Am I to understand that it is your intension to charge the club battery from your car whilst the car is stationary, with the engine switched off. I think you may soon have two partially flat batteries. Modern car batteries are not designed to discharge slowly, they give a lot of energy to the starter motor for a short time, then the alternator takes over. I think your club battery will be a leisure type which gives a smallish amount of energy for a long time, almost the opposite to the car battery. If you want to charge the club battery from your car whilst on the move with the engine running, then you will need a split charge relay as used to be fitted to motor homes. No doubt motor home have an intelligent type charging system now days, with flashing LEDs, many surface mounted chips and plenty to go wrong. Having looked under the bonnet of a few modern car I couldn't even guess which wires are which, so many wires and nothing identified. Also, I do know some cars need to their ECU reprogrammed if a tow bar is fitted, as they are using more current than expected, so be cautious. John

Solar panels are fairly cheap these days. A reasonably sized one on the shed roof, possibly with a charge regulator, would recharge through the working week, ready for the week-end.

I would make up a roster so every member has a go at charging the batteries !

The charging controller from a dual battery system used on a 4wd sense the alternator output and main battery voltage so it will only charge the auxiliary battery when the engine is running and the main battery is fully charged ( alternatirs never fully charge a battery as thier output is typically 13.7v not the 14.4 v needed to fully charge a lead acid battery .)

The controller will also limit the current in two ways :

It ramps the current up slowly as deep cycle batteries don't like a lot of current to start with and they limit the total current as the construction of a deep cycle battery is not designed to deliver or recieve a large current like a starter battery can .

Please also be aware that when charging a lead acid battery it will give off hydrogen gas so if it is locked in a boot of a car that sealed compartment can become a bomb – open the boot and the little switch for the boot light makes contact , gives off a spark and boom !

You can buy what is called a dc to dc charger that connects to your car battery and boost the voltage up so it will charge another battery but they are not cheap and you could set up a solar cherging system for less money .

The light globe idea will work but you really want to hook it up to the car so it will only connect the battery when the engine is running , maybe a relay activated from the ignition circuit ?

I'm guessing that the batteries are for power at the clubhouse ? Maybe time to purchase a generator ?

Dual Split Charge Towbar Relay

All the info you need. Split chargers are fine for charging as you drive, automatically switches off so no current drawn when you start the car. You will only need a canbus system if you take the voltage from the supply at the rear lights afaik. Fitted one to a van conversion with no problems. Colin

Duncan, assuming your club can afford £120, surely the little generator in Nick,s link is the way to go. It has a 12 volt charging facility built in and gets excellent reviews, plus it gives you a 240 volt supply of course.

Just my thoughts.

Ron

Edited By Ron Laden on 01/09/2018 11:50:42

Bigger battery, solar panel is way to go. Solar would charge up during the week and leave plenty for the weekend. I have a small solar setup for a remote garage and use LED lighting for when it's dark.

Very late to this but to ANSWER the original question

No, that will not work.

You cannot properly charge one battery from another of the same voltage. As John F 1 has suggested you will at best end up with two partially charged batteries. Unless one battery is very discharged there will not be enough voltage difference between it and the car battery for any currrent to flow and charging to take place. If you ran the car engine then sme charging wuld happen but it's a vary expensive and poluting way of doing it. Solar panels look like the best solution.

Robert G8RPI.

We have an electric fence on our allotment, it has what I think is a 7 AH sealed battery supplying it with 12 V. It is charged from a small solar panel via a Schottky diode to prevent the battery discharging through the panel at night. The fence unit is continuously connected so always drawing current. The panel (about 300 x 200 mm) keeps the battery topped up and has done for well over a year without attention. You might need to have some kind of charger circuit for intermittent usage.

Assuming the battery is a Li-Ion You probably need something like this.

18650 Charger Protection Module 5S 10A Li-ion Lithium Battery BMS 18.5V 21V Cell | eBay

They are specific to the number of cells. 20V suggest a 5 cell battery.

You will need to get inside to connect the individual cells to the control module. If you can get inside you might find the module is a generic one like this. They are usually installed in the battery rather than the device to allow them to function in a charger.