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My 12 volt battery suddenly fell to 10v, is there a risk in recharging it?

In my boat i have a 80 Ah, 12 v, sealed and vented lead acid battery. Normally charged with a solar panel, with no load. Recently I added a Raspberry Pi and a router to monitor stuff. These has slowly been draining the battery during a week (solar could not keep up). Today it fell from 12.2v to 10.0v, see picture.

I am planning on recharging it. Some rumour on the interwebbs suggest recharging is a risk if a cell is dead (is it?). My charger is a mains connected CTEK multi us 3300, three stage charger. It started alright, delivering 13.4v to the circuit last I checked. Ought i stop it and get a new battery?

Pls ignore the units and titles in the pictures, its voltage. Web-frontend is work in progress :)

pls ignore units and title. Its voltage :)

pls ignore units and title. Its voltage :)

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    \$\begingroup\$ ".. a 80 Ah, 12 V, sealed and vented lead acid battery." How can it be both sealed and vented? Your battery should be fine recharging from 10 V provided the charger current is reasonably limited. (i.e., Don't use a high current charger on a small battery.) \$\endgroup\$ – Transistor Sep 7 '17 at 21:27
  • \$\begingroup\$ Its sealed as in maintenance free, no refilling of the cells. It is vented so as to not accumulate gases when charging. This model: exide.nu/en/product-range/batbatteri/exide-dual \$\endgroup\$ – user156009 Sep 7 '17 at 21:35
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    \$\begingroup\$ A lead-acid battery that "suddenly" loses 2V has probably developed an internal short in one cell. It may be possible to blow out the short by applying short pulses of high current -- preferably to just that cell if you can get access to its terminals. But in the meantime, that cell has been completely discharged by the short, so recharging it relative to the rest of the battery will be challenging. In the meantime, you should be looking for a new battery. \$\endgroup\$ – Dave Tweed Sep 8 '17 at 0:01
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It fell either because it just ran out of juice, or you lost a cell. The rebounce to 13v after you removed the load is an encouraging sign.

Charge it slowly and delicately with the smallest charger you have. Lead-acid and other battery types need a slow tapered recharge when they are in the bottom 20% and top 20%.

Prepare for the battery to not perform as well as it used to. Lead-acid types do NOT like to be hard bottomed, and the way they take damage is with shortened life and shallower capacity. (Notably, NOT significantly lower voltage, that's a bad cell).

Lead-acid types are fragile flowers, and are prematurely aged by overcharging, storing partially discharged, bottoming, half bottoming, excessive heat, excessive use, disuse, or looking at them cross-eyed. If you expect to be abusing this battery regularly, get some nickel-cadmium wet cells (probably a used set no longer fit for use in aircraft, from a FBO)... abuse rolls off their back and they will outlive you. I had one set that was abandoned for 27 years, got some vinegar-water and cleaned off all the schmutz, checked their water, and put a charger on them. Performed well for the last 4 years, though they could use an electrolyte change.

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