# % of Charge by voltage under load?

Discussion in 'Alternative Energy' started by Allan Mistler, Nov 9, 2006.

1. ### Allan MistlerJust a simple man

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Folks, I've looked and looked (googled) but cannot find any information that is useful for determining a battery bank's depth of Discharge based upon loaded DC voltage. I only mention this since the operators manual from my Magnum Energy inverter indicates various cutoff voltages for the inverter based upon loaded battery voltage. The default is 10 volts DC (for a 12 Volt system) but it doesn't seem to take into account how large the battery bank is in amp/hrs or what rate of discharge is being enacted by the load. I feel that 10 volts is WAY too low if the purpose of cutoff is to preserve the battery, but all information I've located speaks only to voltage of unloaded batteries at rest.
I have requested data from the manufacturer but based upon previous requests for information, I'm not holding my breath...
any thoughts??

2. ### 12vmanOffgridkindaguy

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Out in the Boonies.. In Ohio
Open Circuit
State of Discharge
(Flooded Cell)

12.66V 100%
12.45V 75%
12.24V 50%
12.06V 25%
11.89V 0%

Depth of Discharge (DOD) - The percent of rated capacity to which a cell or battery is discharged. It is the reciprocal of a battery's state of charge. Example: a battery that has a depth of discharge of 45% has a state of charge of 55%.

Rated Capacity (Flooded) - The CCA (Cold-Cranking Amps), RC (Reserve Capacity) or amp-hours that a battery can deliver at a given rate of discharge, end voltage and temperature. These ratings are often displayed on the outside of the battery.

To know the depth of discharge, you would need to monitor the open voltage on your battery after a period of time unloaded. (30 min./1 hr.) Monitor the current draw of the inverter for the length of time used to figure your amp hours used. Check the open circuit voltage using the above chart for the level of discharge..

A battery will drop lower than the designed voltage under a big load before it's really dead. The 10 volt threshold doesn't sound too low. Some batteries are tested as low as 5 volts (Under Load) to get the total amp hour rating in reference to to its return voltage with no load. (Open Circuit)

3. ### Jim-miWell-Known Member

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Let your system sit unused--- 6hrs--overnight-- then take the voltage reading and refer to the above readings.

That 10 volts more than anything is to protect the inverter.

4. ### 12vmanOffgridkindaguy

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On an after thought..

How much reserve do you have? How much voltage drop do you have at the battery while using the inverter? (From an open circuit condition with all possible loads on)

If you have a huge battery, I would say the 10 volt threshold is too low. By the time the voltage dropped to 10 volts, the battery would be very low..too low. I know there's some fancy math out there to figure this out..

To rough it, If you can get a voltage reading from full charge (12.66v.) to full load at the battery, subtract that amount from the 75% level (12.45v.) and monitor for that voltage. Set the shutdown voltage on the inverter to this voltage and you should never go below the 25% discharge level.

5. ### 12vmanOffgridkindaguy

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More than likely to help out the battery salesmen..

6. ### Allan MistlerJust a simple man

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Thanks guys,
There's so much going on right now (trying to get Winter ready) that it will be a while before I can do a Fully unloaded to Fully loaded test to measure the voltage drop. What I have done however, until I do get a chance to take specific gravity readings is to raise the cutout voltage to 11.0 VDC I decided this was a bit smarter than the default 10.0 VDC since the inverter is drawing on a 1100 amp/hr battery pack. This many amp/hours at a 20 hour rate would be pretty darn drawn down by the time the voltage got to 10.0 volts!
I guess I've got another lesson to learn relative to what I thought I knew enough about...
:shrug:

7. ### Jim-miWell-Known Member

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If your bat bank is on the low end and your inverter sees a very large inrush current from perhaps a good sized motor, then yes your bat bank could dip to ..? 10 ? volts for the amount of time it takes to get the motor up to speed.
Again the low end cut out voltage setting is to protect the inverter from low voltage--high current damaging the inverter innards. . . . .heat
By raising that voltage setting yes it will be a bit nicer on your bat . . . .but you might now and again get the inverter tripping out from a large current start up of a load.

8. ### Ed_StantonWell-Known Member

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Dec 27, 2004
If your battery bank is 1100 amp/hrs, then I'm not sure that you can actually use that figure for your set up. I was advised to use a figure of 65% or so of the total amp/hrs as a maximum capapcity to use for my 24 volt inverter set up. The 1100 would only be for a brand new set of batteries that stay brand new. But over charge and discharge cycles, the 1100 changes. There is apparently a way to really keep on top of it all, and you're lucky if yours are new to be able to do that. When you get time, perhaps read through some old issues of HomePower magazine, www.homepower.com as I know that they have great articles on this stuff.