Battery buffer to power supply?

Started by gil, March 14, 2014, 12:02:48 PM

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I'm afraid the CW gods will be pouncing on me soon because I will be getting a FT-2900R. Yep, VHF 75W. I'm getting the antenna for it today, a Hustler of some sort  ::)
So, that's 11A to 15A on TX full power. I have a 5A switching power supply.. Of course I can get a 20A PS for only $109.. But if the power goes out, I'm still SOOL. If I put a battery directly in parallel in the circuit, that would work fine, unless the battery is flat and all the sudden draws way too much from the PS when the power comes back on or after much use at full power.

What type of circuit would you guys suggest I use? A battery charger / current-limiter between the battery and PS? Are there dedicated devices for this task?




At these current levels I assume you are talking about a lead acid.  The simplest thing is anything that will keep the battery at 13.8V.  You can just put it in parallel with your 5A supply, assuming it is set to about 13.8V. 12V lead acid batteries are meant to be held at 13.8V which is why most ham gear and their "12V" supplies are set to 13.8V.  But for short money you can get a float charger from the car parts store or maybe Harbor Freight that will really treat the battery right.  A 2A or 3A unit should keep up with your average load.  Once you are done it will bring the battery up to something around 14.5V then let it drop down to 13.8 and hold it there.  Keeps the battery healthier over the long haul. 


FM? 75 Watts? Blasphemy!!! 8)

West Mountain sell the power gate that will do what you want, but at $140 you're approaching the price of a power supply designed to parallel to a battery.

No doubt you could build a circuit utilizing a relay or current limiting but what about just just disconnecting the power supply from the battery if utility power fails and then just use a battery charger to recharge the battery once power is resumed? Sometimes the simplest solution is the best.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin



March 14, 2014, 02:17:44 PM #3 Last Edit: March 14, 2014, 02:44:14 PM by underhill
FWIW, this is what I do, for power.  I float a 12V battery on the output of a 14V 25A switching supply, designed to run radios direct.  I buffer the supply from the battery, with a solar charge controller, (SCC3) so the battery will not back-discharge into the supply when it is off, or pull down the powersupply too much if it is drained low when the supply comes back on.  Has run well for me for almost 8 years now.  2004, maybe going on 10 years?

I modularized my system to make it easy to switch in or out charge sources and batteries.  I use powerpole connectors, have for years, work for me, but ymmv, as in everything else. 

I use a couple 12V lead acid deep cycle batteries, but can switch in or out smaller gel cells I keep around for portable use quite easily.

I wanted to ensure isolation from the charging source to the battery, I ended up choosing a solar charge controller, from Cirkits, model SCC3.  12V/20A and fits into a square electrical outlet box nicely.  I have used powerpole connectors thruout my system for many years, so I picked up a 'rigrunner' fused powerpole powerstrip from West Mountain Radio for final interface to where I want to use the power.

To charge the batteries, I have several choices.  I have about 300W of solar panels, just inexpensive amorphous silicon on glass, picked up from Harbor Freight, for fixed operation.  Unfortunately right now they are not deployed, mounting issues, and other projects that have kept me from getting them up.  They are wired up and ready to go, if needed, just the mount system is not permanent.

I have also a couple portable folding panels, from CT Solar, the 33W backpack panels.  Couple of those, enough to power a full 100W qro digital station in the field I used to have, along with a tiny charge controller (portable use only) I picked up from Buddipole. Cost as much as the SCC3, and not nearly as flexable, but tiny, use is targeted for field/portable use.

Options, each panel section can be removed, I have noticed, if only small power amounts are needed.  Again, very modular.

Normally I have been just running a cheap 13.8V 25A switching supply I picked up from HRO (MFJ-4125), run it on a mechanical 24hr timer so it is only on part of the day, to float charge the battery thru the solar charge controller, which does provide isolation as well as some charge regulation for the battery.  A battery charger may work just as well, I just have the switching supply with the timer, it works, has proven itself that I don't have to monitor it closely, and it's good enough for now.  I have been lucky perhaps, but I haven't found any switching noise in my system from the supply.  Also the little mfj-4125 is buffered by the battery, so any possible power glitches from it have not been apparent.

Other than changing out batteries recently, after 7-8 years the lead acid wet cells do get weak, especially the wallmart ones, the system has been operating reliably with little attention needed for about 8 years now.  Would be nice to have the solar panels up more permanent, just need to get around to mounting them to withstand high wind without having them go visiting the neighbors when the santa ana winds come a-blowing :/

I have not messed with lithium batteries yet, except the standard ones in my ht.  Pricy, and they scare me a little, they do.

Personally, I can't see anything wrong with having radio equipment on hand that will give you information of what's going on around you.  While I have been diligient at my cw studies, ~1/2 hour a day min since Nov, min, seems not everyone is willing to even consider doing cw (sacraledge!).  There have been a couple of times now, that having a shortwave rcvr on a camping trip, or having vhf/uhf available coming back from a business trip, has either been a comfort (collapse of the old soviet union- we were camping out that week), or actually kept me safe and out of the middle of a developing riot (the rodney king riots).

My personal first goto radio, when anything happens, is probably my little VX-6R.  My FT-817 is probably next, but doesn't have the receive coverage the vx6 does, and does use more juice. My HW-8 and the little NorCal-40 are fun, but are not capable of receiving everything I think I need to hear.

The VX6 Works as a great receiver and scanner, fairly low drain (~150mA), receives from .5MHz thru 1GHz pretty continuous.  Will not do trunking, nor digital p25 or whatever the slow migration of public services is to, but gets everything else. Good to use for gathering intel on what just happened (I live in equake country), or whatever else might be going on.  Works well on 2M 220 and 440MHz as well, but the receiver/scanner is proven itself to be really handy. 

I also have a FT-7900, dual band version of your FT-2900.  Works well, makes it easy to reach out to distant repeaters, but doesn't have the receive coverage of the HT tho.



QuoteFM? 75 Watts? Blasphemy!!!

;D Yes, though with a good antenna I'll probably use it at the 5W level, so it's QRP :P
That PWR gate still looks like it would need a 20A power supply, I don't want to have to buy both.
Getting away with using my 5A supply and a battery would be best.

Bob yes, it's for a SLA and I have a trickle charger, but it doesn't seem like the proper solution...

QuoteI ended up choosing a solar charge controller, from Cirkits, model SCC3

I built one of those, it's somewhere in my closet!

I am trying to cover my local ground here in case of a hurricane. HF is great but for local, not so much. So, it's going to be HF CW and 2m FM for me. I don't think I need anything else, and I also have Winlink, so... There are many repeaters I could hit with 75W around here.

Thanks everyone for the advise. I'll post about my installation in another thread.