Can you make an 'ideal diode circuit' with voltage 'bias'?

[No_Shorty]

The 'priority-2' source battery is outside on an enclosure. It is charged elsewhere. It is a primary source of power that would primarily be used when away from AC power for running the entire system it is plugged in to.

The 'priority-3' battery is for a UPS function only, it should only kick in when there is power loss/failure of the primary sources of power (priority 1 and 2)

[Siwastaja]

You are not describing "ideal diode" at all. You are describing a power switch. Typically something constructed from two back-to-back MOSFETs. There are power switch ICs that handle all the ugly details like overcurrent and SOA protection. You still need to provide idiot-proof control logic so that the two batteries are never connected in parallel (well, unless that is intended and the battery voltages are qualified to be very close to each other before enabling the switches). And of course, battery management. All of this is significant amount of engineering work.

[Konkedout]

Read the attached which I wrote years ago.  Let me know if you are interested in building something like this.  It would PROBABLY be possible to add hysteresis so that a few tenths of a volt differential would be needed before it switches.

[No_Shorty]

Thanks @Siwastaja. This is all learning for me, I would love to have a better understanding than I do of electronics as it so often crops up in my desires for completing projects.

@Konkedout
Thanks, am I right to say you are echoing roughly what Siwastaja is saying?
If that is an offer of developing this for me, I would be interested to know what that would cost. My assumption is that the cost would be too great to make it worth it in comparison to the 'keeping it simple' approach of just keeping the internal UPS battery at the lower voltage range to make it work the ideal diodes. But nevertheless I would be interested to know so I can make that decision with facts as opposed to guesses on my part, please do PM me if I haven't go that wrong and I don't sound too much like a dead end to bother!

[golden_labels]

No_Shorty, once again: the forum has an attachment feature. Please us it, instead of 3rd party services.

The image now works, but it doesn’t give the answer to what I asked you earlier about running batteries in sequence. You never addressed that, even despite others also asked you about this. You mentioned batteries above, but this is really just repeating the same statement. A description of how you want it to work, without explaining why you want it this way. And it’s focused on physical appearance and being (or not) in the same enclosure, while we ask you about a completely unrelated thing: the behavior of the circuit.

My guess is that you are focused on some particular picture, which appeared in your head, but you never questioned its sense. So please stop for a moment and rethink it.

Under constant current discharge there is not difference in how long the load will be powered in either scenario:


Even if the backup battery is smaller (2:1 for this illustration).


If it’s not constant-current discharge (e.g. a linear load), there will be some difference, though which will win depends on the situation.

I’m not saying that there certainly is no advantage or no reason to run them this way. There may be. But implementing all this without a good reason is a waste of time and money, making everything less reliable, and even causing a slighly higher energy losses. This is a forum and people try to provide best advice, hence the questions.

[Konkedout]

I am working FT and I do not have the time to do a design.  But if you or anyone else has enough knowledge to engineer a reasonable circuit board, I can offer guidance.

[Berni]

Sometimes there are reasons to switch batteries in and out.

For example in aviation there are often two batteries for redundancy, so that one battery is used at a time, but if that drains or goes bad there is the option of switching over to the other battery. (Similar to how it is done with fuel tanks in aviation)

Another example is if you have two different battery chemistries combined. In a solar installation it could make sense to have a smaller expensive lithium pack that can survive a lot of cycles, so it is primarly used to handle day/night cycles. Then you have a bigger lead acid battery pack that is there to cover bad weather situations, so this way the lead acid pack can sit fully charged most of the time, only getting cycles when it is absolutely required.

[No_Shorty]

@Golden Labels
I'll use the forum attachment feature in the future, I didn't recognise any of the buttons as being picture attachment so didn't realise there was one.

I'm sorry I'm not communicating properly, I am describing it visually to try and be sure that I do...

I am unsure how to describe it differently to clarify my point.

If you imagine a UPS where the main source of power is a large external DC battery (with no AC at all, for simplicity), the UPS would contain a battery that you essentially don't want to use unless there was power loss from the main DC source, at which point you would want to switch to it fast enough to not disturb any load.

I fully understand the rest of your post, but it is not relevant for my use case here.

@Konkedout - thanks, if I have a circuit designed or try to wrap my head around it I'll certainly take you up on that kind offer.

[golden_labels]

No_Shorty: but why would you like to not use this battery? What’s the goal of not using it?

Berni: yes, as noted earlier, I know there may be reasons. Hence nobody is saying that the idea is outright wrong. But it may be pointless and wasteful, if the solution is literally just two Schottky diodes.

[No_Shorty]

@golden_labels:

The goal is to always have a backup. The clearest example in my mind I can refer you to again is a UPS: it has an internal battery that is not used unless there is power loss. This is the purpose of this battery. To provide an amount of time following power loss where the primary source of power can be replaced.