Off Grid Ham: Learning From Off Grid Mistakes, 2

This article comes from Chris Warren at Off Grid Ham – Learning From Off Grid Mistakes, Part 2.

I wasn’t planning a “part 2”. learning from off grid mistakes

Last May’s article about off grid mistakes received a surprising amount of attention. Many months later, it’s still a very popular piece. As a follow up, I thought it would be a good idea to revisit the issue and go over a few points that were not discussed last time. I encourage readers to send in questions and comments because most of the articles that appear on Off Grid Ham are derived from reader input. learning from off grid mistakes

Mistake 1: Mismatched batteries.

Batteries are very exclusive. They don’t like other types of batteries. Just because two batteries are of the same voltage, and maybe even the same capacity, doesn’t mean they play well together. If you are using multiple batteries, they should be the same make and model, and roughly the same age. Most batteries will have a date code on the outer casing for determining age. learning from off grid mistakes

When I went shopping to replace my large storage batteries two years ago, I brought my battery analyzer with me to the store. They had a huge pallet of deep cycle batteries, so I had plenty to choose from. I dug through the pile and picked out a few that were manufactured within a month of each other. From that cohort, I tested each until I found a few batteries that had the same or very close to the same internal resistance. That was the matched set I ultimately bought and took home. Yeah, I must have looked a little weird picking through batteries and running tests, but I got what I wanted. learning from off grid mistakes

When you mix dissimilar batteries or batteries of different ages, the weak one will pull down the strong one. Always Install and remove your batteries as a set. If you must mix dissimilar batteries, wire a battery combiner between them.

Mistake 2: Mismatched solar panels.

This mistake needs some clarification. You should not mix/combine solar panels of differing voltages at any time. Solar panels that produce the same voltage but not the same wattage can be used together, but only if they are wired in parallel. Solar panels are often wired in series to increase efficiency and make better use of MPPT solar controllers. This works only if all the panels in the series are the same voltage and wattage.

If you wire solar panels of the same voltage but different wattage together in series, you will not damage anything or create an unsafe condition. What will happen is that the total power output of the system will not exceed the capacity of the smallest panel. For example, you have one 100 watt panel and one 50 watt panel wired in series. It might seem reasonable to think you’ve got a total of 150 watts capacity. Sorry, but you’ll never get more than 74 watts out of this system.

The reason why is fairly simple: Kirchoff’s Law states that current will always be the same at all points (nodes) in a series circuit. A 100 watt panel will produce about 5.75 amps. A 50 watt panel maxes out around 2.85 amps. Our 12 volt example panels below are wired in series for a system total of 24 volts (in reality, it would be closer to 26 volts).

Since Kirchoff says the current is the same at all points in the series, and the 50 watt panel will never exceed 2.85 amps output under any conditions, the system total is limited to 2.85 amps. Doing some basic math, 2.85 amps x 26 volts= 74 watts. These numbers will vary due to differences between loaded and open voltages, what specifications are used for your calculations, etc., but this gets us pretty close. Think of it like a convoy of ships: The entire convoy cannot go any faster than the slowest ship.

learning from off grid mistakes


Mistake 3: Using automotive batteries.

If someone gives you a car battery, or a car battery is all you have (such as in a SHTF situation), then certainly go with it for your off grid ham radio power needs. But no thoughtful ham would purposely choose a car battery.

Car batteries are designed to deliver a large burst of current over a short period of time, which is needed to start a car. Off grid hams need batteries that can deliver smaller, steady amounts of current over a long period of time. Using a car battery will not hurt your equipment and is not a safety hazard, but you will not see the the level of performance that a correct battery would provide, and the car battery will have a shorter service life too.

Mistake 4: Using automotive “jump boxes”.

Those inexpensive portable battery boxes made for jump-starting cars seem like an easy, ready made power system for ham radio. They are not recommended for ham radio use for the same reason as standard car batteries. They are made for a short power burst, not for a lighter, continuous load. Some hams do use them with modest success, especially for QRP, but they’re not a serious way to power your radio.

Mistake 5: Buying the best, most expensive gear available.

Just as buying cheap junk because it’s cheap is a mistake, so too is insisting on only “the best”. More expensive does not necessarily mean a device has better build quality or will last longer than a less expensive device of the same type. In many cases it only means you get more cool switches and pretty lights. If you cannot justify the extra cost with some clear purpose or practical benefit, buying “the best” is a journey of vanity.

In my experience, mid-grade equipment has always given me the most bang for the buck. Early in my off grid career I spent over $500 on an ExcelTech inverter. They are made in USA. They are practically indestructible. The American military and US embassies around the world use them. They’re the Rolls Royce of inverters. I didn’t know it at the time, but it was unnecessary overkill. As nice as my ExcelTech is, my Samlex inverter is just as suitable for my application. It cost half as much as the ExcelTech and gives excellent performance. I still use both inverters, but if I were doing this over I’d get two Samlexes and spend the extra money on other useful upgrades.

Never buy any piece of off grid amateur radio equipment based solely on high or low price point…(continues)