A beginner to NiMh batteries - questions, questions...

[Jonathan M]

With apologies in advance for so many questions, but having moved from LiPo-fuelled electric foamies (with that whole learning curve!), I'm keen to now understand and look after NiMh batteries. This is for my first IC, an Acro Wot with five Futaba S3003 servos and a Spektrum RX. (I've trawled through many pages of threads in this topic and garnered many useful bits, but thought I'd pose my questions as a complete set - which might then be of use to others passing this way in future.)

Q1. Will a 4.8v pack be sufficient for this application, or will 6.0v be cleverer - more torque, less risk of brownout? If 4.8v is adequate, then what is the dis/advantage of a pack with greater capacity - i.e. a 3800mAh pack made up of four physically larger (but heavier) cells over a 2000mAh Eneloop with its four AA sized cells?

Q2. My 3800mAh 4.8v pack (I chose the heavier option to avoid adding the usual nose-weight to this ARTF) was purchased new and fully charged. After a bit of use during installation, setting throws and then forgetting to switch it off for a few hours, it is now down to 4.9v (five bars out of seven on the battery checker), so about 1.0v per cell. Is there an absolute minimum voltage below which the battery will become damaged or can one just let it drain down in use? (Obviously I wouldn't want to fly with capacity below, say, 25%.) Can I safely charge it back up from half-way, or should I cycle it fully down and back up again, and should I do this every time or just the first few times when the battery is new?

Q3. I have a Fusion Elysium charger, manual here  **LINK**  which gives me lots of options, but I'm unsure what settings to use for safety and longevity of battery pack.

(i) Charge Current. I gather 1/10C is the correct setting, certainly initially,for NiMh batteries, e.g. 0.4A for the 3800mAh pack, which from empty will then take about 10 hours, i.e. overnight. Should I do this every time or can I use a higher C (1/5, 1/2 or 1C) once the pack has had a few cycles?

(ii) Delta Peak Sensitivity. To what level should I set this? The available range is from 3mV to 25mV per cell.

(iii) Discharge Current. To what level should I set this? The available range is from 0.1 to 1.0A. Is 'fast' bad? Just how slow?

(iv) Discharge Cutoff Voltage. To what level should I set this? The available range is from 0.1V to 16.8V in total.

(v) Cycle settings. I don't fully understand the menu options here. I can toggle between D->C and C->D and can, for each, choose between 1 and 5 in 1 unit steps, but I don't know what these mean. Does D->C mean from 'discharged' to 'charged' and vice-versa? Which setting both discharges and then automatically charges, or is this something I have to do manually (there are also separate charge only and discharge only menu screens)? Do the numbers mean total number of repeated cycles or something different?

Specific help with some, most or all of these questions would be hugely appreciated!

Thanks

Jon

Edited By Jonathan M on 24/12/2016 13:05:52

[Jonathan M]

For clarity, this is the NiMh charging options flowchart:

One thing I've noticed is that the Discharge options require you to choose the total voltage of the pack, whereas the charge options don't - I assume the charger simply senses the voltage here?

[Denis Watkins]

To help I hope Jonathan, I fly the same model on a 1000ma 4.8v AA Eneloop pack all day, which is about 6 flights, as I help others during the day, and I use about 300 - 400 ma from the pack, then I recycle and charge this for the next time. During a busy summer, I just charge ready for the next flying day, and recycle the pack monthly. With a large number of servos in a model, namely flaps and retracts, it would be prudent to use a 6v set up with 6v servos and a 6 v Rx. You gain little with a 6v set up if the model has 4 servos. Mathematical care for your Nimhs will arrive shortly from the boffs.

Do not charge overnight or leave unattended. Charge at 1\10 C as you say as Nimhs don't tolerate overcharge or heat

Edited By Denis Watkins on 24/12/2016 13:43:13

[Dave Day]

NiMH batteries are really only useful for transmitters.They can be used as receiver packs but treat them with care. If you use them as an electric power pack, be aware that they have a narrow temperature 'window' and they don't work well until they are warm. In the days before Li-Po's we used them for mini helicopters and used to charge them at 1C. Life varied from as little as 4-5 cycles, They suffer from extreme memory effects. DON'T cycle them. In a nutshell, they are unreliable.

[jrman]

Dave, I think you are confusing them with something else!

[Former Member]

[This posting has been removed]

[Peter Christy]

Nearly all my models currently use 4-cell Eneloops (yes, even with Spektrum receivers), and I've never had any issues with them.

I charge them at 1C (1 hour charge) using a peak detect charger. Generally, the charger will cut off after 15 or 20 mins, when the battery is full. Some of my older models are still using sub-C NiCads (around 1900 mAH), which I treat the same way.

However, AA and AAA Eneloops do have a higher internal resistance than any sub-C cell, which means that if you use a lot of high-powered digital servos, the voltage can sag. For this reason, I wouldn't recommend them for a large scale model, with lots of servos. But for ordinary sports models they are fine.

Whilst Eneloops don't suffer from self-discharge, sub-C NiMhs do, though they have a much lower internal resistance, making them more suitable for heavy duty operations. I would recommend charging sub-C NiMhs within 24 hours of use. Don't charge them and then leave them a week!

If your servos will take a higher voltage, I would recommend a 2-cell LiFe pack over NiMhs or LiPos. They don't self-discharge or catch fire!

Be aware that not all servos will tolerate sustained operation at 6V. They may work fine for a while, but will stop very suddenly and unexpectedly! Check that your servos are rated for 6V before going the 5-cell NiMh or 2-cell LiFe route!

Also I recommend using good quality, heavy duty switch harnesses. The JR ones that have a separate charge lead (rather than a socket in the switch) are good, and I believe the equivalent Futaba ones are as well. Using a cheap harness and switch is a false economy!

--

Pete

[Former Member]

[This posting has been removed]

[John Muir]

I wouldn't worry about discharging or cycling nimhs, it isn't needed IMO, but your understanding of the settings seems fine. D=discharge, C=charge and the number sets how many times the charger goes through the cycle. It is handy to run a discharge at , say, 1A to see what you get out of it for reference purposes later. If the battery's measured capacity drops off at some point in the future it might be time to renew it, but these things last for a long time. You would set the discharge cut-off to about 1V per cell, so 5V for a 5 cell pack, if you really felt the need. So your 3800 pack should be alright but keep an eye on the capacity when you recharge it and check the voltage when full.

I charge my nimhs on a charger with delta peak cut-off left at the default setting at a rate of 0.5C, although 1C would be fine as well (1.9A - 3.8A for a 3800 pack). Using a lower charge rate means the delta peak might not be sensed properly and the pack may over-charge, which nimhs really don't like. I use low self discharge Vapex Instants, which are similar to Eneloops, and charge them after a flying session and they are ready to go for the next time out.

A 3800 subC pack is big and heavy for this kind of model but in other respects will work in exactly the same way as a 2000mAh AA pack. You'll never get through more than 600 to 700 mAh in a long day's flying anyway. I don't use Spektrum myself, in fact my i.c. models are still on 35MHz, so can't help you there.

Hope you enjoy flying with an engine up the front for a change.

[Peter Miller]

I use Eneloops on all my models now. Somewith four, some with five servos, the occasional one with six.

They get charged as needed with a standard rate charger, I have two old SM Services chargers that I can set the charge rates on. so they get charged for 14 hours from flat...but they are never flat unless I forget to switch themoff after flying.

I check my batteries the day before flying witha Hitec battery checker and if they are up over 80% I don't bother to top them up as I only have abut 3 flights. Remember that we used to fly all afternoon on a 500 Mah battery and most Eneloops are 1000 MaH

If I have not flown a model for weeks I will check it and top it up even if it is only downto 80%

Very occasionaly one will find an Eneloop that will not hold its charge so dump it.

I know people who do a fast top up in the morning. They don't have battery problems either

[Jonathan M]

Thanks for all the helpful feedback. So, from this, as well as an earlier thread on this subject, plus a dose of advice from a randomly-found Tamiya forum...

Some charge at 0.1 while others at 0.5 or 1.0C, although low C-rates run the risk of the delta-peak not being sensed, leading to overcharging. If in a terrible hurry then up to 1.5C is possible but 2.0C is cruel. A good compromise between charging-time and battery-life is 0.5C.

Delta-peak defaults (for NiMH) seem to be set, depending on actual charger, at 6 or 7mV per cell (roughly half that of than the more tolerant NiCds). The Tamiya boffin uses between 3 and 6mV per cell, with a max of 8mV for older batteries. He says batts are all different and one should experiment, starting from a low setting: if charging stops but the battery isn't warm, then its picked up a false peak and the setting should be increased. Warm is correct, but if the battery gets hot then its overcharging.

I like the idea that cycling isn't something that's terribly necessary with NiMHs, but once a month certainly seems sensible.

By the way, my only rationale for fitting the 260g Sub-C 3800mAh pack below the fuel-tank in my ARTF Acro Wot was because, despite the 70FS up front, it was coming out tail-heavy (like all the ARTF ones seem to!) but now it needs 50g of tail-weight to balance. Yes, fitting the 110g 2000mAh Eneloop would save 150g up front, and I can then lose the 50g from under the stabiliser, but would need to add lead back to the nose (with its much shorter moment) to re-balance. But, for a model that is bang on spec at 6.25lbs (2.83kg), a net 100g saving only represents 3.5%.  I might yet do this, or maybe I'll just move the 3800 back a smidge into the radio bay - where at least I'll be able to monitor its temperature during charging!

Happy Christmas to all!

Mithering Jon 

Edited By Jonathan M on 24/12/2016 19:34:03

[ChrisB]

I use 3300 sub C ( non eneloop) NIMH's either 4 or 5 cell on most of my non leccy models. On models over 7kg I use twin packs and on smaller models I use single packs where I can. If not I use AA packs.

I rarely have any problems with them, although occasionally they do go but I check them every week or so and before I fly. I have a load checker and usually test at 2amps. Usually they show 6v or thereabouts.

I usually charge on standard wall chargers and cycle on a peak detect charger from time to time.

I've had some Eneloops in the past and they've not lasted very long.

[John Emms 1]

If it helps, I ONLY use electric flight cells for my receiver batteries, and that means that even if there is relatively high discharge current, the voltage remains high under load, and the cells are not going to exceed their current limits. My radio will operate down to 3,2V, so I am not really concerned about volts drop, other than I want my servos to operate at a good speed.

Again, for reliability, I slow charge my radio batteries, normally around the 10hr rate, and that normally means overnight before planned flying. For long service life do not leave batteries (other than lead acid) fully charged, or the capacity reduces quite severely over a season. As the life of NiMh is used up through charge and discharge, I have never cycled NiMh, and never seen the need to. NiCd, did occasionally benefit from cycling. The low voltage limit for NiMh is 0,9V / cell, and in long term storage, they normally reduce to around this point.

Hope that helps,

John

[Denis Watkins]

Morning Jonathan, Merry Christmas; as you are eager to learn so I continue, Peter hinted at it earlier, we used to fly all day on 500ma packs and these lasted years albeit they were NiCad but we were using much of the capacity in the pack then charging for this next day. This turnover helped battery life and still does today. You will barely use use 300 ma from your 3300ma pack each day, so you will fly or store the pack slightly down until next time. After 8 or 9 days out your pack will need rechargeing or like me you top up each day. Can you see we are using the pack inefficiently? Although the pack gives us ballast as well as ensuring maximum power all the time. There you have it, you decide, 3300 gives you ballast and extra backup power, but is inefficient use of cells, or you add lead, fit a 4 cell AA 1000ma pack with charge efficiency.

[ChrisB]

I've got a 4 cell 2200 AA pack, (fully charged it shows 5.5v at 1amp) in a precedent funfly with standard servos. I can have 3x 20mins flights and it still shows 4.2v at 1amp .

[Former Member]

[This posting has been removed]

[Peter Miller]

Posted by Percy Verance on 25/12/2016 12:02:22:

How things change. We used to fly all day 40 odd years ago with a 500mah deac battery for the receiver and servos........

We never worried about running out of juice. No on field chargers then either.........

How true

We NEVER had any larger batteries then. and the batteries were not as good as they are now

And if you overcharged a DEAC it went BANG.Came into the clubroom in Germany to find a mates DEAC in two pieces at opposite ends of the room

Edited by Peter Miller on 25/12/2016 12:30:36

Edited By Peter Miller on 25/12/2016 12:33:32

[Dave Day]

Merry Christmas everyone,

I certainly seem to have stirred things up. I am not confusing them with something else and I stand by every word. Try flying seven days a week and flying a battery until it is flat several times a day. Peter, maybe you remember when I told you I could do an outside loop with a helicopter and you nearly needed a change of underwear when I did it. Percy, I didn't say they were unreliable in transmitters.

[Peter Christy]

Merry Christmas, Dave! And yes, I remember nearly having to nearly change my underwear more than once in those early heli days! - And not all down to you, either!

Good to hear from you again, after so many years!

--

Pete

[Former Member]

[This posting has been removed]

[Jonathan M]

Chris, John, Dennis, et al - many thanks!

The last bit of missing info was setting the low-voltage limit for discharge... now I know.

Might yet get a battery-load tester. This one **LINK** loads at 500mA for 4.8v rather than the 1 or 2amps suggested above, but I'm unlikely to ever go larger than a 40 or 60 sized IC. Interesting however to note that my two batts both show a higher remaining capacity when the meter is put directly on the battery rather than via the switch lead!

Learning absorbed re capacity and charging efficiency. I've removed the big 'un (which required butchery to get out) and will fit the smaller Eneloop (in an easy to remove padded sleeve) plus only as much lead as is needed to balance.

[Jonathan M]

Just an update folks after a few months' flying my Acrowot with a Sanyo Eneloop 4.8v 2000mAh:

Having charged it fully quite some time ago, after several sessions (average 3 flights each, 12-15 minutes each), say a total of a couple of hours flying use, the remaining charge is:

0.3A load checker: HIGH (5.1v after load applied)

0.5A load checker: MEDIUM on no load, LOW when subject to load (voltage fig not given by this unit)

Despite the discrepancy between the two battery checkers, this capacity seems too high really. For reasons of efficiency I don't want to just top it up after each relatively short session, but equally I don't want to risk a flight with batteries which I've left to get too low.

Planning the installation of my Gangster 63 (i.e. similar electrical demands), I therefore reckon on fitting the Eneloop 800mAh ( i.e. 40% of the Acrowot's capacity). Fully charged, I should easily then manage a long day's session, with some to spare, and regularly charge up to full each evening afterwards.

Jon

Edited By Jonathan M on 29/04/2017 11:38:41

[Martin Harris - Moderator]

Just be a little cautious over the power delivery capability of a smaller pack. If you are using a lot of servos or digital servos it might struggle. Eneloops and similar low self discharge NiMHs are probably the best NiMHs overall but they do not deliver high currents as well as standard types.

Edited By Martin Harris on 29/04/2017 11:48:23

[Denis Watkins]

PM sent Jon

[cymaz]

I always have 2000 mAh eneloops. And the genuine packs too from a decent source. Overlander, places like that. I have a battery checker that does lipo batteries as well. You never know if you might get some one day.

Edited By cymaz on 29/04/2017 13:14:11