Battery Bank Voltage Halver

[rbroders]

I have a large 48V battery bank connected to a solar array, but I have several loads (~10A) that require 24V (currently on a small battery bank powered by a SMPS). I'd like to connect the two batteries by tapping off half of the 48V battery bank. Of course that would result in cell imbalance, so I thought I would periodically switch the load from the top half of the 48V battery bank to the bottom half. A DPDT relay on a 50% duty cycle could do it, but I don't think the relay would last very long (and other pitfalls).

I designed this dual half-bridge driver circuit synchronized by a 100Hz 50% duty cycle 555 circuit and opto-isolator. I haven't built it yet, and am hoping to get some feedback here first.

Thanks -- Bob

P.S. In theory this will actually help keep the two halves of my 48V battery balanced as current can run through the switches in both directions...

 

[Easy peasy]

Why not just have a buck converter off the 48V to 24V - much less issues ...?

 

[BradtheRad]

Step-down circuit using capacitors and transistors. 48V supply, load gets 23V. It may be possible to get 24V, if the battery bank puts out 49 or 50V.



The capacitors must carry several Amperes. It's a good idea to make ganged capacitors.

To share the burden you can make a second similar converter, with a clock running at opposite phase to the first.

 

[rbroders]

Thanks for the suggestions. My goal is maximum efficiency, and I’m trying to take advantage of the middle of the big battery to get it.

Seems to me a simple DPDT relay would be essentially 100% efficient. Buck converters (inductive or capacitive) are usually only 90% efficient.

I’d also like the small battery to see *exactly* half the big battery voltage. When I balance the big battery (by using higher than normal charge voltage) the small battery would also get balanced.

My mosfets have Rdson of only 1mOhm and my switching frequency is very low (I don’t need energy storage like a buck converter), so I think my circuit would be extremely efficient...

My only concern is if the half-bridges get slightly out of sync my load could see 48 or 0V instead of 24...

Thanks -Bob