Measure DC usage for future solar setup

stmoloud · November 2018

I have a 85 ah battery which I know to be good. My plan is to connect this directly to the DC circuit in my vehicle and discover how much time it takes to drain it to 50% or 12.2 volts.

I do have a low voltage disconnect device between the battery and load. Unfortunately it is old school / cheap so needs to be set manually.

The plan is to use the DC normally, as I would with an AC/DC converter. Even a little bit frivolously.

And then use the time taken and capacity of battery to figure a suitable solar charging and banking times two days.

That would then take care of the DC for the vehicle.

I would like but do not know if it is electrically efficient to have two banks - one DC, and the other inverted AC

Is this a faulty logic? Is this a waste of time?

Thanks

Photowhit · November 2018

I'm going to be a bit of a jerk here!

stmoloud said:

My plan is to connect this directly to the DC circuit in my vehicle and discover how much time it takes to drain it to 50% or 12.2 volts.

So which are you going to do 50% or 12.2 volts?

Voltage is a VERY difficult way to determine State Of Capacity (SOC) If you are draining a battery with a light load and stop at 12.2 volts, you may have drained 40% of it's capacity. You will want to turn everything off and wait for a while for the voltage to settle out.

If you drain it down using a large load you may drain less than 20%, 2 - 65 watt car head lights should do that, drawing 10+ amps from a 85 amp battery, I'd bet you see 12.2 after half an hour, maybe make an hour, but seriously doubt it.

Don't understand the logic in having 2 banks, you can draw DC and run an AC inverter of the same bank.

I would suggest getting a meter, if you must, to measure the current going in and out of the battery across a shunt. Something like the Trimetric. Then again it's only as accurate as your settings.

stmoloud · November 2018

I guess if I'm draining 40% or 20% that's OK, so long as it's not 60 or 80. I am just taking the published figures that 12.2 volts is equal to 50% discharge.

So if my LVD disconnects at say, 10 hours, can I not then determine from that the amps I have used? If I fully drained a 85 ah in 10 hours does this mean that I have used on average 8.5 amps per hour? Now if my LVD is set to 50% or 12.2 volts wouldn't this then mean that on average I have used roughly 4.3 amps per hour?

Having two banks, if I increase my DC but not my AC usage I would then only need to add to the DC bank. It might work out cheaper in the long run.

mcgivor · November 2018

The amount of current drawn from the battery will have an effect on the rated capacity, one may assume an 85Ah battery would could support a load of 85A for one hour, 8,5A for 10 hours or 1A for 85 hours but this is not the case. Deep cycle batteries generally use a 20 hour rating, so a load current of 4.25A for 20 hours would completely discharge the battery to a terminal voltage of 1,75V per cell or 10.5V for a 12V battery. If the load were doubled to 8.5A the battery rated at 20 hours, would not support the load for 10 hours, it would reach the terminal voltage much sooner. Read this article http://solarhomestead.com/battery-amp-hour-ratings/ it explains this in more detail.

Photowhit · November 2018

stmoloud said:
So if my LVD disconnects at say, 10 hours, can I not then determine from that the amps I have used? If I fully drained a 85 ah in 10 hours does this mean that I have used on average 8.5 amps per hour? Now if my LVD is set to 50% or 12.2 volts wouldn't this then mean that on average I have used roughly 4.3 amps per hour?

Using voltage to measure State Of Charge (SOC) in a working system I would generally say doesn't work. After a while you can get a good feel for it, but it's a Touchy-Feely thing. When you see published voltages for SOC, they are for a battery 'at rest' with no charge or discharge for some time, usually a couple hours!

Low Voltage Disconnects (LVD) are used by inverters to protect there electronics. Sometimes they will also save your battery...lol.

The voltage sag under load has many variables, the size, thickness of the plates used, the alloy of the plates, the size of the load. As @mcgivor has pointed out they have different capacities depending on how much current is used.

A trojan T105 battery has a capacity of 225 amp hours if used over a 20 hour period, but it has 5-Hr Rate capacity of 185Ah 10-Hr Rate capacity of 207Ah 20-Hr Rate 225 100-Hr Rate 250

Estragon · November 2018

Should also mention 20hr rate amp-hour capacity will be room temp. A cold battery in a vehicle could be significantly lower.

If I'm understanding the original question, you're trying to solve for load size by load testing a known good battery. Rather than deal with all the complexities doing that, why not just get a DC clamp meter and measure the load directly?

stmoloud · November 2018

mcgivor , thanks for the link. But would it be correct to say that if my 'battery guardian' (LVD) is set to disconnect at 12.2 V and if I were discharging at the rate of 4.25A, would the battery then be disconnected at approx 10 hours of use, and above the 50% level of discharge generally considered to be the max allowable?

Photowhit , I was hoping that discharge would be more measurable than touchy feely. Perhaps that's why many newbies, it is said, ruin their first set of batteries. Gaining a good working knowledge could be an expensive thing.

Yes, my inverter does have a LVD but is way too low IMO. That's why I was thinking that a separate one on a DC bank would save any potential battery ruin.

Estragon, yes a DC clamp meter would be handy. Not knowing anything about DC clamp meters, do they have a feature that if left reading a cable over say, a period of 20 hours, that a total amount of Amps drawn during that period could then be read?

Estragon · November 2018

There are some meters, such as Trimetrics sold by our hosts, which show current, and also a SOC estimate, ah, voltage, etc. Being shunt based, they're a good way to monitor a system on a day to day basis IMO. They're wired in on a more or less permanent basis.

There are also DC clamp meters which aren't attached so can be used for things like checking individual wires (eg battery interconnects). There are some that are good enough for our needs for ~$45 on Amazon. Note, there are also lots AC only clamp meters out there, which won't work.

BB. · November 2018

There are shunt based battery monitors that have hardware alarm outputs (contact) that can, for example, be programmed to turn off at <50% State of Charge and turn back on at >80% SoC.

https://www.solar-electric.com/xantrex-84-2031-00-linkpro-battery-monitor.html
https://www.victronenergy.com/battery-monitors

Some Victrons may also have a digital comm connection too (as I remember).

But they are not cheap ($250-$300 or so???).

And BMS can "drift".

Setting your inverter to 11.5 volts (or 23.0 volts @ 24volt bus) to shut down under load and turn back on >~12.7/25.4 volts (battery charging again). Some inverters have variable shutdown ranges (i.e., 12.2 volts > 2 hours; 11.5 volts>30 minutes; 10.5 volts > 10 seconds).

Samlex (Cotek and Samlex are sister companies, I believe, and relatively good quality for the price) even has a programmable alarm meter (optional):

https://www.solar-electric.com/samlex-bw-03-battery-monitor.html

Lots of reading ahead...

-Bill

stmoloud · November 2018

Bill, unfortunately my inverter's shut down can't be user determined. The Samlex looks good for my budget.

BB. · November 2018

There are a lot of products out there--And I am certainly not the expert/sales engineer (just a sort of unemployed/retired systems engineer).

Figure out what you need for your installation, and then look (and ask) around for hardware that could support those needs.

-Bill

stmoloud · November 2018

Pleasantly surprised that in 20 hrs I have used 0.8 volts of a 85 ah battery.

My terminal voltage started at 13.2 and at 20 hrs is 12.4. This is 75% of my allowable amount of voltage until the Low Voltage Disconnect of 12.2 is initiated.

Now I can calculate how many amps I have used. 75% of 50% battery capacity (or 12.2 volts at the battery terminals) is 37.5%

So 37.5% of 85 amps is 31.87 amps.

I now have an averaged hourly rate of discharge of 31.87 / 20 equals 1.6 amps or 19.2 watts per hour.

I could hope to get 40 hrs out of this battery before it hits the critical 12.2 volts but at the present rate of discharge there is little chance of that.

Measure DC usage for future solar setup

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