Fair comment. Most of them aren't.

Dougwalkabout's "amorphous (thin film) panels" are also great because of their surface area, so they perform better in cloudy conditions. I'd be careful of saying Blast's batteries were topped-off from a single day's sun from a 30watt panel without knowing the amp-hour capacity of the batteries. Remember we are playing with electrical energy and chemical energy in solar/batter systems.

It's a 20 amp-hour battery which was only 40% discharged (putting out 12.3 V) according to my battery monitor. After one very cloudy day my 30-watt solar panel had brought it back up to 100% charged (12.83 V) according to the same monitor.

My Xantrex Powerpack 400Plus.
My battery monitor.
My monocrystalline 30-watt solar panel.

-Blast

A voltage monitor isn't the perfect fuel gauge of a DC system. I'll try to make a new solar/battery topic tomorrow, and keep kids doing homework no matter what excuse they can find.

Sure, the only real test is to measure the voltage under a load of suitable size for the battery's rated capacity.

(Example: a neighbour can't get his tractor to start. He has a DVOM, and it reads 12+ volts. So the battery must be okay, right? I get him to read the voltage while I turn the key, engaging the starter. 4 volts under load. Time for a new battery. Checked the charging system output after, just in case; it was fine, kicking out 14+ volts.)

It isn't always possible to test with a suitable load. But if you know your battery's general range, you can get a working indication of its overall status. I like to monitor charging with a multimeter. If a lead-acid battery on my solar panels will achieve just over 14 VDC, that's a full charge. The SLAs I've used will typically drop to 12.8 after charging terminates. However, if a batt comes close to full charge and then suddenly cascades to 15+V, it's probably reached the end of its normal service life.

BTW, here's an idea:
A particularly interesting topic for this forum would be "bring out your dead ... batteries." I.e., how to get value/use out of different types of batteries, rechargeable and single use, that are technically dead (hopelessly out of their design range) but can still be used to accomplish useful work in an emergency. I have some field experience in this ...

How does one do this?
-Blast

One measures voltage across the terminals of the battery. If the voltage under load is close to the same as when it isn't under load, your battery is in pretty good shape.

Battery testers that do not measure under load are pretty common, but unfortunately aren't particularly useful. Good battery testers always introduce a load.

I use an inverter and various loads like a hair dryer. Or a dc water pump etc. I find something that develops the load I need. Sometimes I use large wire wound resistors that I find at "specialty" (electronic junk) shops.

I have several shunt operated amp meters to monitor the current and a standard digital meter for the voltage. Shunt meters are easy to make and work well.

The hard part is making sure you are around when the battery reaches the desired "discharged" point. this varies depending on the type of test you want to run. Not hard to make a small arduino (small computer used in robotics) voltage/current monitor that alarms when the set discharge conditions are met.

I can go into it in depth if you are really interested. I live off the grid and do a lot of solar/battery stuff.

What exactly do you want to test??

Nomad

Most of the routers work at either 12 or 5vdc. Check the wall wart output voltage. I run several 5v ones using just the standard "3 leg" 5v regulator mounted on a small heat sink.

Nomad

I'm always leery when doctors o' philosophy ask simple, innocent, open-ended questions. A little alarm bell goes off in my head, warning that I may wind up on the short end of a Socratic dialogue.

Without the manufacturer's specs at hand, it's hard to determine an ideal load. A 20 Ah battery is not that large, and there's a fine line between a load that's large enough to test the battery strength and a load that will overheat/damage the battery.

The safest route is to use the on-board inverter as your load. It has built-in protective circuitry. Plug in an item that's near its maximum capacity, and then measure the DC voltage of the battery. If the voltage holds well, you have a good battery and charge; if it drops like a stone, you have either a weakened battery or incomplete charge.

Another option is to charge your batteries on the solar panel, let them rest a short while, then use the wall charger supplied with your unit. These usually kick off when a full charge is achieved.

As for field expedients, a DC motor (e.g., large bilge pump) would work. I think an old-style sealed automobile headlamp would be quite viable (IIRC, these pull somewhere around 5-6 amps). Connect it, measure the voltage, and disconnect.

A last caution: these batteries are not deep-discharge types. Discharging them too deeply causes damage. If you're using the inverter, it will alarm and shut down when the voltage becomes dangerously low. This is just under 11 volts IIRC. If you're using direct DC, there will not be an alarm or automatic shutdown; you have to stay on top of it.