Charging golf trolley battery whilst in car

AnthonyL pretended :

You could charge it straight off the car battery, when the engine was running, but to be safe - only connected when the engine has been started and running. To achieve that, you could connect a voltage operated relay, exactly as used to connect a caravan's fridge and battery to a car.

Just connected across the car's battery, without the engine running, would not charge it because more voltage is needed. Another issue would be that if your cranked the engine over, a high current could be sourced from the golf battery.

Agreed. I'd charge from the running engine, but not from the battery.

It's a waste of time trying to charge directly from the car battery, car running or not. Charge cable and fuse resistance becomes crucial because of the vary small voltage difference between the car battery and the trolley battery once both are being charged. The charge rate is determined by V1 - V2 and the internal battery resistance. The charge rate would quickly drop to a small fraction of an amp. To make this work properly you need to need to run a proper charger from an inverter that is powered from the car. Given that you are only going to be charging for a couple of hours a 4A or 8A charger would be far better. A 2A charger will soon taper down to less than an amp. The amount of charge in two hours would be negligible. If you really want to charge the trolley in the car you need to run an

Bill

The latter case could be solved by a large capacity rectifier, but you would drop .6v on that. Brian

Yes this was how our old motor home did it, but cheaper options can work as long as you remember to only use the switch when the car is charging the batteries. Mind you, most motor homes ordinary batteries are far more gutsy than a modern cars one from what I can tell.

Brian

From my experience, in a modern car, if you didn't, you might not get home.

I thought that is what I said I was proposing to do :?

I'd get an extra two hours in before I got home, then I'd charge from the mains overnight.

If I had a spare deep discharge battery I might just as well have a spare trolley battery.

Brian Gaff submitted this idea :

The diodes would drop the voltage below the level where any sensible charge would go into the buggy battery. Been there in the 1970's trying to replace charge relays with diodes - it just doesn't work.

It worked on our Survey RRs. There was a big beefy Lucas package which allowed a 2nd battery to charge.

Yep! The car would need to be running for an appreciable amount of time, to bring either the car or trolley battery up to a full charge, but a charge will go into it. It happens all the time with caravan batteries, where they are adequately recharged on the road. Where many fail, is that the cable run through to the back of the tow car is sized for the current, 20amps, rather than sized for the expected volts drop. My car is wired with 8mm all the way to the socket at the rear and volts drop as measured at the fridge and battery is minimal.

I agree, that would be much better, but the OP was probably looking for a cheaper/simpler option to get a bit of charge back in the trolley battery.

AnthonyL wrote on 08/07/2022 :

Yep, that would work lol

Two hours would get a decent amount of charge back into a near flat battery.

The current flavour is to use Voltage Control Relays or Voltage Sensitive Split Charge Relays.

Aren't there any boost converter types?

There are battery to battery chargers, and these are sometimes used when there is smart alternator charging, where the battery is purposely left part charged to be topped up during an overrun.

However they're not particularly cheap:

But solid state FETS are now in vogue to do just that. Very low forward drop.

Fredxx explained :

Yep, prior to that, they used a relay operated by the ignition light - when the light went out, the car battery was charging, so the relay made to charge the second battery.

As said - I did test out using a diode, just as soon as high current diodes became available and the result was not satisfactory.

As I wrote earlier, diodes were perfectly satisfactory in our RR based survey vehicles, A Lucas diode pack (about the same size aa an ignition coil) managed the job from 1971 and 3 generations of vehicle.

It happens that charles formulated :

It would take an inordinately long time to charge up from flat and likely never manage to fully charge the secondary battery. A reasonably quick and full charge is needed for a caravan battery (and for the gold trolley). In the case of a caravan, it is often used for the domestic supply and likely the mover.

As I said, I tried it without success, just as soon as adeqaute current rating diodes became available.