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Old Sep 11, 2009, 02:57 PM
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Where to buy low discharge (1 to 2C; but cheap) Lithium (or NiHM, NiCd) batteries

Hi,

I am looking for many low discharge (at most 1 to 2C rated) Lithium rechargeable batteries (either pack or cell) to make a battery pack for an electric lawnmower (24V at least 20Ah, but only 30A rate). I remember that some online hobby store used to sell them cheap. Does anyone know where I can buy any of those?

Please let me know where I can find surplus NiHM or NiCd cells for this purpose as well.

Thanks.
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Old Sep 11, 2009, 03:03 PM
KC
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12 NiMh for $0.99 on ebay. 30 min left on the sale. Better hurry.

http://cgi.ebay.com/12-AAA-1000mAH-1...d=p3286.c0.m14

KC
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Old Sep 11, 2009, 03:45 PM
Fast and low...
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http:///www.hobbyking.com

Go for the lowest discharge rate which suits your purpose.
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Old Sep 11, 2009, 04:15 PM
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Quote:
Originally Posted by aa78
http:///www.hobbyking.com

Go for the lowest discharge rate which suits your purpose.
I was hoping to find some cells rated for only 1 to 2C that are even cheaper than the ones at hobbyking. I am assuming that the cost of making a 2C cell and a 20C cell are very different; but that could be wrong.
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Old Sep 11, 2009, 04:55 PM
Fast and low...
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Quote:
Originally Posted by smau1
I was hoping to find some cells rated for only 1 to 2C that are even cheaper than the ones at hobbyking. I am assuming that the cost of making a 2C cell and a 20C cell are very different; but that could be wrong.
The lowest is 15C at hobbyking. Have a look at their Tx/Rx lipo packs which are even lower C rated.
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Old Sep 11, 2009, 05:52 PM
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following link is cheapest NiMH (sans the shipping) I've ever seen

http://www.hobbycity.com/hobbycity/s...idproduct=9907

to get 20Ah 24V (around 27.8 to 20V), you'd need 180 of these (20S9P) which will cost around $143 + shipping (6.3 Kg).

Following is the cheapest LiPo I've seen

http://www.hobbycity.com/hobbycity/s..._20C_Lipo_Pack

To get 20Ah 24V (25.2 - 19.8V) you'd need 1S4P of these. that should cost around $240 + shipping (3.6 Kg).

there is just one more option I can see:

http://www.hobbycity.com/hobbycity/s...idProduct=8930

this is cheaper in bulk buy, and gives 36 pieces.
connect 3S2200mAh in 2S9P you'd need 18 of these packs, so you can make a spare one too. Voltage same as 5Ah LiPo above.
$540 + shipping(8.5 Kg)

There may be other alternatives.

If I were you, I'd try to fing some Lead Acids, a trolley and some heavy gauge cables. I think they'd be cheaper (but much heavier) than all the above. Consult your car spares shops.

Either which way, you'd also need to charge those and thats going to take time. Plan about that too.

BTW, I am NOT linked with HK in any way. just giving out what I know.
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Old Sep 11, 2009, 07:11 PM
ProgressiveRC
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Seattle, WA
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I'd look at A123 or larger LiFe cells. Check all-battery.com, they have cheap/low c rated stuff like that.

- David
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Old Sep 11, 2009, 07:47 PM
Boffin
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Victoria, BC, Canada
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Quote:
Originally Posted by smau1
Hi,

I am looking for many low discharge (at most 1 to 2C rated) Lithium rechargeable batteries (either pack or cell) to make a battery pack for an electric lawnmower (24V at least 20Ah, but only 30A rate). I remember that some online hobby store used to sell them cheap. Does anyone know where I can buy any of those?

Please let me know where I can find surplus NiHM or NiCd cells for this purpose as well.

Thanks.
I think you'll find that if you want cheap, you need to go back to the original lead acid batteries. You can get 2 17Ah 12V for around $100. I looked around for NiMH bargains last year for my lawnmower and couldn't get close to the price of Pb. If you were considering 4000 mAh sub-C, you need a 20S5P pack or 100 cells in total. You could get by with 16S.

Rick.
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Old Sep 11, 2009, 09:37 PM
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Rick---Sealed lead acid (SLA) is the cheapest for this application, but I am not happy with its performance. It died after one season. This is also what many other owners' experiences (even when some were using deep cycle), based on my online search. SLA is also a harder to maintain due to its weight and size (harder to move it to the basement during winter, etc).

The NiMH from HobbyKing found by supergps is approaching the price of SLA, but shipping is high, well, twice that of SLA due to the weight of SLA.

By the way, a good way to "normalize" the prices of batteries for comparison is to use their dollar per unit volt-amp-hr because price is roughly proportional to volt (get twice voltage by two packs in series) and charge capacity (get twice capacity by two packs in parallel) for a given product line (i.e., for a given brand, C-rating and product series). One execption is that it will be higher for the most popular configuration like 11.1 V 2.2 Ah LiPo. Note that VAh is Watt-hour (Wh), so this figure is really $/energy-storage, which should be a constant for a product line.

The NiHM from HobbyKing is priced at $0.29/Wh. A deep cycle lead acid at 12V 22Ah costs around $50 and is therefore priced at around $0.2/Wh. The HobbyKing 6S LiPo listed above is priced at $0.54/Wh. Brand name LiPo's are usually more than $1.3/Wh.

This exercise doesn't provide more information, but it does make comparison much easier. You can easily see the premium for higher rate if you compare the $/Wh between product lines of the same brand.

Thanks for all your advices.
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Old Sep 12, 2009, 12:50 AM
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Doesn't look like your price point will be met with Lipos especially if you want to buy a ton of 1 or 2C bats.

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Old Sep 12, 2009, 03:17 AM
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Buy a plug in lawnmower and then you need NO battery. All the rechargeable mowers are just toys and environmental disasters that have been greenwashed.

Better yet, buy a good Honda gas mower if you really don't want a cord.

If you want to be green buy a used mower.

P.S. If you really do want a large format (20Ah 24V) Lipo, you should look at the e-Bike battery suppliers. E.g. http://www.pingbattery.com/servlet/StoreFront ... although because of safety, they all tend to be LiFePO4's now. They're pricey though and will make your rethink using an SLA.
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Old Sep 12, 2009, 07:45 AM
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Or maybe a 30A 24V power supply. That would last longer than any battery!!
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Old Sep 12, 2009, 02:21 PM
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groundfx---What safety concerns led large-format batteries to tend toward LiFeO4?

supergps---Yes, I am considering power supply as well.

I am also trying to get help from some friends working in the IT department. They probably have to-be-discarded laptop batteries and power supplies that I can take advantage of.
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Old Sep 12, 2009, 04:12 PM
Boffin
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Quote:
Originally Posted by smau1
Rick---Sealed lead acid (SLA) is the cheapest for this application, but I am not happy with its performance. It died after one season.
My first battery died within a year as well, mainly because the kids ran it down until it wouldn't move. I only use about half the charge before recharging now and my current Pb batteries haven't lost any capacity after 2 seasons. I rewired them so I can charge and test each 12v separately on occassion with a more capable charger.

I haven't had good success with cheap C and D NiMH. Most test out at less than half the rated capacity at anything more than a C discharge and they don't match very well for making packs.

My yard is too large and has too many trees and flower beds for a corded electric. The rechargeable is a good compromise but its heavier and more cumbersome than an equivalent gas mower. Still, its my small contribution to air quality.

Rick.
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Old Sep 12, 2009, 05:54 PM
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Quote:
Originally Posted by smau1
groundfx---What safety concerns led large-format batteries to tend toward LiFePO4?
Venting with flames on a 2100mah 12V Lipo is one thing, venting with flames on a 20Ah 48V Lipo would be a completely different event.

LiFePO4 don't have this worse case problem... much better.
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