Keeping a caravan battery charged with no mains huck-up

[ericmark]

Years ago I had split charging but today with inverters I wonder if really what is required?

I know I can buy 12v to 12v inverters for boats often used to charge bow thruster batteries but the price is silly.

So looking at the idea of an inverter and battery charger with spare battery idea is in the car cig lighter runs inverter and battery charger giving a 5A charge while driving about with no problem with volt drop because of inverter. Once back at caravan reverse process to charge caravan battery from battery which was charged in car.

Idea seems sound but I am sure some one has already done it so asking if anyone has and what if any problems did they have?

We have 4 users of the caravan all but one can't tow it so want something which can be transferred car to car without having to add split charging diodes or relays.

Intend to test once caravan is back to see how it works but no point re-inventing the wheel if some one already uses the idea.

Aim is to not remove the caravan battery so people left in the caravan still have water pump. Want a box lift into car charge then out of car chained to tow hitch of caravan.

Thoughts please.

 

[westie101]

Most guys are using Solar Panels to keep 12V charged when off EHUs

As long as you can ensure the battery being charged & the car starter battery are not in parallel when the car is being started there would be no need for the inverter set up to charge it.
For the caravan use just put a 7S socket on the box to supply the 12 or 230V to the van

 

[westie101]

Most guys are using Solar Panels to keep 12V charged when off EHUs

As long as you can ensure the battery being charged & the car starter battery are not in parallel when the car is being started there would be no need for the inverter set up to charge it.
For the caravan use just put a 7S socket on the box to supply the 12 or 230V to the van

or you could wait and get one of these

http://www.truma.com/int/en/power-supply/vega-fuel-cell-system.php

 

[SimonH2]

Given how cheap a basic split charge system is to set up, there's nothing really that comes close on the performance/cost ratio.Just add a split charge setup to each car with a connector in the boot to plug the battery into.

Then you'd want a second caravan battery so you can leave one in the van while the other is in a car getting charged.

You could cut it down to just two split charge systems, and fit them into the carry boxes with the batteries - I'm assuming the batteries would be in something convenient to carry. Then just add a permanent live connection in the boot (which is useful for all sorts of things). Don't use a cigarette lighter socket, they are carp. I'd normally suggest a DIN socket but that means you have a plug with exposed live pins connected to the caravan battery which isn't a good idea.
Going back a few years it was common to use a Belling Lee (?) two pin inlet and free plug for caravan batteries - the free plug had recessed terminals, and the fixed "plug" at least had the pins set back in a recess. But these haven't been made for a good few years now.
There must be a good, moderately high current, 2 pin connector where both halves are reasonably safe ?

 

[westie101]

There must be a good, moderately high current, 2 pin connector where both halves are reasonably safe ?

These, http://www.maplin.co.uk/2-pole-speakon-connector-22434

You can now pick up a decent voltage operated split charge for about £20 or less.

 

[bernardgreen]

A small wind driven generator is a good option, but not NOT mount it on the caravan as the noise at night will keep people awake.

The problem of pins on the plug being connected to the battery can be dealt with by using a relay and three pin plug. The relay isolates the two Power pins from the battery. The relay coil is connected to the positive power pin and the third pin. The third pin in the socket is connected to negative, The relay only energises when the plug is in the socket.

 

[TicklyT]

A connector as used for electric forklift chargers etc. may be the answer. The connectors are hermaphroditic, so there's no 'plug' or 'socket', and no exposed pins.

 

[SimonH2]

Ah yes indeed - a 50A size would probably do very nicely and also provide a quick and easy connect/disconnect for the caravan.
It's the sort of thing I keep when I'm scrapping old UPSs and their batteries.

 

[Mursal]

Once back at caravan reverse process to charge caravan battery from battery which was charged in car.

Just wanted to add one battery will not charge the other battery. If you want to full charge a 12volt battery, the charging system will have to go above this voltage. In the car it goes to approximately 14.7 volts.

 

[ericmark]

The problem with most split charging systems is the cars battery keeps the voltage at front of car to 13.8 volts by time one gets to rear of car that has dropped so from bitter experience needed at least 44/0.012 cable which I suppose is around 6 mm sq. The voltage operated relays however have such small terminals one has to reduce the size before the relay.

In the old days of 15-16-17-18ACR Lucas Alternators one could get replacement diode packs which had the same size diodes for the AL output as main battery output so by using three new diodes for the AL I was able to take an output directly back to second battery. The charging was great and no need for relays. However although worked A1 it was a lot of work.

Except when using the CAV 440 regulator which had high medium and low settings the standard blocking diode was useless. Relay was far better. The relay does two things it separates battery so as not to flatten cars own battery but more important it ensures the second battery is not connected when cranking.

There are versions which can take start current used in boats and it was looking at charging in boats which got me thinking about using an inverter.

For boats one can either with an inverter or with a special regulator use the three stage charging method. Units like this allow the engine battery to be charged at normal float rate but the domestic battery to be step charged. Clearly the best option. But then one starts to look at the price. £250 is a tad more than I want to pay.

The battery to battery charger costs even more at £280 however I don't really need to be charging at 45A a mere 5A would be enough. Using a relay I found rarely more than 5A unless I could house battery in engine compartment. And putting two batteries in parallel without the alternator as well and rarely more than 1A as batteries equalised and more often just milliamps.

The inverter to battery charger will give me a good 5A into slave battery in car even with standard cig lighter connection and also will transfer 5A slave battery to caravan battery. Until the slave battery voltage drops when the inverter will stop working.

OK theroy is good but only a theroy. A 5A solar panel will cost around the £100 and the inverter controller

another £125 which is really the same as the battery to battery charger anyway it varies the voltage in to get maximum out of panel and transforms it to step charge the battery.

So options is spend £250 on new equipment or use an existing 150W inverter and a 5 amp battery charger. Clearly the latter has an advantage.

However what happens when it is done for real I don't know. It is doing what the propriety units do except not step charging. The smaller solar panels like at 0.1 amp will stop natural discharge while caravan is not in use but are not really going to charge the battery.

What I was hoping was some one has already done it and so could alert me about any problems. It seems only way is to try it.

 

[Mursal]

The voltage drop across it, would be the main disadvantage with the blocking diode, as someone mentioned earlier. This 0.7V multiplied by the forward charging current will generate heat in watts.

5Amps x 0.7V = 3.5watts

So it will probably need a heatsink and its a waste

No loss across the relay contacts

 

[JohnW2]

The voltage drop across it, would be the main disadvantage with the blocking diode, as someone mentioned earlier. This 0.7V multiplied by the forward charging current will generate heat in watts. 5Amps x 0.7V = 3.5watts So it will probably need a heatsink and its a waste No loss across the relay contacts

True but, FWIW, if one used a carefully-selected Schottky Barrier diode, one could get a voltage drop considerably less than 0.7V, hence much less lost/wasted power.

Kind Regards, John

 

[SimonH2]

The problem with most split charging systems is the cars battery keeps the voltage at front of car to 13.8 volts by time one gets to rear of car that has dropped so from bitter experience needed at least 44/0.012 cable which I suppose is around 6 mm sq. The voltage operated relays however have such small terminals one has to reduce the size before the relay.

As a matter of pedantry, the battery at the front does no such thing
The alternator, and the alternator alone determines the system voltage which should be between 14.0V and 14.4V. The only exception is if the main battery is so discharged as to draw more charging current than the alternator can supply (along with other loads) when the system voltage will droop - you can see this on many cars as the headlights dim at tickover with lots of loads turned on.
Having a fixed voltage across it, the battery will take a charging current which depends on the state of charge - and it will drop off very quickly after start.

Now I know that many people get away with (or did before all these new fangled hook ups) charging the caravan battery in the van - I know we did when I were a lad. The biggest problem is that the charge time is extended - as you point out, there is volt drop in the cable and this will reduce the charging current.

I see no reason a 6mm cable shouldn't be adequate for charging a battery in the boot via a normal split charge setup. it will however require a journey an hour or two if the battery had been well used beforehand. That also applies to the main car battery - it too can take a while to fully charge (my 20 minute commute is far from adequate).
If you are 'lucky' enough to have the main battery in the boot (as some cars do) then you already have a heavy duty feed to the back end.

BTW - you can of course use a split charge relay to control a more powerful relay. Thus you can use a low cost, low current voltage sensitive relay together with a modest cost power relay - almost certainly a lot cheaper than most of the high power split charge units.