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Battery-backed power supplies

posted Jan 8, 2017, 5:32 PM by Charles Boling
This should have been written last February, but I guess I forgot!
I have a bench-top linear power supply (Pyramid PS-36KX) rated 35A 12-15VDC that I use for powering my radios and keeping a car battery charged.  You can get some rather expensive devices (e.g. the $140 PWRgate) that will automatically deal with backup batteries (without overcharging them or sulfating them) from your power supply, but being the cheapskate that I am, when I put this system together in 2008, I decided I could do my own battery maintenance. The battery sits down beside my workbench, with a pair of clips on the battery terminals connecting it to the power supply.  Most of the time I kept the battery disconnected, but when I was using the radio (at least weekly) and had the power supply on anyway, I'd hook it up to keep it topped up.  I made myself a chart (see file attachment) with the pertinent info on the battery, and I adjust the power supply voltage as appropriate depending on what needs to be done to it -- keeping it topped off, regulating the current during a charge-up after it's been used during a power outage, an occasional anti-sulfation session, etc.   I check the output once in a while w/ a fairly accurate voltmeter, and I pretty much know what knob position's appropriate without marking it.

The biggest problem with this was remembering to disconnect the battery.  The power supply doesn't have a diode in the output, so if you just turn it off, the battery will end up running the fan in the power supply, keep the voltmeter up, etc.  As my kids got older, the hams among them would often put the radio on the battery without me knowing it, and occasionally I wouldn't notice it for days.

Schottky diode - 12V bus

This thing needs a diode...

After 8 years of doing this, I decided it was time to get smart & lazy.  I bought a 40A Schottky Diode online for $4 shipped, attached it to a lightweight heatsink from KF7ZIM's junk box, and mounted it to my power-distribution bus bar.  This way, I could keep the battery online all the time, without it backcharging.  The diode has about a 0.9V voltage drop, but with a max of 15V from the supply, there's plenty of room -- if anything, it could actually be an advantage, since it would prevent over-driving any equipment were a little child to come along and crank the knob all the way up.

It works great.  It's nice not to have to worry about connecting/disconnecting the battery; now it's always there, and if there's a surprise power outage while operating, there's no interruption -- the radio (and its lights!) remain on, and you're not left fishing for the battery clip.  Now the biggest challenge is remembering to turn on the power supply when you're using the radios so you don't drain the battery unnecessarily.... 

A Battery as a Buffer

Having the battery online all the time was also an advantage since I had just bought a VHF linear amplifier that can draw over 30A of power.  Couple that to a radio, (not to mention other accessories) and you're getting awfully close to the supply's maximum rating. Cheap supplies in particular tend to have optimistic ratings, so you really want to stay well below, both for the health of the supply and the good operation of your equipment -- you know you're not getting great output when your power supply drops to 10V when you talk....  The battery serves as a buffer; as the power supply's voltage drops due to the load, the battery picks up the slack and delivers more of the current to the radio. Once the demand drops, the supply voltage raises, and begins recharging the battery.  As long as your power supply isn't trying too hard to keep the voltage up -- and it helps if you have the voltage dialed down closer to that of the battery than 14.5V quick-charge -- it won't stress it too much.  That's the trade-off: quicker re-charging vs. less stress on the supply.

In fact, I've often argued that people don't need to have big fancy power supplies to have a radio in the house better than a handheld.  Some people don't want to buy a big expensive supply, yet the same people want to use a battery for backup power.  They already have a nice big power supply (what do you think powers your vehicle radio when the engine's not running?) -- they just need something to charge it!  A little 2-3A supply -- even something like a Battery Tender would work well* for a lot of people.  Sure, if you're going to be running marathon operating sessions w/ 50% transmitter duty, you'd better have a supply that's rated for something close to your radio's max draw or it won't be able to keep up, but if you're operating a 5% duty cycle, you only need 1/10 the charger.

*Be careful: "International" and "high efficiency" versions use switching power supplies that can generate some awful RFI. Stick to the big ol' models.  Incidentally, the battery buffer effect can also help reduce the 60 Hz hum that comes from using a linear power supply without sufficient filtering.  Again, though, it won't likely help the higher-frequency noise coming from a switcher.

Charles Boling,
Jan 8, 2017, 5:32 PM