Measuing Depth of Discharge on Loaded Battery?

Generally no since the items such as lights, water pump, control-boards for refrigerator, furnace, some detectors, exhaust fans may be used some over any 24-hour period. These item might cause a 0.1 to 0.2 amp load on the batteries.

If you are also using an inverter to power some 120v ac items off the house-batteries than thing can get very confusing since they can have a much larger load drop on the battery voltage.

If you are dry-camping more than one day (24 hour period), time to check batteries would be to cut off any unnecessary usage for 1/2 to 1 hour in the afternoon and check the voltage against the chart in the first reply.

You will have a good estimate of what will happen in the next 24-hour period if conditions don't go for the worst (colder weather).

I run my inverter of a separate Trojan 12v battery for my TV, phone, shaver, and Kindle Fire charging. Since my 1000 watt inverter will show voltage and amp readings, I can check the voltage shown initially before plugging items into the invertor. Then I can log the voltage shown while running the items under load. If you know how much the voltage dropped and log the time used, you can have a good idea what the battery state of charge will be if you allowed them to rest at least 1/2 hour under no load. Checking batteries at the battery-terminals with a reliable multi-meter is best. Separate gauges are okay if they compare with a multi-meter reading.

I have 2 new Trojan T105's used for house items which I know what the normal 24 load is, and accounting for furnace of powered vents use. The old one I replaced lasted 10 years.

A Trimetric will tell you everything, a true depth of discharge would be measured by the specific gravity of the solution. You want a Trimetric! I never knew about the state of my batteries like I do now with the meter.

This Rolls doc suggests voltage collection at load, bottom of page!

http://www.rolls-battery.com/content/technical-downloads?q=node/51&phpMyAdmin=2eda4dec0bd69647b9e3cf0f71e01d23&phpMyAdmin=9dec4a269d70t7a63be7c

Some batt spec pages have a discharge curve at various loads.

The chart above is good to 25 amp discharge. How heavy is the load?
Otherwise you need a full battery monitor that counts amp/hrs.

The problem with measuring the battery state of charge under load is it will be different depending on the load. The battery has internal resistance, which causes voltage drop dependent on load.

For example, I often draw 60 amps (for a total of 10 amp/hrs) from a pair of 6V batteries while running an inverter making a pot of coffee. During the draw the voltage drops to 11.75, however as soon as the draw is done it jumps back to 12.5 or better.

For no load, or a very light load the table Roy will give a pretty accurate indication of SOC, but as the load increases it gets less accurate.

There are metering systems such as a Tri-Metric that measure actual battery amps out & in, which will give you a more accurate indication of the battery condition than a simple voltage measurement.

I just look it up on the specification sheet...

This is for the TROJAN T1275 - says this battery will produce 12VDC@25AMPS for 280 minutes or 4.6 hours... At the end of 4.6HOURS this battery will be reading 10.5VDC if you put a 25AMP load on it. This tells me I will only be able to use this battery for 2.3 HOURS of 25AMP DC drain if I want to recharge again at the 50% charge state.

These are the rules I follow at any rate for estimating battery capacity and it seems to pan out pretty good... I will re-charge when my battery voltage drops to appx 12.0VDC according to this chart




Roy Ken