DC-DC charger (B2B) options and general discussion

Zwy

Solar Enthusiast
The problem with large voltage drop is the change of volts that is dependent on the amps drawn. Not a great way of controlling charge input.
You're stretching there in the argument, you stated, " What do you know about voltage drop. It is my opinion you have this backwards in your head."

Nothing in my response said anything about charge input. But I digress, we shall have the discussion on several fronts. First, do you know the most accurate way to measure VD on a wire with no addition/subtraction or any other type of math, no calculator and gives 100% accurate results every time? If you don't, I'd be more than obliging to explain it to you.

Second, charge input is controlled by DC to DC charger which also is a step up converter in this case, 12v to 24v. Input 30a, outputs 15a. Watts in equals watts out minus some loss for the converter but that isn't relevant to the discussion. Output voltage is regulated by the DC to DC charger.

As for your response in the block above, you have voltage drop attempting to control amps from the way you worded it. It can be done, I don't know why you would want to waste energy as heat on a charging circuit but go for it. Take blower motor resistors, the amps drawn by the motor will be less, however the total amp draw with the resistor in the circuit will be the same regardless of speed, note I did not say load. A given load however will increase amps over a lighter loaded condition but still, the circuit with the resistor installed will still draw the same amps on the complete circuit with the increased load, motor will still draw less amps than with the resistor in the circuit. Not a hard concept to understand.

Now, let's look at the DC to DC charger, 12v to 24v step up type and the wiring required. On the 12v side, amps will be double and requires a wire size according to those amps. On the 24v side, the wire size can be 1/2 as only 1/2 the amps will be carried by that wire. Watts across the circuit remain the same, except for converter loss. In the case of a Victron 12/24 DC to DC, input is 12v 30A, output is 24v 15A.

Now. I'll really mess with your head. Take a length of wire and pass amps thru it to power an incandescent light bulb. Record the result. Now, add resistance to the circuit in series at the exact same voltage. Will the circuit draw more,less or the same amps? Correct answers only.

Second, is VD the same on a given length of wire at different voltages? Again, correct answer only.
 

rickst29

Solar Enthusiast
The problem with large voltage drop is the change of volts that is dependent on the amps drawn. Not a great way of controlling charge input.
This is true, but only for a while. "CV" at a high Voltage will start off with high current, and the received Voltage will be low, in comparison to what the DC-->DC battery charger thinks that it is putting at. But the current will drop off towards the end of the charging process, and the accuracy of the DC-->DC "SOC" determination will improve. It might stay in "CV" Absorb mode for a while longer (if its smart), and that is where most of the charging action will occur anyway. (LFP batteries will accept more in float stage than lead-acid batteries will, but most of the job should have been completed earlier.

Less accuracy, but higher efficiency.
 

CindySweden

New Member
LiFePO4 batteries are thirsty and can overtax an alternator if connected directly, The DC-DC charger acts as a middleman between the charging system and the lifepo4 battery and will limit the current to some maximum value (which I believe should be output current + inefficiency).
..and do you have to have a VSR as a middleman between the alternator (indirect the starter battery) and the DC-DC to protect the starter battery from draining? Or am I getting something wrong?

So, alternator - starter battery - VSR - DCDC - LFP ?

Sorry for being noob, still trying to figure this out..
 
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HRTKD

Boondocker
..and do you have to have a VSR as a middleman between the alternator (indirect the starter battery) and the DC-DC to protect the starter battery from draining? Or am I getting something wrong?

So, alternator - starter battery - VSR - DCDC - LFP ?

Sorry for being noob, still trying to figure this out..

Some DC-DC chargers have an alternator sense that lets it know that the alternator is operating and it's OK to draw from the input source.
 

CindySweden

New Member
Some DC-DC chargers have an alternator sense that lets it know that the alternator is operating and it's OK to draw from the input source.
I already bought an VSR before I even knew I had to protect my alternator from my LFP, so I might as well use it... If the DC-DC charger that hasn't got the sense also is cheaper I might go for that! Any tips on model/brand for 12v system? I have an old car, so a DC-DC that doesnt pull too many amps if preferable
 

CindySweden

New Member
New thought! Could I just buy an Victron converter instead, like this one, "Victron Orion-Tr DC-DC converter" and then put the VSR between the alternator and the converter and then get the same function as a DC-DC charger? No?
 

lolailando

New Member
With most VSR you dont need a dc-dc, the vsr will regulate the dc output to your batteries. A dc-dc is another tool to do the same: limit the amount of amps your batteries can draw to overheat and kill your alternator. A VSR is a better option just more expensive usually, but will give you more amps.
 

Mad_Texan

Solar Enthusiast
'It is really important to understand that a VSR is “NOT” a battery charger. It is simply a switch that allows you to connect two batteries in parallel based on a pre-set voltage range. '

 

lolailando

New Member
'It is really important to understand that a VSR is “NOT” a battery charger. It is simply a switch that allows you to connect two batteries in parallel based on a pre-set voltage range. '

My bad , disregard my post. Some alternator voltage smart regulators are also named VSR, I was referring to those. That vsr is the same function as a solenoid or a combiner....lots of names for similar things. My apologies
 

CindySweden

New Member
Just a very simple one. Using only this will cause problem by letting the LFP battery drain amps to fast and overheat the alternator, by the things I have read and understood. So. Putting a converter into the game as I write above.. What do you think?
 

HRTKD

Boondocker
Based on what I've heard of certain Victron DC-DC chargers, they have a built-in VSR. The Victron may be programmable which would allow you to cut off/begin charging at a higher threshold than 13.3v if you wanted to.
 

Zwy

Solar Enthusiast
Just a very simple one. Using only this will cause problem by letting the LFP battery drain amps to fast and overheat the alternator, by the things I have read and understood. So. Putting a converter into the game as I write above.. What do you think?
The problem with that VSR is you shouldn't mix LFP and FLA batteries using it. It's fine if all batteries are the same type.
 

RGeB

New Member
Is it OK to ask about an application that may become very common soon? Hybrid electric vehicles use 12 V for most purposes (other than traction and aircon), and they generally come with a 12 V ‘auxiliary’ battery (lead-acid, about 45 Ah) charged from the vehicle DC-DC converter. For some owners, it would make more sense to use LFP chemistry for the auxiliary battery (which does not crank an engine). The vehicle that I have tested (a 2019 rav4 hybrid) does seem to sense and send appropriate voltage to a 50 Ah LFP battery (14.2 V bulk stage, dropping to 13.6 V once the battery is charged). But at low SOC, an unregulated LFP battery can draw too much current for either the battery (1C) or the vehicle circuitry (~100 A). There is a single cable to deliver charge to and from the battery. Interposing a DC-DC charger would limit current to the battery, but these are generally unidirectional, so the battery can no longer provide power as needed. Bidirectional DC-DC converters exist, but currently they seem to be very expensive. Is it as simple as adding something like a Schottky power diode back from the battery to the vehicle circuit? Is there a better solution?
RavFuses.jpg
 

Chvless956c

New Member
I just converted my RV to a Renogy 40A DCDC charger from a isolation solenoid. My only concern with the renogy is that it starts as soon as ignition is in on/run. I still have the intellitec voltage sensitive delay relay from the isolation solenoid setup, has anyone used this to control the 12v ignition signal to a renovy, seems likes a decent way to delay the start of charging to allow altenator to come up first (waits for 12 seconds of voltage being greater than 13.3v or something like that)
 
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