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Why sudden dramatic voltage drop in 700mAh 4.8v NiMh pack?

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Jonathan M29/05/2020 23:20:34
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Was flying my almost new Ahi on the slope today with DreamFlight's own brand 700mAh 4.8v NiMh pack. After a total of an hour's flying on-and-off from full charge initially, had a tea-break and chat with neighbouring fliers. Checking the battery before flying again the meter reading was 4.9v (under 0.3A load) and the TX telemetry display confirmed this. But a couple of minutes into the next and final flight the model suddenly went into an extreme spinning dive - luckily from a moderate height and only into long grass just slightly down the slope about 10m away. When I got to it all the control surfaces were at extreme limits (full right aileron, full left rudder, full down elevator) but twitching with no clear control response. Put the meter on the battery which then read about 3.7v! Back home an hour later the meter read 4.5v, but after less than a minute with the battery reconnected and moving control surfaces gently exactly the same thing happened - and the TX telemetry showing the voltage down to less than 3.0v!

This was my third flying session from new, and the only one where I've gone on for longer than about half an hour's flying time. Have each time charged the battery at 0.2A (same 0.35C as DreamFlight's own USB charger), and although my charger doesn't for some reason trigger the peak-detect (set at 7mV which works for my normal 2000mAh 4.8v packs) I switch it off after the voltage reaches about 5.8v.

The extreme travel and twitching is nothing to do with failsafe - which was set to neutral on all surfaces.

So what's happened? Duff pack? Poor charging technique? Something in the servos sucking hard on the pack? It is a very aerobatic model on high rates, with four 12g 2.4Kg.cm metal-geared servos doing big travels.

Peter Jenkins29/05/2020 23:54:01
1603 forum posts
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Jonathan, my first reaction is that a 700 mah pack might be a bit small if you are flying an aerobatic slope soarer. I use a 750 mah LiPo in a 2 mtr electric aerobatic aircraft and that consumes around 80 mah per 7 min flight. Assuming your flights are consuming a similar amount then after 60 mins of flying you could have consumed almost 700 mah. I think you are expecting too much from that pack. I only fly 3, sometimes 4, 7 min flights and change my pack. It is usually just under half full and generally takes about 170 mah to be fully charged.

Do you check how much charge you put back into your pack after flying? It is essential to be able to use an average consumption rate so you can work out a safe level of usage of your pack. While I use a LiPo and you use a NiMh, both are only capable of delivering their capacity.

I would also get a proper delta peak charger as just using voltage as the cut off may not be allowing your pack to reach full charge.

However, I would either invest in a larger pack or take a spare pack with you and swap it over if you have less than 50% capacity remaining. NiMh packs are relatively cheap and running them to their limit of capacity causes problems - as you have found.

Geoff S30/05/2020 00:49:47
3669 forum posts
26 photos

Peter's absolutely right. NiMh batteries have a very flat discharge curve and so maintain their voltage right up to the point where they don't as you discovered. Measuring battery voltage to estimate the remaining capacity is very inexact because of the flat curve and should never be considered as more than a rough estimate and never pushed to the limit.

Geoff

Denis Watkins30/05/2020 07:15:43
4463 forum posts
113 photos

Jonathan, it is very simple why things have gone wrong for you here.

You have rightly chosen the smallest, lightest pack for the job but an AAA pack will not cut it?

If they made an AA 700mah you would not have had the problem.

AA packs are heavier and larger around 2000mah, but do provide the punch that you require.

To be clear, you must have chosen AAA and you need AA cells

Edited By Denis Watkins on 30/05/2020 07:16:43

Richard Clark 230/05/2020 07:42:32
295 forum posts

1) Battery is too small for safety.

2) Nimh batteries drop voltage very suddenly if near flat and under load but falsely appear to 'recover' if the losd is removed.

3) Memory effect. If only partly discharged when recharged they can drop voltage under load when they reach that previous partly discharged state. Fully discharge them (most chargers can do this, if not use a torch bulb or car brake light bulb) and then rechsrge them

Jonathan M30/05/2020 07:48:48
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737 forum posts
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Pete, Geoff, Denis, Richard - thank you all, understand it better now.

It was the sudden drop-off that I wasn't anticipating, normally much more familiar with LiPo times in my DLG and RES gliders and 2000mAh AA packs in larger slope-soarers and IC models.

Dream-Flight 700 mAh NiMH Battery Specification
Chemistry Nickel Metal Hydride
Number of cells 4
Voltage 4.8V nominal
Capacity 700 mAh
Charge current range 50 mA - 700 mA
Trickle charge 75 mA for 14 hrs
Weight 57 g
Length 57 mm
Width 31 mm
Height 14.5 mm

The Ahi is ridiculously light with little space in the narrow foam nose, so a 2000 would be way too much. I could switch to a 2s LiPo but would then need to voltage regulate for the servos - and there'd still only be so much capacity available for the size and weight. The sensible move would be to simply carry a spare 700.

In the meantime reckon full charge up then cycle down and up again seems the healthy thing to do.

Re my charger, it does have peak-detect, currently set to 7mV which works to cut off the charge on the 2000s, but doesn't seem to work on this 700. I realised this this when I first charged it (at 0.2C), the display read over 700mA put in and still pumping! Maybe need to experiment with altering the peak-detect for this tiddler, but don't know which way to adjust - up or down?

Jon

PS - Richard, when you say "fully discharge" it, how far down to set the charger?  When I cycle the 2000s, I've got the cutoff set to 1.1v/cell.

Edited By Jonathan M on 30/05/2020 07:50:48

Frank Skilbeck30/05/2020 07:50:35
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4736 forum posts
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Gents, there's no way will you get an AA pack in a Dreamflight Ahi.

Getting back to the battery, stopping charging at 5.8v as the peak detect doesn't work might not be fully charging the battery. Can you say charge it at 70 mah and leave it for 10 hours or so, better still can you do a discharge test on the battery pack, say 200 ma and measure how many mah you pull out before the voltage drops to 0.9v per cell. Try a few charge/discharge cycles you may find an improvement in capacity.

If you still have concerns about the battery then you could always try a 700 mah 2s LiFe battery, I've gone over to these on my more compact slope soarers, but your charger has to be able handle LiFe cells and the servos OK on the higher voltage (same as a 5 cell Nimh)

Jonathan M30/05/2020 08:02:26
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737 forum posts
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Frank, my charger can't go lower than 100mA on charge; seven hours at this equals 700mAh. On the discharge cycle I see you suggest a cut-off of 0.9v per cell; I've been doing 1.1v on my NiMhs as I was previously worried about damaging the packs?

LiFe is an option, which my charger can handle, but can I safely do this on servos rated up to 6.0v without extra gubbins?

Trevor Crook30/05/2020 08:08:27
969 forum posts
69 photos

Agree that taking spare batteries to swap after a few flights is the most prudent solution.

Would this do?

https://www.overlander.co.uk/nimh-batteries/panasonic-eneloop-aaa-800mah-4-8v-config-9-nimh-battery.html

Regarding one of the comments above, I didn't think modern NiMh cells had much of a memory effect, is this incorrect?

John Muir30/05/2020 08:23:34
380 forum posts
1 photos

You need to charge at a higher rate of 0.5 - 1C. Any less and the battery will never get warm enough to cause the voltage drop that the charger is trying to detect. The battery should be quite warm to the touch when the charge completes. It may be that you've been repeatedly overcharging your pack which may have damaged it. I'm not sure about the peak detect cut-off. I normally leave that at the default setting. However, an hour's flying on a 700mAh battery probably just flattened it. I'd go with the spare battery idea if it was me.

Steve J30/05/2020 08:31:14
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1913 forum posts
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Posted by Jonathan M on 29/05/2020 23:20:34:

four 12g 2.4Kg.cm metal-geared servos doing big travels.

That strikes me a big load for a AAA 700 mAh. My Prodij (the glider in my avatar) has less powerful servos and I top off the battery after c. 45 minutes flying to keep the battery in the top half of it's discharge curve.

Re charging, I would be charging a 700 mAh at 500 mAh with 5mV peak detection.

Jonathan M30/05/2020 08:48:00
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737 forum posts
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Thx for the suggestion but the Overlander AAA pack would be too long. The easiest solution is just carry a spare 700 exactly the same as existing, for which the cavity in the foam nose is designed to fit.

500mA is 0.7C, so will try that with 5mV peak detect, then discharge to 3.9v and repeat charge to help recondition.

Good idea to not play below the half-way mark! Now programmed a countdown timer into the model memory: say stop after 30 mins active flying, swap to fresh battery if want to continue flying, then measure amps put back in later.

Steve J30/05/2020 09:00:10
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Charge curves from a AAA NiMH datasheet.

aaa charge.jpg

As you can see, the higher the rate, the more pronounced the peak.

Paul Marsh30/05/2020 10:46:46
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4032 forum posts
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4.9v on a 4 cell pack is flat, reason why your model switched off and went in. Also AAA cells don't like high discharge, in fact all NIMH's are not ideal for high load.

Just carry a spare battery as you can't fit a AA pack in.

Ben B30/05/2020 11:19:02
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1427 forum posts
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If one of the cells is bad and being reversed charged by the other ones it could result in that sudden drop potentially. Just a thought.

Steve J30/05/2020 12:11:51
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Posted by Jonathan M on 30/05/2020 08:48:00:

Thx for the suggestion but the Overlander AAA pack would be too long.

It looks like component shop do cells that are the right size (2/3 AA) for the Ahi, 80p each

**LINK**

Jonathan M30/05/2020 13:38:51
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Those 2/3 AA look just the ticket Steve - never soldered up my own packs but can't be that hard really!

In the meantime the totally flat battery charged back up to full in 94mins, putting in 784mA at a new higher charge rate of 0.7C. It got nicely warm towards the end (showed 6.09v at 85mins) then settled to 5.90v initially after the lower peak-detect of 5mV stopped the charging altogether.

The discharge then took 81mins at the same 0.5A as the charge rate, pulling out 676mA until the voltage was 0.9v/cell then settled at 4.88v, i.e. Paul's "4.9v is flat".

The second re-charge has just now taken 90mins, put in 746mA, settled back down at 5.75v.

Off flying later... with countdown timer programmed in! wink

Edited By Jonathan M on 30/05/2020 13:42:04

Jonathan M30/05/2020 21:30:30
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Back earlier from a session on Beacon Hill and just re-charged the 700mAh NiMh: 330mAh or half its total capacity was put back in return for half an hour's very active flying (11mAh per minute, exactly the same as Peter's LiPo above). I'll therefore leave the countdown timer at 30mins, with safe allowance for a bit of over-run. Just ordered another 700mAh of the same, so any given session can safely run for an hour... with lots of breaks and tea from the flask between flights!

Peter Jenkins31/05/2020 00:53:21
1603 forum posts
266 photos

Sounds like you've got it sorted Jonathan. Good luck with your 2 packs in the future. At least you have a good safety margin built in now.

Jonathan M31/05/2020 08:02:06
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Thx Peter...and now I can start working on the 'safety margin' of the Ahi's foam airframe itself!! laugh

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