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

Registered Users Posts: 11 ✭✭
edited December 2015 #1
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:  `http://imgur.com/X0BdbE0`
Other:
• Air temp 70 degrees F
• 24 volt system, 4 and 4 batteries in series parallel

Tagged:

• Solar Expert Posts: 3,738 ✭✭✭✭
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
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 5,183 ✭✭✭✭✭
Measure the SG, much easier than the math.....
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

• Solar Expert Posts: 8,762 ✭✭✭✭✭
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

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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• Solar Expert Posts: 175 ✭✭✭
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)