Charging the battery from a shared output?

[Artlav]

Consider a battery-backed lighting system:


The lights run on 24V, an AC adapter provide 25V, a stepped-down 30-odd V battery provide 24V.

Due to diodes in series, when AC power is present the battery does not provide any power and does not back-charge either:


If AC power goes out, then the same diodes allow the battery to power the lights without feeding back through the AC adapter:


Makes sense so far, and is as simple as it could be.
But you might have noticed that the battery is not charged from anywhere.
What i want to is to hook up the (36V DC) charger to the same 24V bus (with suitable step-up converter), to be powered by the surplus from the AC adapter.


However, if the power was to go out, then the charger would try to pull it's input from the same battery it's trying to charge:


This would then attempt to produce large amounts of free energy, which is strictly illegal in this universe.

So, the question is - how do you connect the charger to the same bus?
I can think of some straightforward solutions, like an AC relay opening the line to the charger when AC goes away.
Or even less straightforward ways that would allow you to call the whole design Arduino-powered.

However, i can't shake the feeling that i'm missing some very simple solution.
It being 3AM here does not help.

[qwaarjet]

This is similar problem to a student satellite I built. Solar power power computer and charges the battery while in the sun but battery runs system in dark. I based the system on a LTC4000-1 overkill for what you're doing but illustrates the concept. Basically alway run off battery power and then have the battery charger always be charging the battery when the battery is fully charge the current charging it will go to zero the "charger" will be running the lights power goes out battery immediately starts providing power.

[electrophiliate]

Does the charger need to be connected to the bus? What about simply connecting it directly to the 25V output of the AC source. The diode from the 25V source to the bus will prevent the charger from drawing on the battery when the AC goes out.

[rob77]

you don't even need that diode for the charger - just connect the charger to the output of the 25V power supply.
when AC is present - you'll power the bus via a diode from the 25V and power the charger directly. in case of no AC the battery will power the bus via a diode.

[Dago]

Or just forget diodes and use a real power switching multiplexer chip.

[eneuro]

So, the question is - how do you connect the charger to the same bus?

I have a few 12V old car start batteries in pararell and use 230VAC battery charger set for safe 14.2V maximum voltage.
The same can be done with step down buck solar panel converter and the same voltage set 14.2V.
Those chargers are simply connected via diodes, so baterry is charged below 14Vmax-aroung 13.8Vmax depending on load requirements.

When there is no sun, sun charger do not charge battery.
If 230VAC power goes down it does not charge, but during the day solar panel can charge if 230VAC fails.

Load is 12V -> 230VAC power inverter which do not cares if there is sun or 230VAC power line, byt checks for minimum battery voltage and if goes below 11V switches off to do not discharge too much below.
It depends on application and batteries used how much I want them to be discharged while it might affect number of recharge cycles and its lifespan, especially in the case of car starter batteries not designed for deep discharges...

power switching multiplexer chip.

Do you mean something like ideal diode driver similar to ideal full bridge rectifiers which conducts current only in one direction but using synchronized swithing on external mosfets to avoid voltage drops in diodes?

[Dago]

power switching multiplexer chip.

Do you mean something like ideal diode driver similar to ideal full bridge rectifiers which conducts current only in one direction but using synchronized swithing on external mosfets to avoid voltage drops in diodes?

I for example used this (won't suit your application but there are thousands of different ones) in a recent project: http://www.ti.com/lit/ds/sbvs044f/sbvs044f.pdf

[eneuro]

Is it possible to connect together in this sbvs044f  those to two power inputs IN1 & IN2 to get maybe higher output current reverse blocked (2x2A= 4A), so close to ideal diode?

For high current power applications just developing my own analog driver based on Hall current sensor with similar functions like this one, but rather ideal diode driver with a few mosfets in pararell to bypass diode if detected current is inside defined limits including reverse polarity protection.
Didn't looked by the terms "power switching multiplexer" so far   

[Artlav]

What about simply connecting it directly to the 25V output of the AC source.

just connect the charger to the output of the 25V power supply.

I knew i was missing an obvious solution.
Thanks all!

[rob77]

What about simply connecting it directly to the 25V output of the AC source.

just connect the charger to the output of the 25V power supply.

I knew i was missing an obvious solution.
Thanks all!

take it easy it could (and actually does) happen to any of us - sometimes  you simply can't see the obvious solution. i usually put the project aside for a while when it happens.. and later on the solution simply pops up in my head ... and there is light !