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Author Topic: Using batteries in series replacing filter cap in power supply  (Read 322 times)

WB7TDG

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Title pretty much says it all...
wondering if anyone has used 2- 12 volt batteries (sla, flooded lithium or otherwise) in series or 3- 6 volt batteries in series instead of the filter cap in a linear 13.8 VDC power supply to give back up power in case main power goes out.
Proper precautions assumed to be observed...fusing, etc.
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AA4PB

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Re: Using batteries in series replacing filter cap in power supply
« Reply #1 on: April 07, 2021, 11:35:32 AM »

I would think that you will have difficulty limiting the charging current to the battery when it is in a discharged condition. It's simpler to just put that battery at the output of the supply, fed via a diode to prevent the battery from back-feeding the supply when AC power is out. At that point you have a regulated 13.8VDC to work with.

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Bob  AA4PB
Garrisonville, VA

AC7CW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #2 on: April 07, 2021, 12:54:32 PM »

That might be a workable idea. There being several battery technologies to choose from, one might be found that handles the pulsed charging well.
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Novice 1958, 20WPM Extra now... (and get off my lawn)

KH6AQ

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Re: Using batteries in series replacing filter cap in power supply
« Reply #3 on: April 08, 2021, 08:27:00 AM »

That is an interesting idea and I will give my take on why it appears to me to be impractical. Referring to the first diagram below you propose substituting a battery for the filter capacitor. A necessary filter capacitor characteristic is a usefully low ESR (Equivalent Series Resistance). A battery that is partially charged may not have a usefully low ESR when not near full charge. And, battery life might be compromised when used partially charged and never brought to a full charge. As batteries are not specified for use in a highly discharged state I cannot cite ESR or life numbers. However, batteries are commonly used to give backup power as shown in the second diagram below.

AC --> rectifier --> filter capacitor --> series-pass regulator --> regulated DC
                                (battery)

AC --> rectifier --> filter capacitor --> series-pass regulator --> regulated DC --> battery
« Last Edit: April 08, 2021, 08:30:42 AM by KH6AQ »
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KF5LJW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #4 on: April 08, 2021, 10:04:34 AM »

Title pretty much says it all...
wondering if anyone has used 2- 12 volt batteries (sla, flooded lithium or otherwise) in series
That would be a 24 volt battery and that says it all.

What you are describing has been done over 100 years. Every critical mission technical facility has battery standby since the first telephone was installed 100 years ago.

Here is a very small 24 volt cell tower battery plant.
« Last Edit: April 08, 2021, 10:08:50 AM by KF5LJW »
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AA4PB

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Re: Using batteries in series replacing filter cap in power supply
« Reply #5 on: April 08, 2021, 11:54:40 AM »

What you are describing has been done over 100 years.

You missed the rest of his idea: "instead of the filter cap in a linear 13.8 VDC power supply". Of course there is no problem putting batteries in series. The problem is that he wants to use them as a replacement for the power supply filter cap.
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Bob  AA4PB
Garrisonville, VA

W9FIB

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Re: Using batteries in series replacing filter cap in power supply
« Reply #6 on: April 08, 2021, 06:29:43 PM »

I prefer the power supply stay as it is, then use a device that will allow the batteries to charge from the power supply but not discharge to the load unless the main power supply fails. Basically it becomes a 12V UPS. West Mountain Radio has what I use.

http://www.westmountainradio.com/product_info.php?products_id=pg40s
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73, Stan
Travelling the world one signal at a time.

KF5LJW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #7 on: April 09, 2021, 09:05:27 AM »

I prefer the power supply stay as it is, then use a device that will allow the batteries to charge from the power supply but not discharge to the load unless the main power supply fails.

You do not need any device to do that.
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W9FIB

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Re: Using batteries in series replacing filter cap in power supply
« Reply #8 on: April 10, 2021, 04:33:25 AM »

I prefer the power supply stay as it is, then use a device that will allow the batteries to charge from the power supply but not discharge to the load unless the main power supply fails.

You do not need any device to do that.

Then you can cook your batteries. With this, my batteries are safe.
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73, Stan
Travelling the world one signal at a time.

K5LXP

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Re: Using batteries in series replacing filter cap in power supply
« Reply #9 on: April 10, 2021, 07:16:34 AM »

You can't get there from here.  Batteries are not capacitors, so their filtering effect would be limited.  The input voltage is not regulated, so any battery you put in there would meet a quick demise, either from serious under or overcharging.

Mark K5LXP
Albuquerque, NM
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KF5LJW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #10 on: April 10, 2021, 08:03:50 AM »

ce to do that.
Then you can cook your batteries. With this, my batteries are safe.
[/quote]Guess you never heard of FLOAT Voltage like every telephone, data center and radio tower site operates with the same set of batteries for 10 to 30 years huh?
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AA4PB

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Re: Using batteries in series replacing filter cap in power supply
« Reply #11 on: April 10, 2021, 11:21:06 AM »

Guess you never heard of FLOAT Voltage like every telephone, data center and radio tower site operates with the same set of batteries for 10 to 30 years huh?
[/quote]
FLOAT voltage only works when that voltage source is fairly stable, such as after the voltage regulator. The voltage at filter capacitors is before regulation and will vary quite a bit, depending on the power supply load. You'll never be able to control the battery charge rate if you connect at that point. The place to put a back up battery is after the regulator where the charging voltage will be stable. Be sure to use a diode between the battery and the power supply so that the battery can't back feed into the supply when the AC power fails.
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Bob  AA4PB
Garrisonville, VA

KF5LJW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #12 on: April 10, 2021, 12:12:56 PM »

FLOAT voltage only works when that voltage source is fairly stable, such as after the voltage regulator. [/quote]
Exactly and what W9FIB was referring to. Fact is to maximize lead acid battery life requires them to be kept @ 100 SOC at all times floating on a charger. Like I said pros have done it for over 100-years and their batteries last decades. I made a living out of it.

Be sure to use a diode between the battery and the power supply so that the battery can't back feed into the supply when the AC power fails.
Well again I would say make sure you do not use a Power Supply that would require the diode. That is just more overhead and waste heat to deal with.  Example for 5-cents you can convert any Astron Space Heater Power Supply into a float battery charger/Power Supply. All you have to do is add the 5-cent resistor they left out makes the back feed problem go away. Full instructions found in the link below. Smart money buys a DC power supply that has no issues with batteries.

For hams,  I do recommend you spend a lot of money on a usele$$ Interface black box as I have vested interest in you doing so.

https://www.repeater-builder.com/astron/astron-rsbattmod.html

As for using a battery in place of filter caps on the output of a rectifier is no go because the voltage variation and AC nature of the current would not play well with a battery. One example is very real, a discharged battery will take however much current the rectifiers can deliver and then a whole lot more the rectifiers cannot supply without burning up. Even if the batteries were fully charged will still be able to absorb unlimited current by raising the voltage from say no load condition.

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AC7CW

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Re: Using batteries in series replacing filter cap in power supply
« Reply #13 on: April 10, 2021, 05:36:21 PM »

I used to get used lead acid batteries from a surpluser very cheap. If you are free to ruin your batteries you can do about anything you want with them. I used to install mobile rigs [for my own use] by duct taping a battery to the rig, making a cable with a series resistor and running the aux battery from the cigar lighter. That worked fine, rx current came from the cigar lighter and tx current came from the battery, to me it was fine and obviously I don't suffer from perfectionism. The idea of using batteries for filter caps appeals to my minimalism or my ability to heroically make an emergency repair and who knows, there might be a whole new power supply design in the works here... what about a battery paralleled with a super-capacitor and a smaller filter cap or something? Sometimes wild ideas are where good ideas originate, no? We're supposed to advance the state of the art here...
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Novice 1958, 20WPM Extra now... (and get off my lawn)

N8AUC

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Re: Using batteries in series replacing filter cap in power supply
« Reply #14 on: April 10, 2021, 09:57:31 PM »

Using a regulated, LINEAR power supply to float charge a lead acid battery (flooded or AGM) is a perfectly acceptable technique. Don't use a switching power supply to do that, because you risk damage to the switching regulator. In fact, the manual for my switching power supply specifically warns NOT to connect a battery to the output for exactly that reason. I keep a 10A linear supply around just for float charging batteries.

But as Mark K5LXP mentioned, you will cook your battery, unless you limit the time you do float charging. Float charge the battery for a couple of hours once a week maybe. But when the current into the battery drops to near zero, stop float charging. Unless you have an automatic device to do that, like with a Battery Tender.

Totally agree with AA4PB in that using a battery in place of a filter cap in a linear power supply is a bad idea.

Using a West Mountain PowerGate to connect both a battery and a power supply to a radio is a much better solution than just using a diode, because the PowerGate has a much lower forward voltage drop than the diode. The forward voltage drop won't be a problem for the power supply. But it might be for the battery backup, especially as the battery voltage starts to drop as it discharges. Plus, you need a seriously big honkin' diode to handle the current draw from a 100W rig, and you may need to be concerned with heat dissipation in the diode as well.



« Last Edit: April 10, 2021, 10:01:08 PM by N8AUC »
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