-----Original Message-----
From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Jim
Brown
Sent: Thursday, June 30, 2016 3:29 PM
To: Reflector Elecraft <elecraft@mailman.qth.net>
Subject: [Elecraft] Choosing A Battery For Ham Use
Hi Clay,
Because this is of general interest, I'm replying to the list.
First, LiFePO4 batteries are pretty much ideal for ham applications because
of their relatively flat discharge curve, and because their Ah vs weight
ratio is a good compromise. For example, an LiFePO4 pack will be above 12.5
for about 80% of capacity and 12V for 90% of capacity, while a lead-acid
battery will drop below 12V before it reaches 50% of capacity.
Second, LiFePO4 batteries provide a LOT more charge-discharge cycles if
treated properly. The result is that if you're going to use them for a lot
of years, the longer life more than compensates for their higher cost.
Third, LiFePO4 batteries are MUCH safer than Li-ion batteries.
Disadvantages of LiFePO4 are initial cost and the fact that they need a
special charger. However -- Bioenne told me that they can be safely charged
using the West Mountain SuperPWR Gate set for AGM batteries and a power
supply that is adjusted to about 14.5 volts. The AGM setting of the PwrGate
limits the charging voltage to 14.2V, which is what LiFePO4 batteries need.
Also, LiFePO4 batteries do NOT like to be discharged below about 95%, so
care must be taken not to fully discharge them. Good battery packs will have
control circuity that prevents this.
Now, as to how much battery to buy. Start by studying current draw of the
radio(s) and other equipment that you'll use with the battery, taking TX/RX
duty cycle into account. Also look at weight if you're going to carry it. I
bought a 20Ah pack (5.5#) to loan to a friend who was going to pack several
miles uphill to activate a rare 6M grid with a
KX3 and the 100W amp, and I just bought a 100Ah pack (26#) to run my SO2R
shack. If I were going to pack with a KX2 or KX3, I'd buy something much
smaller, like 6 - 12 Ah (2 - 4#), or even smaller. Your application
carrying a K3/P3 around to chase RFI pulls about 1.6A, (1.8A with the SVGA
module in the P3). A KX3 plus PX3 pulls about 350 mA at max screen
brightness.
Bioenne (and other vendors) package their LiFePO4 batteries more than one
way for the same Ah capacity, often to retrofit into existing gear.
With Bioenne batteries, I chose the PVC pack, which is lighter weight than
the rectangular "solid" format.
Bioenne and other battery vendors do NOT say that their chargers are
RF-quiet, which is why you would use a PowerGate and known clean supply.
To charge from solar, buy a Genasun charge regulator, which IS pretty
RF-quiet. I've told Bioenne that they need to find quiet chargers.
We'll see how they respond.
As to voltages -- I would ONLY buy 12V nominal to power ham gear. If you
need other voltages for other gear, look at http://www.batteryspace.com
which carries a MUCH broader range of batteries. They're also good people,
located in the SF Bay area. Bioenne is in Santa Ana, CA.
I've chosen to avoid voltage boost products, which are essentially SMPS, and
noisy. Yes, you can set them to be active only on TX, but if you're running
two radios, the one you aren't TXing on will hear the noise.
As to charging -- LiFePO4 batteries will last a lot longer if they are not
fast-charged. A good rule of thumb is their 4 hour or 10 hour discharge
current. In other words, for a 20Ah battery, avoid more than about 5A charge
current.
If you're sizing the battery to power your shack and will be float charging
it, the charge current can be added to the capacity to figure how much
battery you need. In my application, with worst case of SO2R contesting at
100W, I'll be TX on one radio or the other almost all the time, so I'm
looking at roughly 12A worst case. If I wasn't doing SO2R, I could get by
with a smaller battery.
For non-critical applications like video monitors, router, cable modem, etc,
I'm using el-cheapo lead acid batteries from my hamfest stash, and floating
them from suitably sized linear wall warts. I've found that for most gear,
voltage is not all that critical. For example, my Samsung computer monitors
are sold with a 14VDC wall wart, but were still running fine when my
lead-acid battery had dropped to 10.5 volts.
I've looked around a lot, and so far have not found a better practical
battery chemistry than LiFePO4.
Another important point. For running electronics of all types, we do NOT
want automotive batteries, which are primarily designed to provide a big
hunk of current to start the engine, but which don't like to be deeply
discharged. Instead, we want deep discharge types. Pay attention to this
when selecting a battery.
73, Jim K9YC
On Thu,6/30/2016 12:07 PM, Clay Autery wrote:
Jim,
Looking to get LFP batts for my shack/house/mobile.
Could you recommend sizing guidance? Is it as simple as multiplying
the voltage x amps to get watts and then dividing by 12VDC to get a
12VDC current draw, and then spec'ing a batt based on Ah, etc etc...
calculate run time to 80% discharge (they claim up to 90%).
Or do I need to spec for some % overhead. I usually size stuff for
50% overhead, so that if the device is running flat out it won't push
the supply beyond 50% max continuous. Does that apply here?
Suggestions on how to get the required voltages I need:
5VDC - Probably won't need for now... Can get most of the supply I
need from the laptop docking station via multiple powered USB sockets.
12 VDC - easy (and run the 18.8 VDC K3s and P3 from 12V if I have to?)
13.8 VDC - For radio if possible to do it quietly. (12 VDC w/ boost
circuit or 24 VDC with buck circuit... Boost seems more efficient,
but see below)
19.5 VDC - Laptop (and USB 5VDC) Boost or buck...
Any ideas would be greatly appreciated...
At a minimum, I want to get at least one battery to run my K3s/P3 to
hunt RFI with the house service off and then on one branch at a time.
At some point want to put batteries on all HAM, Broadband, Router, and
switches...
Thanks...
______________________
Clay Autery, KY5G
MONTAC Enterprises
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