How to calculate / estimate how long batteries should take to charge with generator

ryan112ryan · December 2015

I have been living off grid w/ batteries for a year and a half, but this winter we've had a few long runs with very cloudy skies. So for the first time I had to fire up my generator to bring the batteries back up to full charge. My installer told me 1-2 hours with my generator should take my batteries from 50% back up to mostly full, but I ran it for two hours and my voltage was 24.6 to 24.8 Not bad, but I was expecting more. This was what my panel was reading during the charging (http://imgur.com/X0BdbE0)

My question is, does this seem about right? How could I calculate how long it should take?

Also after finishing charging, my batteries seemed to drop voltage a bit faster than normal once I started using them with lighter than normal use, could there be something else going on? I just topped off the batteries a week ago with distilled water.

System details

(9) Canadian Solar CS-6p 250 Watt Poly Black Frame (Spec Sheet)
(1) Schneider SW 4024 (Spec Sheet)
(1) Schneider MPPT 60 Charge Controller (Spec Sheet)
(8) Trojan L-16 6v 370 AH Flooded Lead Acid Batteries (Spec Sheet)
(1) Schneider System Control Panel (Spec Sheet)
(1) Schneider Interconnect Panel (no spec sheet)
(1) Midnight Solar MNPV 80AMP Dinrail Breaker (Spec Sheet)

Generator Details

Troy Bilt Brigs and Straton 6,250 generator Model #: 30594
Connected to my system is 240V through a 30 amp plug

Input readings from my monitor panel during charging

237V coming in from generator
3085 watts
13 amps
image of my panel while it was charging: <a href="http://imgur.com/X0BdbE0" target="_blank" rel="nofollow">http://imgur.com/X0BdbE0</a>

Other:

Air temp 70 degrees F
24 volt system, 4 and 4 batteries in series parallel

vtmaps · December 2015

Your battery capacity is 740 ah X 24 volts = 17760 watthours.

Subtracting loads and accounting for charger inefficiency, I would guess that you have about 2400 watts going into the battery (that means 2400 watthours every hour). 2400 watthours is 13.5% of your battery's capacity, and bulk charging tends to be very efficient, so I would expect that your SOC should rise about 13% every hour during bulk charging.

From 50% SOC, about 3.5 hours of charging should get your SOC above 90% and into absorb stage of charging.

--vtMaps

Dave Angelini · December 2015

Measure the SG, much easier than the math.....

mike95490 · January 2016

Your charging happens via the inverter & controlled by the SCP, which you seem to have a handle on. I don't know if the Combox would be any more useful for you or not.
But, for a 24V battery bank, I think your voltages are quite low. Full charge should be about 25.5V, and to get there, you have to drive close to 30V For 80% (Bulk only, no Absorb) at least 25V - see this chart

Image: http://modernsurvivalblog.com/wp-content/uploads/2013/06/battery-state-of-charge.jpg

To understand some more about charging, this covers some of the basics
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery I think you are damaging your cells by not getting into the upper % of the charge, and could be getting some sulfation / loss of capacity.
Good luck

scrubjaysnest · January 2016

mike95490 said:

Your charging happens via the inverter & controlled by the SCP, which you seem to have a handle on. I don't know if the Combox would be any more useful for you or not.
But, for a 24V battery bank, I think your voltages are quite low. Full charge should be about 25.5V, and to get there, you have to drive close to 30V For 80% (Bulk only, no Absorb) at least 25V - see this chart

To understand some more about charging, this covers some of the basics
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery I think you are damaging your cells by not getting into the upper % of the charge, and could be getting some sulfation / loss of capacity.
Good luck

Some good info in the above but the chart may be low for your specific batteries; it is for mine which are Exide 12 volt. So you will need to go to the your battery manufactures web site for your specific battery(s)

How to calculate / estimate how long batteries should take to charge with generator

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