DC Usage and Battery Monitors

[DavidEM]

There have been a number of questions asking more or less: Do I have enough DC power to do xyz while dry camping.

The answer depends on two things: what are you trying to do and how much battery capacity as well as charging capacity that you have.

Battery capacity is easy- look at your coach batteries and find out how many and what kind you have. Not all battery manufacturer's put the amp hour capacity on the battery label, and that should be a big warning that it is probably not a deep cycle design. But in any case here is rough indication based on size or Group number:

Group 24 70 Ahs
Group 27 80 Ahs
Group 31 100 Ahs
Group GC 220 Ahs for a pair
Lithiums, typically Group 27 size 100 Ah

Knowing how many amp hours you use is much tougher. Looking at the voltage based capacity display on most MHs is approximate at best and often horribly misleading. You should only consider resting voltage, ie no draw or charging for the past hour.

For example if the resting voltage on a battery bank is 12.5V which indicates about 80% charged and you turn on a high DC load like the furnace, the voltage reading may drop to 12.2 V or roughly 50%. But 80% is the right number.

A much better way of knowing how many amp hours you have used is a battery monitor.

All battery monitors are not created equal. The only type that has real validity is one with a shunt- a device that is installed in the negative lead from your battery that senses instantaneous current. The shunt is connected to the monitor's display or in some cases a processing unit that can be mounted near the batteries and read remotely.

Here are two good battery monitors:

Renogy 500A $100 on Amazon
Victron BMV 700 $144 on Amazon

Other Victron models have blue tooth interfaces or other features.

You mount the shunt in the battery box and hook up the negative power lead to one side and the batterie's negative terminal to the other. Mount the display inside in a convenient location and run a couple of low voltage/low current wires to the shunt. Enter the battery bank's amp hour capacity which is used as the base and you are set to go.

The monitor will show voltage, instantaneous current and total amp hours consumed since the last full charge, among other things. It is all encompassing in that the amps and amp hours are the sum of users, as well as charging sources including the coach's converter, solar panels and/or chassis alternator.

If you want to see what just one of these is, note the current reading and turn the other one off, ie turn the converter off for example or turn a large user like the furnace off. The difference is what that source/user supplies or uses.

So if you need to know what your amp hour consumption is while dry camping, maybe to help you decide how much solar to install, keep a log of amp hour usage over a couple of days. The battery monitor can also help you decide when to start the generator to replace the amp hours used. Like I said it is a much better indicator of real battery capacity than the simple voltage reading which is distorted by usage or charging.

David

[astrocamper]

I would recommend the Victron Energy SmartShunt – 500A 1000A or 2000A Bluetooth Monitor for about $130.

https://invertersrus.com/product/victron-smartshunt/

It is easy to use. No wires to run to display. Access data using Victron Energy VictronConnect app on phone, tablet or laptop.

[DavidEM]

One poster on another forum said that the bluetooth range for the SmartShunt was very limited- a few feet maybe. Have you had that experience? Otherwise it sounds like a great device.

David

[profdan]

David, thanks for starting this thread. And astro, thanks for that link to the app. I really need to get serious about monitoring my amp-hours. We are mostly boondockers, and power management is a big deal.

[profdan]

Do these monitors take into account the energy you put back into the battery via solar panels? In other words, consumption of electricity is relevant, but every day my "net a/h available" has to reflect solar production, right?

Please forgive me if this is a newbie question -- I am an experienced camper but know nothing about any monitor more sophisticated than my ten dollar multimeter.

[jcussen]

Originally Posted by profdan

Do these monitors take into account the energy you put back into the battery via solar panels? In other words, consumption of electricity is relevant, but every day my "net a/h available" has to reflect solar production, right?

Please forgive me if this is a newbie question -- I am an experienced camper but know nothing about any monitor more sophisticated than my ten dollar multimeter.

Yes they do take into consideration flow into battery, as well as flow out. I also have the smart shunt and it does work well, have found the range about 10 ft, but this is from the battery bay through the floor to my phone.

[Nickd]

I use a BlueSky solar controller with their Shunt and remote monitor. It monitors charging current, battery capacity in percent and of course amp draw. All info available at the touch of a button and a backlighted LED display.

It’s nice to know how much juice your using at night when there is no sun and the campground has generator hours. For instance my furnace draws almost 6 amps when the fan is running. The coach draws 1.7 amps for sensors and the fridge. That porch light that I thought was LED, draws 1 amp. I wouldn’t have known those draw numbers without a shunt and monitor.

[profdan]

Nick, you can measure draw using a multimeter. It's not hard. But still, knowing the draw of any individual fixture or appliance is not the same as really knowing how much juice you've got available at any given moment!

[jcussen]

A voltage meter does little good if you have lithiums, it will give you a rough estimate on lead acid, but because lithiums voltage varies so little down to 10% SOC, you will need a shunt type that reads amps in and amps out to get an accurate reading.

[Bobby F]

Originally Posted by profdan

Do these monitors take into account the energy you put back into the battery via solar panels? In other words, consumption of electricity is relevant, but every day my "net a/h available" has to reflect solar production, right?

The shunt-type monitors are wired so that there is only one pathway to the ground terminal of your battery bank - through the shunt block. All connections - in and out - are made to the downhill side of the shunt. They thus monitor flow in both directions. If you look at the readout during daylight, what you see is net flow, taking into account both usage and charging input.

[astrocamper]

I actually have the BMV-712 that I bought before the smart shunt version was available. It would work anywhere in the trailer and also in my dining room about 25 feet away. Later I bought the Cerbo GX and now I can see my 4 MPPT controllers, BMV-712 and Multiplus 3000VA Inverter/Charger data from the Cerbo GX touch screen or anywhere I have Internet access.

I recommended the Smart Shunt to a friend and he is happy with his. The price is pretty good and it does have the option to use the VE.direct cables to connect to a GX device.

[AnotherMike]

Originally Posted by Bobby F

The shunt-type monitors are wired so that there is only one pathway to the ground terminal of your battery bank - through the shunt block. All connections - in and out - are made to the downhill side of the shunt. They thus monitor flow in both directions. If you look at the readout during daylight, what you see is net flow, taking into account both usage and charging input.

I'm quoting Bobby F. above to emphasize something.

Once side of the the shunt goes to chassis ground. The other side to battery negative. NOTHING ELSE goes to the shunt connection that is connected to battery negative, or to the battery negative post itself.

I helped a gentleman resolve an issue with the installation of his Victron monitor. He was the second owner of his coach and the battery wiring was confusing. Someone had added a couple of additional wires on battery negative that should have been relocated to chassis ground when the shunt was installed. I also showed him that he should connect his Battery Tender to battery positive and chassis ground, not battery positive and negative.

Mike

[kongmen]

I would like to order one of these but don't know what size. I have 3 agm 100 ah batteries and a 2000 watt inverter. Should I order the larger amp one? I definitely want the one that I can use with my phone. Thanks.

[jcussen]

Originally Posted by kongmen

I would like to order one of these but don't know what size. I have 3 agm 100 ah batteries and a 2000 watt inverter. Should I order the larger amp one? I definitely want the one that I can use with my phone. Thanks.

500 amp shunt should handle anything you have now and probably in the future. 500 amps would cover a 6000 watt load.
The Victron smart shunt is the one I use.