Simplest 2nd car battery charging solutions for rental car, so gizmos can be charged at night.

You could just connect the two using a small cable (e.g. 16AWG zip cord) with large alligator clips.
Since the charging current is relatively low, you don't need the large wire used in jumper cables (especially if you need to carry it on a plane).

To limit the charging current and thus not stressing the alternator or battery for a discharged battery, I recommend adding a small power resistor (e.g. 0.1Ω, 25W, example below) in series with one of the wires.
That will also limit the current from the 2nd battery when starting.
It can be mounted to the inside a small aluminum (not plastic) project box for protection and heat dissipation.

carcharger said:

For example, can I just use a 3 m jump cable between the two batteries all day/while driving and disconnect this cable from the 2nd battery at night when I am charging the gizmos from the 2nd battery?

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That's simplest - as long as the two batteries are in the same area of the vehicle.

eg. It would be difficult if the starter battery is at the front and the auxiliary battery is at the back.

The first critical point is that whatever cables you use to connect the aux battery to the main one MUST be rated at least 100A and fused. If the aux battery is very low, both the alternator and main battery will be providing current to charge it so 100A or more is quite likely.

Any thinner cable or doing it without fuses to protect the cable is risking the entire cable catching fire (and dripping burning plastic everywhere)!
That can happen almost instantly with a severe overload, before you have time to realise what is happening.


There could be 5V or more difference between batteries, if the aux one has been run well down overnight.

Crutschow's resistor idea is not bad, but the resistor would likely have to be 250W rated to stand a seriously flat aux battery... 5V across 0.1 Ohm means 50A charge current..


ps. Also note that normal car batteries cannot be run down and recharged more than a very few times before they are wrecked. You need a "Leisure battery", aka a Deep cycle type (as used in motorhomes etc. to power such as TVs & refrigerators) if you want it to be able to use it repeatedly.

They cannot give the high current to start an engine, but will stand hundreds of charge-discharge cycles.


Note that you can generally buy either 12V input or USB input chargers for everything you mention, so the 110V inverter is not needed and would just waste a good proportion of the power used, before it gets to the target device.

Lind are one of the best known for laptop chargers, but there are many others.
(You can quite often get the Lind ones used on ebay at much lower prices).

I'd personally get something like these - a high capacity "PD" rated power bank:

That should have more than enough capacity to charge multiple camera batteries & phones etc. each night, and it could charge from a high power car cigarette lighter adapter during the day.

Another would have enough capacity to charge a laptop, if it charges from USB-C

They are all a lot more portable than the jumper cable & inverter etc!

You could almost certainly get away with connecting the auxiliary battery to a cigar lighter.

When the lead-acid batteries are discharged, their internal resistance goes up, so it is very difficult to get the large currents that have been mentioned. With a flat battery and a powerful charger, it is very common for the charging current to increase after the charging starts, as the battery's internal resistance drops.

The total resistance in series when you plug into a cigar lighter is going to be around 0.2 - 0.4 Ohms. A discharged lead-acid battery will have a lot of series resistance until it's voltage is up to around 11 V. If the alternator is running, and there is 14 V on the car's battery, the difference is 3 V, and 0.2 Ohms will give around 15 A of charging.

Batteries in caravans and the leisure batteries in campervans are charged like that all the time. You can arrange a lower resistance connection, if you want, and the additional battery will charge faster.

I suggest you have a fused cigar lighter plug wired to some battery terminal clamps. Take some spare fuses.

For most vehicles you can leave that plugged in, as the cigar lighter sockets are turned off when the engine is off.

One problem you can get with modern cars is that the alternator voltage will be reduced one the car's battery is fully charged. That can slow the rate at which an additional battery is charged. Make sure you've got a voltmeter you can plug in to the cigar lighter socket, something like this:-https://www.ebay.co.uk/itm/303956259377?hash=item46c5347231:g:vuQAAOSwObZgdAC7

Diver300 said:

Batteries in caravans and the leisure batteries in campervans are charged like that all the time. You can arrange a lower resistance connection, if you want, and the additional battery will charge faster.

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My issue would be the pathetically thin wiring in caravans - we sold a Grundig Portable TV to a customer, which had an internal 12V inverter, for his new caravan.

The guy was VERY rich (in fact his name was Mr. Money - really!!) and he'd bought a VERY expensive top end caravan. Rather bizarrely, his house (Abbey Lodge) even had an indoor shooting range!.

Anyway - he took the TV and rang back to say it won't work, it comes on for a second and then goes off.

I got sent along to sort it out - already knowing EXACTLY what the issue would be.

So I got a headlamp bulb and soldered a couple of wires on it (as a load), took two multimeters (for amps and volts) and a few lengths of cable to extend the voltmeter leads.

So I measured the resistance from battery terminals to the TV socket, and the resistance was ludicrously high - just much too thin a wiring in the caravan. Even the battery was only connected via 5A flat TV mains lead, and 7 or 8 metres of it coiled up.

Nigel Goodwin said:

My issue would be the pathetically thin wiring in caravans - we sold a Grundig Portable TV to a customer, which had an internal 12V inverter, for his new caravan.

The guy was VERY rich (in fact his name was Mr. Money - really!!) and he'd bought a VERY expensive top end caravan. Rather bizarrely, his house (Abbey Lodge) even had an indoor shooting range!.

Anyway - he took the TV and rang back to say it won't work, it comes on for a second and then goes off.

I got sent along to sort it out - already knowing EXACTLY what the issue would be.

So I got a headlamp bulb and soldered a couple of wires on it (as a load), took two multimeters (for amps and volts) and a few lengths of cable to extend the voltmeter leads.

So I measured the resistance from battery terminals to the TV socket, and the resistance was ludicrously high - just much too thin a wiring in the caravan. Even the battery was only connected via 5A flat TV mains lead, and 7 or 8 metres of it coiled up.

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I agree that the caravan wiring is often too thin. The fact that flat caravans batteries are often charged from cars, and the wiring doesn't usually burn out, is really the point that I was making.

There isn't a big voltage difference between an effectively flat lead-acid battery and the alternator voltage. With a few meters of wire between the two, the resistance of the wire will usually limit the current to what the wire can take.

I could be necessary to add a small resistance, but even 0.1 Ohms will reduce the current a lot.

Great answers, thank you.

crutschow said:

could just connect the two using a small cable (e.g. 16AWG zip cord) with large alligator clips.

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rjenkinsgb said:

cables you use to connect the aux battery to the main one MUST be rated at least 100A and fused. If the aux battery is very low, both the alternator and main battery will be providing current to charge it so 100A or more is quite likely.

Any thinner cable or doing it without fuses to protect the cable is risking the entire cable catching fire (and dripping burning plastic everywhere)!

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Connecting the main and aux batteries to charge the aux while driving -- can you suggest an exact product number or specs for the wire that would do the job safely, but also the lightest, smallest, and portable (I need to use in intl travel in rental cars). US or EU, shopping, either are fine.

crutschow said:

small power resistor (e.g. 0.1Ω, 25W

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rjenkinsgb said:

resistor would likely have to be 250W rated to stand a seriously flat aux battery... 5V across 0.1 Ohm means 50A charge current.

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Which fuse specs for main aux charging while driving?

rjenkinsgb said:

buy either 12V input or USB input chargers for everything you mention, so the 110V inverter is not needed and would just waste

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Not aware of safe USB or 12V charging options for DJI Mavic 2 Pro batteries, Sony L batteries, Sony NP-FV100 batteries, so they have to be 110V.

rjenkinsgb said:

high capacity "PD" rated power bank

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I have these, but they can't power the above mentioned batteries that have no USB/12V charging sockets.

All this talk about hire cars puts me in mind of this:


JimB

carcharger said:

Which fuse specs for main aux charging while driving?

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After some further looking at the problem--
Battery University shows that the charging voltage for a discharged battery starts at about 2.1V/cell or 12.6V for a 12V battery.
Assuming the maximum generator charging voltage is about 14.6V, that would give a two volt drop across the series resistor I proposed.
So a 0.3Ω, 25W resistor (instead of the 0.1Ω I originally suggested) would give a 6.7A maximum charge current.
Thus an 8A fuse should be a good value to use (probably a slow-blow type to minimize transient blowing).

carcharger said:

Not aware of safe USB or 12V charging options for DJI Mavic 2 Pro batteries, Sony L batteries, Sony NP-FV100 batteries, so they have to be 110V.

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OK; then a power bank variant such as these??



They've still got to be better than all the parts and hassle to safely connect a second car battery to a rental car?