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Author Topic: Battery discharge management  (Read 173 times)

KD0VE

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Battery discharge management
« on: May 07, 2020, 10:41:40 AM »

I am setting up an emergency battery for my station.  In reading I find a flooded lead acid battery should not be discharged below 20%, and, preferably not below 50%.

Is there a device that will discontinue the load when a preset discharge level is reached?

thx to anyone taking the time to offer advice.

KD0ZGW
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K6AER

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Re: Battery discharge management
« Reply #1 on: May 07, 2020, 11:11:15 AM »

Discharge level is referenced to the voltage under a mild load. A 100% wet cell charge level would be around 13.3 volts. The 40% charge level voltage would be some whare around 12.4 volts. As a rule of thumb do not discharge the 12 volt wet cell battery below 12.5 VDC (50%) if you want to have a usable battery for many years.

A float charge level might climb to 14.3 volts but this float charge level willll drop to 13.6 volts after only a minuet of operation at 5% capacity ratiing.
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K1VSK

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Re: Battery discharge management
« Reply #2 on: May 07, 2020, 11:16:12 AM »

I am setting up an emergency battery for my station.  In reading I find a flooded lead acid battery should not be discharged below 20%, and, preferably not below 50%.

Is there a device that will discontinue the load when a preset discharge level is reached?

thx to anyone taking the time to offer advice.

KD0ZGW
Yes there are. Lots of us use them on our boat, however, a real battery monitor is far more useful and informative and will show you how your battery supply is working.
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N8AUC

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Re: Battery discharge management
« Reply #3 on: May 07, 2020, 11:57:10 AM »

If you set that battery up as part of a solar charging system with a charge controller, it will automatically shut down the load when your battery voltage under load drops below a value you pre-set. I know that the el-cheapo charge controller that came with my kit from Harbor Freight does that automatically. I know this, because it happened to me once, and it really startled me when suddenly everything shut off. Turns out I had the shut off voltage set wrong. The charge controller did what it was supposed to do, and the only moron in the system was the guy in the operating chair, which was me at the time.

As K1VSK said, it's also a good idea to use a real battery monitor so you can see at a glance what the state of your batteries are at any point in time.

I think some of the products from West Mountain Radio have that function built-in too. Although I can't remember the name of that item off the top of my head.

73 de N8AUC
Eric


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K1VSK

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Re: Battery discharge management
« Reply #4 on: May 07, 2020, 12:19:03 PM »

To elaborate, Victron makes some of the most popular and reliable battery monitors among the boating world. Others exist but I’m not familiar with them. As you say, you want to manage your battery supply. Having a simple voltage cut-off manages nothing.
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K5LXP

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Re: Battery discharge management
« Reply #5 on: May 09, 2020, 08:54:27 AM »

In reading I find a flooded lead acid battery should not be discharged below 20%, and, preferably not below 50%.

There are very few and specific applications that may be true.  A standby battery used for a ham radio station isn't one of them.  Probably your greatest limitation of operating voltage isn't the battery, but the equipment you may be running from it.  Often, transceivers are rated for 13.8VDC, +/- 15%.  That puts the lower limit of voltage to be around 11.73V, which depending on the battery and other loads is only about the 50% point.  So review what the operating voltage range is for your equipment and what the net run time you'd get would be directly connected.  If that's not enough you'll either have to use a greater Ah battery/battery bank or a battery booster that will allow you to realize all the Ah your battery has.

The option I use is an AC inverter, and operating the equipment off of 120V.  Many will exclaim the inefficiency of this but the overall net I get by using all of the battery's capacity instead of just half, it works for me.   Connections, charging and switchover are other benefits to using an integrated inverter unit.  A computer UPS can often check all the boxes for this.

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Is there a device that will discontinue the load when a preset discharge level is reached?

Sure, do a search for battery low voltage disconnect.  I don't think you need one at all.  I don't want something cutting me off when operating in an "emergency", the communications is more important than the battery.  Plus, most equipment is going to crap out before the battery is totally dead anyway, so it's a self solving problem.  Instead of a disconnect, get an inexpensive battery monitor that allows you to see voltage, current and Ah at a glance which tells you all the important things you need to know while you're using the system.

Discharge level is referenced to the voltage under a mild load. A 100% wet cell charge level would be around 13.3 volts. The 40% charge level voltage would be some whare around 12.4 volts.

None of that is true for lead acid.  State of charge can be determined at any discharge rate including static/no load and the voltage representing state of charge is highly dependent on the battery, battery merit and operating environment.  Read that to mean using terminal voltage alone is a highly arbitrary means of determining SOC.

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As a rule of thumb do not discharge the 12 volt wet cell battery below 12.5 VDC (50%) if you want to have a usable battery for many years.

Lead acid batteries have a finite calendar life, no matter how little they're used or how well they're maintained.  Any deep cycle battery is capable of many hundreds of cycles down to 20% SOC.  At a practical level, for ham radio uses you could run a battery down once a week, for years, and never wear it out before the battery expires from age.   

Mark K5LXP
Albuquerque, NM
« Last Edit: May 09, 2020, 08:57:06 AM by K5LXP »
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LA9XNA

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Re: Battery discharge management
« Reply #6 on: May 11, 2020, 01:48:21 PM »

If you are going to use the battery only when atennded just have a voltmeter and ampmeter coneced to the battery. By monitoring the voltage while operating you can decide when the battery is discharged as far down as you want.
There is ready made units avalable at marine supply houses or on Amazon/Ebay.
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=marine+battery+monitor&_sop=15
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=marine+battery+monitor&_sop=15
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