battery problem

Hi

I noticed the battery life had started to deteriorate on my portable speaker

http://cpc.farnell.com/pulse/aps15pa/portable-pa-system-15-usb-sd-fm/dp/DP33511

the battery is one of these

http://www.alibaba.com/product-detail/TAICO-12V12Ah-rechargeable-Lead-Acid-Battery_850045303.html

I removed it and measured the voltage with a good meter and it said 13.5V

popped the caps of the cells and they were all completely dry. so I filled up with de-ionised water and left a small air gap at the top of each cell and put the caps back on. measured the voltage and it was 13V

left it a couple of hours and then noticed that one of the rubber caps had blown off and some water had come out of one of the cells. checked the other cells. a couple hissed as I took the cap off and released gas. but one of them the water level had actually gone down significantly so I topped it up again. measured the voltage and it was now only 9V

checked the speaker charging circuit with the battery out and it is 18V !

put the battery back in and turned the charging circuit on. voltage measured as 12V across the battery but within a few seconds I hear hissing noises so I disconnect the battery.

I've ordered a battery protection circuit to stop overcharging which will hopefully stop the battery going dry again:

http://www.ebay.co.uk/itm/12V-Battery-Controller-Anti-Over-Discharge-Against-Overcharge-Protection-Board/141749332295?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

does the battery I have sound knackered or should I keep trying to recharge it?

cheers
Phil

ah, spec sheet says 'Separators – Advanced AGM separators for high pressure cell design'

probably shouldn't have filled them with water LOL. what now? guess I will try and charge it with the caps off and see if I can gas the liquid back off. probably best done outdoors...

Lead acid chargers can have quite a high open circuit voltage but it sags under load down to more reasonable values. My 5A model finishes charge at about 13.5v but reads about 17v unloaded.

Hi, I've had quite alot of experience with agm and know about gel batteries.
Agm's and gels are very sensitive to charge voltage, if a gel is charged beyond 14.6v the gel gets lifted off the plate due to gas forming. This ruin's the battery. As the battery charged and the voltage rises the charger See's less load and voltage climbs to dangerous ( for the battery) levels. Gel charger's have very specific cutoff point's.
Agm's are slightly more forgiving but prolonged high voltage at the end of charge premotes gasing which eventually gets released by the valves (vrla battery) in the battery case. This then causes electrolyte loss and premature drying. Flooded batteries need high voltage occasionally to mix the electrolyte. Absolute max for agm is 14.8 v
Hope this helps

I use 16 175ah agm batteries for my solar set up and absolutely hate them. Expensive, heavy and prone to failure. No wonder Excide Corp shelved better tec (such as NiFe batteries) in favour of the limited life span crap we have to buy. But yeah, agm and gel really could do with BMS like lithium cells require. Can't advise on a charger circuit tho, sorry. I think the suggestion to go lithium is a good one tbh, less weight, better power density and good life span if treated well. Comment about limited life span of lead acid batteries is for stuff we use, not huge telecom back up gear which lasts 20years +....

I've decided to give up fitting a charge-protection circuit inside the speaker. It has 12V terminals on the back which I have attached a solar charger to. so rather than plug a kettle lead into it I can just parallel a decent mains powered battery charger to the 12V terminals.