12v sealed lead acid drops to 6v then revocers fully, why?

[unn]

Hello, I am using a 7AH 12v lead acid sealed, to run a USB/ 5v fan (normally runs on 2X 18650 batteries or USB), through a car USB adapter. Fully charged battery reads 13v. After running the fan 2.5 hours I notice that the fan has slowed down. So I disconnect and check the battery. It reads around 6.5v. Disconnected battery. After leaving it overnight without charging, it reads 12.6. I connect the fan again. Run it for 4 hours. Battery reads 6v. After just 30 minutes, it reads 9.8. I am waiting to see what the final recovery number will be. Is this behavior normal or is something wrong with battery? It is an old used battery.

If I were using a battery low voltage controller that shuts the system down at say 10v, the fan would have shut down long time ago. Or will it go off and on, off and on as it drops to 10v and then recovers etc. Should I use such a controller? Thanks.

 

[GXMnow]

That battery is certainly damaged. It will not supply anywhere close to it's rated capacity after being discharged like that. Lead acid batteries should not be pulled below 50% capacity, which is still up at 11 volts or so. All batteries will rebound some, and when given a chance to rest, they even may give a little more power, but once it falls below 11 volts, it should be properly charged. Do you know how much current the fan draws? Measure it at the input of the 12V car adapter. That should be about half of the 5 volt current at the USB connector. Standard USB devices should not exceed 0.5 amps, so it should be about 0.25 amps at the 12 volt battery. Your first discharge on gave you 2.5 hours at probably under 0.5 amps. That is under 1.25 amp hours out of the 7 amp hour battery. That second run, without charging, pulled the battery down to 6 volts. At what point did it go below 10 volts? But even if it made the 4 hours, it would still only be another 2 amp hours. So the battery is toast, and each time you pull it under 10 volts, it is dying even more.

When you get a new battery, you should certainly use a low voltage shut of of some kind. Check the specs. Some need to be manually reset, some reset after a charge, and some will auto reset if the voltage does float back up above a certain setting. I bought an oversized power bank unit. It has about 100 watt hours of 18650 cells in it. They are wired 3S4P so the raw DC is about 12.3 volts when fully charged, and the BMS shuts it off at about 9.5 volts when discharged. It also has a regulated 9 volt output, and 2 USB charge ports on it. It comes with a charger all for $60 bucks on Amazon. This is nearly double the usable power of a new 7 amp hour gel cell. I actually bought a second one because my girlfriend took it over for charging her phone, wireless headphones, tablet, etc. There are all kinds of these on the market. 100 watt hours should run a .5 amp 12 volt fan for over 15 hours. Here is the Amazon link to 20 pages of USB power banks. Some come with chargers, some don't. Some have just USB output, some have 12 volt, some have 9 volt. Some are junk, if it is too cheap, run away.

Amazon.com : USB power bank