Expansion battery current limiting

[BerndL]

Does anyone know for sure how power station expansion batteries limit current when attached in parallel? It's been discussed here before, but with no resolution.

Some people said that the expansion ports are directly wired to the battery of the main unit, which means the expansion battery itself must have some sort of DC-DC converter with current limiting. The Bluetti ones can't charge from the main unit, so it's not bi-directional and they also have a separate charger built in.

I ask because I'm in that typical situation of "buying another cheap battery to parallel with my milk crate, but not sure I want to lug around a complete 70lb system all the time."

I can see a few tard-rigged DIY alternatives:
1) A Victron Orion 12-12 DC charger, like van guys use for alternator charging. Limits the expansion battery output to 18/30A, costs far more than the battery itself! (Or a Renogy 20A for $75)

2) A manual switch selecting either the main battery or the expansion port. Use and charge one battery at a time. Awful!

3) Same manual switch, but have the AC charger on the expansion battery and the SCC on the main. Add second port for connecting them in direct parallel once you've confirmed they are both fully recharged. Seems fine but not idiot-proof.

4) Stop posting nonsense on the internet and go lift weights until 70lbs stops bothering me. Bonus: nobody will want to carry off my batteries.

I know a lot of guys here make their own portable power stations, and since most have big inverters for the battery Whr I'd guess many end up thinking about adding capacity.

 

[Kremer]

Bluetti expansion batteries link directly to the dc bus of the head unit. Each battery does have a way to disconnect itself though.

 

[BerndL]

Bluetti expansion batteries link directly to the dc bus of the head unit. Each battery does have a way to disconnect itself though.

Do you know how the current-limiting works? Is it something in the expansion battery?

 

[meetyg]

I have asked a similar question in the past:

Thread 'Solar generator expansion batteries - how do they work ?'

Jul 29, 2024

So how do these off-the-shelf expansion batteries work in portable solar generators / power stations?

I mean if they connect directly to the battery of the main unit, the two batteries will most likely have a voltage difference and a large inrush current will flow from one to another.
How do the manufacturers mitigate this issue?

Do they have some sort of current limiting circuit?

Also, which battery is used first ? Or are they truly in parallel?

The reason I'm asking is mainly educational, for planning expansion batteries in my DIY solar generator builds.

• meetyg
• Replies: 3
• Forum: Solar Generator and Micro Systems

• 

I didn't get a definitive answer on that post.

In hindsight, I assume that they do some logic so that the battery with the highest voltage gets discharged first, until it reaches the voltage of the lowest battery. Then they can parallel them. The opposite goes for expansion battery charging, if that's available from the main unit (lowest battery charges first).

I am assuming this because I saw a YouTube of a powerstation teardown (it was an Ecoflow, if I recall correctly).
They had quite alot of electronics on the PCB connected to the battery.

I'm no electrical engineer, but I think that my assumed solution would be the easiest and most efficient to implement, rather than current limiting circuitry.

To implement such a solution (voltage based) DIY, you would need some kind of microcontroller, that would sense the voltages of both batteries (internal/main and external/expansion) and enable a contactor or FETS of the highest voltage battery only (when discharging, or lowest when charging) untill it is within a certain threshold of the other battery's voltage.

 

[meetyg]

So just for fun, I thought it would be interesting to ask ChatGPT what it thinks the working principal is with expansion batteries.

So in short, it mentions the BMS alot, but I doubt it's actually talking about BMSs as we DIYers know them. I think it's actually referring to additional circuitry. But my assumption from the previous response was one of the possible solutions ChatGPT proposed, so I guess I wasn't far off...

Here's the full response I got:


My question:
‐-------‐------
How do common portable power stations enable the use of expansion batteries with the main unit, when there is a voltage or SOC difference between the two?
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ChatGPT answer:
----------------
Here’s how it works:

Portable power stations enable the use of expansion batteries with the main unit through sophisticated battery management systems (BMS) that handle the integration of both the main power station and the expansion batteries. These systems ensure smooth and safe operation even when there are voltage or state-of-charge (SOC) differences between the two.

1. **Battery Management System (BMS)**: Both the main power station and the expansion batteries typically have their own BMS. The BMS is responsible for monitoring voltage, current, temperature, and the SOC of the battery. It ensures that both the main and expansion batteries remain within safe operating limits and prevents overcharging, overheating, and other issues.

2. **Voltage Balancing**: When the expansion battery is connected to the main power station, the BMS in the main unit will ensure that the voltage of the main power station matches that of the expansion battery before allowing power transfer. If there’s a significant voltage difference, the BMS may prevent charging or discharging until the voltages are more closely aligned. In some cases, the system will allow for a gradual charge/discharge process to bring the voltage levels into balance.

3. **Current Regulation**: The BMS also regulates the current flowing between the main unit and the expansion battery. When there is a SOC difference, the BMS can adjust the current flow to ensure safe and efficient charging or discharging. For instance, if the expansion battery has a lower SOC than the main unit, the BMS will ensure that the main unit’s power is used to charge the expansion battery gradually.

4. **Charging and Discharging Logic**: Some power stations allow for expansion batteries to be charged while the main unit is either charging or discharging. The BMS will manage the flow of energy to ensure that the main unit or expansion battery doesn’t become overloaded. In some systems, the expansion battery may be prioritized for charging or discharging based on the needs of the power station.

5. **Communication Between Units**: The main unit and the expansion battery may communicate over a proprietary protocol to share information about their SOC, voltage, and health. This communication ensures that both units are operating in sync and that energy is transferred efficiently between them.

By using these mechanisms, the portable power station and its expansion batteries can safely and effectively operate together, even when there are voltage or SOC differences. The BMS acts as a critical interface to prevent damage, optimize energy flow, and ensure the system's overall safety and efficiency.


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[BerndL]

My god, that would actually be a genius way of doing it! A much smarter and automated version of my 3rd idea.
I saw your thread and was frustrated nobody picked up the topic, because it's really interesting. Almost all the youtube videos are annoying people copying the basic "expansion battery hack" with the PV input

 

[Kremer]

Do you know how the current-limiting works? Is it something in the expansion battery?

I know the expansion batteries have charge and discharge mosfets/ solid state contractors.

 

[meetyg]

I know the expansion batteries have charge and discharge mosfets/ solid state contractors.

Yeah, but those don't/can't limit current. So voltage based control of those FETS/contactors/relays still sounds logical.