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.
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.
Mistake 6: Using batteries of unknown provenance.
This is as bad as using mismatched batteries. Suppose you’re walking through a swap meet or a yard sale. You come across a few awesome-looking batteries at a great price. There’s only one problem: They’re an unknown brand you’ve never heard of, or maybe are not branded at all. You’re not even sure how old they are or if they are new or used. The seller doesn’t know anything either. They’re just random batteries. learning from off grid mistakes
“Mystery batteries” may be an acceptable choice if the price is right and you plan on using them only for experimental, low-risk purposes. For critical off grid communications, this is not the time to roll the dice and hope for a win. Ask yourself: What are the consequences if the mystery battery fails unexpectedly or underperforms? How much hassle and money will it take to get the correct battery you should have bought in the first place? If you’re not comfortable with the answers, then take a pass.
This brings us to related Mistake 6A: Used batteries. As before, they are ok for fooling around or if you have no other options. For anything important, there are just too many unknowns to fully trust them.
What we learned today.
Experience may be a great teacher, but learning from someone else’s experience is even better. Some of these common mistakes were sent in by readers, others are based on my own misadventures. Of course, this list could be a lot longer but I think we hit on all the big ones. If you see yourself in any of these points, don’t be discouraged. No matter how long you do this, you’ll never be perfect, so use your own slip-ups as a reset to do better next time.
Correction December 2020: The article text and graphic has been updated to fix a math error.
Good ibformation, thanks de wb5eat
Thanks for checking in, Bob! I’m glad I can be helpful.
Excellent discussion ???
Hi Julian, thanks for stopping by! I’m a fan of your YouTube channel.
Good article, Chris 🙂 I’ve had the discussion about why you shouldn’t use automotive batteries for things like this far too many times, alas. Some people just can’t seem to grasp the issues involved, the different battery capacities, different chemistry, etc. I won’t bore you with tales of the bulging batteries, leaked electrolyte, burned out chargers, etc. that I’ve run into over the years from people using the wrong batteries. I’ve seen it all, but it still shocks and surprises me.
That is a great point about chargers. I should have included “using the wrong charger” as part of my article.
Foe some reason those automotive “jump boxes” are popular with hams. Yeah, they’re cheap but for about the same money you can build your own box with the proper battery. I think people are just lazy and want something that is “plug-and play” even if it’s not a good application.
Thos jump boxes do have a lot of problems. As you pointed out, they’re intended to only do one thing, provide a brief burst of energy to boost a car’s electrical system just barely enough so it will start. They’re generally very low capacity to begin with, made as cheaply as possible, often don’t retain a charge for very long even when not being used, take a long time to recharge and often fail after just a few recharge cycles.
This reminds me of a boss I had many years ago when I worked in building maintenance. The cleaning crew had a battery operated floor scrubber that ran on 4 six volt industrial deep cycle batteries wired to put out 24V. When the batteries failed he congratulated himself on saving a small fortune by replacing them with 2, 12V car batteries. And using the same 24V, high amperage battery charger to charge them. I’m sure you can imagine how that worked. The scrubber only ran for about a quarter of the time it should have before the batteries died. We had boiled over batteries, bulging batteries… He’s lucky he didn’t burn the building down or get someone injured. I never could get him to listen. He believed the cleaning crew was somehow deliberately sabotaging the batteries, tried to get them fired, tried to get me fired when I tried to explain what was wrong… He did the same thing with a big floor sweeper.
Everything you say about those jump boxes is true. I know some hams use them and claim success, but with so many better options out there, why bother?
Your learning points are excellent, but I have an issue with the Solar panel connection chart.
Mismatched panels in series will limit the current but the resultant voltage is still the sum of the panels output.
Assuming each panel shown is 12 volt panel ( to get the 50/100 wattage ratings shown the calculated open circuit voltage of the panels appears to be 17.39 volts)),
In the series circuit amperage is as shown is limited — 2.85 amps –but the voltage will be 34.78 volts, (the sum of the two panels) — so the wattage is 99 watts –and this 99 watts feeding a MPPT controller can “all” be used.
That said, this is series method isn’t as efficient as the parallel arrangement where the output to the controller will be 17.39 volts at 8.6 amps or 150 watts.
Then the question is — is the MPPT controller “more efficient” with a 34 volt input than a 17 volt input when charging a 12 volt battery
Or alternatively, the mismatched panels could be used with a MPPT controller to charge a 24 volt bank — not the most efficient method, but one may have to run with what they have on hand
Well, how ironic that I make a mistake in a article that talks about making mistakes!
You are mostly correct. I say “mostly” because for your calculations you used the open circuit voltage of the system. Under load, the “24 volt” system will be somewhere around 26-28 volts. I updated the graphic with correct calculations and edited the accompanying text as well.
I appreciate you taking the time to point out my error and will also update another article on this website that used the same graphic.
Since setting up my first station in 2013, my comma have been completely off grid. Your advice today would have saved me a few headaches and money back then. What I thought was a pioneering adventure in the beginning was actually just a learn-the-hard-way exercise.
Keep up the good work!
Hello, Jim and merry Christmas! Making mistakes is just part of the learning process. I have no problem admitting my imperfections and accounting for them in a blog article in an effort to help others. Thanks for stopping by!