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For a few weeks now I have been charing a deep cycle battery using a 80W solar panel on the roof and a charge controller.

The frist few days the battery voltage would settle at 12.8V once the sun went down. Now days it only has a rest charge of 12.6V and I am no longer able to have it settle at 12.8V. Is this common?

I have tried charging the battery on a long and sunny day. The voltage goes above 12.8V easily but at night it settles down to 12.6V and not 12.8V again.

I am worried I am not charging the battery properly. It is often cloudy were I live and the battery is mostly charged quite slowly. I do not have any charging equipement for condition charging the battery using electricity from the wall.

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    \$\begingroup\$ Are you in the northern hemisphere which is transitioning into winter now, and the battery is now colder at night that it was before? \$\endgroup\$ – Olin Lathrop Nov 10 '12 at 17:33
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    \$\begingroup\$ Hi Olin. The battery is inside the house so the temperature is relatively constant. My apologies for not filling out my profile. I just joined the electronics part of the stack exchange 10 min ago. Will add the info now. \$\endgroup\$ – Paul Nov 10 '12 at 17:36
  • \$\begingroup\$ Presumably, you're discharging the battery, too. Tell us more about the load that you're applying to the battery. How old is the battery? Has it ever been run completely flat? I couldn't find much technical detail on this battery - is it "flooded" or "maintenance free"? \$\endgroup\$ – HikeOnPast Nov 10 '12 at 23:09
  • \$\begingroup\$ 0.2 is only 1.6% of 12.6. \$\endgroup\$ – Kaz Nov 11 '12 at 6:41
  • \$\begingroup\$ It is a new battery and lately I have not had any load on it as I have only aimed at reaching the 12.8V I had before. \$\endgroup\$ – Paul Nov 11 '12 at 12:08
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These flooded carbon fibre cells like other lead acid batteries are prone to sulphation which raises internal plate resistance and lowers the capacity. Try to adapt your charger method to rejuvenate the battery. Sulphation can be reversed by pulse charging if the plates are not warped from excess heat and a steady narrow inductive pulses help reducing the buildup.

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  • \$\begingroup\$ This answer makes a lot of sense. I think the plates might be a bit sulphated. I think what is needed is a battery charger. A good one is expensive (cost the same as the battery) and that is why I was hoping there was something I could do to avoid the added cost. \$\endgroup\$ – Paul Nov 11 '12 at 12:09
  • \$\begingroup\$ You can put a low ESR capacitor across the PV and use a fast switch to stimulate desulphation while storing the current in the Cap. Then use a PWM control to regulate the filtered average voltage. The array may be undersized for that battery. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 13 '12 at 16:40
  • \$\begingroup\$ There are also devices that only do desulphation for around 40 bucks. \$\endgroup\$ – Gunnish Dec 13 '12 at 5:28

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