Battery to Battery
Over the years of accumulating renewable energy-related gear, I've evolved several variations of the battery-in-a-box concept. Basically, some sort of portable system with a battery and a charge controller, and other wiring, to be used with solar panels as the primary charging source, for powering whatever. That could include charging electronics, running amateur radio stuff, etc.
In the past I've hard several of these sorts of systems I could grab, usually a smaller one and a larger one. And, enough other crap to throw one or two more together. Right now, I've settled on having a larger more permanent system, which lives in my basement. It's got a 105 Ah AGM battery, charged by 164 watts (nameplate) of solar from three old panels. I might see 10 A into the battery on a sunny day if I've discharged it, but it's a non-useless amount of power – I've run lighting and laptops with it during power outages, for instance, and shift some loads to it on sunny days such as fans.
The recent portable system, however, has a 50 Ah LiFePO4 battery, and a couple of 130 watt semi-flexible photovoltaic panels. (The panels and battery are from Eco-Worthy, and the charge controller is a Morningstar SunSaver MPPT, of which there is another in the previous system I mentioned.) This is technically more powerful, with a little less storage than the permanent system. (Technically you can better utilize the LiFePO4 battery compared to lead acid, but it does work out being a little less capacity.) It's great for ham radio, backup power, etc., and excels of course when the sun is shining – if you have loads you want to run during the day, it's great.
I do have a mishmash of solar panels and batteries, not all of which are used, but I definitely have more smaller batteries sitting around. Sometimes it's nice to be able to grab one for something, and I rotate them for charging. Usually, I do this by putting them in parallel with a battery in one of the other two systems. It's not the most careful way of charging, but if you keep an eye on it, it works. As long as you don't have something set up for an absurdly high charging voltage, like if you wanted to equalize a flooded lead acid battery, you don't really damage anything.
Example: 4 Ah Gel Cell
I have a small gel cell, about 4 Ah, as kind of a lab bench battery. If I parallel it with the big system, with the 105 Ah AGM battery, it might see 14.3 V when that system is in Absorb (meaning, the charge controller is letting the battery sit at constant voltage). That's not too bad, and with wiring drop the smaller battery might pull a quarter of an amp from the bigger system, more if it's more discharged.
Example: Car Charging
I have a cord I made with a power diode in series on the positive side. It has Anderson PowerPoles on both ends, and I also have a cigarette light cord I made with PowerPoles. This actually works out for charging any of these batteries in the car, since the diode introduces a drop of about 0.4 V, and when the car is running the alternator can get up to 14.8 V or so based on my observations. (Actually, this is a guess, I've measured about 14.3 V after the diode while the car is running.) The diode not only keeps the battery from discharging, it also sort of keeps the car from being powered when it's off. This is because I don't really know how it's shutting the cigarette lighter outlet off, I assume it's a relay, but I don't know what else is on that circuit.
This works great for portable radio applications. You just go cigarette lighter –> fuse –> diode –> fuse –> battery –> fuse –> radio. When fusing, keep in mind where energy might come from in the event of a short. For instance, a short in between the diode and battery means current could dump in from both the car and the smaller battery.
Battery To Battery Charger
There are commercial DC-DC chargers out there, and constructing one of my own is on my list. The advantage here is a) having a proper charging algorithm for your battery, and b) being able to charge even if the source battery is at a slightly lower voltage.
That said, this is more complex, and sometimes simplicity is more the way to go. The DC-DC charger is a good idea in something like a camper, however.
Find me at @toroidalcore@masto.hackers.town