March 2024 Issue - advice on replacing a Nimh RX pack with a Lipo

[leccyflyer]

Have just been reading the March 2024 issue of the magazine and there is a segment in Dave Goodenough's column regarding replacing an expired NiMh receiver pack with a lipo and voltage regulator or UBEC. The stated reason for this is due to the superior very low self discharge rate of the lipo pack, versus the  substantial self discharge rate of the NiMh pack. At no point in the article does it make it clear that the lipo should be removed from the model when not in use and that it really should not be charged in the model either. Furthermore, for that to be relevant it would essentially require that the lipo pack is stored at the fully charged voltage, which is not best practice for the longevity of the pack. These factors make the low self discharge characteristics on the lipo pack largely irrelevant, as the pack should not be stored in the model fully charged in any case.

[Stuart Z]

Leccyflyer

 

Very good point and it does seem unnecessary if the Nimh will do the right voltage.  The capacities available now seem to be more than adequate.  

[GrumpyGnome]

Lots of opinions are quoted as facts nowadays, often with minimal/zero context.

[Zflyer]

I think the LiFe are the better option.

[Peter Jenkins]

1 hour ago, Zflyer said:

I think the LiFe are the better option.

+1 on LiFe.  They can stay in situ for charging.

[John Lee]

And according to Panasonic, makers of the Enerloop NiMh batteries:

 

"We've improved the metal-hydride alloy lattice inside your eneloop battery so that it retains 70%* capacity even after 10 years in storage. The design also resists voltage drop during discharge and minimizes loss of capacity. Once charged, you can trust eneloop to work just like a dry battery, and its long storage life is great in emergencies.
* Capacity based on testing method established by IEC 61951-2 (7.3.2) when stored at 20 °C (based on Panasonic's estimation) and compared with minimum capacity. Varies according to conditions of use."

 

The Enerloop pack I use for setting up models on the bench was bought 14 years ago retains its capacity & gets charged maybe once a year.

 

Whilst I use LiPo's or LiFe's in most of my models quality NiMh's tend to be my 'go to' for slope soarers and are still a viable option in many cases.

[simon barr]

I swapped from NiMhs to lipos as receiver packs simply for practical reasons... It only takes less than an hour to charge, and I would be charging flight packs anyway, so can be out flying (power or glider) with just a couple of hours notice. 

[Phil Green]

Many of mine use a single 18650 Lithium cell, direct without a booster.  Same for transmitters but 2S.
No need to remove and a charge lasts absolutely ages.

The cells are bought as Lidl 'Parkside' drill packs, these are really good cells that I also use on my ebike:

https://mode-zero.uk/viewtopic.php?p=5800

Edited February 16 by Phil Green

[Nigel R]

2 hours ago, simon barr said:

I swapped from NiMhs to lipos as receiver packs simply for practical reasons... It only takes less than an hour to charge, and I would be charging flight packs anyway, so can be out flying (power or glider) with just a couple of hours notice. 

 

Given that the low discharge Nimhs lose so little charge over time anyway, and can be fast charged in an hour from flat, I'm not certain I see a practical advantage of a lipo.

Edited February 16 by Nigel R

[Dave Brooks]

Hi all, 

Just after a little advice on this topic please. I tried using a 2s 1000mah Lipo with a small UBEC as an alternative RX power source, however, I think I made a mistake in that when disconnecting the the battery from the plane I left it attached to the UBEC. After a week or so, when I came to check the battery it was completely drained and not recoverable. This happened on two occasions, ruining two separate lipos (the first time I assumed the battery had failed for other reasons). I assumed that as there was no current draw it wouldn't matter if the UBEC remained attached to the battery but perhaps this isn't the case and the UBEC circuit continues to drain the battery over time. Has anyone else experienced similar? Thanks   

[PeterF]

Even if the UBEC is not drawing any current the device will still keep the output wires at the correct voltage so the voltage control and monitoring circuits will still be drawing current and there will be some leakage current through the power circuit creating a standby current draw. A 6 milliamp current draw over a week will flatten a 1000mAh battery.

[Peter Jenkins]

1 hour ago, Dave Brooks said:

Hi all, 

Just after a little advice on this topic please. I tried using a 2s 1000mah Lipo with a small UBEC as an alternative RX power source, however, I think I made a mistake in that when disconnecting the the battery from the plane I left it attached to the UBEC. After a week or so, when I came to check the battery it was completely drained and not recoverable. This happened on two occasions, ruining two separate lipos (the first time I assumed the battery had failed for other reasons). I assumed that as there was no current draw it wouldn't matter if the UBEC remained attached to the battery but perhaps this isn't the case and the UBEC circuit continues to drain the battery over time. Has anyone else experienced similar? Thanks   

Rather than using a UBEC why not use a Powerbox Digiswitch?  https://nexusmodels.co.uk/products/digiswitch-v2-electronic-switch-user-adjustable-regulated-output-jr-jr-connectors-from-powerbox-pb-6430?pr_prod_strat=pinned&pr_rec_id=143cf3104&pr_rec_pid=8026108199155&pr_ref_pid=8025941049587&pr_seq=uniform

 

You still need to disconnect the LiPo after flying though.   You can select the output voltage to suit.

[Dave Brooks]

Many thanks for the information. Lesson learned. Might also be a comparatively safe way to discharge old lipo batteries prior to disposal I suppose! Thanks also for the digiswitch recommendation. These do look good but at c.£45 a time are probably beyond the reserve of most of my airframes. 

Assistance much appreciated.  

[Manish Chandrayan]

On 15/02/2024 at 17:19, leccyflyer said:

Have just been reading the March 2024 issue of the magazine and there is a segment in Dave Goodenough's column regarding replacing an expired NiMh receiver pack with a lipo and voltage regulator or UBEC. The stated reason for this is due to the superior very low self discharge rate of the lipo pack, versus the  substantial self discharge rate of the NiMh pack. At no point in the article does it make it clear that the lipo should be removed from the model when not in use and that it really should not be charged in the model either. Furthermore, for that to be relevant it would essentially require that the lipo pack is stored at the fully charged voltage, which is not best practice for the longevity of the pack. These factors make the low self discharge characteristics on the lipo pack largely irrelevant, as the pack should not be stored in the model fully charged in any case.

And the point that by introducing the voltage regulator/UBEC one is introducing an additional piece of equipment with multiple individual component that can fail. Had one yesterday at the field. Post recovery of the model it was investigated and found that the voltage regulator had no output. Radio was a Jeti so logs are awaited for a conclusive proof.

  

[Peter Jenkins]

On 16/02/2024 at 11:05, simon barr said:

I swapped from NiMhs to lipos as receiver packs simply for practical reasons... It only takes less than an hour to charge, and I would be charging flight packs anyway, so can be out flying (power or glider) with just a couple of hours notice. 

You also need to make sure that your Rx and servos are capable of taking the full 8.4 v of a 2S LiPo or use a voltage reduction device.  While many Rxs and servos will cope with 8.4 v quite a few do not.