31 replies to this topic

[rando]

The fuse/breaker size depends on the size of the wire you run.   It needs to be smaller than the max ampacity for the wire.

[lmwilco1]

I have a slightly different problem that could use a similar solution. I have the Renogy DCC50S DC-DC charger, 2 170W Rich Solar panels, and a Progressive Dynamics Lithium changer for my 190AH LifePO4 battery. The Progressive Dynamic charger is effectively a 14.6v power supply. I was thinking about connecting it in parallel with the solar cells so when the converter had power it would be able to charge the battery using he charging profile of the DC-DC charger. 

 

This seems to be a pretty similar use case to what is done in the thread, and thoughts if this is workable?

[rando]

I have a slightly different problem that could use a similar solution. I have the Renogy DCC50S DC-DC charger, 2 170W Rich Solar panels, and a Progressive Dynamics Lithium changer for my 190AH LifePO4 battery. The Progressive Dynamic charger is effectively a 14.6v power supply. I was thinking about connecting it in parallel with the solar cells so when the converter had power it would be able to charge the battery using he charging profile of the DC-DC charger. 

 

This seems to be a pretty similar use case to what is done in the thread, and thoughts if this is workable?

 

Which solar charge controller are you using?

[lmwilco1]

The Renogy is both a DC-DC charger as well as a MPPT solar controller.

[rando]

I am not familiar with Renogy, but is there a way to set the maximum current in the solar charge controller?  Otherwise it will try and draw 50A from your charger.  Can the charger source 50A continuously?    This is one of the design parameters in this setup - the boost convert (or charger in your case) must be able to handle the current the MPPT will try to draw. 

 

The other issue may be the solar panel open circuit voltage, which is likely higher than the PD charger output voltage.  The PD charger may or may not like having it's output pulled up to 18-19V.  You could address this with an appropriately sized diode between the PD charger and the solar panels. 

[lmwilco1]

The charger has to be current limited, otherwise the battery would pull unlimited current out of it. Would feeding the MPPT controller be different?

[rando]

The difference is the I-V curve for the battery vs the MPPT.   

 

You are right that the charger will hit its current limit with battery, then the output voltage will drop, the battery will draw less current and a stable equilibrium will be established.   

 

With the MPPT, the current it tries to draw will actually increase as the charger output voltage drops, as it tries to maximize the output power.   The MPPT algorithm may be smart enough to figure this out and back off on the current draw, but maybe not as the I-V curve for the charger is much different for the charger than for a solar panel.  

 

Assuming the charge is rated at less than 50A: Worst case scenario, it causes an overload situation for the charger and it shuts down, or pops a fuse.   Best case scenario, it works correctly and finds the maximum power point for the charger and will run it there until the battery is charged, but in this case it is going to run the charger at its absolute maximum until the battery is charged.  Depending on the design margins for the charger, it may not enjoy this. 

 

This is all a theoretical discussion without really knowing the details, so it could be totally wrong.  However, I completely agree that charging the lithium batteries at a constant 14.6V is not a good idea, so something should be done.   Ideally you want the charger (or power supply) to have a power rating significantly above the MPPT power rating, then you know it will work. 

 

Also, unless you really need to charge fast, I don't really like the idea of such high currents, they just stress everything.   Could you get a smaller say 10-20a shore power charger with an adjustable charge profile? 

[lmwilco1]

I will look into alternate chargers. Thanks for your input.

 

Louis

[lmwilco1]

One other question, with a 190AH battery, 50A charging is roughly .25C, doesn't that seem like a safe charging limit?

[rando]

Generally 0.25C is safe for the battery, but it depends on the BMS and battery specifics.

 

However, unless there is a need to charge fast it is almost always better to charge slowly - less heat, lighter components, less risk of catastrophe if something goes wrong.   

 

I can see where you may want to charge faster from the alternator or solar, as you may only drive for a few hours or have a limited amount of sun.  But for shore power, it seems like 99% of the time you are either plugging in the driveway for days - months at a time, or plugging in at a camp site for at least 12 - 24 hours.   In either situation, there is not really a need to charge over  ~0.1C.  In these situations it seems more important to charge carefully than quickly.