Power supply

[sarahjane77]

We are having 12v battery issues, but what we can't understand why and need a simple explanation to our heads around is,
The lights, toilet flush, the control panel etc that we are told run from 12v should still work even if there is no battery connected, does this mean they run from both ehu and battery if no ehu, we just need to understand how it works so we can get our heads rounds it to be able to understand and figure out where the problem is, hope you understand what I'm trying to say
Thanks

 

[borderbilly]

Lights will work on EHU and 12v.

 

[ProfJohnL]

Hi and welcome to the forum

You haven't told us the age of your caravan which does affect the interaction between systems.

Touring caravans 12V dc systems need a power source to run the 12V appliances and lights. Traditionally a caravan is expected to have a 12V leisure battery fitted to be the power source.

It was in the 1970's when caravans began to sold with Mains power options, which included a 3 to 4 Amp battery charger. the type of chargers were not neither smoothed or regulated and they needed a 12V lead acid battery to help limit the wild voltage swings these chargers had. These could easily over charge a battery.

By the mid 1980s The chargers began to be configured as "voltage regulated" to protect batteries from overcharging, but they still only had around 3 to 4 Amps of current capability. and were unsmothed so they also were still expected to have a 12V battery to provide the smoothing and extra power for higher current devices.

As time has gone by, we have adopted more items of equipment that need 12V dc power so the demands on the 12V supplies in caravans has grown and outstripped the contemporary chargers capabilities.

The 90's saw advent of Switched Mode Power Supplies. (SMPS) these were capable of supplying about double the current 1980's units, and now you might get away without a 12V battery, but you still needed to be frugal with 12V powered items. With two or three lights on here may not be enough current available from just the SMPS by itself, so it was still expected to have a 12V battery.

Since circa 2000 standards have changed a little. Caravans now have "Smart chargers" or Power Supply Units (PSU) these have greater current capability (typically 12 to 16A a few with 20A) but are optimised towards recharging a battery and managing them better. So the advice is you may get away without a battery by using the SMPS's output but it is still advised you may need a battery even when using an EHU. And of course you will definitely need a larger battery if you do any off grid activities. or have a caravan mover.

 

[Woodentop]

Sorry BB but your response is just as confusing.
Lighting works from 12V only (unless you have a very old van which may have a couple of mains powered lights) and is sourced from the caravan battery or instead from your car battery if it is connected by the 7pin/13pin plug..
The EHU feeds a power supply unit which provides a nominal 12V output, so if you have no battery in situ and the connector cables are isolated the output of the power supply will also drive the 12V-powered items.
The power supply usually puts out 13.6V or thereabouts and is connected directly to the battery, so it provides low voltage for appliances such as lighting but also keeps the battery charged. The power supply - despite being physically quite small - is of a type known as a Switched Mode Power Supply and can and does provide in excess of 20A so it is well capable of supplying all of your 12V kit and keeping the battery charged.
As an aside, the battery also provides a means of soaking up 'ripple' from the power supply output which can, say, make your lights flicker.

You will get more help if you provide details of your caravan and what 'issues' you are having.