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Author Topic: Question about max safe load on a small battery  (Read 5315 times)
AJ4IK
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Posts: 16




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« on: March 22, 2013, 12:49:49 PM »

Wasn't sure which forum was best for this question, but giving it a try here:


I want to use a small (10ah) 12v AGM sealed deep cycle battery to power
my FT-857D abroad.

I believe max current draw is 20amps at 100 watts TX.

I realize running a 10ah battery at 100 watts on the radio is a bad
idea, but I don't know how to judge where I should actually cap it.
What's the max power setting I can use without damaging the battery?
Let's assume current draw is linear, e.g. at 50watts out it pulls
10amps, etc.

Please help me determine how much is too much. PLEASE NOTE -- run time
is NOT the primary consideration right now. I know the more you draw,
the faster the discharge. I want to know how much I can run without
hurting the battery. Thanks so much!
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NO2A
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« Reply #1 on: March 22, 2013, 01:18:52 PM »

The main thing is to watch your voltage on the `857. If it gets below about 11-11.5 volts it will shut itself off. The built in voltmeter helps monitor that. I wouldn`t let the battery get below that voltage. I`d try about 20 watts. Not sure if you`re using ssb/cw. Current draw isn`t always linear,but i think you`re about right with 50 watts drawing about 10 amps. Not sure with gell cell type batteries,but with car batteries I`ve heard they can be damaged if they get below about 10.5 volts.
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AJ4IK
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« Reply #2 on: March 22, 2013, 02:09:28 PM »

Thanks for the reply. Not sure if your voltage guidelines are at rest or under load. Radio seems to run fine down in the low 10's under load, FYI. I've been playing with about 20 watts (going by the fact that its bigger brother, the 897's optional battery is 4.5-5ah and limits to 20watts).

And just to differentiate, this is an SLA AGM, not gel. (Gel's are most sensitive to things.)
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WN2C
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« Reply #3 on: March 22, 2013, 08:38:42 PM »

Once you use drain the battery down, how are you going to charge it?  You might be better off buying a 25 amp switching power supply that you can use abroad.  Input Voltages vary country to country depending on where you are going so you would need to look into that.

Have fun on your trip.

73 de Rick  wn2c
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K5LXP
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« Reply #4 on: March 23, 2013, 06:42:57 AM »

Not sure if your voltage guidelines are at rest or under load.

Always under max load.  That's when you'll see the most voltage drop at the battery terminals.

Another factor is just how long you expect this thing to run.  Suspect that RX is around an amp, and TX with no modulation is around 5A with modulation power on top of that.  10Ah will "work" but if you do any amount of transmitting, maybe not for long.  An AGM has a slight advantage over gel in high current applications but Ah is still Ah and you only have so many.  As a data point I've run my Icom 706 on a 7Ah gel cell during Field Day and it's good for about an hour and half at 5W TX CW.

It would probably be worth it to characterize your rig under controlled conditions to see what the absolute minimum operating voltage is and the actual impedance (voltage drop) your battery exhibits.  Once you're out in the field it's more difficult to come up with alternative solutions.


Mark K5LXP
Albuquerque, NM

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AJ4IK
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« Reply #5 on: March 23, 2013, 12:45:56 PM »

Once you use drain the battery down, how are you going to charge it?

Solar rig will charge during the day.

Quote
Another factor is just how long you expect this thing to run.

Again for my purposes, I will only be operating long enough to get a message home via a sked. I'm not DXing, not contesting, not ragchewing, just checking in to say all is well. That's why I want to run the most power without damage to the battery. We're talking 15-20 minutes of operating per night, so Ah in terms of longevity is of minimal importance to me.

Quote
It would probably be worth it to characterize your rig under controlled conditions to see what the absolute minimum operating voltage is

The 857 doesn't actually cut out until it drops below 10 volts (at least on my rig). At a pretty full charge, the TX battery voltage drop is in the low 11's, high 10's running 20-25 watts.

I'm guessing there's no simple answer here. I was hoping there was some type of guideline, such as "with a 12V 12Ah AGM battery, never load it so voltage drop under max load is less than XX.X." Or "never draw more amps than the Ah rating of the battery" or something straightforward like that.

Thanks very much for the replies so far guys, I appreciate you taking the time to respond!
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WB6BYU
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« Reply #6 on: March 23, 2013, 07:55:06 PM »

My guideline is to limit the draw to 1/4 of the Ah capacity of the battery.  I don't always
follow that for short operation:  I've used my 33Ah battery at 100W before on SSB, but
generally only run it at 50W or so.  On SSB the voltage will drop on the peak currents
more than on average, which may cause some distortion, but you'll have to test it with
your specific rig to see where that becomes a problem.

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W8JX
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Posts: 6682




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« Reply #7 on: March 24, 2013, 10:22:59 AM »

My guideline is to limit the draw to 1/4 of the Ah capacity of the battery.  I don't always
follow that for short operation:  I've used my 33Ah battery at 100W before on SSB, but
generally only run it at 50W or so.  On SSB the voltage will drop on the peak currents
more than on average, which may cause some distortion, but you'll have to test it with
your specific rig to see where that becomes a problem.

A easy way to fix this is to put a 100,000 ufd or so electrolytic in parallel with load. It stabilizes voltage on peak draws and limits average peak draw on battery during SSB modulation and extends battery life/efficiency since lower peak draws means less energy lost to internal resistance. I even use this trick with HF mobile and with station power supplies.
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K0BG
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« Reply #8 on: March 24, 2013, 03:56:13 PM »

The ultimate current source when a battery and a capacitor are in parallel is the battery. So the question remains, what does hooking an external capacitor to say a gelled battery (a capacitor of sorts) do with respect to voltage drop—the supposed reason for using one in the first place. The answer is, not much if it isn't being charged at the time.

The internal resistance of a LA battery averages around .003Ω. A bit more for a flooded one, perhaps a bit less for an AGM type. The internal resistance of a good quality electrolytic might be a bit less, but not by much. A super capacitor's internal resistance hovers around .001Ω. So the result of paralleling a cap with any battery is a slight reduction in output voltage ripple (perhaps 50% during an SSB transmission), but it doesn't do a darn thing for voltage drop at the battery's terminals.

In hybrid vehicles, super caps are often used to temporarily store a charge from say regenerative braking, and then used to recharge the propulsion battery which can't react to as quickly to charging current as the caps can.

Anyone contemplating using batteries to supply current for any application, should perhaps purchase the book by Isidor Buchmann entitled "Batteries in a  Portable World". It is available from the ARRL store for $29.95.
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W8JX
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Posts: 6682




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« Reply #9 on: March 24, 2013, 04:54:40 PM »

The ultimate current source when a battery and a capacitor are in parallel is the battery. So the question remains, what does hooking an external capacitor to say a gelled battery (a capacitor of sorts) do with respect to voltage drop—the supposed reason for using one in the first place. The answer is, not much if it isn't being charged at the time.

The internal resistance of a LA battery averages around .003Ω. A bit more for a flooded one, perhaps a bit less for an AGM type. The internal resistance of a good quality electrolytic might be a bit less, but not by much. A super capacitor's internal resistance hovers around .001Ω. So the result of paralleling a cap with any battery is a slight reduction in output voltage ripple (perhaps 50% during an SSB transmission), but it doesn't do a darn thing for voltage drop at the battery's terminals.

In hybrid vehicles, super caps are often used to temporarily store a charge from say regenerative braking, and then used to recharge the propulsion battery which can't react to as quickly to charging current as the caps can.

Anyone contemplating using batteries to supply current for any application, should perhaps purchase the book by Isidor Buchmann entitled "Batteries in a  Portable World". It is available from the ARRL store for $29.95.

The flaw in you logic is first in a gel the internal resistance is higher than you claim and second its energy is created by a chemical reaction and has measurable delay in milisec to produce power for load changes and efficiency decreases with load increases. Batteries are best with a constant load. A large capacitor stores electrons and can respond to massive demands on peaks far quicker than a gel cell and with higher efficiency too. Same with your desktop power supply, a CAP in parallel with it can respond to quicker to a sudden peak than any supply's regulator. Same with your cars electrical system. Try it and you will not be disappointed. Very simple to do and so simple many spend time trying to explain why not to do it rather than trying it too. Granted the cap will not help with key down carrier drain but will soften surge load on power source and it works great with SSB.
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W9MMS
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Posts: 121




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« Reply #10 on: March 29, 2013, 12:46:24 PM »

The ultimate current source when a battery and a capacitor are in parallel is the battery. So the question remains, what does hooking an external capacitor to say a gelled battery (a capacitor of sorts) do with respect to voltage drop—the supposed reason for using one in the first place. The answer is, not much if it isn't being charged at the time.

The internal resistance of a LA battery averages around .003Ω. A bit more for a flooded one, perhaps a bit less for an AGM type. The internal resistance of a good quality electrolytic might be a bit less, but not by much. A super capacitor's internal resistance hovers around .001Ω. So the result of paralleling a cap with any battery is a slight reduction in output voltage ripple (perhaps 50% during an SSB transmission), but it doesn't do a darn thing for voltage drop at the battery's terminals.

In hybrid vehicles, super caps are often used to temporarily store a charge from say regenerative braking, and then used to recharge the propulsion battery which can't react to as quickly to charging current as the caps can.

Anyone contemplating using batteries to supply current for any application, should perhaps purchase the book by Isidor Buchmann entitled "Batteries in a  Portable World". It is available from the ARRL store for $29.95.

The flaw in you logic is first in a gel the internal resistance is higher than you claim and second its energy is created by a chemical reaction and has measurable delay in milisec to produce power for load changes and efficiency decreases with load increases. Batteries are best with a constant load. A large capacitor stores electrons and can respond to massive demands on peaks far quicker than a gel cell and with higher efficiency too. Same with your desktop power supply, a CAP in parallel with it can respond to quicker to a sudden peak than any supply's regulator. Same with your cars electrical system. Try it and you will not be disappointed. Very simple to do and so simple many spend time trying to explain why not to do it rather than trying it too. Granted the cap will not help with key down carrier drain but will soften surge load on power source and it works great with SSB.

>>> Batteries are best with a constant load. A large capacitor stores electrons and can respond to massive demands on peaks far quicker than a gel cell and with higher efficiency too. <<<

I guess that the truck/car manufacturers has got it wrong all along!
Maybe they should have been using Capacitors to turn over their engines via the starter.

The starters on engines requires massive amount of current to turn over an engine.
No other component on a car/truck has such instantaneous demand as a starter.
SLI Batteries has been meeting that demand since when.
Has anyone ever seen Capacitors used for such a task?
BTW, they do make gelcel SLI batteries.

((((73)))) Milverton   
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W8JX
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Posts: 6682




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« Reply #11 on: March 29, 2013, 01:29:43 PM »

>>> Batteries are best with a constant load. A large capacitor stores electrons and can respond to massive demands on peaks far quicker than a gel cell and with higher efficiency too. <<<

I guess that the truck/car manufacturers has got it wrong all along!
Maybe they should have been using Capacitors to turn over their engines via the starter.

The starters on engines requires massive amount of current to turn over an engine.
No other component on a car/truck has such instantaneous demand as a starter.

Main reason the are not used in cars to assist is the cost of the size of capacitor needed. Battery is cheaper and its all about cost. When car starting systems were developed capacitors were very primitive in physical sizes needed and cost a lot. There was a move a while ago to switch from 12 v to 42 volt systems in cars but it was shot down. Shame it would make cars more efficient electrically by reducing current loads and reduce amount of cooper wire needed and have room to grow. Aircraft have long been 28volt and some new ones are moving to 42v.   

SLI Batteries has been meeting that demand since when.
Has anyone ever seen Capacitors used for such a task?
BTW, they do make gelcel SLI batteries.

You seem to be fixated on either/or here. The point is to augment with a cap to stabilize voltage not replace battery (though you could in theory with a big enough CAP) Plain and simple any gelcell or lead acid battery will see a big drop in efficiency as load increases. It is the nature of the beast. At starting power loads you might recover 30 to 40% tops of power stored in battery vs what you would get with a constant lighter load of 1/20th of rated capacity per hour.

Some seem to forget that a battery is not like a glass of water that releases same amount no matter how fast you empty it. Neither is a capacitor but it is a LOT closer to it than a battery.
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K1CJS
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« Reply #12 on: March 30, 2013, 10:17:36 AM »

The ultimate current source when a battery and a capacitor are in parallel is the battery. So the question remains, what does hooking an external capacitor to say a gelled battery (a capacitor of sorts) do with respect to voltage drop—the supposed reason for using one in the first place. The answer is, not much if it isn't being charged at the time....

The flaw in you logic is first in a gel the internal resistance is higher than you claim and second its energy is created by a chemical reaction and has measurable delay in milisec to produce power for load changes and efficiency decreases with load increases....

I think the best indicator of whether something is useful or not is the number of people who do/use it.  So far putting a capacitor in parallel with a battery is done by....  one?  Half a dozen?  Not such a good number when considering that there are thousands of mobile operators and other battery powered electric users out in the world.  

Also, something else to think about is this:  What about leakage current?  True that there is little in a good capacitor, but how about if the cap were to start to fail?  If it DID fail/short?  Not so good.....
« Last Edit: March 30, 2013, 10:19:52 AM by K1CJS » Logged
W8JX
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Posts: 6682




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« Reply #13 on: March 30, 2013, 07:15:14 PM »

I think the best indicator of whether something is useful or not is the number of people who do/use it.  So far putting a capacitor in parallel with a battery is done by....  one?  Half a dozen?  Not such a good number when considering that there are thousands of mobile operators and other battery powered electric users out in the world.

It is not because it does not work. Sometimes the most painfully obvious is ignored as to simple to work.  

Also, something else to think about is this:  What about leakage current?  True that there is little in a good capacitor, but how about if the cap were to start to fail?  If it DID fail/short?  Not so good.....

Never had one fail yet and as far as a short, in mobiles I always use a fuse/circuit breaker on feed to rig before cap.
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K7RBW
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« Reply #14 on: March 31, 2013, 08:21:03 AM »

Been there, done that, got a switching power supply.

A 10a battery will work fine with an FT-857 if you keep it on low (20W) power. At 100W, a 10A battery will last about 30-45 minutes. the 10 AH rating is down to 10.5 volts; however, the 857 starts to cut off at 11.0 v so you can't use the rated capacity--only about 70% of it. Then, you'll need a big (or slow) charger before you can use it again.

I thought about carrying a battery like that around, but the heavy weight and low operating time made that seem like a lot of work for a little benefit. I ended up getting a switching power supply that weight 1/2 as much as the battery and would run all day (depending on the local power grid, of course). If you shop right, you can get a power supply that works on 110/220 (if that's a concern).

If you want to go battery-powered, you'll need a bigger battery, a LiPo battery (lighter and more power than an AGM), or stick to QRP.
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