAh it pumped into the battery. Do not run NiMH batteries all the way down in a dumb device like a flashlight. Not good for them at all.

 

Edited to add: Or use your smart charger's capacity test, as g-o-cashers suggests.

Edited December 10, 2009 by twolpert

[+ras_oscar]

I have 3 pair. all duracel and all placed into service within the past month. I have numbered the pairs as 1,2,and 3 to rotate them equally. i've completed 1 full rotation. the 2000's show full charge when placed in the unit after a full charge followed by a "rest" of 3 weeks. The 2675s by comparison show 1 bar low. All 3 sets are used under the same conditions, which is caching more or less daily during lunch. Backlighting full bright (I'm old and the screen is small).

 

I had assumed that since none are advertised as "low self discharge" and all come "precharged" that they'd have the same self discharge rate and the higher cap units would have more available charge under the same conditions. do the higher cap batteries have a higher corresponding self discharge rate naturally due to chemistry or did i get a bad 4 pack?

[+Chrysalides]

I have 3 pair. all duracel and all placed into service within the past month. I have numbered the pairs as 1,2,and 3 to rotate them equally. i've completed 1 full rotation. the 2000's show full charge when placed in the unit after a full charge followed by a "rest" of 3 weeks. The 2675s by comparison show 1 bar low. All 3 sets are used under the same conditions, which is caching more or less daily during lunch. Backlighting full bright (I'm old and the screen is small).

 

I had assumed that since none are advertised as "low self discharge" and all come "precharged" that they'd have the same self discharge rate and the higher cap units would have more available charge under the same conditions. do the higher cap batteries have a higher corresponding self discharge rate naturally due to chemistry or did i get a bad 4 pack?

AFAIK the only way they can sell them "precharged" is because they are low self discharge batteries.

 

And also AFAIK there are no LSD NiMH with capacity higher than 2100 mAH.

[ZeMartelo]

The timing for this thread is perfect.

I just got myself a lacross Bc-9009.

 

The number of options for this thing are puzzling to me.

I have a brand new set of Duracell 2650mah. WHat mode and current should I use the first time I put them in the charger?

 

I have other duracell 2650 that are not performing well.

I put a charged set on my Oregon 550 yesterday, it showed full, used the oregon for a couple hours yesterday and again today and the Oregon is already complaining low battery.

Should I use the discharge mode on these?

 

Is it okay to use 700mah on these batteries?

[+Chrysalides]

I have a brand new set of Duracell 2650mah. WHat mode and current should I use the first time I put them in the charger?

The regular 200 mA current recharge should work just fine.

 

I have other duracell 2650 that are not performing well.

Switch the charger to refresh mode for these cells. You need to do it within a few seconds of inserting the batteries. It will continuously discharge / recharge until it detects no improvement in capacity. I usually turn it off after 5 cycles if it did not complete by itself, and just run regular recharge to top it up. Read the manual to see how to switch the mode (it involves pressing the battery button, followed by the mode button twice, I believe, but I don't have perfect recall...

 

Is it okay to use 700mah on these batteries?

You can, but faster charging than 1/10 capacity might result in shortened battery life and lower maximum charge. I try to keep it to a minimum, and only for emergencies when I'm out of batteries.

[+ras_oscar]

Duracel model aa/hr6/dc1500 NiMH 2650MaH

also says standard charge 270 ma for 16 hrs.

 

FWIW the charger im using came with the 2000 MaH batteries. Next time i charge the high cap units i'll put it in the GPS immediately and see if the meter says its full. I just charged until the red light turned green (overnight)

 

Duracel charger CEF-20

Edited December 11, 2009 by ras_oscar

[+Curioddity]

Is it okay to use 700mah on these batteries?

You can, but faster charging than 1/10 capacity might result in shortened battery life and lower maximum charge. I try to keep it to a minimum, and only for emergencies when I'm out of batteries.

Modern NiMH cells will take charge rates of .25C to even .5C in stride if you can do it without getting them hot and the limiting factor is typically the delta-V (peak detect) setting, not the charge rate. This will be hard to explain without getting into painfully boring details but I'll try:

 

Near the end of a charge cycle when NiMH cells are sneaking up on their fully-charged state, the cell voltage will peak before the cell is fully charged. Then the voltage will actually drop off a little as the cell reaches maximum charge. That voltage dropoff after the peak is called the delta-V and it's what a peak detect charger looks for to decide when the cells are fully charged. Once the peak and proper delta-V has been detected, the charger will either terminate the charge entirely or drop back to a maintenance charge rate.

 

A typical delta-V setting for consumer-grade NiMH chargers will be around 5mV (.005V) per cell. Just for information purposes, a typical delta-V for NiCd cells will be 8-10mV (.008-.010V) and this is the primary difference between NiMH and NiCd charging algorithms. Consequently you can charge NiCd cells with a NiMH peak detect charger and the only consequence will be slightly undercharged cells, but you don't want to charge NiMH cells with a NiCd peak detect charger. Anyway, NiMH cells will generate the most heat during the dropoff after the peak and the heat is what does the damage to the cell chemistry. Manage the heat and you can charge at higher rates without significant consequences.

 

This is where most of you are probably scratching your heads and wondering how you can "manage the heat" and the unfortunate answer is you have very limited means with consumer-grade chargers. About all you can do is reduce the charge rate until the cells don't get hot at the end of the charge cycle. Warm is unavoidable and not harmful, but hot kills NiMH cells.

 

And that really is the crux of what I'm trying to explain. With peak detect chargers like the La Crosse and Maha models you can charge at higher rates if it doesn't overheat the cells. I use the Maha C-800S for my loose AA cells for my consumer electronics and it only has 2 rates: Regular and Soft. Regular gets the cells too hot, so I use Soft and my cells love me for it. Those of you with the La Crosse BC-9009 can experiment with several charge rates, but pay attention to cell temperature near the end of the charge cycle.

 

I happen to make a regular habit of huckin' composite gliders worth [literally] thousands of dollars off of jagged coastal cliffs and they all have packs of NiMH cells in them to run the receiver and servos. My transmitters also have packs of NiMH cells in them and if only one of those cells in the plane or transmitter fails, the glider either goes in the ocean or makes a smoking dent in the ground. For those NiMH packs and the LiPo packs for my powered planes and helicopters I use some pretty specialized (and expensive) programmable chargers which give me complete control over every aspect of a charge cycle. With chargers like that I can "manage the heat" in several ways including reducing the delta-V setting to 3mV per cell and automatically taper the charge rate during the voltage dropoff after the peak. I can even use temperature probes and automatically vary the charge rate based on pack temperature.

 

And I can safely charge NiMH packs at rates up to 1C with these chargers, but I do my best not to. I will if I have to, but that typically means I screwed up. Under normal conditions I charge my NiMH packs at .3 to .4C which means a 600mA to 800mA rate for AA Eneloops, and Eneloops are my battery of choice for transmitter and receiver packs. I've never lost or crashed a plane due to battery failure in the 2+ years I've been using them and I can't say that about any other brand that I've used.

 

Excellent comparison of Low Self Discharge batteries: http://www.stefanv.com/electronics/low_self_discharge.html

 

Pete

 

Addendum:

 

Please note that the information I shared in this post only applies to peak detect chargers which are generally going to be the "better" grade of aftermarket chargers. Standard chargers which come with most "store bought" rechargable batteries are typically constant current chargers and they are notorious for overcharging cells which reduces their life. A decent peak detect charger like the La Crosse or Maha isn't that expensive and it's a good investment if you want your rechargable batteries to last.

 

rePete

Edited December 11, 2009 by Curioddity

[+Searching_ut]

Is it okay to use 700mah on these batteries?

 

.......... I've never lost or crashed a plane due to battery failure in the 2+ years I've been using them and I can't say that about any other brand that I've used.

 

.....

rePete

 

Ah, you're not having any fun if you're just running batteries in your transmitter and receiver. Wait till you miscalculate the current of a motor/prop combo, and you torch a 12V lithium battery in a full power climb. You'll get all sorts of excited sounds from the crowd, and quite the smoke and flame display as several hundred dollars goes literally up in smoke.

 

As to the initial question, you really need to be able to put a fixed load on the battery, and track current and voltage during the dishcarge cycle to see what the real capacity of the battery is. You'll also find it varies with the amount of current you're trying to pull. If you don't want to buy a charger that will do the testing for you, any friends you might have that are into electric RC aircraft will probably have the equipment to test them for you.

[+ras_oscar]

As to the initial question, you really need to be able to put a fixed load on the battery, and track current and voltage during the dishcarge cycle to see what the real capacity of the battery is. You'll also find it varies with the amount of current you're trying to pull. If you don't want to buy a charger that will do the testing for you, any friends you might have that are into electric RC aircraft will probably have the equipment to test them for you.

ok, i uderstand.

 

sooooo when I plug in the batteries and fire up the garmin it shows me a battery meter. What is that really telling me??

[+Minimike2]

The voltage of your batteries, compared to what a fully charged battery of the type you have selected in System happens to be.

[+Curioddity]

Ah, you're not having any fun if you're just running batteries in your transmitter and receiver. Wait till you miscalculate the current of a motor/prop combo, and you torch a 12V lithium battery in a full power climb. You'll get all sorts of excited sounds from the crowd, and quite the smoke and flame display as several hundred dollars goes literally up in smoke.

I've never done that with a powered plane because I don't rely on calculations. I use an EagleTree eLogger and get pre-flight and actual in-flight measurements and that takes any guesswork out of it. If you haven't already guessed, I fly electric aircraft that can make their own wind too.

 

I have smoked a couple of big LiPo packs, though. The biggest was a 6S (22.2V) 4350mAh pack which I pushed to the published limit. Turns out that the published limit was, erm..., a tad optimistic to say the least. I got the pack out of the plane before it began "venting with flame" but I did considerable damage to the plane in the process. But the plane did eventually fly again and that would never have happened if the pack had gone off in it.

 

And yeah, the crowd loved it.

 

As to the initial question, you really need to be able to put a fixed load on the battery, and track current and voltage during the dishcarge cycle to see what the real capacity of the battery is.

Do you think one of these will do? (I actually have the CBA II):

 

 

I also have two of these:

 

 

And two of these:

 

 

And all three will do logged discharges at any load and to any final voltage that I program into them. The EOS0615i and Accu-cycle Elite are also fully programmable logging chargers and the CBA and EOS0615i will interface with a computer via a USB cable.

 

Needless to say, I take my battery maintenance quite seriously.

 

Pete

Edited December 13, 2009 by Curioddity

[John E Cache]

Sanyo says," Capacity is not Capacity".

[+Curioddity]

Sanyo says," Capacity is not Capacity".

Holy cow! They charged these cells at 1C (2000mA) and terminated the charge cycle at a 10mV delta-V. They certainly weren't interested in long cell life at those rates.

 

 

Pete

[+Egnix]

The only reliable way to measure the actual capacity used is to fully charge the batteries, discharge them in your unit and recharge them in a smart charger that will show you how many mAh it pumped into the battery.

 

A more accurate measurement of cell capacity is to discharge a fully charged cell at a constant discharge rate.

[+embra]

Agree; some of the charging mAh recorded will be lost to heat.