Rising internal resistance of 9v NiMH batteries

jderimig said:

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).

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So add a 1 ohm resistor in series. That will make it look more like the NiMH in regards to short circuit current without loosing all the benefits of LiPo.

jderimig said:

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.

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10A is not a problem for me because the altimeters that I am designing and using have huge and strong FET (the smallest I have used are 17A)

davel said:

So add a 1 ohm resistor in series. That will make it look more like the NiMH in regards to short circuit current without loosing all the benefits of LiPo.

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Putting in series resistance looses most of the benefit of a LiPo.

bdureau said:

10A is not a problem for me because the altimeters that I am designing and using have huge and strong FET (the smallest I have used are 17A)

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What about the power dissapation rating of the FET? Are you using heatsinks? 3oz or higher copper?

Have you dead short tested your 9V lipo's yet to test for protection circuits?

You have to be mindful of the power rating of the transistor as well as the maximum current. You may be able to handle the 10A, but at 9.6V it's 96W continuous, which will blow out just about any low-cost FET you're gonna find (or bipolar power transistor, for that matter). For 100W, you need a TO-3 package with a proper heat sink, that would be way too big and heavy to fly with under a J-K motor. ON Semi and other vendors make some FETs that have current-limiting build into them, that would be a good option if overcurrent is a possible issue; you'll have to go hunting for one that will handle the current/power.

The series current-limiting resistor isn't a bad idea, you just have to make sure its power rating is sufficient, and that it doesn't sink so much current that your igniter won't fire.

Use a couple FETs in parallel if you're really worried about heat management. Unlike BJT's, they don't experience thermal runaway really easily.

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

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They make NiMh and LiPO/LiFe smart chargers that aren't that expensive. Somewhere in the $30 range. Standard issue for Airsoft guns, since the wall chargers that they come with are absolute garbage. I'm sitting next to my LiPO/LiFe charger, small light, and can handle up to a 4S (14.8v) battery. Let me know if you'd like me to price one for you

cerving said:

You have to be mindful of the power rating of the transistor as well as the maximum current. You may be able to handle the 10A, but at 9.6V it's 96W continuous, which will blow out just about any low-cost FET you're gonna find (or bipolar power transistor, for that matter)..

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The point of this exercise is to deliver power to the igniter. If all of it is being dropped at the power FET, you are doing something wrong.

I dragged up a datasheet for a SOIC-8 package device (very small and power limited) in the form of an IRF8910. At a Vgs of 4.5V its Rds is 0.0183 Ohms max. So I^2*R at 10A is a very reasonable 1.83 Watts. Not something it would tolerate continuously but a 1 second pulse shouldn't be a problem.

Agreed. I think the concern was that the airstart igniter may short out after ignition and cause the LiPo to dead-short through the FET, but if you are either dumping a capacitor or limiting the "on-time" to one second or so then it's not an issue.

UhClem said:

The point of this exercise is to deliver power to the igniter. If all of it is being dropped at the power FET, you are doing something wrong.

I dragged up a datasheet for a SOIC-8 package device (very small and power limited) in the form of an IRF8910. At a Vgs of 4.5V its Rds is 0.0183 Ohms max. So I^2*R at 10A is a very reasonable 1.83 Watts. Not something it would tolerate continuously but a 1 second pulse shouldn't be a problem.

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jderimig said:

Putting in series resistance looses most of the benefit of a LiPo.

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Doesn't affect the ease of charging. Doesn't affect the much lower self discharge rate.

So the testing has started. Not enough cycles to see any trends yet, any variation is just noise. But here is sample data.

Vo = Voltage after recharge
Ao = delivered current into 1ohm load after recharge
V1 = Battery voltage after 30min on 50ohm load (~200ma)
A1 = Current delivered into 1ohm load after 30m

I don't know why the picture below is so small, click on it to see it readable.




So when relatively new all these batteries will deliver plenty of current for altimeter duty. The surprise here is even though the LiPo has almost unlimited current >20A into a short, it show some internal impedance into a 1ohm resistor only being able to source 6A. The highest of the bunch but still in the neighborhood of the NiMH.

jderimig said:

So the testing has started. Not enough cycles to see any trends yet, any variation is just noise. But here is sample data.

Vo = Voltage after recharge
Ao = delivered current into 1ohm load after recharge
V1 = Battery voltage after 30min on 50ohm load (~200ma)
A1 = Current delivered into 1ohm load after 30m

I don't know why the picture below is so small, click on it to see it readable.

View attachment 137731


So when relatively new all these batteries will deliver plenty of current for altimeter duty. The surprise here is even though the LiPo has almost unlimited current >20A into a short, it show some internal impedance into a 1ohm resistor only being able to source 6A. The highest of the bunch but still in the neighborhood of the NiMH.

Click to expand...

V/I=R so if V=8.3 volts and I=6 amps then R=1.4 ohms. Take away 1 ohm and R,battery<0.4 ohms. The short circuit current is I=V/R=8.3/0.4=21 amps. No surprise.

bobkrech said:

V/I=R so if V=8.3 volts and I=6 amps then R=1.4 ohms. Take away 1 ohm and R,battery<0.4 ohms. The short circuit current is I=V/R=8.3/0.4=21 amps. No surprise.

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Bob,
Well no surprise now....

So I have modified my table to include that magical little known Ohm's law thingy you mentioned and calculated Rb (battery internal resistance after partial discharge).



The MAHA 8.4v 300mah looks to be pretty decent so far (as compared to the LiPo). It will be interesting to see how long it can maintain that internal resistance over cycles. The jury is out on the Imedion, will have to determine if that second cycle number is an outlier or not.

davel said:

Doesn't affect the much lower self discharge rate.

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Technology marches on.

Low Discharge NiMH

This has been a very helpful thread, thank you Bob & John. Looking forward to your results.

As a long time user of the old Nicad Sanyo N-6PT 9 volt batteries, I've never really found a good replacement in a 9 volt form factor. For most of my altimeter needs I've moved to an iPower 530 mAh 9 volt li-polymer in a traditional form factor for av-bay compatibility (spec's here: https://www.batteryjunction.com/ipowerus-9v-520.html). I've also moved almost exclusively to Q2G2 igniters for black powder ignition which alleviates 90% of my need for high amperage.

As for the few motor air start projects I have, I connect the altimeter/timer/wrc+ to a relay board in which I use a hand-made battery made from the same nicad cylindrical cells the old sanyo's were made from (I think Bob gave me the link years ago). When this dies, I'll probably go with a few NiMH in series, or a lithium-poly without high-current protection (or uses a relay or quality FET for high current protection).

After the initial round of testing some leaders and laggards have distinguished themselves

So the lesson to be learned is not to generalize batteries especially NiMH's, there are differences.

The talley so far is shown below (click on the picture to enlarge)



Based on internal resistance alone, I would eliminate the MAHA9.6 and the Imedion 8.4v for rocketry altimeter duty.

The two remaining are the surprising robust MAHA8.4 300mah and the Krech recommended 180mah Turnigy NanoTech 2S Lipo.

From a safety standpoint (charge and forget it) the MAHA8.4 NiMH 300mah looks to be decent battery. Good capacity, healthy current supply and a decent internal resistance for a 9v NiMH. The drawbacks are price (~$11 ea) and with 70-90ma recharge rate for the MAHA490F charger means a couple hour recharge time for a drawn down battery.

The NanoTech Lipo with the least internal resistance can supply healthy current as well. But with it slightly higher internal resistance it also gives some FET protection on short circuit loads. The disadvantage is the little more care you need to take with a LiPo. The advantages are lower price (~$4 ea) and with the ability to charge at 2C allows you to recharge it in a half hour or less at the field.