Charging batteries and caravan electrics ?

[Roger_374296974]

This is more a query than question

I will break it in to parts as to explain> I have working understanding of electronics and Electrics

1 Theory states you dont parallel charge batteries of different AH as the battery of lower AH will get charged but the bigger battery wont. So when charging leisure battery up in van via the 12s connection on car while driving does the leisure battery get charged properly as usually the car battery as about 45Ah and the leisure is about 85 or 110 Ah. so in theory the leisure battery would not be charged properly.

2 Ive noticed with some caravans the 3 way fridge is only powered from 12v then connected to the car and the vis the red wire pin 6 on the 12s socket and is not connected to the 12v of the caravan so if you did need to the 12v on the fridge while on site for what ever reason it wont work. But a simple change over relay would solve this if fitted in the caravan. But Ive never seen this

3 and finally Ive seen with some charging relay fitted to cars that come on via the ignition feed.ie when you ingition lights ar on on the dash you are powering your caravan up.If your car battery is a little low when starting it will try and pull current from the caravan battery and blow the fuses or worse burn out wiring if not fused.If lost count the times Ive had to hunt out the fuses and change them.So if your having electric fitted to your car.ASK them to show you where the fuses are for the split charge relays so if they do blow you know where they are

How many on here know where they are in their car.Are they under the bonnet,under the dash or in the boot.They could be any where

 

[rob_jax]

Hi Roger,

1. The car and caravan batteries are effectively not in parallel.

The car battery is monitored and charge controlled by the car. The 12V (about 13.4V) supply to the caravan battery is unmonitored, and it doesn't supply enough voltage to 100% charge a GEL Leisure battery anyway - that's why our Eriba has the German Schaudt WA1214 battery booster / charge controller - boosting the car 12V to the right 14+ volts to fully charge the caravan battery.

2. Current draw from a fridge on 12V is around 10 amps - this would kill a 100 AH battery in between 6 and 8 hours. The battery ratings are for a discharge rate of Capacity/20 - or at 5 amps in this example.

Take the battery below 25% and you'll seriously shorten it's life

3. Charging relays are (often) Voltage sensing and can be adjusted (with a trim potentiometer) for correct operation.

Robert

 

[ProfJohnL]

Hello Roger,

Rob Jax, has given the bare bones, but perhaps a little more detail will help.

Point 1.

The charge status of a battery is assessed by checking the terminal voltage. If both batteries are wired in parallel, the terminal voltage of both batteries will be the same, thus the charge status will be determined by the battery that takes the longest to charge. The charge current will be directed to the cells with the lowest internal resistance, so both batteries will take the same time to charge even though they have different capacities.

There are potential problems with parallel batteries, if one battery has a greater terminal voltage than the other before they are connected, when they are connected, a large transfer current between the batteries may occur, and that could cause one or both of the batteries to overheat and cause an explosion.

In Cars with a split charge relay, the relay should only closed when the alternator is producing more than about 14V This voltage will only occur when the engine is running, so there is no danger of the car or caravan battery discharging into the other, or for either battery to be the sole power source for both the car and the caravan. As back up, a fuse usually protects the charging circuit.

Point 2

As the 12V circuit of a typical caravan fridge can use 8 to 10A, as Rob Jax points out, that level of consumption would flatten most batteries in a few hours. To prevent this a similar relay to the split charge is used or a combined unit, so that the 12V circuit for the fridge is only powered when the tow vehicle is producing enough power to run it.

There are very few situations that I can think of where it is necessary to have uninterrupted power to the fridge. The relatively short time between arriving on site and making the electric hook up, or being able to turn the gas on would be unlikely to damage the content of the fridge.

Point 3.

In the vehicles I have seen, the split charge has been set to only come on when the alternator is producing power. This can be achieved by using a voltage threshold sensor, or by using the alternators self-excitation circuit.

If you are experiencing blown charging fuses for the reason you explain, it suggests that there may be a problem with the original installation of the relay.