How Can You Tell When a NiMh Receiver Battery Is No Longer Serviceable?

[David Davis]

I have a number of 2000 and 2200 six cell rx batteries, a Hobby King X120 charger and a couple of those little black LiPo testers.

 

How can I test these batteries to ensure that they are still in serviceable condition?

[Peter Christy]

I would set the charger to discharge at 1A down to 1V per cell (from a full charge). You won't get the full 2 (or 2.2) Hrs, even with a new pack, but you should get between 1 1/4 to 1 1/2 hrs. Even 1 Hr would be useable, with caution.

[Martin Harris - Moderator]

Unusual receiver packs - 6 cell?

 

Anyway, there are two checks you can do fairly easily - first is capacity…fully charge and then use the charger’s discharge facility set at a typical discharge rate (say 200 mA if you’re using standard servos on average models) to see how much capacity you have. I’m assuming your charger will give you the amount of mAH taken at the end of the discharge. 
 

If your models are using high voltage servos on large surfaces at high speed as suggested by your pack description, up the discharge rate to a more typical rate. 
 

Another indication of pack condition is self discharge which will depend on the exact type - you should notice very little on Eneloop for example.  If the capacity check gives dramatically different results after repeating the discharge test after the pack is allowed to stand fully charged for a few days then the pack should be regarded as suspect. 
 

With so many variables it’s not really possible to give simple figures - maybe aim for 80% of new capacity but if you keep an eye on capacity you could probably safely accept lower. 
 

A regular test of remaining capacity after a flying session is a useful guide. 

[Jon H]

I charge mine up using my normal 'off to the field' delta peak charger. I leave it for 2 days, then cycle down to 1v/cell at .5amp. I then charge back up at .5amp and see the numbers. From a 2000 i would expect something like 17-1800 in/out. If its down below 1500 i would probably start treating it with some suspicion, although if its holds the .5amp discharge for the full 1500mah then its probably not dead, i would just keep an eye on it. 

 

So similar to Peter's method just with lower current draw as .5 constant draw is way more than we use in flight when averaged out.

 

Edit: my .5amp draw is also based upon using AA cells for many years. I could push my subc cells harder but choose to keep my testing constant. 

Edited April 17, 2023 by Jon - Laser Engines

[Nigel R]

Can't add much to the advice already given. 

 

On AA cells I use 2a for the capacity check and would expect to see 80% of the nominal capacity in a 2a  discharge from full down to 1v power cell. 

[Brian Cooper]

The suggested discharging regimes are all good and should give an indication of battery health and stamina. 

 

If buying a new battery, give it the same tests/regimes. . . Many people assume a new battery will be 100% perfect.  In reality, some new batteries don't quite make the grade. 

It pays to be cautious. 

[Peter Christy]

One final comment from me, in light of other answers above: Jon is quite right about the average current draw over a flight being around 0.5A - possibly lower depending on the use (or not) of power hungry servos. However, some servos - especially "digital" ones can draw quite high peak currents. If the cells have developed a higher internal resistance over time, this can result in voltage sags under load.

 

Using a higher current draw will help uncover this, at the expense of providing a lower than expected capacity result. As long as you tailor your expectations to this, and don't panic if a 2000 pack only produces (say) 16/1700 mAH, then the higher discharge current will reveal possible issues with a pack that lower currents may not. I've found that for 2000 mAH (or so) packs, a discharge current of between 1 and 1.5 A provides a realistic check, though I do tend to re-charge at a slightly lower rate (1-1.25A depending on capacity).

 

The reason for the lower charge rate is that most modern peak chargers have a safety cut-off after 2 hours, so I calculate the charge current to give a full charge in 2 hours.

 

With NiMh cells, the "peak" will not be so easily detected at low charge rates - even if you extend the charge time cut-off. A 2hr rate will give a good "peak" without the cells getting too warm.

 

Most NiXX cells thrive on being exercised - just like humans - as long as its not taken to excess! 😉

 

--

Pete

 

[leccyflyer]

Concur with what Peter says above. Having measured the current drawn by some microservos several years ago the results were surprising, with some examples drawing up to 1.5A peak current in use during the test.

[Cuban8]

I'd recommend finding a way to monitor the voltage of individual cells during the discharge test. In my experience, RX battery packs never deteriorate uniformly across all cells and dud packs are usually caused by a single cell becoming weak and causing an early fall in full voltage across the rest of the otherwise good pack. Warning of  trouble can be detected long before a crashed model. Found plenty of dodgy packs over the years with suspect cells that might otherwise have cost me a model. Balancing a pack as with lipos does nothing to improve matters.

A good reason to go with a 5 cell nimh to give redundancy.

Edited April 18, 2023 by Cuban8

[Nigel R]

Wise words from Pete Christy.

 

 

21 hours ago, Jon - Laser Engines said:

So similar to Peter's method just with lower current draw as .5 constant draw is way more than we use in flight when averaged out.

 

Just a thought.

 

I fly IC - mostly around 60 size, mostly sport aero types. They all seem to use approx. 80mAh per 10min flight. Given that a single flight is 1/6 of an hour, that's about equal to 500mA average over the duration of a flight. If you're flying bigger warbirds, I'd expect the average to be higher (although I have no data points to back that up). Pete's point about checking the pack at a 'much higher than average' discharge rate is well made.

 

 

[Cuban8]

I always smile and no longer pass comment where I here a flying buddy say something like " this XYZ brand pack has been wonderful, never let me down and is now fifteen years old".

 

What ya gonna do with 'em?

[leccyflyer]

Used to have a clubmate many years ago who would never charge a receiver pack from one month to the next. He would just have a plastic bag with a bunch of old spare NiCd packs, if the RX pack in the model would show as flat, he would just  find one with some charge left and put it in the model to fly.

[Jon H]

4 minutes ago, Nigel R said:

They all seem to use approx. 80mAh per 10min flight.

 

Does that include the 4 minutes the radio is on during pre flight, start, carry out and taxi/shutdown after landing? Im not trying to be difficult, just we tend to focus on the flight when talking about power/fuel consumption etc and forget that the flying part might be only 2/3 of the whole flight from switch on to switch off. 

 

Its been a while since i tested power consumption over a flight but i seem to recall no more than 100mah for my La7, although in truth i cant remember and my test equipment was decidedly lacking. 

 

In any event, a constant draw of .5amp is far more than the model will see in flight. It would be low load and then higher load when something is moved. The argument about cells falling over under high current draw is fair enough, but you could smash it with a 3amp discharge load on a charger. If it lasts 30 seconds like that without the voltage crashing, its fine. Dragging 3amps out of a 2000mah AA pack all the way down from full to flat wont do it any favours and will shorten its life, especially an AA pack. 

 

I just did some checking and in fairness it seems nimh cells are more robust than i gave them credit for, at least there are racing/airsoft packs on the component shop website with 3c constant discharge and up to 7c peak on a 5000mah sub c pack. However what is their lifespan when used that way? For an Rx pack, where life and reliability are more important than peak discharge performance i would not want to exceed 1c discharge and .5c charge as maximum values. I use 500/600mah charge as its plenty fast enough to get a decent peak (on my chargers anyway its been fine) and wont cook the batteries. I do have some that are getting pretty old, 8-10 years, but they still perform well and i see no need to replace them yet. They are under close observation though as they cant have long left i wouldnt think. In fact my models are all due for a battery health check and i need to get on that pretty soon. Maybe i will do some science and try the same model with .5 and 1.5amp cycles and compare the results. 

[Nigel R]

1 minute ago, Jon - Laser Engines said:

Does that include the 4 minutes the radio is on during pre flight, start, carry out and taxi/shutdown after landing? Im not trying to be difficult, just we tend to focus on the flight when talking about power/fuel consumption etc and forget that the flying part might be only 2/3 of the whole flight from switch on to switch off. 

 

It does, but, consider the load on a servo with zero airspeed over a control surface - essentially idle current. Engine running? Yes, the servo now has some vibration to contend with, a little more draw. So yes, that will take up some of the total draw, but, I would suggest, not that much. All that said, I lack a telemetry recorder to fully answer the question in proper detail, I am afraid.

 

As for robustness. A 1.5C load from full to flat should present no significant difficulty for a (good) low discharge nimh. If done as an occasional health check (once per year?), it will make next to no difference in terms of long term lifespan. Good cells (I do keep using the word good, quite deliberately) are rated for hundreds of cycles. Any health checks, the likes of which we are talking about here, will take up only a very few of those cycles. In real terms, our daily use of RX nimh cells is only the lightest of partial cycles, and a (good) cell can tolerated thousands of those. I think the early generation of nimhs prejudiced many people against them, they were somewhat high capacity but fragile by comparison to the nicads of the day. Modern nimh cells are as good and in many ways better than nicads.

[Jon H]

28 minutes ago, Nigel R said:

A 1.5C load from full to flat should present no significant difficulty for a (good) low discharge nimh

 

This is where we are into the nitty gritty. We all use different cells of different quality, age, capacity, cell size, have different chargers etc. Its not easy to generate a one size fits all solution. Its why i stick to my tried and trusted .5amps as i know it works with my equipment and gives me consistent results. Like you i dont have power data logging on my model as, frankly, i just dont care. Its not important to me and i operate all of my equipment well within its maximum capability as that is a great defence against failure. 

[Frank Skilbeck]

This is where I find telemetry is useful, you can set an alarm to warn you of low pack voltage in flight. Saved a plane of mine, even though it was a faulty switch (New supposedly Heavy Duty one), operating the servos caused a big voltage drop. 

 

On other planes it's often electric retracts that take the most load, but on one other electric plane we had the BEC cutting out, turned out to be a faulty servo drawing quite high current, it worked fine just for some reason was drawing a high current, changed that servo and all was fine. Had 3 of those servos in the model and the other two were fine.