Running around with ham radios and 12v DC devices, knowing how much power is left in the tank can be pretty important.
For simple lead acid batteries, the voltage of the battery can give you a pretty good idea how much capacity is left. But nowadays a lot of people have moved to the LiPo4 batteries. They’re smaller, last for more cycles, and can be discharged much deeper without damaging the battery.
They maintain a VERY stable voltage output, though, which makes it more difficult to estimate capacity based on the voltage like we used to do! One option for this is a DC Coulometer. Basically, these sit inline with the load just like the your AC utility meter on your house, and measure the power going in (charging) or going out (discharging). If set to the total capacity of the battery, many Coulometers will show the percentage of battery discharge, as well as the estimated remaining runtime or amount of time until the battery finishes charging. It situates itself “inline” by being in the path of the negative connection to the batteries.
I installed this model in my camper, but there are lots of options in various sizes and capacities. It is readable both on its own display as well as via Bluetooth to a cell phone or tablet.
Now it’s easy to find out exactly how long I can be disconnected from AC without losing power. It’s also effectively an ammeter so you can see how much power you are using.
My next “quest” with this, is to see if it’s possible to hook up RV power to the tower trailer and solar panels at the trailer hitch. Potentially, it could displace the need to use the generator to charge things up. A field day operation with no generator noise is appealing, and even if solar isn’t able to fully displace power usage, it could slow down power consumption enough that it could make it through the weekend anyway.
The challenges, though, include the length of the 12v DC run and the likely voltage drop. And the limiting of charging current through what is sure to be a long run of wire.
Testing with the coulometer will be able to show whether or not enough charging current will make it through to make it worthwhile, under typical/expected uses.
Coulometers are another tool in the ham radio toolbox for battery management.
-Tim N0TJN
Ham Radio Tower Trailers 
If you have multiple parallel batteries will a single meter set for total amp hrs be accurate?
LikeLike
Presumably if the batteries are identical and in parallel, the sum of the batteries AH ratings should be accurate. The device itself measures the power crossing the sensor (often a metal bar with an inductive loop sensor going around it), so it shouldn’t be able to tell the difference between two 100AH batteries or one 200AH. That being said, I never go far enough down in capacity to find out. I might get more adventurous if I ever get LiPo4 batteries in the coach, but they are crazy expensive (and require new charging controllers to get full capacity because of the different charging voltages needed)… So I try to never go below 50% with traditional lead acid.
LikeLike