Charging a caravan battery

[ericmark]

In the narrow boat it does not switch off the alternator when accelerating, and back on with over run, and the alternator is not connected to the engine management, so yes the Sterling unit works great allowing you to use both 70A alternators to recharge the domestic batteries and use stage charging, however this is not an option for many tow cars. Also on the boat the engine start battery and the domestic battery are next to each other so no volt drop problems. Even then for the bow battery to work the bow thruster it is common to use a DC/DC charge controller.

Engine starting is very different to domestic use. If you can charge a battery for 10 minutes with a set of jump leads from another car you will likely transfer just 10% or less of the batteries capacity but that is enough to start and restart the engine. To run heating fans and lighting over night however if it takes 10 Ah per night the 88 Ah battery will only last 8 nights and on the 9th the heating will stop. Bang in the same 10% which got the car started and running, and with the caravan battery you don't even get one night.

The question which can't be answered is how much power is required to maintain the battery? If 0.6 amp then at 0.8 amp in the end it will charge, if 0.9 amp then it will never fully charge, I think my 3.8A is sailing close, but at 0.8A it is possible the battery never will fully charge, if my energy meter drops below 14W then I know battery fully charged and the charger is big enough, until it does drop, don't know if big enough or not.

 

[aesmith]

The question which can't be answered is how much power is required to maintain the battery?

Although it's not a universal answer, I can confirm that a 0.8A charger is well able to maintain a 120AH battery, and to fully charge it for that matter. I say well able because it sits on the Float state most of the time, during which time I think it doesn't provide any current at all, just monitors until voltage drops enough for it to want to put a bit more charge back in. I might try measuring that.

 

[Keithmac]

I accidentally left the control panel on in our Sterling Moonstone last year in storage.

The 85ah leisure battery was dead flat.

Optimate 3 brought it back into usable condition in less than 3 days so I don't really know how it works/ charging strategy.

I was under the impression it charges then goes through a check cycle, if not up to scratch it goes for another charging cycle?.

 

[aesmith]

I just did a quick test, looks like it pushes a hundred milliamps or so through while the battery's on float, it varies so may depend on voltage rather than being a fixed current. The Optimate will be quite gentle since a large part of their market is for motorcycles and similar with small batteries.

 

[Keithmac]

We have a battery "reclaimer" at work, when that gets going depending on the battery selected it pushes 20+ volts through it (20ah battery will get 20 amps thrown at it!.).

That has brought dead flat AGM batteries back. Not all will be usable but it's saved a few..

 

[JohnD]

Optimate is one of those chargers that has a recovery cycle. Some will not charge a fully discharged battery. I have an idea the recovery includes sending brief high-power pulses to break up sulphate whiskers.

 

[ericmark]

Optimate is one of those chargers that has a recovery cycle. Some will not charge a fully discharged battery. I have an idea the recovery includes sending brief high-power pulses to break up sulphate whiskers.

Even the cheap Lidi charger has that feature at under £15 however most including Lidi also have a 6 volt and reverse polarity protection which means in real terms the battery must be over 7.5 volt at start for this to work. I had to use a standard battery charger before the Lidi one to get it over this 7.5A threshold. The battery reached stage 5 as shown in 16.5 hours, but it to date has not reached stage 6 and started charging on Thursday at 11 am. I intend to give it a bit longer however last time I looked it seemed to be stuck at 13.7 volt which is displayed on the charger. It seems likely the self discharge is too high for it to reach the 6th stage when the charge rate drops to 0.1 amp, since supplied through an energy monitor if it drops into stage 6 then goes back to stage 7 it will record it.

I am sure with a new battery stage 6 would be reached and likely it would never reach stage 7 but as batteries get older the self discharge increases, as with the Ctek which most of these battery chargers seem to copy there are different versions designed for different jobs and sizes of battery, the Optimate model linked to states two things, one is 55 Ah limit second is the 48 hour timer.

The cheap Lidi charger has two options, up to 14 Ah selected for motorcycle batteries and 14 to 120 Ah it does seem there is no time out, which could be seen as a bad point, as with battery under test it can stick at the 0.8A output for an extended time, but to be far there is a huge price difference between the two chargers, and unlike the expensive Optimate the Lidi one actually displays the voltage so even if I didn't have a energy monitor attached I would know the mode, stage of charge, and voltage from the display. The Ctek 3.8A costs around £57 against under £15 for Lidi one, and Ctek 0.8A around £40 against £50 for Optimate 0.8A the Optimate 5 which has 6 volt option like the Lidi one with a 2.8A output is nearly £90 and still does not display battery volts.

Now back to reason for post which was charging from vehicle, the Ring rscdc 30-30a dc/dc smart battery charger cost just under £200 from Screwfix and this is likely the best option for a car battery to battery charger, at 30A not too big and too much strain on car alternator and not too small so battery will never fully charge in the normal time taken between caravan sites. The main feature is "Will work with smart alternators to deliver accurate charging voltage" which is why the old blocking diode and split charging relay no longer work. Tow cars tend to be larger and so tend to have smart alternators.

With the twin feed taken to a caravan regulation A721.55.2.6 in the old BS7671:2008 I don't have up to date copy, there is no reason why the DC to DC inverter should not be in the caravan rather than the car. To comply with the regulations A721.55.5 one has to have relays in the caravan as you have two items wanting to only run with ignition on and only one ignition feed, so it would seem best place in in caravan. OK the regulation does seem outdated, I know our tow car does not have an ignition switch as it is compression ignition so has a fuel cut off switch instead.

The old spit charging relay worked from alternator warning light and the new from the battery voltage neither will work with Smart alternators, the whole idea of relay is when engine is cranked the caravan battery is not connected so there is no flow from caravan to car during cranking which if permitted would be rather high, the not causing car battery to go flat is rather secondary as we don't tend to leave caravan connected to car for extended times. With the auto engine stop and restart some thing to stop car drawing on caravan battery is essential so with modern car we are forced down the inverter route.

So a 10A battery charger will cost around £30 and 500W inverter around £50 so for £80 you can charge a caravan battery at 10A which is more than the old split charging would do and you have an inverter which can be used for other things. Lucas years ago did a caravan battery box, the idea was to charge it in car and use it in caravan so battery charged on day trips out, at that time 40 Ah was ample for a caravan, as motor movers came in we moved to heavier batteries so it became a problem moving battery from car to caravan, however with an inverter you can charge a small battery in car and then use inverter to transfer charge into caravan battery, you don't actually need to carry battery the inverters have a min voltage cut out so simply plug in and leave.

Likely better if a portable battery as some sites do not allow tow car to stop with caravan. This one is not British but gives general idea instead of a generator running to keep battery charged one uses a car, does not even need to be the tow car if a size which will work on cig lighter.