Hints for over-discharged SLA battery??

Lee · #1 02-03-2014, 04:06 PM

After some obs maintenance yesterday I left the lights on, so found my two 9Ah 12V SLAs running at 8.5V or so....
One of them seems to be trickle charging off the solar, the other seems to be getting nowhere, ie still reading 8.5V. I've put it on a 'Battery Fighter' charger, and the voltage seems to jump to 10.5, then back to 9V every 30-60s or so. Is it dead? Can it be revived?
I'm going to make some timer switches I think to avoid this, I replaced one last year after the same thing happened - so before I buy some more batteries, any tips to rescue this one??

Merlin66 (Ken) · #2 02-03-2014, 04:42 PM

I'd give the charger at least 24hr.......

pdalek (Patrick) · #3 02-03-2014, 05:36 PM

Buy a low battery voltage alarm. Common in boat and battery shops or on ebay.

Some smart chargers just will not charge a really flat battery.

What I would do is use a lab supply set to a voltage limit of 14.4V and a current limit of 0.9A (capacity/10) and see what happens to the voltage over a few hours. Maybe you could borrow a suitable supply.
If the voltage rises, keep supply at 14.4 until the current drops to about 90mA. Then set voltage limit to 13.6 and leave for a day.
Don't try charging at over capacity/10.

Even if charging works, the battery will never regain full capacity and will have a shortened life.

Lee · #4 02-03-2014, 05:42 PM

I actually have two variable lab supplies in the obs, might retask one for a bit tonight.... thanks for the tip.

rally · #5 02-03-2014, 07:31 PM

Try a multimode battery charger
They usually have a Battery reconditioning mode which would help revive a "dead" battery
But even the normal charging modes can intelligently kick start the charging cycle - I think its done by pulsing with a higher than normal charging voltage - above 13.7v - like maybe up to 20v

You can buy battery monitors form p;aces like eBay or HobbyKing for under $5 - the simplest ones sound a buzzer if the voltage drops below a certain level - I'd suggest about 10.5v or even just above 11v if you want to be conservative

No battery should be discharged below about 70% of its rated AH capacity of you want longevity !

You could also invest in one of the inline battery monitors or Wattmeters for around $25 - so you can see the voltage, current consumption, total watt/hours consumed . . . etc
So if you get into the habit of checking it you are less likely to forget or maltreat your battery

cometcatcher (Kevin) · #6 02-03-2014, 08:55 PM

Are you using a solar regulator? These have low voltage cutoff on the load output to save the battery from being over discharged.

Lee · #7 02-03-2014, 09:14 PM

Quote:

Originally Posted by cometcatcher

Are you using a solar regulator? These have low voltage cutoff on the load output to save the battery from being over discharged.

The solar charger I have sits on the PV side of the battery only, doesn't influence the load at all...

Lee · #8 03-03-2014, 05:25 PM

Well.... after two chargers running all night and day, I've come to the conclusion one battery is dead, with an open-circuit voltage of 11.2V. The other seems OK, although the extent of its injuries may become clear later on, it is back on the job, voltage 13.0V.

The replacement battery sits behind one of these now....

Merlin66 (Ken) · #9 03-03-2014, 05:49 PM

Bumma!

Lee · #10 04-03-2014, 07:45 PM

I should get one of these, would stop the problem of overnight over-discharges....
http://www.jaycar.com.au/productView...UBCATID=997#12
the charger would be worth a few dollars too....

mithrandir (Andrew) · #11 04-03-2014, 07:54 PM

Quote:

Originally Posted by Lee

I should get one of these, would stop the problem of overnight over-discharges....
http://www.jaycar.com.au/productView...UBCATID=997#12
the charger would be worth a few dollars too....

For that price you could buy a Honda generator and run it for a few years.

BPO · #12 06-03-2014, 08:09 AM

At those voltages, you can safely assume the cells are toast, even if you manage to recover some charge and capacity in either of them.

Lee · #13 06-03-2014, 09:12 AM

One of them seems to have survived, at least in a capacity that suits my needs - it only runs a 12V LED light fitting, has been charging to 13.4 or so with the solar panel, then reads 12.9V overnight, under load it reads 12.7 or so as it did when new.... I suspect its capacity has been dented, it never really gets drained much though, so doesn't need that much...