[Elecraft] Internal LiFePo4 for KX3

[Henshaw, Andy]

" AA LiFePo4 cells appear to have a rating of about 600 mA-hours."

I've noticed this also, which seems odd, as my understanding is that LiFePO4 has (at least) comparable energy density as NiMH.  However, I did run across these cells:

http://www.aliexpress.com/store/product/High-Capacity-LITELONG-AA-1200mah-14500-3-2v-lifepo4-Rechargeable-Battery-Consumer-Battery-Free-Shipping-4pcs/816574_1302272496.html

which would be better than the NiMH solutions that I've found.

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Wayne Burdick
Sent: Wednesday, February 11, 2015 5:52 PM
To: Jim Wilkie
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Internal LiFePo4 for KX3

The charger could in theory be redesigned to work with a 12.8-V LiFePo4 pack (4 cells), but there are several disadvantages to this:

- AA LiFePo4 cells appear to have a rating of about 600 mA-hours. As a coarse metric, 4 cells at 13 V would provide about 7.7 watt-hours, which is very poor compared to 8 NiMH cells; the NiMH pack rovides roughly 2500 mA hours at 10 V, or 25 watt-hours. There's room for 8 cells, so NiMH makes better use of the existing space.

- Even though you'd get 13 V from LiFePo4 cells, and could thus run full power, you'd only be able to operate for a third as much time (roughly). The KX3 can put out as much as 5 W from 10 V, and since this is only 3 dB down from full power, it's probably a better trade off for a weekend of casual operation. Your antenna and propagation will generally outweigh this 3 dB power factor.

- Being able to use just about any type of AA cells in a pinch is important for ad-hoc field use of the radio. You can then borrow AA cells from other devices in an emergency. So if you used 4 LiFePo4 cells, you'd want to leave the other socket in place and insert some sort of placeholder cells (easy to lose) or a bypass switch.

- LiFePo4 cells are not as readily available. You can get NiMH cells a drug stores, supermarkets, Radio Shacks [before they close], etc.

Wayne
N6KR



On Feb 11, 2015, at 1:03 PM, Jim Wilkie <santini at verizon.net> wrote:

> Andt, will wait for the experts to answer, but I think the nimh charger vs the lopo charger would be a problem.
> 
> Jim Wilkie WY4R
> 
> 
> On 2/11/2015 2:18 PM, Henshaw, Andy wrote:
>> Has anyone looked into hacking a replacement battery case for the KX3 that featured LiFePo4 cells?  I've seen external LiFePo4 packs being used, but it seems like an internal solution might be workable.  With a 13.2V supply (four cells in series), one can run the KX3 in full-power mode.  In addition, the discharge curve of LiFePo4 is much better than NiMH.
>> 
>> I'm asking, because I'd like to take my KX3 on some backpacking trips and it just seems wasteful (of space) and slightly awkward (for setup) to have to carry an external battery pack in order to get full-power.
>> 
>> It appears that AA size LiFePo4 cells are available.  So, wouldn't it be feasible to arrange them in a 4S2P configuration that could replace the existing battery holder?
>> 
>> Andrew Henshaw
>> 
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