Rising internal resistance of 9v NiMH batteries

[jderimig]

I have used the MAHA 9.6v rechargeable batteries for 4 years now and still recommend them highly. However I want to share a recent experience that may be useful to all that use any of the 9V NiHM.s

I had a successful flight at URRF in a 2-altimeter rocket both powered by the MAHA 9.6v.

When I got home I used those batteries for some desktop development work and noticed one of my altimeters under development was complaining of low voltage conditions even though the battery was fully charged. So I check the current of the battery and sure enough it only was able to deliver 240ma into a dead short. I pulled the other one I used on that URRF flight and it only delivered about 150ma. When new these batteries would source 4-5A into the same test. At 15ma load the battery voltage was pulled down almost 2volts! I haven't kept track of how many charge recharge cycles were on these batteries but it was probably less than 100.

So the lesson here to all my fellow NiMH battery users to frequently check the internal resistance of your batteries with a short or high current load test before sticking them in your rocket.

 

[bdureau]

I now use only lithium 9V batteries because I can do more than 10 flights without recharging them ( even with a buzzer beeping away)
Try the following and you will enjoy them
https://www.ebay.fr/itm/2-pcs-9V-9-...ronics_Batteries_Chargers&hash=item5af0855433

 

[jderimig]

I now use only lithium 9V batteries because I can do more than 10 flights without recharging them ( even with a buzzer beeping away)
Try the following and you will enjoy them
https://www.ebay.fr/itm/2-pcs-9V-9-...ronics_Batteries_Chargers&hash=item5af0855433

Thanks, those look interesting but I didn't want to turn this thread into a battery recommendation thread. Have you checked what the discharge current is on those and the internal construction (welded or just pressed)? Most of the LiPoly 9v batteries I have seen are 7.4 volt nominal (only 8.4v fully charged) and discharge current rated to only 1C (~500ma). If there are protection circuits in those batteries, (I bet there is), that will cause some problems with some altimeters when the pyro fires (like shut off and reset the altimeter).

My point is if you use rechargeables, ANY rechargeables, it is good practice to check the internal resistance or high load current occasionally because that can degrade and it is undetectable by a simple battery voltage check.

I am hoping that Bob Krech will chime in as he knows something about battery chemistries and discuss if NiMH is more susceptible to developing high impedance over time than other chemistries. A quick search of the web says that this happens with all chemistries.

 

[COrocket]

Most LiPo batteries I use are rated to 25C continuous discharge and 40C burst. A 180 mAh LiPo gives off in excess of 7 amps with these specs, more in a short circuit. They are common in RC aircraft, but I've had good success powering rocket electronics. Using a 7.4V battery with less voltage isn't necessarily a bad thing, as long as it is within the altimeter spec. (The strattologgers I use can have anywhere from 4-16V) so you could even use a 3 cell 11.1V LiPo battery. I know the ravens are powered by a single cell LiPo. They do have very low internal resistance and put out quite a lot of power for their size. In the example i used, the batteries found at https://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=28648

 

[jderimig]

Chris, all true.

But this thread is specifically about "9V" batteries and I'd like to keep it confined to that subject and not turn it into another general battery thread. The batch of 9V LiPo marketed batteries appear to be 1C rated at best and their spec sheets are hard to find (common for China originated consumer components).

 

[bobkrech]

I'll try to explain what going on with the current variants of "9V" transistor battery NiMH batteries. There are 3 generic types of "9 v" NiMH transistor batteries: 6 cell - 7.2 volt variants which are getting rare; 7 cell - 8.4 volts units; and 8 cell 9.6 volts units. The 6 cell variants may use either (6) AAAA cylindrical cell, or 6 prismatic (flat) cells, while the 7 and 8 cell variants use prismatic (flat) cells. The 6 cell variants typically have 300 mah capacity, the 7 cell variants typically have 250 mah capacity and the 8 cell variants typically have 225 mah capacity. The variants using prismatic cells are not designed to delivery high currents and really aren't designed to deliver an amp. They have a relatively high impedance and are designed for low current draws. The 7.2 v with (6) AAAA cells are designed to deliver high currents and are more suitable for altimeters as they will deliver several amps. Unfortunately the 7.2 volt, (6) AAAA cell NiMH variants are getting hard to find, and IMO the only "9 volt" transistor battery size NiMH that should be used in an altimeter that does not use a pyro energy storage capacitor (not to be confused with an anti-brownout capacitor used to keep the CPU voltage up when the pyros fire) when using e-matches that draw more than an amp. The Quest Q2G2 igniters will work with all variants of "9 volt" NiMH batteries because they draw less than an amp, as will Christmas tree bulbs. Although you might want to keep the discussion to "9 volt" NiMH transistor batteries, 2s1p LiPo packs are a better alternative today. A good "9 volt" NiMH battery weighs 57 grams in a 47.5 mm x 26.5 mm x 16.5 mm package. This 2S1P 180 mah LiPo pack weighs 13 grams in a 35 mm x 20 mm x 10 mm package will source 7.2 amps which is a better performer but won't overcurrent most altimeter output transistors https://www.hobbyking.com/hobbyking...180mah_2S_25_40C_Lipo_Pack_US_Warehouse_.html

 

[jderimig]

Bob,

So when I test a fairly fresh MAHA 9.6v (8 cell prismatic) and it delivers 4A into a short is that "only when new" performance and it shortly will go to the several hundred ma range that I am currently seeing with my 'seasoned' MAHA's? If so what causes this observed impedance rise? Age, use, recharge cycles, high loads, all of the above?

BTW I have ordered several more and plan to take some testing and take some data to see if I can measure an impedance rise over time.

 

[bobkrech]

Bob,

So when I test a fairly fresh MAHA 9.6v (8 cell prismatic) and it delivers 4A into a short is that "only when new" performance and it shortly will go to the several hundred ma range that I am currently seeing with my 'seasoned' MAHA's? If so what causes this observed impedance rise? Age, use, recharge cycles, high loads, all of the above?

BTW I have ordered several more and plan to take some testing and take some data to see if I can measure an impedance rise over time.

The trend in NiMH batteries is to reduce the self-discharge current. You can do this by reducing the electrode area and several other design method mentioned in this battery add. https://www.thomasdistributing.com/...ow-Discharge-Rechargeable-Battery_p_2457.html When you implement this electrode design in a small prismatic cell, you will prematurely degrade the electrodes if you attempt to draw 4 amps by shorting the cell which would be 16C when compared to the mah rating of the battery. Although there is not a real data sheet in this add, I believe this electrode structure doesn't want a draw greater than 1 C for long periods of time. The increase of impedance with charge cycle is shown here. https://batteryuniversity.com/learn/article/battery_performance_as_a_function_of_cycling BTW Battery University is a good general reference for most battery chemistries.

 

[jderimig]

The trend in NiMH batteries is to reduce the self-discharge current. You can do this by reducing the electrode area and several other design method mentioned in this battery add. https://www.thomasdistributing.com/...ow-Discharge-Rechargeable-Battery_p_2457.html

Cool. I just ordered one of the Imedion and a conventional MAHA 300mah 8.4 cell. I'll test these in an altimeter use environment and see if I can generate data like in the Battery Univ link you posted.

 

[bobkrech]

https://www.batteryspace.com/prod-specs/SPECIFICATION_9V250[1]_1.pdf datasheet state 750 ma or 3 C is maximum continuous draw. https://www.batteryspace.com/prod-specs/SPECIFICATION_9V200[1].pdf.pdf datasheet considers 1 C a high discharge rate. https://www.batteryspace.com/prod-specs/1403.pdf datasheet considers 0.5 C to be the maximum discharge rate. Current draws not very high if you want good cycle life with high capacity.

 

[wetmelon]

I now use only lithium 9V batteries because I can do more than 10 flights without recharging them ( even with a buzzer beeping away)
Try the following and you will enjoy them
https://www.ebay.fr/itm/2-pcs-9V-9-...ronics_Batteries_Chargers&hash=item5af0855433

LiPo's should never be run completely dry, by the way. For non-system critical applications, always use a battery monitor that can turn off the draw before the battery runs completely dead. Doing so can cause permanent damage to the battery.

In addition to not having a "memory", LiPo's are more energy-dense than NiMh batteries and can deliver SIGNIFICANTLY more current. Take for example this small 250mAh 7.4 LiPo. It only weighs a few grams at most, can deliver power at 20C iirc (5 Amps peak) and has a tiny form-factor. I have never seen an equivalent battery in NiMh. IMO, there is no reason whatsoever to run a NiMh unless you're afraid of Lithium poisoning from CATO or battery explosion.

 

[COrocket]

In addition to not having a "memory", LiPo's are more energy-dense than NiMh batteries and can deliver SIGNIFICANTLY more current. Take for example this small 250mAh 7.4 LiPo. It only weighs a few grams at most, can deliver power at 20C iirc (5 Amps peak) and has a tiny form-factor. I have never seen an equivalent battery in NiMh. IMO, there is no reason whatsoever to run a NiMh unless you're afraid of Lithium poisoning from CATO or battery explosion.

NiMh seem to be phasing out and marketshare being replaced by lithium polymer/ion, at least according to the Wikipedia article. I think both have built in features to vent instead of exploding.

 

[bdureau]

NiMh seem to be phasing out and marketshare being replaced by lithium polymer/ion, at least according to the Wikipedia article. I think both have built in features to vent instead of exploding.

With my altimeter I have stopped using NiMh battery because they are not reliable, the discharge very quickly when they get older (espacially if you have a continuity test beeping when waiting on the pad) and then you end up with ematches not firing.
The lithium 9v batteries that I am using are in fact around 8.4volts but can deliver a hight current which is better because the altimeter does not stop working when firing charges (which could be a problem if you want to store the flight altitude and do double deployment). Those can do several flights without being charged and are a lot lighter.

 

[jderimig]

Altimeters draw about 10-20ma on average and their high current draw (if no energy storage capacitors) last a few tens of milliseconds. Lipo for current capability are way overkill for this application. Now size and weight in another matter though but selecting LiPo for current capability for firing an ematch is gatuitous.

For the record I have never seen a NiMH battery drained down by the altimeter beeper even with my worn down NiMH's.

With my altimeter I have stopped using NiMh battery because they are not reliable, the discharge very quickly when they get older (espacially if you have a continuity test beeping when waiting on the pad) and then you end up with ematches not firing.

What are the units of quickly? Time? Number of Recharges? Please show your data. Do you have data or is this just anedote?

I WILL have data on 3 models of NiMH 9v batteries but it will take some time to get the few hundred charge-discharge cycles on each. I plan to discharge them at 0.5C and recharge to get probably near worst case plots of capacity, internal resistance and short circuit current versus time. It should be interesting and we won't have to speculate anymore.

 

[jderimig]

Bob,

For altimeter use max continuous draw is not a concern right? Most altimeters average 10-20ma. The pyro firing event lasts only tens of milliseconds. My question is are these pulse current draw of 4-5a for 30ms damaging to a NiMH rated for 3C?

 

[bobkrech]

Bob,

For altimeter use max continuous draw is not a concern right? Most altimeters average 10-20ma. The pyro firing event lasts only tens of milliseconds. My question is are these pulse current draw of 4-5a for 30ms damaging to a NiMH rated for 3C?

Batteries are a commodity business. Not only do new batteries have to be better, but they also have to be cheaper. NiMH batteries are so well developed in the industrial form factors, especially 18650 aka 4/3 A, where they still give Li batteries a run for the money, e.g. the Prius NiMH battery pack versus the Volt LiPo or the plug-in hybrid market. The cylindrical NiMH batteries frequently last over 2,000 charge cycles with no memory and have high current capability. The 7.2 volt NiMH batteries use 6 cylindrical AAAA batteries and do not have any issues with current or cycle life, but they have a relatively low power and mah capacity compared with the latest 8.4 V and 9.6 V prismatic versions. The lack of actual technical datasheets make comparisons difficult if not impossible, so I have to go with my instincts on the internal electrode structure to pinpoint the flaws with the latest versions of the higher cell count NiMH transistor batteries. Certainly the 2 to 20 ma continuous current draws of the modern altimeters should not be an issue, but the igniter current pulse could attempt to draw several amps for 1 second if the bridge wire doesn't burn out. Again, I would not consider this a major problem, but apparently it is, or folks are carefully tracking the number of cycles they put on their batteries. Additionally, NiMH transistor batteries are a commodity item where price is king. Since most devices probably get thrown out before the batteries get totally used up, cheapness counts. Unlike LiPo battery chemistry where batteries either work or don't, NiMH battery chemistry is more forgiving so marginal manufacturing techniques will produce batteries that work for a while before they go south. Sorry I can't be more definitive, but without datasheets and life cycle data, all you can do is speculate.

 

[bdureau]

Ok let me clarify
I was using NiMH 9V rechargeable batteries and each time I wanted to fly a rocket the battery was flat. As for the charge-discharge cycles ; well after 10 your battery is dead (actually it depend of the charger you are using I guess)
With those 9Volts batteries I have had them for 3 years and recharged only 3 times and they are working like they were on the first day.
For one of my rocket I flew it 10 times in one year without recharging it.
I guess we could plot a curve to be able to compear it or I could ask the manufacturer one

 

[jderimig]

Ok let me clarify
I was using NiMH 9V rechargeable batteries and each time I wanted to fly a rocket the battery was flat. As for the charge-discharge cycles ; well after 10 your battery is dead (actually it depend of the charger you are using I guess)
With those 9Volts batteries I have had them for 3 years and recharged only 3 times and they are working like they were on the first day.
For one of my rocket I flew it 10 times in one year without recharging it.
I guess we could plot a curve to be able to compear it or I could ask the manufacturer one

Yes I understand. I have been using NiMH for years (5+) with no problem. Just recently I have seen problems and now I am curious. Perhaps it is what Bob said with quality marching backwards.

 

[bdureau]

Yes I understand. I have been using NiMH for years (5+) with no problem. Just recently I have seen problems and now I am curious. Perhaps it is what Bob said with quality marching backwards.

Find below the spec for the batteries that I am using
Litelong 9V 780mAh Li-ion Rechargeable Batteries

Specifications:

Size: 9 Volt(6F22)
Voltage: 8.4v
Capacity:780mAh
Brand: Litelong
Chemistry: Lithium-Ion(Li-ion)
Dimensions: (H x L x W): 48mm x 26mm x 16mm
weight: 24 g

Features:

No memory effect
Reliable service life
Long-Lasting Performance
Low Self Discharge
High Discharge Rate
Recharges up to 1500 Cycles

 

[jderimig]

I bet it has a protection circuit. Short it with a voltmeter attached and see what happens.

 

[COrocket]

Yes I understand. I have been using NiMH for years (5+) with no problem. Just recently I have seen problems and now I am curious. Perhaps it is what Bob said with quality marching backwards.

It could also be what bob said about the NiMh's possible marginal manufacturing techniques, rather than lipo's which either work or don't. This would make me nervous, especially for batteries without spec sheets. I also wouldn't think that asking a battery to exceed is specs for deployment is a good idea, even if it is for a short period of time. That could be good test, to see if multiple "deployment" events shorten the battery life or affect impedance. I'm curious as to why you wouldn't consider the lithium batteries considering their discharge capability and low internal discharge advantages.

 

[jderimig]

I'm curious as to why you wouldn't consider the lithium batteries considering their discharge capability and low internal discharge advantages.

Discharge capability of a LiPo can be a negative. I prefer a battery whose maximum current is less than the current rating of the power fets on my altimeter (and the polarity protection diodes). To get than in a lipo requires a fairly low capacity.

However, if my testing uncovers a systemic weakness in the 4-5 NiMH's I will test, then I certainly will consider a LiPo.

 

[lmt56]

What type of multimeter is required for this type of testing?
I have quite a few Powerex300 & PowerexTrue 9.6v I would like to test. All have some age on them.

 

[jderimig]

I just use a multimeter that has a 10A current measurement setting (most do).

Here is the test protocol I am going to use when my new batteries arrive.

1. Top off charge using MAHA490F. Measure voltage.
2. Measure dead short circuit for 1 second and record.
3. Dscharge battery at approximately 100ma for exactly one hour.
4. Measure voltage drop at battery plus terminal to determne internal resistance. Record.
5. Measure no load voltage.
5. Recharge and repeat.

Plot results versus recharge cycle.

 

[dixontj93060]

I have not done any formal testing of the MAHA Powerex batteries, but I have been using them for 8+ years (both the 9.6V and 8.4V versions). Before I started using them I had a friend that is a "battery expert" (QA for Motorola) take one apart and compare to a Duracell. His conclusion was that it was top quality and should be as durable, if not more so than the "gold standard" Duracell. I was mostly interested in durability under extreme conditions and didn't have him comment on the chemistry. Saying that, I have some batteries (at least 3) left from my original purchase (I can tell because their connector configuration has changed over the years) and they still work great. When I have had a problem it has been due to a CATO/crash where the battery has taken a beating. At that point degraded operation was evident and flagged by the MH-C490F Stealth charger.

 

[bobkrech]

I just use a multimeter that has a 10A current measurement setting (most do).

Here is the test protocol I am going to use when my new batteries arrive.

1. Top off charge using MAHA490F. Measure voltage.
2. Measure dead short circuit for 1 second and record.
3. Dscharge battery at approximately 100ma for exactly one hour.
4. Measure voltage drop at battery plus terminal to determne internal resistance. Record.
5. Measure no load voltage.
5. Recharge and repeat.

Plot results versus recharge cycle.

A few questions. 1.) Are you planning to let the batteries rest between charging and measuring the no load battery voltage in step 1?; 2.) I assume you are measuring the short circuit current in step 2.; 3.) Would it be worthwhile to measure the no load voltage after the short circuit test after step 2; 4.) What are you discharging the battery thru: a 3 terminal current regulator or a resistor. If it is a resistor, it should probably be a 10 ohm, 10 watt resistor so the temperature doesn't change with time, https://www.digikey.com/product-detail/en/RH01010R00FE02/RHRB-10-ND/1166275 is a good inexpensive one with a low tempco; 4.) Would it be useful perform step 4 at the beginning as well as the end of step 3; 5.) Would it be a useful to repeat step 2 after step 5.

 

[jderimig]

A few questions. 1.) Are you planning to let the batteries rest between charging and measuring the no load battery voltage in step 1?.

Probably a good idea. What is a good wait time?

2.) I assume you are measuring the short circuit current in step 2.

Yes

3.) Would it be worthwhile to measure the no load voltage after the short circuit test after step 2;

Yes, Easy to add

4.) What are you discharging the battery thru: a 3 terminal current regulator or a resistor. If it is a resistor, it should probably be a 10 ohm, 10 watt resistor so the temperature doesn't change with time, https://www.digikey.com/product-detail/en/RH01010R00FE02/RHRB-10-ND/1166275 is a good inexpensive one with a low tempco

I was thinking 80ohm wirewound resistor to average around 100ma.

4.) Would it be useful perform step 4 at the beginning as well as the end of step 3; 5.) Would it be a useful to repeat step 2 after step 5.

Possibly, but I was expecting to see long term trends. Maybe step 4 should be done after step 2. As for 2 dead short tests per cycle (one fresh, one after a discharge) that may interesting also. I'll add those.

Now I am thinking of automating this....nah.

 

[bobkrech]

https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride might explain a bit about why some NiMH batteries die young. Also I slipped a decimal point with the resistor value. The 2 inexpensive in stock 1% 10 W wirewound resistors with 20 ppm tempco in the desired value range are 75 and 100 ohms at digikey.com by Vishay Dale at $3.78 each. P/N https://www.digikey.com/product-detail/en/RH01075R00FE02/RHRB-75-ND/1166289 and https://www.digikey.com/product-detail/en/RH010100R0FE02/RHRB-100-ND/1166273

 

[jderimig]

I am often guilty of leaving batteries in the charger too long, days sometimes...

 

[jderimig]

I have the 4 batteries that will be tested they are.

1. MAHA 9.6V 230mah
2. MAHA 8.4V 300mah
3. MAHA Imedion 8.4v 250mah low-discharge
4. HobbyKing 2S1 180ma hour LiPo that Bob Krech linked to.

I have modified the test procedure after some dry runs.
1. Recharge
2. Measure Vo
3. Measure current delivered into a 1ohm load for 1 second.
4. Discharge across 50ohm resistor for 30 mins.
5. Measure voltage and current delivered into 1 ohm load for 1 second.
6. Repeat

Initial dry runs indicate the following.
1. The new MAHA 9.6 source about 5A.
2. The 180ma LiPo source greater than 10A (my meter rails at that point) making that LiPo NOT FET safe in the event of a short (which can frequently happen in an airstart). But I really like that battery, I will post a picture in a bit.

More data will be posted over the next few days as I get some charge-recharge cycles under way.