Charging a 1.2Ah 12V lead-acid battery with a bench power supply

[3lectrokid]

Hello everyone,

I have changed the battery (np1.2-12) inside the alarm recently and now I have a spare old battery **broken link removed**. Because I do a few experiments in my room I made a small power supply on the side of my breadboard consisting of a L7805 regulator and LED. I have been using my battery to power this to power some small circuits. Connecting an ammeter in series with the power supply tells me I only use about 50ma which is not much at all but this makes me ask the question when is my battery flat. even if my battery drains to 7 volts it will still easily power my circuit. however I have heard that if I drain my battery too much I will permanently damage the battery so I would like to know at what voltage at what current I need to charge the battery.

I have this bench top power supply **broken link removed**

I was wondering if it was possible to charge my battery with this power supply and if so how without using much or any circuitry. What voltage should I use.

These datasheets may be helpful
https://docs-europe.origin.electrocomponents.com/webdocs/0111/0900766b80111548.pdf
https://www.yuasabatteries.com/pdfs/NP_1.2_12_DataSheet.pdf

Thanks for reading and any help would be appreciated

 

[MrAl]

Hello there,


Here is a rough guide to the state of charge (measured after the battery sits for at least 24 hours):
12.65v 100%
12.45v 75%
12.24v 50%
12.06v 25%
11.89v 0%

From this we can see that the battery is discharged when the voltage reaches around 11.9 volts. You would want to start charging it at least when the voltage reaches that level. You probably dont want to let the voltage get much below that 11.9 volts either before you start charging again. 7v is much too low.
As to charging, a voltage of 13.8 to 14.1 is considered to give maximum service life. The current limit should be set to around 1/5 of the capacity which comes out to 200ma for your 1.2 Ahr battery. After the battery is charged it is disconnected.
This means to charge your battery you would set the open circuit voltage to say 14 volts (no battery connected yet) and set the current limit to 200ma (perhaps using a resistor load or just set it low and then connect the battery and turn it up to 200ma).

It just so happens that i have a 12v LA 7.5Ahr sealed type too and charge it like this (higher current though). I have an old one too (replaced the battery in a device) and although the capacity is much much reduced (from when it was new) it still works to some degree. I intend to get a new one soon anyway.

 

[3lectrokid]

is the state of charge voltage with or without load?

 

[MikeMl]

The quick answer:

1. Set your Power supply voltage knob to 1V open circuit.
2. Short the leads of your power supply, set the current knob to 0.25A
3. Unshort the leads, and Set the voltage knob to 13.8V
4. Connect the supply to the battery; + to +, and - to -
5. Go for a long walk
6. Battery is charged when the charging current drops below 0.05A

 

[MrAl]

is the state of charge voltage with or without load?

Hello again,


The state of charge voltage is measured after the battery sits for at least 24 hours without any load or any charging. The voltages are approximate.

 

[3lectrokid]

Thanks for the help

 

[3lectrokid]

Iv'e left the battery for 12hours now and if I measure the voltage it gives me 9.94 volts. If I leave it much longer will it go up to 11.89v. I wouldn't have thought so because its nearly 2 volts so what should I do?

 

[MrAl]

Hello again,

Oh sorry to hear that it's only at around 10v after 12 hours at 200ma. Well, i've never had a LA that went that low and didnt come right back up once i started to charge it so all i can say is try another 12 hours as long as the battery doesnt start to heat up. Are you sure it's only 1.2Ahr too? If it is more it will take longer to charge so the current would be stepped up a bit.

 

[MikeMl]

A 6-cell SLA that has been discharged below 11.0V has become a great device to keep papers blowing off your desk.

 

[3lectrokid]

well iv'e just measured it at 24hours and its up to 10.5v. however I shall try charging it anyway to see what happens. I have connected my power supply up next to me now because then i can keep checking it to make sure it doesn't blow up or over heat. I haven't smelt the smell of rotting eggs yet

If it isn't successful I may buy a new battery just for my experiments because since iv'e had the battery it has become very useful for me to use. they're only about £8 and I will be able to get them for about £5 because my dad is good friends with a wholesaler.

 

[MrAl]

Hi again,

Oh ok, well it doesnt sound too good then but i guess we'll see what happens next. Would be interesting to find out how it works after this. As Mike said, it may end up being a book end (ha ha). I have two around that make nice paper weights too.

You're lucky if you can get the batteries for a good price. Since i've had mine i've found them very useful too. It's amazing how handy they come in when you need one. I use two of mine for 'cordless' drill, with a small cable...it's still portable just have to take the battery too. Lots more power then those little tiny sub C NiCd's though.