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1-9 of 9 messages
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Battery Backup
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by KB9RLE on December 28, 2003
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I am looking for some information on batteries. If any one knows the formula to determine how long you can run a rig off of a gell cell battery would you please let me know. My example is running a 50W all mode radio at 13.8 volts using a gell cell with a capacity of 20AH, how long will I be able to operate with out recharging? Please include the formula in your reply so I can play with different scenarios. Thanks Much
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RE: Battery Backup
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by KE4SKY on December 28, 2003
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A 20ah battery is a bit light to operate a 50w rig for much more than about 4-6 hours of voice ops unless you limit transmitter output to about 20w and limit operating duty cycle to not more than 20%. Then you may get a few hours more IF the battery is fully charged and is providing its full rated capacity.
The latter can be determined only by load testing the battery and measuring its capacity. Presuming that the battery is capable of its full rated capacity and that your rig will continue to operate down to the 10.6 volts floor level at which battery depth of discharge ratings are normally determined (which it probably will not - most ham rigs cease to operate normally below about 11.5VAC), then you can approximate battery operating life by:
Sum (current load - Amps)
(Tx load typical 10A for most 50w solid state)
(Rx load w/squelch open typical 1A if solid state)
Times Operating Duty Cycle %
(use 25% for voice operation)
(use 50% for CW, or digital)
Times safety factor 150%
(200% if rig is unstable below 12VDC
or for critical systems with no other backup)
A rule of thumb which we teach for RACES deployments, which has been found adequate in "most" instances, is to allow one amp-hour of battery capacity for each watt of transmitter output, for each 12-hour operational period. We also recommend using the 200% safety factor if you don't have solar, generator or other means independent of AC mains for recharging.
The best thing is to measure the actual capacity of your battery system, as well as the operating loads and duty cycle for all battery powered equipment you will use, radio, laptop, TNC, external DSP, station lighting, etc.
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RE: Battery Backup
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by KB9RLE on December 29, 2003
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Thank you KE4SKY for the useful information. I have a lot of work ahead of me on the layout of my new QTH, this is just the first step. Again thank you so much.
KB9RLE
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RE: Battery Backup
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by W4LGH on January 11, 2004
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Another interesting thing you can do with your battery backup, is to insert a diode in-line with the B+ line coming from the battery to your equipment. This diode will need to be rated high enough to handle your current requirements. I use one that is rated at 60amps@600PIV, more than enough to handle my equipment.
What this diode will do is isolate your battery from your 13.8VDC power suppy you use to run your equipment. Then you can put an inexpensive "Float Charger" on the battery to keep it fully charged without "cooking" it. When the AC in your house goes out, the radios will continue to play just fine without any intervention from you. Yes there is a .5volt drop across the diode, but a fully charged battery (lead-acid) is about 12.77volts so the .5V drop to 12.27volts is really no big deal.
I have been using this kind of setup for many many years, and built these for many others thru out the years.
PS..I use a marine deep cycle trolling motor battery, which is good for about 90 to 100AH.
73 de W4LGH - Alan
St.Johns County, FL. ARES EC
website: http://www.w4lgh.com
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RE: Battery Backup
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by VE3TMT on January 12, 2004
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Go to Wal-Mart and pick up one of their 12V utility batteries. Mine is a 220 CCA (cold cranking amps). They also sell a 1.5 amp trickle charger that mounts right to the side of the battery. It has auto charge and shutoff so you can leave it connected all the time. It will automatically charge the battery as you use it. I have been running my TS850 on this setup for months now with no problems at 100W. When the next ice storm hits and the power goes out I'll be all set.
Max VE3TMT
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RE: Battery Backup
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by WCO3103 on January 26, 2004
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Typically rechargeable batteries are rated at their "one tenth C" rate. "C" being the rated charge Capacity, and 1/10th C meaning the point where they can supply that energy over ten hours of constant load.
So a 20AH battery usually means a battery capable of supplying 2Amps per hour for ten hours--and then being totally consumed.
That's going to vary with the manufacturer and the chemistry and the construction but it is a good starting point. At less than 1/10C you should get more total power out of the battery, at more than 1/10th C you will get considerably less total power out of it.
AGM (glass mat), gel, and wet acid batteries all have different characteristics and trade-offs of durability, charge cycles, recharge time...but the makers of the batteries are pretty good about publishing specs for them. And NiCad and NiMh of course are different too.
With any lead based chemistry you will usually get the best life from the battery by not cycling it more than 50% discharged, i.e. if it is good for 300 cycles at 50% discharge cycling, it may only be good for 80 cycles at a 100% discharge rate, or for 500 cycles at a 25% discharge rate.
Fun, isn't it?<G>
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RE: Battery Backup
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by WCO3103 on January 26, 2004
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Your specific numbers:
50 Watt @ 13.8V pulls 3.6 amps, probably will pull closer to 4 amps as soon as the voltage goes down. So with a 20AH battery, designed to supply on 2Amps of power at rated capacity...you're pulling twice what it is rated for when you transmit. Much less when you receive.
At a 50% duty cycle? You might get 4 hours out of the battery, I wouldn't count on more than 3 before the voltage was own low enough to hurt you. Less as the battery ages of course. And less if it is cold (below 70F counts as cold!) outside, hot outside (above 80F<G>) or the battery hasn't been charged in a month.
When temperatures go above 85F and batteries get a year or two old...you can figure they have only 1/3 of their original power. You derate about 25% for heat above 85F, and derate about 20% for every year the battery has aged. Three years old? Replace it, used or not!
The duty cycle (amount of tx/rx) is going to determine what you can expect. Best thing would be to clock it and try it.
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RE: Battery Backup
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by KC8VWM on February 10, 2004
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Whatever the formula might apply here, it may not account for lost voltages or current from the battery simply sitting around.
Equally important are the operating conditions the battery will be subjected to.
Colder temperatures will further reduce the batteries output efficiency etc.
Also the age of the battery will contribute to a reduced and diminished charge capacity over a period of time.
Therefore, I am not so sure if any single formula or theory equating amp hours and current draw to equal power output time - would be very accurate in terms of a REAL case scenario.
I would use these figures cautiously and only as a baseline or approximation. This formula should not be used as the definitive & determining factor of a batteries longevity in the field.
A general rule of thumb would be to have at least an additional 25% battery power than you actually require to act as a buffer.
73
Charles - KC8VWM
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RE: Battery Backup
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by KC2MMI on February 10, 2004
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Charles-
<Therefore, I am not so sure if any single formula or theory equating amp hours and current draw to equal power output time - would be very accurate in terms of a REAL case scenario.>
Right and wrong. The ballpark rule of thumb is just a ballpark rule. But you could fairly easily put together a more precise formula if or as you need it.
Derating a battery for operating environment (temperature) is easy. The battery makers all have the numbers for this. Derating it for storage, same thing. And when all is said and done all the details don't matter, you can apply the simple rule of them and just know there will always be a certain amount of variation.
If I remember correctly, from specifying computer UPS supplies for runtime, a gel type battery used in the small consumer grade UPSes is derated 25% for every 10F that the temperature goes over 75F or 80F. And then derated another 25% or so for every year it has been simply on a float charge, i.e. not being used.
So a two-year old backup supply on a 90 degree summer day may have only 50% of the nominal power it was rated to supply. If that's "grossly" wrong and the number is 40% or even 60%, that doesn't matter--you're still stuck with it at some point in time, unless you replace batteries every six months in an effort to get tighter spec compliance.
The solution is that you ignore the details, plan for the larger variations (temperature, age, capacity) and then add a safety factor to cover the rest.
Any battery manufacturer can supply the full details for every type they make, and if you really want to, you can plug all those numbers into a spreadsheet (or a long but simply equation) and run the numbers. AFAIK that's a waste of time because size, weight, or money is going to be the limiting factor and then you simply live with the rest--unless you're in a critical industry, with the time and budget to deal with it.
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