DC - DC converter options , build or buy , prefer build ? - Page 1
Forum section: electronics / projects

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 05, 2017, 12:39:48 pm
content: Adding post here , had in renewable energy section , but no response .I have looked at store bought stuff , but sometimes just too many choices & not much with the spec.s I'm looking for .Also , info for just building one , or a couple , adjustable ?I am collecting old 18650's to start to experiment with using on medium size and then maybe something like home-brew Tesla power wall .The mid size is for ham radio gear , most of that runs on 13.8vdc & to maintain high RF output , many rigs should have a steady voltage close to 13.8 .Then putting together a 18650 block , so using 4 in series & maintaining , lets say 13.6 , a fully charged 4.2 x 4 = 16.8Image result for 18650 state of charge chart - not sure if the copy/paste is going to work ?But keeping the battery block from discharging too much to extend battery life and trading off with that to maintain useful voltage .The main question is controlling voltage high point [ 13.8 ] WITHOUT dissipating capacity [ using resistor ] ?I am guessing some type of DC-DC auto regulator .A side note , I have been using solar for decades with Trojan L16 x4 , 5000 watt inverter / charger and now around 1,200-1,500 watts in solar panels , the FLD - flooded lead acid batteries last over 12+ yrs. and need replacing , and now about double the cost $1,000 - $1,400 .

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 05, 2017, 02:28:20 pm
content: What, precisely, are you looking to do? It sounds like you are currently using a 1.5kW solar panel to keep a bank of flooded lead acid batteries charged for powering both a 5kW inverter and some ham radio equipment, correct?And that you want to replace the FLA batteries with Li-ion but need to address the difference in both charging and output voltage, correct?It should be possible to modify a few component values in the 5kW inverter to allow it to run directly from 16.8V. The RF gear - particularly the PA - won't like that much of an increase in voltage, however, so a buck converter would be the best solution.Even better, though, would be to increase the battery bank voltage even more and replace the 5kW inverter with something that works on 24V or 48V nominal input, as well as a buck converter for supplying 13.8V to the ham equipment.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 05, 2017, 06:10:17 pm
content: The working with 13.8vdc for ham gear is building a battery bank with the 18650 lithium cells , that may or may not be connected to solar ,in a bank 4series and enough parallel to give the capacity needed , so a controller that good keep the voltage at a steady around low 13vdc to 14v ?The inverter / solar setup , is a separate situation , that could have ham gear included , it is going to be the same as my old [ FLA battery system ] , just use the lithium bank to avoid spending too much on the FLA again , I am accumulating 18650's now .I lived full time RVing , running AC / furnace every thing for over 12 yrs. with the old setup , but have 3 1/2 times more solar now .The inverter/charger/tranfer-sitch I have now costs about $1,500 new , when I can afford , I would upgrade the inverter to something like 4-6 kw sine wave , grid tied 240vac , but that would cost more than I can afford at this time .And a friend is putting together a new class A motor home with large solar , and maybe follow me with lithium battery , he's got the money to buy a Tesla car battery [ or part of one ] , and also looking at the voltage ranges to be covered , the RV has lots of 12v only stuff , and most the solar is going to 24-48v , and we are both hams , so being able to draw 20-40 amps for ham gear - at 13.8 .When you have loads at 13.8 , a healthy draw of 20amp ++ , it does not make sense to go through an inverter , loss's .So kinda looking at the possibility of power supply's that allow for multiple voltages , both in & out [ have you CAKE & eat it too?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2017, 07:53:13 pm
content: Why not keep ONE flooded Lead Acid battery to power the ham gear, and float charge it via a DC-DC converter with adjustable voltage and current limits off the Lithium bank. That way all the rigs can be run at the 13.8V they like, you only have to find a DC-DC converter that can keep up with the average daily current, not the peak transmit current, and if the DC-DC converter fails shorted, the Lead Acid battery will hold down the terminal voltage long enough for the input fuse to blow without damaging the rigs.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:00:38 pm
content: I see a few problems with that suggestion , There are not any real FLA deep cycle batteries [ that I am trying to avoid - because of cost & weight - and lack of 13.8 ] so that 3 there negatives .Next is attaching 2 battery banks of different chemistries , is at least 2-3 more issues - drawing down of Li to lead , again weight issue , trying to charge both [ 2 different chemistries ] .Creating a failure point - 2 battery banks , DC-DC converter waiting for fuse to blow .Kinda breaking the #1 rule of engineering - KISSThen the primary question - if a DC-DC converter is use - no suggestion as to finding one with the match spec.s ?At least that's what I see , maybe not grasping your perspective ?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2017, 10:44:47 pm
content: My perspective is: maintaining 13.6V out to a reasonable tight tolerance, with pulsed loads >20A with an input voltage range of 12.0 to 16.8V and without HF ripple that could interact badly with a transmitter and cause serious intermodulation problems is difficult, expensive and potentially unreliable, especially if you want to design and build it yourself.You could start from a significantly higher voltage e.g. a nominal 48V so you could use an off-the-shelf telecom DC-DC converter then add some serious filtering on the output to prevent undesirable interactions between the transceivers and the DC-DC converter's control loop, but that's going to cost you big $$$$.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 05, 2017, 10:55:19 pm
content: Ok , I wasn't taking RFI into account .My friend can afford more and is leaning to higher voltage .Maybe some day in future when I can up grade , and I have a system now [ minus a battery bank , old one dead after 12+ yrs ] , my inverter is 12v , I have a mix of 12v & 24v solar panels , I started in solar about 35 yrs. ago when 12v was the most common , now 24v is it , the future may be mini inverters as part of panel .Thanks