Another battery voltage question

[force9BOAT]

Hello,

My system has been operational for seven days now.  It consists of four deep cycle 115Amp/hour 12V batteries wired for 24V (2 pairs of 2 in series).  They are charged by two 100W solar panels connected in series (for 40V maximum charging voltage).  The load on the system so far is relatively light, constant draw of 1/3 amp (to power some small electronic devices).  

I've been taking voltage measurements twice a day, morning and night.  The morning voltage is slightly depressed (~24.5V) as expected after a night of power drawdown.  The evening voltage is higher (~25.1V), after a day of charging.  What I have found though is that if I take another voltage measurement much later at night, say five hours after sundown the battery voltage is often higher than it was minutes after sundown.  Somehow the battery voltage rises even though no charging is taking place and the power draw remains constant.  

Is the above description expected?  Is it the result of chemical reaction in the batteries?

Thank you,

Rob

[willib]

Rob , actually i've never heard of that happening..

the batteries should be at a max at sundown ,

i cant explain what you are getting.

[drdongle]

Where do you have your batteries, inside or outside?

[electronbaby]

yes, it could have to do with the temperature the batteries sit at. Also, if there is a slightly heavier load just before shutdown, over the course of the night the batts could be recovering from that draw and the voltage might inch up slowly.

RoyR

KB2UHF

[kitno455]

can you try testing V of each individual battery? i have seen a bank with a weak cell do something similar, current draw seemed to drain that one the most, then after sitting a couple hours, the other cells brought it back up. is there a change in current draw after sundown? (ie, you turn something off)

allan

[force9BOAT]

The batteries are in my garage.  The nighttime temperature has been around 40F.  I'll measure each cell to see what I get.  

Thanks all,

Rob

[richhagen]

If the batteries were unloaded at night, or very lightly loaded, I would guess a surface charge, that would rapidly drop when load was applied.  If the loading is constant, then I dunno.  Rich

[Ungrounded Lightning Rod]

Here's a thought.

You measured them after sundown and under load, then again a while later, also under load.

If there was some gas, left over from the charging, trapped on the surface of the plates, it would produce a raised resistance and a voltage drop from the load current.  With time under discharging conditions the gas would dissipate and the resistive vlotage drop would be reduced.  If that change in voltage drop is greater than the drop in voltage from the discharging, the result would be an observed raise in voltage at the terminals.  The actual cell voltage would be dropping, but the resistive voltage drop would be overcompensating for it.

Another possibility is that the cells are warming a bit just after sundown (perhaps from resistive heating), which would actually raise the cell voltage slightly.

[wpowokal]

Rob, firstly what is this elevated voltage?

Secondly for a 24V system, 25.1 V at end of day/charge is seriously undercharged. From my experience bulk charge does not start until 26.2v, and will stay in bulk charge until passing 27-27.5v which is the upper end of the bulk charge.

You then go into topping off (my term) then equilisation after 28.8V.

Homepower magazine have some good articals on battery charging.

regards allan down under

[wpowokal]

The Homepower link

http://www.homepower.com/files/battvoltandsoc.pdf