# Is 70 mA Sufficient for a SLA Float Charge ?

#### ViReN

##### Flashlight Enthusiast
I have built a simple charger with a Bridge Rectifier & Filter Caps to Charge 12 V 7 AH SLA Battery.

When the battery Voltage Drops below 11.8 Volts, I Start Charge. The Primary Charge Current is about 250mA I let it charge for about 18 - 24 ..... leave it idle for about an hour or two, after that I measure Voltage across the battery When I find Voltage to be about 12.6 to 12.9 ... I take for granted that the battery is fully charge.

The charging voltage is slightly above 13.5 Volts (unloaded)

The Question that i want to ask is, After monitoring the currents (initially and after fully charged battery)

I found out that Initially (for a fully discharged battery) 300 mA Current flows.....

70 - 50 mA ... after no load battery reaches 12.7 - 12.9 .....

So Is it OK if i let the things connected ?

The Charger can supply 650 mA (to a short circuit across 10A Shunt of DMM)

#### wptski

##### Flashlight Enthusiast
[ QUOTE ]
ViReN said:
I have built a simple charger with a Bridge Rectifier & Filter Caps to Charge 12 V 7 AH SLA Battery.

When the battery Voltage Drops below 11.8 Volts, I Start Charge. The Primary Charge Current is about 250mA I let it charge for about 18 - 24 ..... leave it idle for about an hour or two, after that I measure Voltage across the battery When I find Voltage to be about 12.6 to 12.9 ... I take for granted that the battery is fully charge.

The charging voltage is slightly above 13.5 Volts (unloaded)

The Question that i want to ask is, After monitoring the currents (initially and after fully charged battery)

I found out that Initially (for a fully discharged battery) 300 mA Current flows.....

70 - 50 mA ... after no load battery reaches 12.7 - 12.9 .....

So Is it OK if i let the things connected ?

The Charger can supply 650 mA (to a short circuit across 10A Shunt of DMM)

[/ QUOTE ]
Not sure if this helps but I have a 1A Battery Minder/Pulser on a SLA battery right now. It's fully charged reading 13.15V at 40ma with my new Extech 380942 clamp probe which matches my chargers within .01A or less.

#### snakebite

##### Flashlight Enthusiast
you want to get it to 13.5-13.8 to ensure full charge.
that is the recomended float voltage.

#### Doug Owen

##### Flashlight Enthusiast
Panasonic lists two values for end of charge voltage, one for 'float' (full time charging) one for 'cycle' (charge then disconnect), both *temperature dependant:

Temp 0 *** 25 *** 40
Cycle 15.4 V 14.7 14.2
Float 14.1 V 13.7 13.4

Cheers.

Doug Owen

#### ViReN

##### Flashlight Enthusiast
will the voltage readings be true even for a depleted battery ?

My battery is taken out from an 8 year old UPS Supply....

I use this battery to drive a Computer Fan to cool Human being (i.e. myself), we have regular 4 hour daily load sheading (Power cut) ... the typical drain of battery is of about 120 mA ... for 4 hours...

Want to charge the battery for rest of the time ...

Doug Owen,

The voltages mentioned .. but what about the current ? ... do we need to regulate the voltages to exactly those values while charging ? ....

#### VidPro

##### Flashlight Enthusiast
some opinions based on what you said

""When the battery Voltage Drops below 11.8 Volts, I Start Charge.""

* why do you wait till its low? you know that lead acid doesnt like to stay discharged. i dont understand, are you saying that you ONLY drop it that far, or only bother to charge it once it has dropped that far?

""The Primary Charge Current is about 250mA I let it charge for about 18 - 24 ..... ""

*sounds great for a SLA, its probably loving that.

""leave it idle for about an hour or two, after that I measure Voltage across the battery When I find Voltage to be about 12.6 to 12.9 ... I take for granted that the battery is fully charge.""

*well it would be mostly charged if that is a RESTED voltage (the charge is no longer on it).
depending totally on the chemistry, the starved electrolytes seem to get higher.

"The charging voltage is slightly above 13.5 Volts (unloaded)"

*well with that you could leave it on forever (about). like a emergency light.

""The Question that i want to ask is, After monitoring the currents (initially and after fully charged battery)

I found out that Initially (for a fully discharged battery) 300 mA Current flows.....

70 - 50 mA ... after no load battery reaches 12.7 - 12.9 .....

So Is it OK if i let the things connected ?""

* YES, it sounds OK for a lead acid trickle charge, as the voltage gets higher the current will drop to a minimum, that way you should not get bubbles in the (blasted) gel, which ruins it.

The Charger can supply 650 mA (to a short circuit across 10A Shunt of DMM)