backchannel: Victron MPPT + Renogy DC DC

2+2 = 4

from this thread on Will’s DIY solar forum.

(OP) … I have… one 100W solar panel, Victron 75/10 SCC and Li-Time 50Ah battery. … I would like to add the ability to charge the battery via alternator when driving. …So if I were to get the Renogy 20A DC to DC charger and connect its output to the battery (with solar from SCC also connected directly to the battery) it would be ok, correct? One 100W solar panel produces up to 5.5A, the Renogy DC to DC outputs 20A and the Li-Time 50Ah battery has 50A maximum charging current. So 25.5A < 50A and all should be working right

Yes, that is how it would work.

If we want to dig a little deeper: how comfortable are we with a sustained charge rate of 0.51C? After much reading and biased by my meager budget, my approach is:

  • charge ≤0.4C (20A in this case) **at moderate voltages like 3.45Vpc (13.8v)
  • charge <0.2C (10A in this case) at the Li profile voltages in the devices, which I think are too damn high

If we are comfortable with 0.51C at (IMO) excessive voltages then perhaps we should examine the alternator charging setup. See below.

(knowledgeable poster whose opinion I respect) Connecting the litime lithium battery to the vehicle battery via a relay of some sort is not recomended

This position is a double standard at best, and FUD at worst. If we wanted to inform the OP we would mention other things in this class that are not recommended:

  1. using an oversized DC-DC
  2. relying on a shoddy Li profile
  3. spending much more money than necessary to get similar results in a given use case

(knowledgeable poster) The 50Ah lithium has a recomended charge current of 10 amps, and a maximum of 50 amps, thus some current limiting device is needed.

Thus? Are we suggesting that a relay could provide greater current than the BMS will allow?1 Are we suggesting overcurrent is unavoidable, or that DC-DC is the only limit on current acceptance?2

a thought experiment

What if a relay could provide analagous results for 1/3rd the price? Would that be acceptable?3

if “yes”, continue

What if a relay passed voltage to the bank within the battery’s charging spec? Would that be acceptable? 4

if “yes”, continue

What if a relay passed current to the bank within the battery’s charging spec and alternator’s comfort range? Would that be acceptable? 5

if “yes”, continue

It is quite common for all those things to be true in actual use. People don’t know that because they haven’t tried it. Or Brand Fandom has given them brainworms.

I truly don’t care how people charge their battery bank. I care when people mislead beginners.

how direct charging would work in this scenario

There are differences:

  1. current acceptance will be inversely related to bank SoC: more current at low SoC and less current at high SoC. If one were feeling charitable they might say “more current when you need it and less when you don’t”
  2. because the relay is just a pass-through from the alternator, current through the relay will decrease when solar charging is present. This happens because the solar is pushing up bank voltage. From the relay circuit’s PoV the voltage between apparent “resting” voltage and alternator voltage is reduced. I=V/R again, so it’s more of a 2+2=3.5 situation..

In the dataset LiFePO4 banks average 0.32C in the lower knee, quickly popping up to 0.2C in the flat portion of the voltage curve, then fall off to 0.1C in the upper knee. There is an odd pattern I do not yet fully understand where larger banks tend to have lower charging C rates than small ones. I suspect it’s due to more voltage sag when direct-charging the bigger banks. So very small banks like 50Ah tend to have higher acceptance. I’ve seen up to to 0.8C in the lower knee before leveling off at 0.4C in the broad middle, and 0.10C in the upper knee. Most of the time the relay scenario would have lower current acceptance: 5.5A solar charging would depress alternator charging for the reasons given above. A fraction of the time it would be higher, or lower.

combo chargers

(OP) Any issues with such approach besides having two boxes instead of one that combines SCC and DC to DC charger?

OP has a nice Victron MPPT so I would not obselete that by installing a combo charger. But since they asked, here might be some marginal benefits for some people who are starting from scratch. Using the DCC30S as an example:

  • ability to limit the combined output to 15A or 20A or whatever.
  • starter battery charging (maintenance)
  • voltage/temperature sensing at the bank (the Vic controller will talk to a BMV or Smart BatterySense but the [Orion-Tr 12 12 18]( will not, AFAIK) for more accurate charge and low temp charging cutoff.

I don’t like combo boxes because of their limitations, but 100% of the Renogy combo units I’ve had my hands on worked to spec.

  1. I’m not saying it’s impossible, but in the 49 documented direct charging setups I’ve recorded, ZERO reported getting so much current that the BMS tripped. experience > FUD 

  2. current is limited by resistance in the circuit and by voltage sag. I=V/R, people. 

  3. this question reveals who is making rational decisions and who is rationalizing a position. 

  4. bank voltage stays in the 13s in the broad middle of SoC. I shut down my alternator charging at 13.7v to let solar finish up, and have maybe had to do so 5 times in as many years. I bought a HVD to automate it but it’s so rare an occurance I haven’t wired it up yet. 

  5. experience and data from others demonstrate that LFP banks charged by chassis-grounded relay average 0.2C in the middle SoC and 0.32C at low SoC. In the latter case, the steep lower knee means the voltage comes up into the flat part of the charging curve quickly and current levels off to the 0.2C range. The main drawback of a relay setup isn’t excessive current, it’s insufficient current at higher SoC when the average drops to ~0.1C.