Switching batteries from parallel to series and back automatically

[santaishere]

Hello, I hope, someone can help me. I have two battery banks. Bank 1 has 48 V (consists of two 24 V batteries in series) and Bank 2 has 24 V (consists if two 24 V batteries in parallel). Let's say, Bank 1 is 85% SOC and Bank 2 is fully charged.

If Bank 1 is at SOC 85%, Bank 1 und Bank 2 (which is fully charged now) should switch jobs, so Bank-1 is like Bank-2 and Bank 2 is like Bank-1. That means, that Bank 1 should wired in parallel and Bank 2 should wired in series. If Bank 2 now is at SOC 85%, Bank 1 (which is fully charged now) and Bank 2 should switch jobs. Bank 2 is now Bank 1 and Bank 1 is Bank 2.

A generator is supplied by Bank 1 and is charging Bank 2. I know this doesn't make sense to the most of you, but it's just a prototyp. How to do this fully automatically? Thank you so much for your help.

 

[amberwolf]

You can use relays / contactors to do the switching itself, and an MCU of some type (arduino, etc) to detect the voltage of the packs (via whatever necessary buffer circuitry) and control the relays as needed. I don't know of anything off the shelf to do it, so you'd have to design and build it, and write the code for it. You can probably find code bits you need for various parts at places like arduino.cc in other people's projects that do parts of what you want, if you need a jumping off point.

However,

A generator is supplied by Bank 1 and is charging Bank 2.

is just a complicated way to drain batteries, since there are so many losses in the system. (there is no way to get enough power out of such a system to ever fully recharge Bank 2, if it's batteries have the same Wh as the Bank 1 batteries...and if they have less, then when you switch them the lower capacity set will definitely not have enough to recharge the other one.

If you tell us exactly what you're trying to accomplish, with schematics / diagrams, we may be able to help you design what you need to accomplish it, assuming it's physically possible (which fully recharging one battery from another and then swapping back and doing again is not).

 

[lnanek]

A generator is supplied by Bank 1 and is charging Bank 2.

Charging a 24V volt battery from a 48V voltage battery takes a DC-DC buck converter at the minimum, not a generator. Just connecting the two directly would transfer energy too fast and cause heating and the 48V battery would be higher than the max charge voltage of the 24V battery even at 85% charge.

There are lots of chargers that will take a DC input instead of AC and charge a DC battery for you as well, doing the voltage conversion and charging logic for you. Those are called DC DC chargers. E.g.:

https://amzn.to/455Qdyl

 

[amberwolf]

the 48V battery would be higher than the max charge voltage of the 24V battery even at 85% charge.

It would be higher even at negative percent charge.

(really zero percent charge would be something like 13s x 2.5v = 32.5v, and you wouldn't want to run it below 13s x 2.8v = 36.4v.)

 

[santaishere]

Let me ask a different way. I think, a better way to switch, is driving a motor by Uc-signal to operate a knight switch. Would you agree?
But how exactly to do that?

 

[amberwolf]

Let me ask a different way. I think, a better way to switch, is driving a motor by Uc-signal to operate a knight switch. Would you agree?
But how exactly to do that?

I don't know what a "knight switch" is so you'll have to link one.

Other than big clunky rotary switches, I don't know of any switches that would be easy to drive with a motor; you'd have to setup limit switches on the switch, so it can't be pushed or pulled too far by the motor and break the motor or the switch or the motor controller or the power supply, you'd have to write code to drive the motor, etc. Rotary switches you could gear a motor down to make it easier to control, or use a stepper motor with steps of the right size to equal switch positions or some perfect multiple of them.

It'd be a lot easier both to build and to code to just use relays (or contactors), which you can buy ready-made boards for various switching configurations, voltages, and currents, that already have the relay drivers built in so you can easily command them from an MCU (arduino, etc).

The rest of the stuff you'd need regardless of which way you control whatever switches you use: you need something to buffer each battery voltage and convert it down to the MCU analog inputs' voltage, so the MCU can read the batteries voltages to know what it's at, and you make code to read it and compare that to whatever you've preset as the switching point. Then you code to do the switching as needed.