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dc-dc CHARGING

You will definitely need a "power assist" from somewhere. It's not good and here's why...

After setting the Renogy DC-DC 20A charger to a lead acid battery profile, as you recommend, it will only put < 60A of charge to your LiFePO4 battery bank over the 3 hour bulk/absorption time. That's fine if that's all you need to bring the battery up to near full capacity but if you are down 100A at the start you still require >2 hours to charge to 100%. But now your battery charger has entered "float" or "power assist" [sic] since you aren't using the correct charge profile. Because of this, your charger is trying to charge at a lesser rate with a parasitic load adding more stress to the charge cycle. Not good but probably not going to cause any fires. There are some other detrimental things shortening the life of your batteries if you use the wrong profile for your battery chemistry. I can go into those things if you would like.

In the LiFePO4 charge mode you are still charging batteries with a load but at a much more optimal voltage for the battery chemistry. You don't need a "float" for LiFePO4. They take what you give them. That's why you are protecting your alternator with a charger/isolator in the first place.

If you are charging LiFePO4 batteries then you should use a LiFePO4 charge profile for optimum charging. The charger will still "load share" (a more precise term) much like charging your cell phone or laptop while using it. It will cause less stress on the batteries this way.

So, although a lead acid battery profile will work it is not the best way to charge LiFePO4 batteries. If you are charging with a parasitic load, like I am most of the time, you want to get to ~85% and then let the batteries carry the load. That's what they are there for, not the charger. With LiFePO4 you can always charge again. They work well like that. They love to cycle through between ~30% and 85%.
Just stumbled upon this DC/DC charger thread. I will be hooking up a Renogy 20A DC/DC charger to charge up my DIY 120A Lifepo4
S4 12v battery, with Daly BMS. After going through the Renogy manual, I was planing to set the charging profile to Lifepo4, with DIP switches 3,4 set to ON and 5 to OFF. Now can't decided what voltage to charge at. Was thinking of playing it safe and setting the charge at 14.0V or 14.2 to keep the battery topped up at around 85%-90%. That would keep the 4 cells at 3.55 max. I want to avoid over charging the battery since the Renogy keeps charging even if battery is full, albeit with low current. I do wish the Renogy would actually stop charging, but since it doesn't, the next best thing would be to keep the charge voltage under 14.6V. What do you think?
 
I do wish the Renogy would actually stop charging, but since it doesn't, the next best thing would be to keep the charge voltage under 14.6V.
According to pages 20-22 of this document https://www.renogy.com/content/RNG-DCC1212-60-BC/DCC1212-204060-Manual.pdf it does stop if SW5=off.
Float is omitted and absorb is limited to 3 hours.
This 3 hour time limit is the charge termination logic that I referred to earlier in the thread.
Its also safe to use profiles with float as long as the float voltage is below full resting voltage of the battery.
 
According to pages 20-22 of this document https://www.renogy.com/content/RNG-DCC1212-60-BC/DCC1212-204060-Manual.pdf it does stop if SW5=off.
Float is omitted and absorb is limited to 3 hours.
This 3 hour time limit is the charge termination logic that I referred to earlier in the thread.
Its also safe to use profiles with float as long as the float voltage is below full resting voltage of the battery.
So with SW 5 OFF, and SW 3,4 set to ON, the charge profiles will go to the TYPE 2 voltage range, 14.0V-14.6V. If I select 14.0V, then the charge will be limited to 3 hours according to page 23. With SW 5 OFF, there will be a combined Bulk/Absorption charge only, and the float charge will not occur. If cells are balanced, the 14.0V charge should work out to each cell being around 3.50V. Well under the 3.65V max allowed.
 
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I want to avoid over charging the battery since the Renogy keeps charging even if battery is full, albeit with low current.
Your BMS will prevent overcharging per the linked video (at about the 5 minute mark).

I recommended earlier to just put a on/off switch on the D+ line to turn the DC-DC charger off. Others in this thread have a more automatic solution. Also, Renogy respond to another poster that the charger does not shut off automatically. It continues to charge until the D+ signal is turned off. Don't confuse "float" with a "no charge" state (or "off" if you like). It's not the same and if you use the LFPo4 profile the charger will continue to charge at the voltage setting you select as long as the charger is on.

So, using the D+ line is the best method for shutting off the charge cycle. There's a lot of good information about DC-DC charging in this thread.
 
Your BMS will prevent overcharging per the linked video (at about the 5 minute mark).

I recommended earlier to just put a on/off switch on the D+ line to turn the DC-DC charger off. Others in this thread have a more automatic solution. Also, Renogy respond to another poster that the charger does not shut off automatically. It continues to charge until the D+ signal is turned off. Don't confuse "float" with a "no charge" state (or "off" if you like). It's not the same and if you use the LFPo4 profile the charger will continue to charge at the voltage setting you select as long as the charger is on.

So, using the D+ line is the best method for shutting off the charge cycle. There's a lot of good information about DC-DC charging in this thread.
People are mixing up LA profile with the LFP profile.
 
Alot of people in here seem confused about how battery charging works.

A battery charges when the applied voltage is higher than the resting voltage of the battery at its current state of charge.

The higher that voltage the faster it charges.

A Lifepo4 Cell at 100% SOCs resting voltage is typically 3.35v or 13.4v in a 12v nominal battery. So by setting your float at or below 13.4v effectively stops charging it and thus preventing you from overcharging.

The advantage of floating instead of turning off the charger entirely is that any loads will run off the charger instead of draining the battery.

Note: Lead acid batteries typically float higher than their resting voltage because they are less susceptible to damage due to overcharging.
 
Hi, I already install the Renogy 40A and connect the D+ to the BMV700 relay but now I am curious how to setup the relay in the BMV700.
Can someone advice me how to ?

This is the diagram how I connect it and how I did the setup for the relay
 

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Does anyone know if it works like I suggest ? Any advice is welcom (y)

@DJSmiley do you use the relay of the BMV700 to switch the Renogy 40A booster ?
 
That's what I think. But the widely used Victron Orion-Tr Smart has a Float stage same as the CTEK D250SE. The only DC-DC charger I could find where the float stage could be disabled is the Enerdrive DC2DC but I can't find a European distributor for that one.
Have you looked at the Votronic kit in Europe?
We use it very effectively (based in UK)
 
Does anyone know if it works like I suggest ? Any advice is welcom (y)

@DJSmiley do you use the relay of the BMV700 to switch the Renogy 40A booster ?

No, I don't. If needed, I can, but I need to run a seperate wire to the BMV (which is a PITA since the majorty of my cable ducts are either full or hard to reach.)
I'm not planning to, unless I really need to. I have a switch on my dashboard in the cab so I can manually do it, and the Renogy is set to 14.2V which seems to work fine, I noticed the current is dropping once the 14.2V is reached, so it doesn't seem to stay at 14.2V, just as it should. Haven't fully monitored the full process yet.

Based on my BMV, and the weather predictions, I can estimate if I need the DC-DC at all, or if the solar panels are likely to topup the batteries when needed. Also, with 280Ah I don't have the cells to be full each day, since I can coop with multiple days without or with less sun before they are completely flat.
Since my van will be use primary in the summer (and slightly in the spring/autumn) I think the DC isn't needed on a daily base and I think the manual setup will do fine.

I was thinking of using the BMV relay to switch on my heater to use the excess of solar in the future. Seems more usefull than switching the Renogy which is supposed to - and does as far as I have seen - switch off once the cells are full.

I also could use something like a voltage adjustable relay, with adjustable set-points: Enable relay when voltage is eg <13.4V, and release if 14.2V is hit. That would do basicly the same, which, in my case, is easier, since I don't have to run wires from the BMV, and can add the relay in a small box near the Renogy. Something like https://nl.aliexpress.com/item/4000617345994.html
 
The relatively harmelss thing about "cheap Solar chargers" which perform all decision based on battery Voltage and not charging current, combined with an insistence that they WILL run "Float with in infinite run time is this:

The "Boost Return Voltage" (that's the EpEver name for a parameter which restarts the higher-voltage "CV" charging mode) must be set below the "Float Charging Voltage".

And that is basically the only issue which I see. "Excess Solar" should be ENTIRELY disallowed by the Solar Controller. (I don't know the operating logic of the Renogy PWM-type battery chargers but assume that they do fall into a float stage - based on either "detected battery Voltage" or charging Current falling to near zero during CV charging).

If there zero phantom loads, then your batteries (example: "12.8V" 4S-LFP batteries) WILL end up reaching full "Float Voltage". But a low Float Charge Voltage (e.g. 13.6V, or 13.2V, or even 13.0V) should only cause very low Voltage "stress" on the BMS, and the cells will never be charge all the way up to 14.2V (the minimum for balancing the cells) until you FORCE that higher-Voltage "CV" stage to occur, for an adeqaute time, by pulling the battey(s) down to "Boost Return Voltage" *13.4V, or 13.0V, or even as low as 12.8V for those 3 respective "low Float Voltage values.)
- - -
In my own RV configuration, I have an EpEver SCC with "Boost/CV" max Voltage of 14.5V, and a minimum boost time of 180 minutes. That nearly assure the battery string (3 x LFP batteries in parallel at "12.8V") to reach 100% charge AND balance the cells at nearly maximum voltage allowed by the 3 BMS boards. My "Boost-Return Voltage is pretty high, at 13.2V, restarting whenever the battery bank falls to even 93% SOC.

That's great for camping - not so great for long term storage. But my RV Trailer also folds down and stores in the garage, without sunlight. In long-term storage, charging is done through an even less dynamic PD Power Converter (basically 14.6V continuous, AFAICT). In storage, I only plug in the PD once a month, with "phantom loads" (a low temp "heating thermostat, and Coulomb Counter meter) pulling it much lower between recharge cycles. (The coulomb counter isn't visible to me when the Trailer is folded up -- all I can see is Voltage, at roughly 12.8V).

I'm basically doing the manual thing, charging back up to 100% from somewhere in the 20%-75% SOC range, and then pulling the plug on the charger. A "five fingered Relay" with a voltmeter. ;)
 
OK thanks DJSmiley for the explanation, then I suppost you can´t help me with the setup I made for the relay.
 
The relatively harmelss thing about "cheap Solar chargers" which perform all decision based on battery Voltage and not charging current, combined with an insistence that they WILL run "Float with in infinite run time is this:
Also unless you live in an extreme latitude the sun rises and sets.
 
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