charging SLA battery

[Frosty_47]

Hello, I have two 6V 12A/hour SLA batteries. I've been having issues trying to recharge them using a linear PSU. The PSU supplies 7.5V to the battery. However, the battery draws a mere 300ma-600ma of current. This has nothing to do with the PSU as I tested it, and it can supply 2Amps of current easily. Do these small SLA batteries have high impedance that causes the charging current to be this small? Or am I doing something wrong?

I would really like to charge these batteries faster...

Thanks


**broken link removed**

 

[Hero999]

7.5V sounds quite high for a 6V lead acid, you shouldn't charge them past 7V.

You say it can supply 2A; into what load?

How old is the battery?

It sounds like it's knackered to me.

 

[Frosty_47]

7.5V sounds quite high for a 6V lead acid, you shouldn't charge them past 7V.

You say it can supply 2A; into what load?

How old is the battery?

It sounds like it's knackered to me.

I am not sure how old. But it hasn't seen much action from me ( no pun intended ). Psu is current limited and can supply 2Amps into various loads. I am not the only one who is having this issue. I just talked to 3 other people in my school and they all claim to have the same problem with this type of battery.

 

[Frosty_47]

I just spoke to my prof and he recommended that in order to achieve best results, the charging current should not exceed 1/100 * A/hour ratting of battery. In my case this is 120mA since the battery is ratted for 12A/h. Oh, and yes 7.5V is a little too much so I am going to lower it to 7.2V. As that’s roughly the typical voltage of a fully charged 6V SLA battery.

So maybe the battery is fine and it's just my impatience?

 

[Hero999]

1/100Ah sounds far to low to me at that current it will take ages for your battery to charge - about 5 days 5 hours.

 

[Willbe]

Supposedly a fully charged unsealed lead acid battery with no surface charge should read 12.76/2 = 6.38v and can be charged at 13.3/2 V(hot ambient) to 15.5/2 V (cold ambient), but use a current limit.
Charging at a C/20 rate, 1A, shouldn't hurt anything.

Check the battery!
12Ah at 20 hrs = 12/20 A, so at 6v you need a 120/12 = 10 ohm, 7w load resistor. A 100w 120v lamp coming in at a cold resistance of 10 to 14 ohms should do it.
The "cutoff voltage" may be 5.5v.
Since your load is not a constant 600 mA current you may have to do some math.

 

[k7elp60]

This is the recommended charging voltages and currents for SLA batteries:
There are two recommended charging ways. The cyclic or the Float or standy method.


1. At a temperature of 77F or +25C
Cyclic 2.4 to 2.5 V per cell 6V= 7.2 to 7.5 V 12V= 14.4 to 15V
Initial charge current limited to C x .20 amps, when charging current drops to less than 0.01 x C amps the charger must be removed or stepped back to float level.

Float 2.25 to 2.3V per cell 6V= 6.75 to 6.9V 12V= 13.5 to 13.8
Initial charge current limited to C x .20 amps. This method the charger can be left on the battery terminal indefinately without damage.


The amount of current a battery draws during charge depends alot on the state of charge or the condition of the battery. The batteries should never be allowed to discharge below 11.8 for a 12V battery or 5.9 for a 6V battery unless they are recharged immediately.

The above information was extracted from the Powersonic Technical Data Handbook.

 

[Frosty_47]

when charging current drops to less than 0.01 x C amps the charger must be removed or stepped back to float level.

thanks for your advice!

 

[Frosty_47]

1/100Ah sounds far to low to me at that current it will take ages for your battery to charge - about 5 days 5 hours.

Sorry I think I forgot to mention that my prof recommended this method for batteries that are completely depleted. In my case, one of my batteries was at 3V and it could not even supply 40mA of current to a shorted load. Someone also mentioned to me that when you try to recharge a depleted battery with high current, you can cause permanent damage to the chemistry.

 

[Frosty_47]

Supposedly a fully charged unsealed lead acid battery with no surface charge should read 12.76/2 = 6.38v and can be charged at 13.3/2 V(hot ambient) to 15.5/2 V (cold ambient), but use a current limit.
Charging at a C/20 rate, 1A, shouldn't hurt anything.

Check the battery!
12Ah at 20 hrs = 12/20 A, so at 6v you need a 120/12 = 10 ohm, 7w load resistor. A 100w 120v lamp coming in at a cold resistance of 10 to 14 ohms should do it.
The "cutoff voltage" may be 5.5v.
Since your load is not a constant 600 mA current you may have to do some math.

Thanks for your advice.

 

[ki4ymd]

Float?

I have a few of these 12v 7Ah batteries and want to keep them in great shape.

I also have some Chicago Tool float chargers. In the manual of the charger there is a caution of useing these on SLAs. can someone tell me why it is not advised? I've seen a few charger on eBay that look like nothing more than a wal-wart with clips. What I want to do is leave my batteries on float 24/7/365 so that when I need them I can just grab and go

Thanks,
Chris

 

[tcmtech]

How many million of those batteries did they make? I have 3 of them also. But they are in the battery recycle pile. The plates are shot. I had one hold a charge for awhile but it eventualy died and would not recharge.
I built a small float charger out of a 12 volt 1 amp wall charger. (actual output is around 17 VDC open circuit) I just put a 4 ohm 5 watt resistor in line with the output. When ever I need to top of a battery I put it on for a day or so. Its never given me problems.

 

[Skywatcher]

Hi folks, If you are having trouble charging a lead acid with a normal charger, you can use pulsed charging from the flyback of a coil and it will desulphate the battery and with a few cycles of this will bring the battery back to life. Check out John Bedini solid state charger circuits for a start. Myself, I am using a joule thief circuit which is a bifilar coil that self oscillates and i use Bedinis one diode method and extract the flyback off the main coil into the battery. Good luck, any questions let me know.
peace love light
Tyson