Would that three-constant power equation (V=IR?) tell how much the battery is going to draw, say the battery is 11.8V and the socket is 13.8V then I = ? (not sure what resistance is) - I'm not going on this trip but I can send the team with a tester to check the battery voltage.
It's slightly more complex with things like inrush current, slew rates, etc etc etc. But I'm only a biologist.?ÿ At least my degree says i am.?ÿ 😉
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@bill93?ÿ Agreed.?ÿ To be more precise the voltage drop in wiring to the cigarette lighter is enough to limit charging to only a few amps near the upper part of the charge cycle and once the aux battery is above 50% charged it means it will take many hours to fully charge.?ÿ The key in this setup is the alternator is sensing the car battery voltage say, 14.2v but the thin wiring to the aux battery may depress the aux voltage by 5-10% if delivering 5-10amps worst case the voltage drops to 13.0-13.5v which is near float voltage and doesn't deliver much charging.?ÿ Also, a cigarette receptacle is not designed to handle 10-15 amps for hours; it gets hot, is contaminated with tobacco shards, may introduce another 0.5v to deliver below 13volts and degrades (melts) the plug if not of high quality.?ÿ Of course when the aux battery is near dead (10.8volts) the lighter circuit will blow its fuse when presented with such a heavy load.
I'm not being theoretical, I have Jeeped for up to 6 months for years (pre-Covid) in the remote West with the cigarette lighter wiring and had nothing but problems which was fixed by providing the aux battery with breaker protected 8gauge direct wiring and it's golden, charge a near dead aux battery in 3 hours of driving without overheating and failures.?ÿ I've noted that some recent trucks and SUVs provide high amp Anderson connectors in the truck bed/rear storage which is the perfect factory solution.
Every car battery I have ever purchased new is charged when purchased. And, if it isn't there is a problem.
It's slightly more complex
... for those reasons and also that the chemistry going on in the battery will cause variable offset voltage and non-linear resistance to the current during the charge process as those ions generated at the plates diffuse through the electrolyte.
@jitterboogie?ÿ I tried an DC-AC inverter/to Autoparts DC charger to charge the aux battery when underway but one that could deliver DC 40 amps (500 watts) was bulky and clips to the car battery were stressed and running wires through the passenger compartment for hours when driving was concerning.?ÿ I do have a an 8amp cheapo AC-DC charger which I use to fully charge the aux battery when at home if it's been stored for months (Covid) before going on long trips, or a boost If I've not been driving, it's been cloudy and I have overnight access to AC.
Commercial flying with significant batteries is these days a no-no; better arrange rentals locally.
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@dmyhill Yep, that's a fact.?ÿ They regularly trickle charge the on the shelf batteries and only maintain an inventory no older than a year or so.
Rent 2 cars. Leave one with the base, using it's battery for power. It's probably cheaper to rent an extra car for a day than to buy a battery and abandon it.?ÿ
You are correct, it would be cheaper to do that - we are getting the car for US$40. The catch is while two guys are going, only one has a drivers licence, also we are going to go there at least 3 times on this project, so the battery will be stored with the contractor (or we'll bury it somewhere) and we'll use it again.
To be honest, didn't think of that and probably it is the thing to do - probably got blinded by the fact that 99% of our clients are not battery/charger equippers so we usually go self sufficient, including the double battery job in the same area that we did last month. It's the weekend now and we go on Monday so too late to change plans.
