Connecting a powersource directly to a SLA Battery

[Bdman32]

Hi there,

Im looking at a project at the moment, and am wondering what would happen if i left a 12v power supply from mains connected directly to a SLA 7.2AH 12v battery.

there would be a small load continually drawing on it (about 1mA) .
would this damage the battery?
If so, how would i go about building a "UPS" Circuit to solve this

any help is appreciated

 

[Colin Mitchell]

12v will not charge a 12v battery.

 

[Bluejets]

Burglar alarms and such run like this all the time........ they do however have a charge controller in between the power supply and the battery.
Assuming (as most power supplies are) the unit puts out slightly more than 12v.

 

[Bdman32]

so how would i go about building a charge controller?
and i have a 13v power supply

 

[Bluejets]

Perhaps Google " SLA 7.2AH 12v trickle charger"

 

[duke37]

Car batteries are charged between 13.8 and 14.4V. For continuous charge with no load I would think 14.0V would be best. The burglar alarms I have seen use a simple voltage stabiliser such as a 7812 or 317T raised to 14V.
An input of about 16V would be necessary unless a low drop out (LDO) regulator is used.

 

[mark_101]

I use a small off the shelf (Mascot) 3-stage charger to keep an SLA battery fully charged when it is connected to the mains. When the battery is fully charged the charger keeps it at the float voltage, which I can't remember at present, but its about 13.8v. I don't draw any current from my set-up in this state, but I would not have though drawing 1mA would be problem, as long as your load can handle the elevated voltage.

 

[Alec_t]

A 12V supply would maintain a '12V' battery in a partly-discharged state.

 

[Bdman32]

i am aware that i would need about 14v to charge the battery.
the question i am asking is basicly, if i connect the supply directly to the battery (i.e. without a charge controler) will the battery be damaged?

 

[duke37]

Yes !
Overcharging a Pb/H2SO4 battery will boil off the water and make the acid far too strong. I do not know how much overcharging a SLA battery can stand.

In my early motoring days, a battery would last two years if you were lucky. When dynamos gave way to alternators with accurate voltage output, things improved greatly.

 

[(*steve*)]

SLA batteries do not like overcharge at all.

The battery may have the "float" charge voltage printed on it. If so, this is the voltage of a power supply that could be connected to the battery.

However you need to check that the power supply can handle both of these cases:

1) being connected to the battery when power is removed from the power supply.

2) being connected to a battery under load (or discharged) so that the load presented to the power supply wants to draw more current than the power supply can provide.