Precharged NimH batteries

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Re: Precharged NimH batteries

Postby RCModelReviews » Fri Oct 15, 2010 9:14 am

The internal resistance is a measure of how well the cells can deliver current.

The lower the internal resistance, the more current you can draw from the cell without losing voltage.

Most NiMH cells are relatively high internal resistance when compared to other types, such as lithium packs of an equivalent capacity.

That's why I use A123 (LiFePO4) packs for my receivers/servos. They have a very low internal resistance and even the 1100mAH packs can deliver over 30A continuous and 50A for a few seconds. There's no way you could get that much current out of a 1100mAH NiMH pack.

Why is that important for a receiver pack?

Well although most of the time a receiver and servos are only going to be drawing a tenth of an amp or two, there are times (such as when you command several servos to move at once) that the instantaneous current draw can surge to several amps or more. This is because when the servo first starts to move -- the inertia of the motor, gears pushrods and control surfaces all work to place a very heavy load on it -- almost as if it were stalled.

If your receiver pack has a high internal resistance then the sudden load of several servos staring up simultaneously can cause the voltage to drop so low as to cause problems with the receiver (especially if you're running a 4-cell pack with a DSM2 system).
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Re: Precharged NimH batteries

Postby disco stu » Fri Oct 15, 2010 10:35 am

Thanks for that.

I was hoping to find out what the 3-5m Ohm difference between the eneloop and the hobbyking LSD would mean (if answering, feel free to get technical and educate me on this at the same time if willing. Hoping this doesn't sound pushy :? ). I know people put a lot of faith in the eneloops for highish current situations, packs in planes with higher drawing servos etc, and don't trust anything non eneloop basically, so it would be good to know the real comparison. Was stoked when I saw this review, first I've seen

Would love to try A123, just price putting me off at the moment. Pity you can't get them as cheap as lipo, wouldn't consider lipo in that case for what I do

Cheers, Stu
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Re: Precharged NimH batteries

Postby RCModelReviews » Fri Oct 15, 2010 8:17 pm

The 3-5mOhm difference won't be that important for low-current applications.

As for the price of A123 packs, you can buy single 1100mAH A123 cells from HK for under $8 which means you can build your own 2S 1100mAH pack for under $20 (including connector and lead).

That's pretty reasonable -- probably less than a genuine Enerloop 5-cell pack actually.

And don't be put off by the 1100mAH capacity. You can charge the A123 packs at 4C so they can be charged from empty to 90% in under 15 minutes. What's more, you can get a genuine 1100mA out of them, even under high loads. Once you start drawing high currents from an NiMH pack, the rated capacity (usually measured at 1/10th C) falls well below the claimed figures so a 2000mAH pack may only be able to deliver 2/3 that amount.
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Re: Precharged NimH batteries

Postby disco stu » Sat Oct 16, 2010 12:04 am

Definitely less than any eneloop pack, for sure. Cheapest I have seen genuines is $20 for 4, add connector and what not and that price jumps a lot.

Didn't realise they handled 4c charge, thats really handy actually. Will go and have a look for them again.

Do you drop down the voltage somehow, or just run the 6.6v (or more of a full charge of course) straight into standard reciever?
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Re: Precharged NimH batteries

Postby disco stu » Sat Oct 16, 2010 5:10 am

Are you pilfering cells out of the hobbyking LiFePO4 packs or do they have single cells at $8 that I just can't find on there??

Edit: Don't worry, figured it out. Add the single cells to cart and the price miraculously changes to $7.95!! Thats more in my price range, pity as I just put an order in. Will get a few to try next order. Any validity in the claims of dodgy cells some people put in the feedback?
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Re: Precharged NimH batteries

Postby RCModelReviews » Sat Oct 16, 2010 8:04 pm

I use 2S A123 packs in all my models without regulation and never had any problems at all.

I've also had no issues with dud cells either -- the ones from HK come in plastic bags with an "A123-Racing" label on the bag so I'm pretty sure they're kosher.

If anyone's worried about the higher voltage, they can simply put a single suitably rated silicon power-diode in series with the pack (although you'll need a separate charge-lead then) and that'll bring the nominal voltage down to exactly 6V.

I have used the generic LiFePO4 cells from China but they have a much higher internal resistance -- still fine for smaller models but given that they're still $5 per cell, the genuine A123s are just "better value".
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Re: Precharged NimH batteries

Postby disco stu » Sat Oct 16, 2010 11:47 pm

Thanks for that, will pick some up next order (if they are in stock :roll: something I need is always out).
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