555 Solar Charger - Flip Flop Initial State always the same. (SOLVED)

[hgg]

Hello,

I am building a small solar charger to power a weather station console and I am using the
following circuit:




It works well but I need to make a change.

I will be using a 500mA solar panel together with a 12V SLA deep cycle battery.
The circuit will stop charging the battery when the voltage rises above 14.9V and
will restart charging when the level drops below 12V.

The problem is that whenever the circuit gets powered up, it always starts in its
charging mode.

I know that SLA batteries need a constant charging voltage but because I don't
have a powerful solar panel I will do the following:  I will charge the battery with
17V from the solar panel until the battery level reaches 14.9V and then I will
apply a constant floating voltage of 13.6V for the rest of the day.  When the sun
sets, the whole charging circuit will switch off, dissipating only 140uA.   I have
already made another circuit for that.  Now, when the sun rises again,  I don't want
to charge the battery again until it discharges down to 30%-40%, so I need the
charging  circuit to be in the 'dumping' state. Here is the problem because it always
starts in the charging state.

- Is there a way to start the above circuit in its 'dumping' state instead of the charging?

-  Another way would be to short pin 6 to 5V for a fraction of a second, when the circuit
is first powered on.  Can you think of a component or a small circuit that would short
pin 6 to 5V for a fraction of a second automatically and then disconnect?  (Pin 6 is at 2.8V)

Finally, charging voltage for an SLA should be 15V at around 25 Celsius.
Do you think that applying a 17V voltage until the battery reaches 14.9V would damage it?

Thank you!
George.

[Simon]

No, the reason is that at rest the voltage will drop back to 12-12.5V, why is starting in charge mode a problem ? the voltage will soon go up if it is already charged and cut it out.

Why not just use a window comparator ?

[hgg]

I have already charged the battery twice and the rest voltage is at 13.3V after 48 hours.

I 've seen from experience that deep cycle SLAs like a mild discharging cycle and last a longer time
if you cycle them like that.  I am not using a window comparator because it needs a flip flop as well.
The 555 already contains all of that.  Cheap and simple.

So, there is no way to start this 555 circuit in its 'dumping' state?
What about the automatic shorting of pin 6 to 5V at power on?

[Simon]

It's the flip flop that causes the problem, all you need is two comparators one for each condition, one will not work if the other is anyway.

[hgg]

Correct me if I am wrong but a window comparator gets triggered when input voltage is off limits,
but it does not stay triggered when the voltage returns back in the window range.  Some kind of
latched window comparator is needed or a window comparator with hysteresis,  but this might
complicate things.

[Simon]

Yes you can do 2 comparators for you upper and lower limits with hysteresis.

[hgg]

It was an easy fix !
A large enough capacitor (330uF) from pin 6 to 5V solved the problem!

[Simon]

It was an easy fix !
A large enough capacitor (330uF) from pin 6 to 5V solved the problem!

Yes that would have worked as it's equivalent to pressing the overide switch. Why the 5V reg ? I suppose it's for when the panel hits 17+ V ? use something more civilized like 9-12V

[hgg]

I am powering all the switching circuits from 5V, but more importantly the weather station
needs a regulated 5V supply to work, so I needed that anyway.  Thanks for you help.

Only thing that is left now is to replace the relay with something else that eats less power...
An SCS maybe.  Haven't played with these yet.

[Simon]

or you can use an N and P channel mosfet........

[hgg]

Indeed, the N type P Type would be perfect.

Only thing that remains to be discovered is if the 17V will destroy the SLA battery...   
On the positive side, the solar panel is a constant current source with a maximum of 500mA.

[Simon]

Yes it will, I have had a battery boiling at 16V, as the battery charges and required less current the panel will compensate by raising it's voltage, this will take the battery voltage up and boil it dry or if it is a dry one cause certain irreversible damage (A car battery can have water topped up, a gel / sealed battery can't)

[hgg]

But did you stop charging it when it reached an upper limit like 14.9V?
At 14.9V, I will drop the charging voltage to a floating 13.8V voltage until
sunset when the solar panel will be completely disconnected.

[Simon]

err 14.4 is usually the maximum. I was just messing about with a panel connected directly. I used a SMPS to try and be somewhat efficient in converting the power but I think most of it was lost in the 2V drop because it was an all in one chip with a BJT.

[hgg]

It depends on the ambient temperature.  You can go higher when the weather gets colder.
At 30 degrees Celsius I think maximum is 14.9V but yes if you connect it directly on the solar
panel it will be destroyed.  I did the same with a car battery that I was trying to save...

An MPPT charge controller would be the optimum solution, but they are very expensive.

[Simon]

yes if you include temp control you can play a bit but that is the standard unmonitored voltage