Please check my auxiliary power supply design?

[KC2ELS]

It's too late for this year, so I'm trying to design an auxiliary power supply for next year.

The equipment I want to run is limited -- a 5W HT in a base station charger, an AEA PK-232 TNC, and an old laptop.  Unfortunately, the only way I can power the laptop is through an inverter so that alone is 75W of power.  The other equipment comes up to 65W for a total of 140W, or 10A at 13.8VDC.

I found a nice (but expensive!) DC-to-DC converter which produces a regulated output of 13.8VDC at 80% efficiency from 20-28VDC.  Here's where I'm confused, though.  Do I need 600Ah of capacity (10A*48h*1.25) in order to run this thing for 48h straight?  If so, is that at 12V or 24V?  Since I'm supplying roughly twice the output voltage, wouldn't I need roughly half the capacity?  Either way, that's an *expensive* amount of batteries.  The largest battery I found in a quick web search costs over $150 and... well, I'd need a few of them.

Am I doing something wrong, or is it really this expensive?

[KD6JLS]

There's a point where a small used generator is cheaper and more useful than a big array of batteries. You might be there.

-Mark KD6JLS

[KD5MSY]

Here is what I use and it works good.  With the help of a DC Engener friend of mine. I run a 12V 150 ah. bat, with a 20a 13.8vdc p/s, and a 1500W converter.  I run the feeds off of the P/S in PARALLEL with the BAT. To a Fused power distribution unit, and the inverter fed off of that. The bat Stays charged by the P/S and stays condidtioned from use by the radios and equipment. and when there is a brown out or a outage the equipment dosent even flinch.  as recomended I fuse both leads coming from the P/S and to the PDU, one not form normal reasons just incase the leads ever shorted together to keep the bat. from blowing up! I run HEAVY insulated 6 ga. wire for all the main leads with 6 ga. crimed lugs. This sounds weird but it is the same way we run our communication sites and is the way most telco companys run there sites. With it in this configuration "apparantly" all the equipment is run off the battery, and the battery is suposed to act like a line conditioner.  Here is a link the the bat i have and dont know how much the curent cost is new, but they are suposed to last around 10yrs if taken care of. http://www.cdpowercom.com/cd/products/batteries/sealed/fa12_383.htm
Just rember if you have a long period with no power your P/S will be working over time to charge the bat up if your talking on the radios and using the laptop.
I have ran for many hours this way during black outs, and have not have any problems. My system is fused with 100 amp circuit brakers so i can disconect the P/S if the power has been off for a while and i still need to talk, and just turn them back on when im done and dont plan on being on for a while.
Hope this helps.
Robert Howell
KD5MSY

[KD5MSY]

Two quick questions 1)Why would you need a "DC/DC" inverter your laptop sould run off of a standard 110 Vac out of the wall outlet, to the  inverter/power cord or you would have to build a conector. and if like mine is suply it with something like 16V @? amps. Go with a 12V DC to 110Vac inverter and keep every thing standard.
2) is your inverter "using 75W just sitting there?" more than likely that is what it will provide at max draw AC power. I dont think you would include the inverter in the Ah calculation.

And just as a matter of standard i would go with 12V battery system. Plus i think you will find even just regular 12V deep cycle bat's ALOT cheeper than you will 24V deep cycle bat's.
Best of luck,
Robert Howell
KD5MSY

[KD5MSY]

Oh, forgot to tell you if you do hook the P/S in parallel to the bat. you MUST use diodes in line to prevent possible feed back into the P/S incase of power outage.  The diode's MUST be able to handle the FULL out put amps of the P/S!

[WA9SVD]

I'm puzzled why you are considering operating from 24-28 volt DC, unless this is some sort of aircraft system.  The H-T and other equipment should be able to operate off a 12 V auto or preferable marine deep-cycle battery.  If the laptop won't work directly off 12 V, than an inexpensive 12V-120 VAC inverter is a possibility, but try it first, to amke sure the RFI noise from the inverter is not objectionable.
    Give us more info, and we can be more specific.