Frequently connecting a "portable" battery with a different charge level to the main battery pack

[jeffc2021]

Hi guys, I was wondering if you can combine two battery banks, each with the same nominal voltage (24v) and a BMS, but different charge levels and capacities. So for example, I have a 24v, 230ah LiFePo4 battery with 8s BMS that I built myself as my main stationary power source. I want to have an additional "portable" 24v 100ah battery bank that I can charge somewhere else when the main battery is low, and then connect them in parallel so I can use more power without draining the main battery.

My goal is when my main battery is running low and the solar panels can't provide enough power from the cloudy weather, I can take a portable battery somewhere else to charge it and then come back with more power to use. Is this possible and what do I need to make sure both batteries are safe? Or can I not combine two batteries in parallel that have different charge levels or capacities at all, even if both of them have a BMS?

 

[cs1234]

The only real concern is that the voltages are very close to the same between them when you connect them. Otherwise whichever one has the higher voltage will dump current into the lower voltage one to even them out. It can be a significant amount of current. If they have the same or very close voltages, it will be fine.

If that's not possible because of how drained the main one will be, consider using a dc to dc charger between the two of them, at least until the voltages are close again.

 

[jeffc2021]

The only real concern is that the voltages are very close to the same between them when you connect them. Otherwise whichever one has the higher voltage will dump current into the lower voltage one to even them out. It can be a significant amount of current. If they have the same or very close voltages, it will be fine.

If that's not possible because of how drained the main one will be, consider using a dc to dc charger between the two of them, at least until the voltages are close again.

I see, so I do plan to charge the portable battery to as much as I can each time and then connect it to the very drained main battery regularly. It would be more inconvenient to have to connect a DC-DC charger every time and wait for them to equalize, because I would need to use one of the batteries immediately.

Would it be possible to connect them both to my off grid electrical system but only use either one at a time? Maybe use a isolator switch to choose between either batteries that have very different charge levels?

 

[cs1234]

Sure, if you don't mind turning it off first to switch batteries. I assumed your mobile bank wouldn't be large enough to power your stuff all by itself though.

You can get a pretty hefty DC-DC charger.. it would supply a significant "boost" to help the larger stationary battery. Or use multiple smaller ones at the same time.. they don't mind working as a team.

They don't need to be completely equal.. just close enough.. how close.. it's up to the size difference I suppose. I don't know the math.

 

[DIYrich]

Lots of sparks when you connect to a live bus.

You can do it if:
1) set charged battery bms to not discharge, and not to charge.
2) connect to bus.
3) set depleted battery bms to not charge.
4) enable discharge on charged battery.
5) enable charge on charged battery.
6) when depleted battery starts discharging, voltages should be the same, and you can enable charging of depleted battery.

Not something I would do on a regular basis. Mess up one step, and...

 

[rcguymike]

Interesting video on this topic