Charging 48v battery from another 48v battery

[The Good Package]

I've built a portable 48v lifepo4 power station with inverter, and I'm looking for the "best" way to charge it from my house 48v bank while parked without having to be paralleled.

Here is initial info / questions / ideas:

- I'll need to frequently "detach" and "reattach" the portable to the main bank for charging and don't want to deal with SOC mismatch, amp surge, etc. Paralleling is not an option.
- While parked, it will be in continuous use running some devices off its own inverter, so charging needs to be fully automated and continuous.
- Grid power is not available.
- The portable has its own separate SCC, but I don't want to have to run a separate PV array to keep it charged. Plus, I'll mainly be reconnecting the portable to the house bank in the evenings, so it would need to charge overnight when PV is unavailable and ready to detach in the morning.
- I'm currently using a standalone AC to DC 10a charger off the house inverter to keep the portable charged. The relay on the portable's Victron BMV-712 is connected to a 40a relay that opens / closes the AC charger's output based on specified voltage. This is working fine, but there are substantial efficiency losses with this method.
- Are 48v DC-DC chargers even a thing? The Victron Orion series seems to be available only in 12v / 24v.
- I know a 48v SCC can't work on its own, but I've seen some inexpensive adjustable DC-DC step up converters that take in up to 60v / 15a and output up to 80v / 10a (some models even higher). Could the output of one of these be connected to the PV input of the portable's MPPT SCC to give it 80v so it can charge the portable? It probably wouldn't charge as quickly, but it would be way more efficient than double AC conversion. Here is an example device: https://www.amazon.com/Booster-Converter-Constant-Current-Voltage/dp/B07KDQ2VY2/

 

[DIYrich]

This?

https://www.victronenergy.com/upload/documents/Datasheet-SmartSolar-charge-controller-MPPT-75-10,-75-15,-100-15,-100-20_48V-EN.pdf

Or

Customizable DC/DC Converter | DEMKE ELECTRONIC

Customizable DC/DC Converters with ultra-wide input voltage range. Electrically isolated and non-isolated buck boost converters for industrial applications.

demke-electronic.com

 

[The Good Package]

The Victron requires Vbatt + 5v on the PV input to start the controller (1v thereafter). This would require a PV input of 59v to *start* charging a battery sitting at 54v, hence the DC boost converter's input connected to house battery then feeding (just) high enough voltage to the PV input on portable's SCC to charge.

I need this to be an inexpensive off the shelf solution, so custom design is out.

I'm going to test one of these 1500W models and report results in a week or two: https://www.amazon.com/Converter-Constant-Voltage-Current-Equipment/dp/B07X3H85X2/

 

[danmc]

I'm in a similar situation myself. I've spent the last few days trying to find an adequate solution without much success. I was going to make a separate post, but as we have the same problem I may as well just post here.

I have a Seplos Mason 48v 14.3KW 200A battery bank which handles all the house loads and was as good as gifted a 48v 5.2KW 100A battery with a built on inverter module. As I have over 10KW of solar I don't want to limit my charge limit to accommodate the 100A battery and parallel the batteries so I need a means to charge the smaller battery from the larger one. I have the added restriction of limited space. Also I'd prefer to keep my system Victron only as much as possible.

Potential I've found so far - :

Victron Orion DC to DC charger
Info: https://www.victronenergy.com/uploa...DC-DC-converters-isolated-100-250-400W-EN.pdf
Issue: Limited to 8A charging power

Victron Argofet Battery Isolator
Info: https://www.victronenergy.com/uploa...olators-with-alternator-energize-input-EN.pdf
Issue: Limited space, unsure whether it will work for this scenario, can only find examples of use in vehicles and boats used in conjunction with an alternator.

Both of which aren't cheap.

The boost converter looks interesting, although won't the 30A input limit be an issue? Please report back with your findings. If I find a better solution i'll be sure to report back here.

 

[danmc]

This looks promising. More substantial than some of the other similar offerings. Price is reasonable enough. Just not sure whether I'd trust installing a no brand product in my house.

DC-DC Boost Converter Step-up Power Supply Module 1500W 50A 12V-48V to 24V-70V

 

[The Good Package]

The boost converter I ordered arrives tomorrow so I'll test it this weekend. I will be connecting its output to the portable battery's internal Victron 100 | 20 SCC, so I'll be limited to 20a charging no matter what. The load I'll be putting on the portable when not in use will be under 20a continuous, so the charging will still be a net positive. If I find that it can't charge enough, then I can always add a second external boost converter and cheap 20a SCC to give it more juice and some redundancy. I have several XT60 direct connections to the battery on the portable's box so easy enough to scale up if needed.

 

[danmc]

The boost converter I ordered arrives tomorrow so I'll test it this weekend. I will be connecting its output to the portable battery's internal Victron 100 | 20 SCC, so I'll be limited to 20a charging no matter what. The load I'll be putting on the portable when not in use will be under 20a continuous, so the charging will still be a net positive. If I find that it can't charge enough, then I can always add a second external boost converter and cheap 20a SCC to give it more juice and some redundancy. I have several XT60 direct connections to the battery on the portable's box so easy enough to scale up if needed.

My first instinct was an SCC, but I was put off by some of the comments in the following threads.

https://diysolarforum.com/threads/f...dc-into-the-input-of-a-mppt-controller.37330/

https://diysolarforum.com/threads/using-a-mppt-as-a-battery-charger.41303/



Really interested to see how you get on as I have half a dozen SCC's lying around.

 

[The Good Package]

From one of the threads:

"I use a well-regulated DC->DC "boost" controller from more-or-less "unlimited" source (the car alternator and battery) to run into my MMPT controller, and the total power consumed by the MMPT is limited by its maximum output current parameter."

The DC boost converter I ordered current limits to 35a on the input and delivers a clean (up to) 20a output, so the MPPT SCC is unlikely to be killed. I definitely wouldn't go directly from a battery bank into an SCC without some form of current limiting. If the MPPT SCC does fine, I'll test it with a cheap PWM SCC as well.

Crossing my fingers...

 

[Jennifer]

I may be mistaken but isn't charging a 48v from a 48v called 'balancing' not charging? LOL, remember in high school, the old two water jugs and a straw between them experiment...... I guess it would be charging to a point, at least one of them, while the other discharged.

 

[cs1234]

I would say it's charging if the device you are using uses a battery type specific charge routine to charge the battery. It's balancing if it's just making the voltages between the source and destination the same.

 

[The Good Package]

The DC step up converter will take the battery bank's 51-55v range and boost it (with current limiting) to a constant 61v to simulate 24/7 sun for the MPPT's PV input.

 

[danmc]

The DC step up converter will take the battery bank's 51-55v range and boost it (with current limiting) to a constant 61v to simulate 24/7 sun for the MPPT's PV input.

Sounds like a solid plan.

Unfortunately the AIO I'm trying to charge has a string MPPT charger built in, specs say 120～450V input voltage range, so I guess that's out of the question in my case. I've plenty of Victron SCCs lying around but they all have a max output of 24v so I'd have to buy one especially which could be expensive. Starting to think I may be as well just selling it and using the money to just buy something I can parallel with my main battery bank. Pity as it has a 5KW inverter built in which would have proven useful.

 

[The Good Package]

I don't see why this method wouldn't also work with a cheap 48v PWM SCC ($40-50). I'll be testing a PWM this weekend as well.

 

[danmc]

I don't see why this method wouldn't also work with a cheap 48v PWM SCC ($40-50). I'll be testing a PWM this weekend as well.

I should have read your earlier post properly, I have a spare 100/20 that can output 48v. I'm back in the game. ?

Let me know how you get on.

 

[The Good Package]

I've received the boost converter and did some initial testing last night:

Looks like PWM controllers won't work (or at least the 20a BougeRV test model I have). I was able to get clean voltage to the PWM's PV input and it powered up properly. Once connected to a 12v battery, it kept blowing the 25a fuse between the two even with the boost converter's onboard current limiter turned down all the way (RIP Blue Sea fuses). I decided to not pursue PWM testing any further because (excuse my French) those devices are "le crap".

Once I felt comfortable that it wouldn't destroy the Victron 100 | 20, I was able to power it up on the PV input side and stabilized the battery output's *contacts* to a constant voltage. I had to call it a night at that point, so later this evening I'll test it with the 12v battery again so I can do some proper testing of the onboard current limiter.

 

[The Good Package]

Had to temporarily put this project on hold to handle the honeydo list, but I'm planning to resume ASAP. I've purchased a small project box, a surface mount 50a on / off switch for the input, two 12v 80mm low profile fans, and small voltage displays for in / out so I can give this thing a proper fancy and distinguished look. Also fitting in a small 48v to 12v / 5a step down converter to handle the fans since the tiny joke of an onboard fan doesn't run full time (my fans will).

I also realized it's pulling 30a on the input for 12v instead of (what I thought would be) 20a. At 48v it pulls 25a on the input. This explains why I blew the 25a fuse when doing some testing with a 12v battery. This is unrelated to the crappy SCC fuse nuking issue and charging should be handled fine by the Victron 100/20 since the boost's output is 20a max.

I'm going to wire in two separate flush mount female xt60 input connectors with 12 awg to split the load (25-30a is pushing it for 12 awg and 10 awg won't fit in an xt60). On the output a single xt60 with 12 awg should be fine for 20a.

 

[wpns]

This is an old thread, but did you ever get anywhere with this? I have found with boost converters that if they are not (also) buck converters, if the input voltage is any higher than the output voltage, it just acts like a diode and sources infinite current, which is to say it blows the fuse

 

[danmc]

I ended up just paralleling them in the end. The only solutions I could find were either too expensive (Victron) or solutions I wouldn't have been able to trust running unattended in the house (No name Chinese equipment).

No help to you though, sorry.