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Author Topic: Question about max safe load on a small battery  (Read 5318 times)
KB1GMX
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Posts: 830




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« Reply #15 on: March 31, 2013, 10:34:11 AM »

Maximum safe load is variable number depending on what your trying to accomplish.

They use 7AH gell sometime 10 or 12AH cells for starting assist depening whos you buy
and the peak current is in the range of 50A or more (at 7-8V).

Generally gell cells can supply peak currents high enough for short periods to melt wires
or terminals.  We are talking 30-100 amps or more for seconds.

So with those two things in mind we have no meaningful "whats the max current" number.
What does happen is the more current you draw the lower the terminal voltage and
you'd like to stay above 11V.  At around 10.4-10/5V the battery is working too hard
or exhausted.  Keep in mind that over discharging the battery is very bad for its life
but brief large current usages generally is not a problem.

What you want to know is run a 100W radio (full or throttled) and for how long?

One caveat you want to keep the total Depth of Discharge (DoD) below 50% for
best battery life (measured in discharge charge cycles).  That mans you can take 5AH
out of a 10Ah battery.

On receive that radio wants an amp maybe 1.5 (give or take) so we are just listening
for around 3.5 hours for less than 50% DOD.   On transmit with no modulation that
radio sits there likely consuming about 2.5A with voice making for 20-23A on peaks
(at 100W).  Since averages are what count we are around 1/3 of the 20-23 A (at 100W)
for the period your transmitting or around 10A (weighted and rounded up).  So that
battery is good for less than 0.5 hour transmit (maybe more) depending.  So if your
total time on of an hour RX (-1.5AH) and 15minutes of transmit (-2.5AH) you have
used around 4AH or about 40% DoD and have some room left.  So the 10AH gell cell
can certainly run full power for short periods of time (minutes at least).  At a more
moderate 25 or 50W that time goes up.  So when you intersperse mostly RX with
short TX operations that little battery can last hours.  Add enough solar power to
offset the RX (20W panel) and that time grows greatly.  A larger panel say 30W
would not only offset the receiver it would also replace some of what the TX used.

A good example is I wanted to demonstrate my new Eagle, 1.3A on RX and 2.5A on no
modulation TX.   At a full 100W it's over 22A on voice peaks.  The battery was a handy
7AH gell cell.  I was able to run RX for over an hour and three 4 minute transmissions
at 50W and the battery was still above 12.5V while receiving.  That was enough for brief
check in to a net.  I don't advise this only it does show what can be done as those gell
cells can deliver a huge amount of current for short periods of time.

NOTE: the assumption is on transmit the no modulation current is 2.5A and at 100W the peak
is around 22-23A take 1/3 of the difference or about 7A and add that back to the 2.5A for around
10A as an average.  It will vary with radio, voice, pauses without speaking, and if compression
is used make the 1/3 average value 2/3 (about 14A).   If you throttle back the TX to 50W
the numbers are better but not twice better as the TX still needs 2.5A to run things and at lower
power the efficiency of the transmitter stages are not as good so the average current does
not drop as fast.  Based on what some of my radios do at 50W the example would only improve
a small amount to maybe an average current of 8A in the first case.

Also the 857 if memory serves has a low power high efficiency 20W mode to help that. 
Check your radios manual on that.

So yes portable ops with a 10AH gell cell are doable and for reasonable times.  However you still
need to replace the power used and charge the battery fully when done if you expect to repeat
that over several days.

FYI: if you discharge that battery to 50% and use a 20W panel (1.2a charge current) you need
no less than 4.1 hours of FULL sunlight on the panel.  That would be hard to do in the winter
and with areas that have trees it may be tough to do.  More solar if you can afford it and carry it
would be better.  Note there are foldable solar arrays but they are very expensive per watt
compared to conventional arrays.


Allison
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K5LXP
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Posts: 4536


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« Reply #16 on: April 01, 2013, 07:33:14 AM »

Keep in mind that over discharging the battery is very bad for its life but brief large current usages generally is not a problem.  One caveat you want to keep the total Depth of Discharge (DoD) below 50% for best battery life (measured in discharge charge cycles).

I think it's important to consider the application in regards to cycle life.

A typical quality gel cell is good for many hundreds of cycles down to 50% DOD.  In a portable amateur radio scenario you would be hard pressed to amass 500 or more cycles before the battery reaches end of life through time.  I would take the "better to burn out than fade away" approach, running the thing to 80% DOD or even more if the equipment will still operate at the lower voltage.  The idea is that if you're only running down to 50% you're carrying around twice as much battery as you need, only to throw away unused cycles at end of life.  There is a tradeoff of life vs cost vs utility in any given portable scenario.  Ah and weight may be more important factors than cycle life.  It would be more practical to say, use a 7Ah gel and run it dead 100 times and replace it than to have to carry around a twice as heavy 12Ah gel and only discharge to 50%, if weight and portability is a concern.


Mark K5LXP
Albuquerque, NM
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W8JX
Member

Posts: 6686




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« Reply #17 on: April 01, 2013, 08:26:30 AM »

Keep in mind that over discharging the battery is very bad for its life but brief large current usages generally is not a problem.  One caveat you want to keep the total Depth of Discharge (DoD) below 50% for best battery life (measured in discharge charge cycles).

I think it's important to consider the application in regards to cycle life.

A typical quality gel cell is good for many hundreds of cycles down to 50% DOD.  In a portable amateur radio scenario you would be hard pressed to amass 500 or more cycles before the battery reaches end of life through time.  I would take the "better to burn out than fade away" approach, running the thing to 80% DOD or even more if the equipment will still operate at the lower voltage.  The idea is that if you're only running down to 50% you're carrying around twice as much battery as you need, only to throw away unused cycles at end of life.  There is a tradeoff of life vs cost vs utility in any given portable scenario.  Ah and weight may be more important factors than cycle life.  It would be more practical to say, use a 7Ah gel and run it dead 100 times and replace it than to have to carry around a twice as heavy 12Ah gel and only discharge to 50%, if weight and portability is a concern.


I do agree if you a carrying it, I would not worry about the 50% discharge for maximum life cycle. I would carry as big a battery as possible and run rig till it quits. Even if you only got 50 charge cycles doing this it is still less than a dollar a cycle. 
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
K5LXP
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Posts: 4536


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« Reply #18 on: April 01, 2013, 02:00:03 PM »

Even if you only got 50 charge cycles doing this it is still less than a dollar a cycle. 

Exactly my point.  Often in these forums users are admonished to "never" discharge below 50% when in nearly all amateur use cases these batteries are subject to sporadic cycling at best.  Even to 80% DOD most gel cells are rated for 300 cycles or more, which translates to about a cycle a week before an optimistic end of life at 5-6 years.  Even at one cycle a day to 80% DOD you're still looking at nearly a year of useful life.  Yuasa spec's their NP-7 to around 180 cycles to 100% DOD, about 6 months of daily abuse.

Ideally you size a battery so that cycle and calendar end of life occur at the same point.  But that is rather difficult to do except under controlled conditions.  So rather than buy "too big" and take cycles you paid for up front to the recycler (and needlessly carried around the extra weight), better to use up a smaller battery and trade a bit of replacement cost for better portability.


Mark K5LXP
Albuquerque, NM
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