Food for thought: "Volts is Volts...."

WilliamA

When I get together with my oldest son Ian and his band of metropolites (my word: means those who dwell in urban environs and eschew the hazards of the natural world) the primary gnashing of teeth comment is the amount of volts carried in their smart phones. It seems to the outsider that their world is a post-apocalyptic one where the masses wander the wastelands in search of charging facilities so they can use said devices to then contact friends and have long diatribes about how unfair it is that they are reduced to a hunter-gatherer existence by the evil forces of foraging for volts. Well, as my good friend and fellow electrical troubleshooting instructor (military advanced school) used to tell his students: “People, the first thing to learn here is that volts is volts.”
What he meant by that is, like matter, volts are not created from nothing, they are traded for other forces and then used, stored and ultimately, go back to the resting state of energy from which they came. As I have penned previously, they are similar to any fuel in that they can be stored and transported, but must be renewed occasionally. The downside to electrical storage as compared to a fuel tank is that an empty fuel tank weighs next to nothing relative to its capacity whereas a battery weighs exactly the same, empty or full. Another disadvantage of batteries is that, unlike fuel tanks, they can only be filled at a fixed rate. Fuel tanks can be filled in proportion to the size of the opening in their fillers. Want to fill faster? Just put in a bigger opening. Relatively straightforward. Batteries, on the other hand, will only allow filling at a fixed rate. No matter how big the battery, filling it is a fixed law of its design. You can’t put on a bigger wire and expect to fill it faster. It doesn’t work like that. There is a design maximum that refuses to be exceeded. Push beyond it and you’ll do nothing but swell the plates within and short out the battery.

There is a pernicious misunderstanding of that particular law that runs through every automotive and RV forum I am on. “Bigger is better”. Well, no it isn’t. For every amp carried, there is a cost that must be paid. Lots of thought and conversation is applied to how many days a battery will run device A or device B before it runs out of juice and needs to be recharged. That particular mule gets whupped on a daily basis. Almost none of the discussions spend important time working out how long it will take to replace all of those amps. To be fair, there is a lot of peripheral chatter about how much a bank of solar panels or generators or etc will put out, but little about how long it actually takes.

A simple rule of thumb is that charging times are not exponential, but linear. One battery takes X hours to charge. Two batteries of the same capacity will take twice as long. Try to “push” voltage and you just get waste heat and ruined batteries. Another thing to tuck away for reference, half isn't half. The closer to fully charged a battery is, the less time it takes to fill it. For example, let's cut the capacity of a battery into quarters, with half full equaling two quarters. The bottom of two quarters (the one between half full and three quarters full) takes much longer to charge than the last quarter between three quarters full and full. It's because the amperage flow becomes less the closer to full the battery is and therefore, the less heat is generated in the charging process. That rule doesn't change the total charge time of the battery but does make a big difference in the life of that battery. Heat is bad. Less heat equals longer battery life. Even though you beat the crap out of your car battery daily, it will last a long time because its life mostly exists in a nearly full charge state. It rarely gets drawn down to less than 3/4 charge.

There are other considerations that get little more than lip-service. For example, if one has the charge line from the tow vehicle to the trailer hooked up and working well, it will still take X hours to charge however many batteries are in the trailer and how big the alternator is. The alternator is an amazing, powerful device but that doesn’t mean just because it’s capable of producing 130 amps it will charge the batteries any faster than a good trickle-charger. If your battery is a 130 amp job that doesn’t mean it will take one hour for a 130 amp alternator to fill it. It’s possible to cram a lot of surface charge into a battery in a short period of time, but it’s hard on both the battery and the alternator. Running a modern alternator at it’s full rated output will quickly destroy it. There’s maximum output and then there’s duty cycle. Usain Bolt can do the 100 in under 10 seconds, but he can’t maintain that for the length of a marathon. Just rename your alternator, “Usain Volt.” Same principle. If you were to try and get 130 amp hours out of your battery in one hour, it'd split like a melon in about 2 or 3 charge cycles. Let’s not forget either that the electricity produced isn’t free. It takes fuel to turn an alternator. They use a surprising amount of horsepower at high amperage. No free lunch kids. Another factor with tow vehicle charging systems is that they work best when the batteries are more closely matched. During the charging cycle, if one battery (or set of batteries) is grossly larger than the tow vehicle battery, the result is that power will be transferred from the charged battery to the discharged one. It’s a parallel circuit. In addition to moving electricity to the trailer batteries from the alternator, you’ll also be transferring energy from the tow vehicle battery to the trailer batteries as the natural transfer of energy wants to even out the voltage of all of them. That will ultimately reduce the life of the tow vehicle battery.

Another way of looking at the whole principle of voltage storage is if I told you that it’s possible to put an auxiliary fuel tank in your car so you could haul 500 gallons of gas. You could drive from Chicago to Nevada and back on a single tank. You’d say that’s ridiculous! In addition to being dumb, it’s dangerous. Just pull off and fill up, right? Batteries are, in principle, no different. As with all vehicles from the Space Shuttle to a lawn mower, the more fuel you haul, the more fuel it takes to haul it.

Solar energy
I have my own thoughts on solar panels and will keep them tucked in where no one can see them for now. If that’s your bag, and you want to use them, consider that so far as keeping your battery/batteries charged, it will not apply more power simply because you have more batteries. If you have one battery and it will last a day or two on a charge, your solar panel array of a manageable size output will do a better job of keeping up with one than two. Because it takes less power to “fill” a smaller battery it will more quickly fill it to capacity, keeping the charge level much more efficiently than a system that struggles to provide a minimum charge. The short version of that is if you have adequate capacity with 200 watts of solar panels, then you don’t need 800 watts of storage capacity unless you plan a long trip across the south pole in winter. If that’s the case, you’ll probably want a generator anyway. If you plan to use solar panels, then it's a straightforward computation to decide how big they must be. How much power do you use. Short of setting up an electronics testing lab on site, it's easy to determine how much you need. Go camping. (Of course, that's pretty much my go-to answer to every problem). Get a piece of paper and jot down your beginning voltage after charging, then keep a log of how much the voltage drops over a certain period. Say, 6 hours or so. Keep in mind that, the lower the voltage gets, the quicker it will drop per hour. That's because as the voltage drops, the amperage must increase to accomplish the same work. When the battery gets to 12.3 or 12.4, charge it and do it again. Over the course of a few times, you'll quickly see about how long your battery will reliably keep all your widgets whirling and drumming and illuminated.

Second only to shore power, the most reliable, easy to access, long lasting power source you can have is a good charging system from your tow vehicle. It works in the rain, at night and on the road.

The greatest of all variables, you....and me....and everyone else....
If you’ve read this far, it’s possible that you’ve come to the conclusion that I am advocating for a single battery system as opposed to a dual, or advocating any particular system at all. That’s just not true. It is always, first and last, up to the individual user to determine which kind of system best fits the needs. My needs on any particular trip are rooted in my “Johnny Appleseed” lifestyle. I move frequently on my wanders and seldom plant for more than two or three days in any one spot. If your modus is that you plant your trailer and use it as a base of operations for days at a time, your needs will differ. The only personal observation I’ll make is this:
In all of my traveling, in the diverse circumstances of my trailers’ use, I have never, not once, been in a situation where my trailer battery went so dead that the refrigerator shut off. Not once. I guess then it's fair to say that if you asked me, I'd have to admit that I don't actually know how long my battery will go on a single charge. I know that I've gone the lions' share of three days and everything was still working. I still use the single, anemic small battery my trailer came with and if it goes boots up, I’ll get another one just like it. It’s been my experience in both RV’s and Auto’s that the best way to maximize battery life (lead/acid battery, that is) is to use it often and charge it often to its capacity. My battery is probably more than 2 years old and still works like it did the day I picked the trailer up. I have my tow vehicle wired to charge the battery anytime it’s connected. The small-ish RV battery charges quickly on short trips and doesn’t seem to need any auxiliary charging between them. When I go out on day trips (often) I just turn on the refrigerator before I leave, put my sandwiches and drinks in and within half an hour, it’s cool and frosty in there. I just leave it on and rarely check to see what the temp is. My trailer will go 2 days or more on a single charge without adding voltage. That’s my standard. After 2 days, I’m usually down to around 12.4V or so (using everything, not just the fridge) so if I’m away from shore power and plan to spend more time on site, I back up my tow vehicle and plug in. If I’m somewhere that I can use it without being shot, I’ll fire up the gennie and let it run for half hour a day or so, usually around lunchtime. I obsess about weight. So much so that I spent a couple hundred bucks getting rid of 20 lbs of weight by going to a smaller propane tank and shedding the landing legs. Weight is the enemy of what I want to do with my trailer for many reasons, both structurally and in consideration of fuel used. Space is the next consideration for me and I hoard any free space with a vengeance. I won’t carry anything that has not proven to be useful. As far as groceries are concerned, to me it’s not much different than gas for the mule. Every couple of days or so, I’ll inevitably wander by a place to restock my fruit and veggies and things. I have dry goods to insure I won’t starve but even in the back country of the 4 corners region, I seldom have gone more than a couple of days without accidentally going by somewhere that sells bananas or milk. My trusty mule, Hurricane (my Jeep) can’t stand being away from its trough (gas station) for more than that anyway, so what’s the point of me stocking up for an apocalypse if I have to brave the zombies for fuel.
As with any logical discussion, it is inevitable that one must come to a logical conclusion, usually involving the appearance of “giving advice”. As we aren’t particularly logical creatures, that can cause lots of friction and smoke. I’ve dug this hole so I might as well finish by filling it in. The short version of a long discussion is that, if you already have a dual-battery setup, keep it. If you are a newbie and your rig is relatively stock, then try it as it is before throwing out the baby along with the single battery bathwater. In my very humble opinion, the very best money that can be spent on this topic is to wire the tow vehicle to best accommodate high energy capacity transfer between the tow vehicle and the trailer. That’s also, by coincidence, easy to do. Simply make sure that you have two good, dedicated 10 gauge wire circuits that go between the tow vehicle battery and the trailer connector. I mean dedicated as in, not connector to frame, but connector to battery in both directions. I’ve covered the specifics of amperage through the ground elsewhere but it stands repeating that the vehicle connector on your tow vehicle must run a dedicated 10 gauge wire between the trailer plug-in and the tow vehicle battery. I’ll sum up: 10 gauge, red wire with a 30 amp fuse. 10 gauge ground wire (black or white) between the trailer plug-in and the tow vehicle battery. This simple, one time step also goes many miles toward warding off future problems with lights, brakes and pretty much any other plug-related problem. A 16 gauge (or even 12 gauge) ground wire that goes to the frame of the tow vehicle simply won’t do. It’s a gunny sack of problems, just waiting for an inopportune time to appear. 90% of all lighting, braking, charging problems originate there.

WilliamA

Jumper

I think I just read the longest post on any forum that I have ever been on. I actually read the whole thing and may have learned a thing or two. Very good info! Thanks for taking the time to explain all of that!

Brian

Thank you, William. Well worth the read.

LuckyJ

Ok, I must confess, I did began, then I jump to answer, to post my reply. I will comme back later to finish or anyhow. Continue to ready this!

OutdoorEd

Not only is that the longest post I've ever read, it's also one of the most articulate, informative and enjoyable posts I've ever read. Well done, WilliamA. Thank you.