Page 1 of 2

Precharged NimH batteries

PostPosted: Tue May 11, 2010 4:29 am
by JMP_blackfoot
okay, someone has to start :P

I have fitted all* my transmitters with low self-discharge batteries, commonly sold as "precharged".
My first experience with them was when I devised the "4-cell mod" for the FlySky transmitters.
I find similar results with Sanyo Eneloop, Kodak precharged, and Duracell precharged. The transmitters now have over 10 hours operating time on a single charge, they all do retain charge for weeks if not months, and I do not need to recharge the night before flying as before.
On the receiver side, I have only used the Sanyo Eneloop, either 2100mAh or 800mAh, and am quite happy with them.
My Canon S3-IS and my wife's A720-IS are also very happy with the Eneloop and Kodak 2100's.

*On last count, this involves 12 transmitters, from Ace Pulse Commanders and Controlaire Galloping Ghost (remember these?) to Futaba 6EX 2.4GHz and Supertef

Re: Precharged NimH batteries

PostPosted: Tue May 11, 2010 4:52 am
by RCModelReviews
Yes, the low-self-discharge (LSD) NiMH technology is good but I'm unsure as to how it stacks up from the perspective of the cell's internal resistance.

I may grab some and do a few tests.

I know that the older hi-capacity NiMH cells (> 1850mAH AA cells) were *awful* and they sagged badly when any real current draw was attempted. This made them quite bad for receiver packs if you were using anything other than a few standard servos.

In fact, the Sanyo 1650mAH AA cells would give much longer flight-times when used in a receiver pack and also provided better torque and less voltage drop under load.

Still, things are changing quickly in the battery world. I'd like to see more in the way of LiFePO4 form-factors and capacities, they (right now) are the ultimate technology for RC use.

Re: Precharged NimH batteries

PostPosted: Fri May 14, 2010 4:20 am
by Roger
Bruce the consumer magazine did a cell review of these a little while back, the Sanyo cells came back on par with the top cell which was the low self discharge cells put out by Varta. Not sure exactly what their testing criteria was, but they all faired better than the standard cells with regard to "high drain" devices... Now you know as well as me, that doesn't mean a heck of a lot as each cell has its own unique characteristics. Given today's battery technology, I'm not sure how many people are still using NiMH or NiCads as their main model power batteries - as Lipos have taken over in quite a way... They do still have their place in other applications though, plus the fact that they don't have the tendancy to explode on the odd occasion...!

Re: Precharged NimH batteries

PostPosted: Fri May 14, 2010 5:06 am
by RCModelReviews
Roger wrote:plus the fact that they don't have the tendancy to explode on the odd occasion...!

That's another advantage of LiFePO4 (A123) cells.

I'm surprised that nobody's made an LiFePO4 cell in a "double-length AA" size. That would enable a single double-length cell to replace two AA NiMH/Zinc-carbon/alkaline cells and deliver the same voltage. Would be great for all those AA-powered devices that require us to stick with older technologies.

Re: Precharged NimH batteries

PostPosted: Wed May 19, 2010 4:04 pm
by hillflyer

good to see the new forum board up and running. Even my DNS server has finally caught on...

I am testing a bunch of Low Self Discharge cells for their suitability in RX applications.
I have tested the HobbyKing AAA ones as well as Eneloop AAA.
My testing involves my homebrew constant current sink (up to 100W)
I switch it between 0.5 Amps and 1.5 Amps at 100Hz and measure the voltage difference across the battery terminals.
Here the intersting bit:
HobbyKing AAA 45 milli Ohms Ri (not 115 milli Ohms as stated before!)
Eneloop 800 AAA around 40-42 milli Ohms Ri
Eneloop AA 30-35 milli Ohms Ri
GP Recyco 2100 AA 35 milli Ohms
'normal' GP 2500 AA 170 milli Ohms!(little used, 2years old)

I have amended the above values, because of a testing error with the HobbyKing batteries, they are far better than I previously measured!

I will try the AAA Eneloops in a flying wing with 2 digital 2,5Kgservos in the wing.

Re: Precharged NimH batteries

PostPosted: Wed May 19, 2010 9:11 pm
by RCModelReviews
Thanks for the information -- the AA Eneloops seem quite good. What state of charge were they in when you performed your tests?

Can you test them once they've been discharged to their nominal (1.25V/cell) voltage rather than full-charged?

And how do they handle a constant load of (say) 1.5A over a period of (say) 10 seconds. I've seen some NiMH cells that appear to have a low internal resistance short transient loads but seem to show a marked drop when the load is sustained over a few seconds (as might be the case with a faulty servo).

Re: Precharged NimH batteries

PostPosted: Thu May 20, 2010 6:17 am
by hillflyer
Hi Bruce,
have a look at my amended infos above.
The tests were done with fully charged batteries, but with the HK AAA's the charging took place 2 weeks ago.
My current sink is still under construction, I will do your suggested tests in the next few days.

Re: Precharged NimH batteries

PostPosted: Thu May 20, 2010 6:30 am
by RCModelReviews
Generally I avoid AAA and AA packs like the plague (for receiver use). The best I've found are the Sanyo 1650mAH and all the others I've tried with more capacity than that have been too soft for my liking.

The 1500mAH 2/3A cells have a much lower internal resistance but now I almost exclusively use 1100mAH A123s on my glow-powered models and the 2300mAH A123s on bigger planes. Now *that's* low internal resistance! :-)

Re: Precharged NimH batteries

PostPosted: Thu May 20, 2010 9:51 am
by hillflyer
I agree with you about the dangers of using the standard NiMH AA's or worse AAA's. Combined with cold weather it spells disaster.
My problem is space. I am building the Moth, a fast 4 ft flying wing for slope soaring.
I can fit 4 AAA's or alternatively two 400mAH LiPo cells with an external switching or linear regulator.
I need to run the load test on the LiPo setup too, so I can compare it to the NiMH results.

Re: Precharged NimH batteries

PostPosted: Fri Oct 15, 2010 5:05 am
by disco stu
Is someone able to put those figures from hillflyer in context for me? Still fairly new to the world of batteries on this scale.

Been using the hobby kings LSD batteries a fair bit lately, mostly due to price. Just replacing everything around the house with these, same price as buying alkalines and they do seem low self discharge (from memory they lost 10mv over about 3 months from brand new, as new charge state)

So far just used them for Tx packs (still don't like the idea of lipo in Tx) and Rx packs in slope gliders.

Great site and reviews to Mr RCmodelreviews. Bit thumbs up there