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Internal Impedance of Li-Ion Battery Packs

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Jim Brown-10

Internal Impedance of Li-Ion Battery Packs

I'm in the planning stages for a backpacking operation using a KX3 and
KXPA amplifier. The application will be 6M JT65 from a rare grid. A key
element of that is deciding what battery pack to buy. Because weight is
a primary consideration, Li Ion is the clear choice, but the decision is
between 3-cell and 4-cell packs.

Real data on LiIon battery packs is damn near impossible to find. I've
found discharge curves that make sense, but it isn't clear whether the
voltages shown are open circuit or under load values. The difference, of
course, is the internal resistance, and that's what I'm trying to get a
handle on.

I'm considering batteries in the 20-25 Ah range, with the Ah rating
based on 10A. So my question is, what sort of internal resistance should
I expect for a battery rated like this? Yes, I know if I had the battery
and a sufficiently accurate voltmeter I could measure it, but these
batteries sell in the $250 - $300 range, so I would like to know before
I buy.

Here's a pack that I'm considering.

http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh14arateli-ionwithcarsocketsmartcharger.aspx

A 11.1V (3-cell) battery is clearly the right choice to run the KX3, but
it's the KXPA100 (100W amp) that I'm concerned about. I suspect that it
isn't going to be happy much below about 10.5V. That's OK if the
published discharge curves for Li Ion batteries represent the terminal
voltage under load -- they show about 90% of capacity with the rated
discharge current, but if that voltage is reduced by the internal
resistance, I've probably got at least 10% less.

The link below shows the discharge curves for a single cell, followed by
my own plots of the same data multiplied by 3X and 4X for 3-cell and
4-cell packs. The KX3 and KXPA100 are rated for supply voltage up to
15V, and I've been told that 16V is an absolute maximum. That means the
output of a 4-cell pack is too much, so I think it has to be 3 cells.

k9yc.com/Lithium Ion Discharge.pdf

Can anyone point me to some real data or relate some experience on this?

73, Jim K9YC
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Greg Troxel-2

Re: Internal Impedance of Li-Ion Battery Packs

Jim Brown <[hidden email]> writes:

> I'm considering batteries in the 20-25 Ah range, with the Ah rating
> based on 10A. So my question is, what sort of internal resistance
> should I expect for a battery rated like this? Yes, I know if I had
> the battery and a sufficiently accurate voltmeter I could measure it,
> but these batteries sell in the $250 - $300 range, so I would like to
> know before I buy.
>
> Here's a pack that I'm considering.
>
> http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh14arateli-ionwithcarsocketsmartcharger.aspx
>
> A 11.1V (3-cell) battery is clearly the right choice to run the KX3,
> but it's the KXPA100 (100W amp) that I'm concerned about. I suspect
> that it isn't going to be happy much below about 10.5V. That's OK if
> the published discharge curves for Li Ion batteries represent the
> terminal voltage under load -- they show about 90% of capacity with
> the rated discharge current, but if that voltage is reduced by the
> internal resistance, I've probably got at least 10% less.
>
> The link below shows the discharge curves for a single cell, followed
> by my own plots of the same data multiplied by 3X and 4X for 3-cell
> and 4-cell packs. The KX3 and KXPA100 are rated for supply voltage up
> to 15V, and I've been told that 16V is an absolute maximum. That means
> the output of a 4-cell pack is too much, so I think it has to be 3
> cells.
>
> k9yc.com/Lithium Ion Discharge.pdf

In my experience, every plot labeled "discharge" has been the terminal
voltage under a specified load, and they are often given as a family.
For lead acid, it's typically at C/20h and then higher rates. A plot
showing open-circuit voltage after some amount of usage should be
labeled "stage of charge" rather than "discharge".

In the case of the plots in your PDF, the first plot shows three lines,
with 0.2C, 0.5C and 1.0C. This is sloppy, because C is in Ah, and 1.0C
therefore has units of Ah, which makes no sense. But surely that means
C/1h and is in amps. 0.2C is C/5h or 0.2 1/h * C. And I see this all
the time; the RC people know what it means :-)

It's interesting that the curves are so close, for 0.2C and 1.0C. As I
read it, there's only a few % (from 100 to 95 at 3.2V cutoff) loss. For
lead acid, there would be a far greater difference at these rates.

For JT65, what power output are you running, and what input voltage is
needed for the KXPA100 to be happy? I would think you'd be down around
50W for linearity and heat, but I don't yet have any JT65 experience.

It's unfortunate that equipment designed around lead acid batteries (our
nominal 13.8V notion) wants the first half of a 3S pack and the second
half of a 4S.

I don't think this is useful, but you could consider different batteries
for the kx3 and kxpa100. But with the same max, the kx3 should be fine
with what the amp needs.

I find "25 Ah at 10A" to be unusual, but I think it's the difference
between lead acid standby/cycle use, where capacities are ~always given
in 20h rate, and LiPoly RC culture, where the 1h rate is a slow
discharge. I wonder if 10A is a standard discharge current for
multiple batteries - it's a fair metric if the question is "how long can
I power a 10A load".

Have you thought about DC-DC converters? I am about to order an
AnyVolt3, which isn't big enough (3A) to power a KXPA100. But a
converter could dodge the max/min issue, and may be worth it in
weight/price, presuming it's quiet (which some are said to be).

73 de n1dam

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WB4SON

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Jim Brown-10

I wouldn't mess with Li-Ion. The voltage from the pack is either too low
(3-cell) or too high (4-cell), and they are quite dangerous if physically
abused.

Instead I would go with 4-cell LiFePO4, another form of Lithium battery.
The iron stabilizes the lithium, and at no part of the discharge or charge
process is there any free metallic Lithium in the cell (that's what makes
traditional Li-ion so dangerous - Lithium burns violently when exposed to
water, even moisture in the air). The energy density of LiFePO4 is almost
as good as a traditional Li-Ion.

Check out the LiFePO4 batteries offered by BioennoPower.com

I have plenty of information and discharge curves for various LiFePO4
batteries on my website, www.wb4son.com For example, check out a
discussion of a 30 AH battery here: http://wb4son.com/wpblog/?p=1250

As for your specific question, you can expect almost any LiFePO4 batter to
remain above 13 volts under load for about 80% of its capacity. It will
start about 14.3 volts (open circuit), and won't drop below 13.0 volts for
a long time.

You will need to figure out your desired capacity, and also make sure you
select a battery that will support the maximum current you need.

I've run IOTA operations for a weekend (30 AH), and ran our GOTA station
for Field Day off of LiFePO4 batteries (60 AH). Plus I've been using them
and keeping track of there performance for almost 4 years now.

73, Bob, WB4SON

On Mon, Aug 4, 2014 at 2:01 AM, Jim Brown <[hidden email]> wrote:

> I'm in the planning stages for a backpacking operation using a KX3 and
> KXPA amplifier. The application will be 6M JT65 from a rare grid. A key
> element of that is deciding what battery pack to buy. Because weight is a
> primary consideration, Li Ion is the clear choice, but the decision is
> between 3-cell and 4-cell packs.
>
> Real data on LiIon battery packs is damn near impossible to find. I've
> found discharge curves that make sense, but it isn't clear whether the
> voltages shown are open circuit or under load values. The difference, of
> course, is the internal resistance, and that's what I'm trying to get a
> handle on.
>
> I'm considering batteries in the 20-25 Ah range, with the Ah rating based
> on 10A. So my question is, what sort of internal resistance should I expect
> for a battery rated like this? Yes, I know if I had the battery and a
> sufficiently accurate voltmeter I could measure it, but these batteries
> sell in the $250 - $300 range, so I would like to know before I buy.
>
> Here's a pack that I'm considering.
>
> http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh1
> 4arateli-ionwithcarsocketsmartcharger.aspx
>
> A 11.1V (3-cell) battery is clearly the right choice to run the KX3, but
> it's the KXPA100 (100W amp) that I'm concerned about. I suspect that it
> isn't going to be happy much below about 10.5V. That's OK if the published
> discharge curves for Li Ion batteries represent the terminal voltage under
> load -- they show about 90% of capacity with the rated discharge current,
> but if that voltage is reduced by the internal resistance, I've probably
> got at least 10% less.
>
> The link below shows the discharge curves for a single cell, followed by
> my own plots of the same data multiplied by 3X and 4X for 3-cell and 4-cell
> packs. The KX3 and KXPA100 are rated for supply voltage up to 15V, and I've
> been told that 16V is an absolute maximum. That means the output of a
> 4-cell pack is too much, so I think it has to be 3 cells.
>
> k9yc.com/Lithium Ion Discharge.pdf
>
> Can anyone point me to some real data or relate some experience on this?
>
> 73, Jim K9YC
> ______________________________________________________________
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>

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73, Bob, WB4SON

Bob N3MNT

Re: Internal Impedance of Li-Ion Battery Packs

Another KEY consideration, especially since you indicate it is a rare DX location ( I read as air travel required) is what you will be allowed to take on the plane. Even if the battery meets your electrical and weight requirements it will be of little value if you can't get it There. I echo the LiFePo selection as they a are safer and more closely alighed with the operating range of ham equip.

Jim Brown-10

Re: Internal Impedance of Li-Ion Battery Packs

On 8/4/2014 6:02 AM, Bob N3MNT wrote:
> Another*KEY* consideration, especially since you indicate it is a rare DX
> location ( I read as air travel required) is what you will be allowed to
> take on the plane.

Thanks for all the comments and ideas. Please keep them coming.

This is NOT a DX location, no air travel. It's activation of a rare grid
on 6M in Northern California. The guys doing the climbing will drive
several hours north from San Francisco. They have to hike several miles
up a mountain, making at least two trips to get the gear they need up there.

73, Jim K9YC
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WB4SON

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Bob N3MNT

Most LIFePO4 battery discharge curves are based on the full C rate

This is an example of a battery you might be interested in. 20 AH, 20 Amp
Max CONTINUOUS Discharge Current. There isn't a good "standard" for what
the capacity is based on. You need to check with each vendor. LiFePO4
batteries are very Coulomb-efficient -- what goes in is what goes out, and
this tends to keep your capacity very close to rated values no matter what
the load is. It also means that unlike SLA batteries, a LiFePO4 doesn't
get hot charging or discharging (at least not at 20 Amp values) -- this is
because the internal resistance is so darned low -- so there is little
wasted energy in a charge/discharge cycle.

My Bioennno 30 AH rated battery, tested at C/4 rate of 7.5 amps (the limit
of my test equipment), provided me with 29.88 AH of usable capacity, and
that was stopping the test at 12.00 volts, which is several volts higher
than absolute minimum (8.8 volts). Speaking of which, I would highly
recommend you stop using your battery when the terminal voltage falls below
12.00 volts -- there will be very little remaining capacity at that point
anyway.

http://www.bioennopower.com/collections/12v-series-lifepo4-batteries/products/12v-20ah-lifepo4-battery

That 7 pound battery is $198 including the charger, and will outperform a
40 AH SLA battery (will last 5 to 10 times longer, will weigh 1/5th as
much).

73, Bob, WB4SON

On Mon, Aug 4, 2014 at 9:02 AM, Bob N3MNT <[hidden email]> wrote:

> Another *KEY* consideration, especially since you indicate it is a rare DX
> location ( I read as air travel required) is what you will be allowed to
> take on the plane. Even if the battery meets your electrical and weight
> requirements it will be of little value if you can't get it There. I echo
> the LiFePo selection as they a are safer and more closely alighed with the
> operating range of ham equip.
>
>
>
> --
> View this message in context:
> http://elecraft.365791.n2.nabble.com/Internal-Impedance-of-Li-Ion-Battery-Packs-tp7591903p7591907.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> Elecraft mailing list
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> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
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>

73, Bob, WB4SON

WB4SON

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Jim Brown-10

By "(will last 5 to 10 times longer, will weigh 1/5th as much)." I mean a
LiFePO4 will have a total cycle life that is 5 to 10 times longer. In
round numbers 2000 charge/discharge cycles vs. 200. The total life cycle
cost is lower too. 5x 40 AH SLA will set you back about $300.

73, Bob, WB4SON.

On Mon, Aug 4, 2014 at 10:25 AM, Jim Brown <[hidden email]>
wrote:

> On 8/4/2014 6:02 AM, Bob N3MNT wrote:
>
>> Another*KEY* consideration, especially since you indicate it is a rare DX
>>
>> location ( I read as air travel required) is what you will be allowed to
>> take on the plane.
>>
>
> Thanks for all the comments and ideas. Please keep them coming.
>
> This is NOT a DX location, no air travel. It's activation of a rare grid
> on 6M in Northern California. The guys doing the climbing will drive
> several hours north from San Francisco. They have to hike several miles up
> a mountain, making at least two trips to get the gear they need up there.
>
> 73, Jim K9YC
>
> ______________________________________________________________
> Elecraft mailing list
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>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
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>

73, Bob, WB4SON

Bob N3MNT

Re: Internal Impedance of Li-Ion Battery Packs

I suggest using two 4S1P LiFePo packs, each being half of the desired capacity. This has several advantages.
More purchasing options therefore better price
on other outings where possible, one can be charging while one is in use.
A cell failure ( yes they do happen) will be less painful financially and not put you off air.
More choices of charger and therefore lower cost.

Phil Hystad-3

Re: Internal Impedance of Li-Ion Battery Packs

Good recommendation. I have two 4S1P LiFePo packs (purchased from Buddipole) and this is an excellent combination. I operate with one pack at a time and of course when power is available I can be charging the backup pack while using the other. I use mine with my KX3, KX1, and sometimes my Norcal 40A.

73, phil, K7PEH

On Aug 4, 2014, at 7:54 AM, Bob N3MNT <[hidden email]> wrote:

> I suggest using two 4S1P LiFePo packs, each being half of the desired
> capacity. This has several advantages.
> More purchasing options therefore better price
> on other outings where possible, one can be charging while one is in use.
> A cell failure ( yes they do happen) will be less painful financially and
> not put you off air.
> More choices of charger and therefore lower cost.
>
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Internal-Impedance-of-Li-Ion-Battery-Packs-tp7591903p7591915.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
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Jim Brown-10

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Bob N3MNT

On 8/4/2014 7:54 AM, Bob N3MNT wrote:
> I suggest using two 4S1P LiFePo packs, each being half of the desired
> capacity.

Thanks -- but this application is to run both the KX3 and KXPA100.

73, Jim

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Ignacy

Re: Internal Impedance of Li-Ion Battery Packs

I use 13V 20AH LiFePO3 for IC 7000. Very low internal resistance and good enough for a few hours of operating at a 100W power level. But the weight is 7 lb.

Li-Ion can be two times lighter than LiFePO3.

I use 4S1P Li-Ion for KX3. About 2.4 AH and max voltage 16.8V. Weight 6 oz. I use it in two ways:

1. Charge to 16.8V and use with two diodes in series.
2. Charge via one diode in series to 16.2V and attach to KX3 directly. About 15.3V max shown by KX3 as it has another diode internally.

The pack has about 0.8V drop at 2.5 A. It is good for a few outings.

There are low and high current Li-Ion packs. See http://www.batteryspace.com/4cellspack148v.aspx.

I am considering adding 10A Li-Ion to IC7000 for short outings. The battery should weight < 2 Lb.

Ignacy

Ingo Meyer, DK3RED-2

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Jim Brown-10

Hello Jim,

> ... So my
> question is, what sort of internal resistance should I expect for a battery rated like
> this? ...

...

> Here's a pack that I'm considering.
>
> http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh14arateli-ionwithcarsocketsmartcharger.aspx

Thats easy! Following the link the description says:
"11.1V 25.2Ah Polymer Li-Ion battery pack is made of 2 pcs of 11.1V 12.6Ah Polymer Li-ion
battery modules in parallel connection"

Following the next link to the 11.1V 12.6Ah Polymer Li-ion battery modules says now:
"11.1 V 12.6 Ah pack is made by 3 pcs Polymer Li-Ion cell of 3.7V 12.6Ah"

Following again the lin, at his time to the Polymer Li-Ion cell of 3.7V 12.6Ah brings the
value:
"Internal Resistance: 14 m-ohm"

3 times 14 milliohms in each row (= 42 milliohms) with 2 rows parallel results in 21
milliohms for the whole pack.
--
73/72 de Ingo, DK3RED - Don't forget: the fun is the power!
www.qrp4fun.de - [hidden email]
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Craig Smith

Re: Internal Impedance of Li-Ion Battery Packs

In reply to this post by Jim Brown-10

It seems to me that using the two smaller packs would have some advantages for your application. They would each be able to power both the KX-3 and the amplifier, just for about half the time of the single larger battery. So you would have to switch over to the other battery at some point. But using two does offer redundancy in the event of a battery failure. Also, dividing the weight into two chunks may be advantageous for the folks who are humping them up the mountain.

73 Craig AC0DS

On 4, Aug2014, at 10:01 AM, Jim Brown wrote:

> On 8/4/2014 7:54 AM, Bob N3MNT wrote:
>> I suggest using two 4S1P LiFePo packs, each being half of the desired
>> capacity.
>
> Thanks -- but this application is to run both the KX3 and KXPA100.
>
> 73, Jim
>
> ______________________________________________________________
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WB4SON

Re: Internal Impedance of Li-Ion Battery Packs

I would seconds Craig's opinion (having two batteries is better than one).
I did build a 60AH LiFePO4 battery a couple of years ago. If I had to do
it all over again, I'd go with two 30 AH batteries. It isn't that the
original battery was that heavy (20 pounds), but two 10 pound batteries
would be easier to carry than a single 20 pounds. Also it gives you
redundancy -- if you kill one battery you have one left. And the cost of
replacing it is lower when you do.

There is often a price premium for higher capacity cells anyway.

73, Bob, WB4SON
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73, Bob, WB4SON