Can I charge my caravan battery from my car alternator with a DC to DC while idling?

[Kenelect]

Can I charge my caravan battery from my car alternator with a DC to DC while idling?

 

[davesparks]

Probably not whilst idling, you'll need to put a few revs on to actually generate enough power for it.

 

[mainline]

Can I charge my caravan battery from my car alternator with a DC to DC while idling?

A modern car will charge at idle.
Even at idle the engine is turning at about 800 revolutions a minute.
Most are ecu controlled and will adjust idle to compensate for alternator load on the engine.
If it didn’t charge at idle you wouldn’t be able to leave it very long idling with the lights on etc.

 

[davesparks]

A modern car will charge at idle.
Even at idle the engine is turning at about 800 revolutions a minute.
Most are ecu controlled and will adjust idle to compensate for alternator load on the engine.
If it didn’t charge at idle you wouldn’t be able to leave it very long idling with the lights on etc.

Whilst this is true for the vehicle battery it is my experience that charging additional leisure batteries through a DC-DC charger will require more revs to be effective.

 

[brianmoooore]

Why would this require a DC-DC charger? Simple wire should suffice, and yes, it should charge while the car engine is idling.
The battery in my caravan is charged by the car alternator with nothing between them other than wire and N/O relay that closes as soon as the engine is started.

 

[davesparks]

Why would this require a DC-DC charger? Simple wire should suffice, and yes, it should charge while the car engine is idling.

Modern lithium batteries do not present a load to the alternator in the same way that a lead acid battery does and so a DC-DC charger can be installed to aid in charging.

 

[brianmoooore]

Lithium batteries have found their way into caravans now? Think I'll stick with a good old deep discharge lead acid.

 

[timhoward]

Why would this require a DC-DC charger? Simple wire should suffice, and yes, it should charge while the car engine is idling.
The battery in my caravan is charged by the car alternator with nothing between them other than wire and N/O relay that closes as soon as the engine is started.

That's exactly how I do it. I've wavered over the years whether a switch + relay/ ignition live source + relay / or if really pushing the boat out, a voltage sensing relay is best.

Modern lithium batteries do not present a load to the alternator in the same way that a lead acid battery does and so a DC-DC charger can be installed to aid in charging.

Aha, it sounds as though modern leisure batteries are lithium?! I can see that adds a new factor. I've never purchased a leisure battery in my life, as I've invariably got a serviceable spare car battery kicking about.
I've always thought there's a lot of hype about leisure batteries, it's in the caravan folk-law that a motor mover needs a 120Ah battery which is great for sales. I had a 50Ah car battery in my caravan for years.

 

[brianmoooore]

Relay switched by the D+ output of the alternator is best, and what I've used with previous tow cars, but my current barge has a rear mounted battery and no ignition switched lives in the rear that I know about, so uses a voltage switched relay, which has worked perfectly for 12 years.
I always fit a caravan battery that is compatible with my car battery, so that in an emergency they can be swapped over. Paid dividends once, when my car battery suddenly died for no apparent reason ,at a French campsite.

 

[littlespark]

I take it an Aussie towing socket has a +12v line like the UK?

If I recall, is it still 7 pin, but with flat blades rather than round pins?

 

[mainline]

The op actually wasn’t specific in his post as to whether he meant dc dc as in battery to battery or whether he’s using a dc to dc charger or what type of battery he’s using.

A modern car will still charge at idle whether he has a dc to dc charger or not most dc dc chargers draw from the main battery and step up or down even if the main battery is down a bit.

It’s without doubt better to use a dc dc charger splitter as it will not only deal with ecu controlled charging but also with the mixing of different types of cells.

Some cars fitted with Agm for instance need to be reprogrammed to the ecu when changed.

Those days will soon be gone where a simple relay will suffice.

Lithium-ion doesn't suit for use as a car battery

 

[davesparks]

Lithium batteries have found their way into caravans now? Think I'll stick with a good old deep discharge lead acid.

Aha, it sounds as though modern leisure batteries are lithium?!

Lithium leisure batteries are available yes, and are quite impressive in their performance.

My experience with them is from live-aboard narrowboats, rather than caravans, which is generally very hard on the batteries as they are in use 365 days a year and rarely, if ever, plugged in to shore power for a full charge.

 

[Zerax]

Modern lithium batteries do not present a load to the alternator in the same way that a lead acid battery does and so a DC-DC charger can be installed to aid in charging.

From my experience with lifepo4 cells on a boat... the 'standard' way to charge them is via a DC-DC converter off the Lead Acid engine start battery (or a dedicated 'intermediate' Lead Acid battery). This is done because the super low internal resisitance of LifePo4 cells creates a huge load on the alternator, it gets hot, then the dreaded smoke appears. So it's more a matter of too much load, rather than not enough.

Having said that, it does depend on what regulator you have on the alternator, whether integral or external.

Also... the OP should make sure they have some sort of BMS for safety. (Lithium cells have a different charging regime to LA)

 

[mainline]

From my experience with lifepo4 cells on a boat... the 'standard' way to charge them is via a DC-DC converter off the Lead Acid engine start battery (or a dedicated 'intermediate' Lead Acid battery). This is done because the super low internal resisitance of LifePo4 cells creates a huge load on the alternator, it gets hot, then the dreaded smoke appears. So it's more a matter of too much load, rather than not enough.

Having said that, it does depend on what regulator you have on the alternator, whether integral or external.

Almost all cars after 2013 will have smart charging and won't allow too much current.

Also... the OP should make sure they have some sort of BMS for safety. (Lithium cells have a different charging regime to LA)

Almost all lithium batteries will have battery management built in, some will also have heater, Bluetooth etc.

 

[mainline]

Aha, it sounds as though modern leisure batteries are lithium?!

It's only an option, I would have thought most people wouldn't want to pay the £300 - £1000 for a battery.