How to detect if battery is charging or not (with low voltage drop)

[gohilurvish]

Hello Guys,
This is going to be my 1st post on this thread. I am doing my best to follow the rules of thread, but if I miss at any place, I would be glad to take that into notice.

My question looks rather simple.
-For my uC work, I am using a battery(12v, 1.7AH) to power uC.
-Battery is being charged by a ready-charger of 12v, 2A.

Just to get idea of simple connection, +/- of charger goes to +/- of battery and that only goes to regulator (7805) of circuit.

-Arrangement is such that if mains power fails, battery will keep circuit alive for a few hours.

-Normal output of charger is around 12.7v and battery withstands (and also requires) that voltage.
With a battery fully charged with that voltage , my circuit lasts around 4hrs.

-Now, I want my circuit to check if power has failed or not so it can run some routine.

-If I connect +/- directly to charger, and if mains fails, it will still get 12v from battery.
So to get away with it, I connected a diode in + line from charger to battery.

-While this thing is working well, charging voltages now are 11.7v only and with that battery last only for ~1/2 to 1 hour.

-Using schottky diode will definitely reduce the drop but would still remain in range of 0.4v.

Any better idea to implement this?

 

[MikeMl]

...

Any better idea to implement this?

Get a proper charger!

If this is a Sealed Lead Acid Battery, your charging voltage is too low by about 1.0V. At only 12.7V, your battery recharges to less than 30% of its rated capacity. A partially discharged battery sulphates, and looses its ability to hold a charge. To prevent sulphation, any lead-acid chemistry battery must be maintained at a terminal voltage >13.2V for the long term. During recharging, the battery voltage should be >13.8V. A proper charger would switch to a higher voltage during recharging, and then to a lower voltage during long term maintenance (floating).

If your "charger" was adjustable, you could increase your charge voltage to include the forward diode drop.

Here is recent thread that deals with recharging/floating sealed lead-acid batteries. One of my posts says:

"A lead-acid battery is fully charged when the current it draws from a constant-voltage charging source drops to a few tens of mA while the battery terminal voltage is held at 2.4V per cell (7.2V for a three cell battery).

At the point the battery first reaches 7.2V during charge, it is NOT fully charged; not even close. "

 

[gohilurvish]

Hi MikeMl,
First thing first : Thanks a lot for a very very quick reply.

Second thing : You solved one issue that I didn't mentioned but I was facing (As per my calculations battery should last for ~6-8hrs but actually it was very less).

Checking charging will be easy now because I have to make a charging ckt from 24v adapter(ready chargers of ~14v will be difficult to find here).

Again, thanks.