Renogy 20A dc to DC report

theoldwizard1 wrote:

BFL13 wrote:
#8 might not be enough. I had to change my input neg path to use the frame (as a really fat wire) to get my voltage drop small enough to keep input voltage in the 13s.

The whole point of a DC-DC charger is that it will accept low input voltage and still provide the correct output voltage.

Yes, it has "boost" (and "buck" if needed) but you cannot control the input, only the output setting voltage. The 20 amps is built in. Once you set the output voltage, you now have the output watts it needs to "supply"

So now the unit will "demand" the watts it needs for input. You have no control over how many amps that will be. All you can do is tinker with input voltage to try and keep the amps demand from being too high for the alternator. Also you don't control the vehicle's voltage, just the size of the input wiring.

The alternator should not have more than about 1/3 of its rating demanded ("pulled") from it, so you have to know your alternator's rating before you pick your DC-DC "size" in output amps. We have had several threads about that on here.

Some on here have reported how they use really fat wire on their inputs and the truck frame as needed, and have their DC-DC pulling only a few amps more than their rated output amps "size". The Renogy unit says it can pull as much as 30 amps to output 20 amps, but you can use fatter wire to get higher input voltage and thus have fewer amps being pulled to get the same input watts.

Going back to the example output setting of 14.7v and 20 amps for 294w and spec 90% efficiency, it will "demand" 327w input.

If the vehicle is running at 14v after warming up, and you have 13 amps showing at the DC-DC input (1 volt drop) it will pull 25 amps. ( if you did get 14 at the unit, amps would be 23.4.

My truck alternator is rated at 105 amps and 1/3 of that is 35 amps, so I am good with that pull of 25 amps. That leaves a few amps to run the truck too (as was mentioned by one of the guys on here recently)

People with vehicles that have alternators rated at over 200 amps can use the Renogy 40 amper which might pull 60 amps and they will be ok as long as they have fat/short enough wiring for input.

On the output wiring, you can set the unit for your battery spec of say 14.7v for charging. But how many volts reach the battery of that 14.7? You want it all, but if the battery can still get charged with 14.5v (the specs usually give a range of ok voltages eg 14.5-14.8) So you have to use fat/short enough output wire to get 20 amps at 14.5v at the battery posts, allowing a 0.2v drop.

PT--I thought you did a bench test and it was not installed yet. Confusing. Anyway, same thing--try it out and if it does not hold 20 amps at the set output voltage, you must improve the input so it will.