Batteries recharging themselves???

SoulRaven · Jun 3, 2015

I have heard and read that deep cycle batteries that are fully charged read 12.7 volts. Yesterday I ran my microwave (about 10 mins total) and small Silverstone fan (.4 amp for 1 hour). When finished, I read my batteries (two 12 volt 35 ah deep cycle batteries wired in parallel) which showed 12.54 volts. I hooked up my 100 watt solar panel but it immediately turned very dark and cloudy so I only left it connected for about 15 minutes, then unhooked it and put it away. Today it is sunny so I thought I would take advantage of that and hook up the solar panel to top off the batteries. Before I did I took a reading on the batteries and they read
12. 75 volts. How is that possible? I am pretty sure the solar panel didn't put very much back into the batteries if anything at all. So overnight the batteries came back from 12.54 volts to 12.75 volts. It's like......they recharged themselves back up. Which is a dream, but doesn't happen. I know when they read 13 or more volts after being just charged fully they come back to rest at a lower voltage after sitting for a few hours. What is the answer for this?

ccbreder · Jun 3, 2015

simple lead acid battery chemistry. The voltage is not a reliable test for state-of-charge.

Spaceman Spiff · Jun 3, 2015

Anytime you add or remove charge in a battery, you are messing up the electron distribution on the lead plates (among other things). Takes a while for the battery to equalize (and the voltage across the terminals to stabilize). To get an accurate voltage reading, the battery should sit with no activity for ~24 hours.

-- Spiff

SoulRaven · Jun 3, 2015

ccbreder said:
simple lead acid battery chemistry. The voltage is not a reliable test for state-of-charge.

Everything I read says it is. When I charge my batteries with the solar panels it stops charging at around 12.7 ( i have checked this with my volt meter), and the fully charged green light blinks which means it is full. I have then taken the solar off the battery and hooked up my smart charger set to AGM, it reads full immediately. So are they both wrong? Why do the solar charge controllers and the smart chargers cut off at 12.7 volts and start a float charge if the batteries are not really fully charged? I don't understand. Plus, everything I have read says that a battery is fully charged when it reads 12.7 volts. Are all these websites wrong?
I also know about the hydrometer test to see the specific gravity of the acid. But these are sealed AGM batteries. The only way to test them is with a volt meter. Is there another way to test them?

SoulRaven · Jun 3, 2015

Spaceman Spiff said:
Anytime you add or remove charge in a battery, you are messing up the electron distribution on the lead plates (among other things). Takes a while for the battery to equalize (and the voltage across the terminals to stabilize). To get an accurate voltage reading, the battery should sit with no activity for ~24 hours.

-- Spiff

That makes sense, I hadn't thought about it that way. So I probably didn't take that much out of the battery. These must be pretty good batteries. The microwave is a power hungry thing. So I guess the 10 minutes of running it barley used much. When I put the 100 watt solar panel on today to top the battery off, it only took about 15 minutes before the charge controller started blinking green on the battery light signaling it was fully charged.

SternWake · Jun 4, 2015

All batteries have slightly different fully charged rested open circuit voltage. I've had some wet/ floodeds that levelled out at 12.62, and My current Northstar AGM rests at 13.06volts

With AGMs, voltage generally sags less, under higher loads, than flooded batteries.

How quickly they rebound, and how high voltage rebounds is an indicator of how healthy, and how charged the battery is. With age it rebounds slower and less.

With AGM's you need to know the amps flowing into them at absorption voltage to know when the battery is fully charged. Once it tapers to under 0.5 amps for every 100 amp hours of capacity, then it is fully charged, or very nearly so.

After 24 hours, unused, uncharged, then the voltage will have settled to the point where voltage can equal state of charge. Any other time, voltage is not a great indicator of state of charge. The more you watch your voltmeter under loads, and when charging, the better feel you will get for where the battery is, in terms of state of charge.

It is good to note how high they rest after 24 hours rested fully charged after meeting the 0.5a for every 100AH parameter, to have a baseline fully charged voltage, rather than expect they line up with your 12.7v expectation. They could be 12.92v or higher, and 12.7v would then represent about 75% state of charge

AGMs like the occasional higher amp recharge if deeply cycled day after day, and as always, like all lead acid batteries, They want to stay at as high as possible state of charge at all times.