# 13.6v to 13v fast is this normal?

Registered Users Posts: 4
I finally have my first solar system up and running and tonight I had it hooked up to a minor load.  My batteries (3 of them at 125W each) float around 13.6v to 13.7v.  I was just pulling 1Amp out and they went from 13.6 to 13 in just 45 minutes.  (night time so no charging current)

Is that normal?  My hope is that there is an initial fast drop and then the drop from 13v to 12v is much much slower.  At this rate I'll be close to 12v in around 2 hours and 15 minutes on a minor load.  That can't be right, so is this fast initial drop normal or do I have an issue?

Thanks for the help!
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• Solar Expert Posts: 1,547 ✭✭✭✭
A fully charged 12 volt battery, when not charging or being discharged is up to 12.8 volts. After your system isn't doing any meaningful charging or supplying a float charge would naturally will drop to something around these full or close voltages when presented with a load

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2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Solar Expert Posts: 3,487 ✭✭✭✭✭
Even without a load, left overnight the voltage will drop to around 12.7V, this is normal with a healthy battery. In addition when you have a load connected the apparent voltage will climb once the load is disconnected. The initial voltage when a load is connected may at first be low but will rise slightly after a while, this is known as coup de fouet. Your load of 1 amp is small, so the 13.6v is the surface charge, once that is removed the voltage would stabilize to around 12.7V and take much more time to drop from there, it's a logarithmic curve not linear. Read the attached document to gain more insight, although primarily about UPS batteries much of the information is relevant.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Registered Users Posts: 374 ✭✭✭
> @n_yohan said:
> I finally have my first solar system up and running and tonight I had it hooked up to a minor load.  My batteries (3 of them at 125W each) float around 13.6v to 13.7v.  I was just pulling 1Amp out and they went from 13.6 to 13 in just 45 minutes.  (night time so no charging current)
>
> Is that normal?  My hope is that there is an initial fast drop and then the drop from 13v to 12v is much much slower.  At this rate I'll be close to 12v in around 2 hours and 15 minutes on a minor load.  That can't be right, so is this fast initial drop normal or do I have an issue?
>
> Thanks for the help!

Yes it's normal. The 13.6 is the voltage that is put into them to charge. They will retain that level only a short while with or without a load as they reach their point of equalibrium again at 12.6 or slightly higher depending on the age and condition of the plates. That will be the full charge voltage. Try not to let them get below 12.45 to 12.5 if you want them to last as long as they can. That's about 80 to 90% and will allow you the most cycles and longest life.
• Registered Users Posts: 4
Thanks everyone!