Bi-directional DC/DC Converter

Hi All,

I have a 48V 18650 6kwh battery running for the last 3 years in my garage. Ive recently upgrded them to Lifep04 cells for safety reasons (dont want lion in the house anymore).

I want to couple up the 48V 18650 6kwh battery with my shed solar install but my battery voltage is 24Vdc.

Ive done a couple of searches and quite a few people have asked the same question.

Is there a way to safely do this via a bidirectional dc-dc converter? People have suggested using one step up and one step down dc-dc converter which I could do as a little frankenstien project by would be great to do it properly.

Any help or sugestions would be much appreciated.

Cheers

Gareth

Can you not split the 48V 18650 pack in half and parallel the two halves?

You would need something that can control which direction is on at any particular time. How would it know when to charge (convert 24v to 48v), and when to discharge (convert 48v to 24v)?

@SeaGal unforunately I cant. I bought aload of those 18650 pcb boards. I did have a look into how I could split each pcb trcks to make 2 x 7S but its impossible.

@DIYrich I use node to control how much I charge and discharge to my house depending on house load, solar generation and cheap off peak night charging.

I think a bi-directional inverter would be great but they look quite special and rear as rocking horse .........

Maybe I will try 1 Buck boost and one buck step- down converter and control them via a couple of relays that are interlocked.

Something like this - note I will including fusing both sides of both converters and maybe diode block if required.

cheers

gjonesy84 said:

Maybe I will try 1 Buck boost and one buck step- down converter and control them via a couple of relays that are interlocked.

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I'm guessing ... I think you need relays rated for 100 amps or more. Maybe a 3 position switch, break-before-make. You only need to switch the plus side.

What current are you considering? Heavy duty 3000W+ buck converters aren't cheap.

Re switching dc... mosfet would be better than relay for longevity, IMHO. If the sun is going in and out of clouds it could be switching between charge and discharge many times an hour.

gjonesy84 said:

I want to couple up the 48V 18650 6kwh battery with my shed solar install but my battery voltage is 24Vdc.

Click to expand...

To clarify, what is it exactly that is in the shed that you want to use with the 48V battery bank? Is it just a charge controller? Inverter? all in one or what?

rhino said:

To clarify, what is it exactly that is in the shed that you want to use with the 48V battery bank? Is it just a charge controller? Inverter? all in one or what?

Click to expand...

Hi,

Thank you for your replied. Much appreciated.

Functional description of my shed system is I have an 400W solar array connected to a Victron MPPT Solar Charge Controller. Connected to the dc bus is a 3KWh 7S 24V lion battery pack. An adjustable 500W inverter and 30A charger. I only use this for base loading. 500W max. My other battery system supplies the bulk of the power.
It's quite a home made system but very versatile. My off peak starts at 00:30. My control logic I have wrote looks at the SOC and solar forcast at 00:29. It then decides weather to top the batteries up off the cheap off peak using the 30A charger if the weather is crap the next day. Or it runs and doesn't charge if the weather is good.
If the weather is good it charges the cells using the victron SCC in the day.

Obviously it would be great to utilise this 48V 6kwh battery I have laying around.

I have done another drawing. Rev 2. Hood shout with the change over switch and only switching to positive side.

Bit messy but hopefully this drawing makes sense. ?

These are the 2 buck converts I have ordered.

Vicor has a great 12V to 48V bidirectional converter - the BCM6123. They also have a 48 to 24V part - the BCM48BF240. They work VERY well but are pricey ($100 for 300 watts and that's the module only.)

Keep in mind that a true bidirectional converter does not regulate - it's like a DC transformer. So if your 24V side is at 26V, it will try to make the other side 52V.

billvon said:

Keep in mind that a true bidirectional converter does not regulate - it's like a DC transformer. So if your 24V side is at 26V, it will try to make the other side 52V.

Click to expand...

Which is presumably a good thing if you actually want to charge the batteries

For the 48v to 24v side, consider the Victron Orion-tr 48/24-16. They can be parallel stacked, and has a control line (relay part built in to it). Adjustable output voltage.

If your load is 500 watts max, you could get by with one at 80% of the 384 watt (24v x 16a) rating. It will provide a constant 300 watts for 20 hours from a 6kWh battery.

For the other direction, the 24/48-8.5 model can charge 6kWh in 20 hours. Also can be paralleled for more watts and has a control line.

Both are designed for what you want to do.

DIYrich said:

For the 48v to 24v side, consider the Victron Orion-tr 48/24-16. They can be parallel stacked, and has a control line (relay part built in to it). Adjustable output voltage.

If your load is 500 watts max, you could get by with one at 80% of the 384 watt (24v x 16a) rating. It will provide a constant 300 watts for 20 hours from a 6kWh battery.

For the other direction, the 24/48-8.5 model can charge 6kWh in 20 hours. Also can be paralleled for more watts and has a control line.

Both are designed for what you want to do.

Click to expand...

Brill. Thanks for your reply.

That seems the way to go.

cheers

Gareth

billvon said:

Keep in mind that a true bidirectional converter does not regulate - it's like a DC transformer. So if your 24V side is at 26V, it will try to make the other side 52V.

Click to expand...

This is not accurate. Buck / boost regulators will maintain their set output voltage regardless of input voltage.