intelligent leisure battery charger for motor caravan

[VanMan]

I've searched and there are quite a few threads on battery charging. But I haven't been able to find one that gives me all the help I need. My plan is to build an intelligent leisure battery charger for my self build motor caravan .
I want to arrive at my destinations with my battery as fully charged as possible.

Can you help?

Here's the brief.

1. To connect to the alternator when the engine is running and the vehicle battery has been recharged.

2. To control the voltage and the current to optimise the charging operation of the leisure battery.

3. To seperate the leisure battery from the alternator when the engine is off or the battery is fully charged.

4. I also want to be able to determine how much charge is left in the leisure battery.

 

[BeanieBots]

WOW!
Major project with major hurdles to overcome.
What is your skill level? Do you understand the problems with car electrics?
Do you need a charge controller or just switch in/out with relay?
Do know about op-amps?
Have you seen my capacity meter in the finished projects section?
(it can be adapted for bi-directional measurements)

I'm currently designing a PICAXE bi-directional battery charger/dis-charger but it won't be ready for publication for a few weeks yet and can only cope with about 5W power transfer but might give some inspiration.

 

[VanMan]

Thanks BeanieBots I didn't realise that this was so major. I've just had a look at your Simple Battery Capacity Meter and found it very interesting but quite complicated. Maybe measuring how much charge is left in the battery is not the most crucial part of my project. I shall leave that part on hold for the time being. The main part is switching the relay off or on at the best times.

Yes I do know about op-amps. Originally I was planning on using an op-amp to compare the voltage of the vehicle batterry with that of the leisure battery and use that information to switch the relay accordingly. Currently I am using the relay and switching it manually after the engine is started. How simple can you get!

The leisure battery becomes quite flat after a couple of days running pumps and lights and I have become worried that when the engine is started up and the relay is switched on there might be a damaging amount of current passing into the leisure battery. Also I want the battery to be disconnected when it is fully charged to prevent gassing off and I need some automation to prevent me from forgetting to turn the relay on or off.

 

[manuka]

Many mobile motor homers have been this "Watts up with my Joules" path - what are others doing in your region? Is that leisure battery a deep cycle one? What sort of load do you have ?

Just for the record, & even though you're in gloomy UK conditions, perhaps you should mention if placing a suitable (~20W?) PV or small wind gene is intended when based in one place. PVs & motor homes are of course made for each other & you'd be silly not to have at least a modest PV. I've recently done testing with new CIS/CIGS types & found their overcast * low light performance superior to normal mono/poly-X Si PVs.

EXTRA: Check the simple Ah meter based around a PICAXEd bike computer => http://picaxe.orconhosting.net.nz/bikeamph.jpg This approach has been modified by electric bikers especially => http://www.picaxe.orconhosting.net.nz/bikepcfb.jpg As you no doubt know Lead Acid battery capacity is influenced by many things-temperature, current drain, battery age etc- so achieving a pin sharp Ah reading is pretty futile IMHO. Stan. in sunny NZ.

 

[BeanieBots]

Hi VanMan, the major part is protecting your electronics from the hostile environment of a car. Long duration +20v surges, 15kv spikes and all sorts.
Then there's the current handling. If just a relay, then probably not too bad as long as the contacts can cope.
As for capacity, there are two approaches.
One is to measure current and integrate by incrementing a variable every set period of time. The other is to integrate with an op-amp integrator and reset before it saturates. The later is how my capacity meter works. The former is easier but has much lower resolution. If your leisure battery is likely to have long duration small loads AND high current loads, then the op-amp solution should be considered otherwise loads below the ADC resolution won't be counted.

Have you ever worked with switch mode supplies?
Maybe consider a buck regulator for charge control.
What capacity is your battery?

 

[meridian]

Battery regulation

I have also thought about doing this on my boat. I thought of using readacd12 to monitor the voltage through a divider and use that to switch relays/resistors (approx 2 ohm 10W) in and out of the field circuit of the alternator. When V=14.4, disconnect them. On our boat we only run at around 1500RPM, so the normal output is quite low. This boosts output up to about 40A from a 120A Bosch. Note also, the house bank is 600Ah.

I do this manually, watching the voltage on the battery monitor, have to make sure it doesn't go over 15V, as the tracks in the alternator have been known to melt!

paulr

 

[VanMan]

Thanks guys,
My basic worry is :-
Do I need to control the current that flows into the leisure battery and if so what is the best way?

Here are the details:-
The battery is a deep cycle 85Ah leisure battery , the alternator output is rated at 150 amps so I'm using a180 amp relay for the connection.

I've measured the current flows out of the leisure battery 2 amps for the CD player, 0.75 for the heating system 0.5 amps for the LED lighting system and 5 amps for the water pump. So the normal useage is 3.25 Ah with short bursts up to 8.25 amps. This means the battery gets pretty flat over the weekend.

I'd like to put a solar panel on the roof but it would have to be quite a large one, say 50 W, to recharge that much useage (estimate 4 amps * 12 volts * 4 hours) on the dull winter days we get here in the U.K.

I've used a 600W SMB Transient voltage suppressor (SMBJ15A) to protect my LED lighting system and have two spare which I could use along with some big capacitors to protect my new circuitry, also I'm going to power the pic from its own 4.5v supply.

I'm thinking of using the voltage divider method to measure the voltages of both batteries and when the vehicle battery is at, say, 13.8 volts and the leisure battery is less than this I'm thinking of turning the relay on to connect the leisure battery into the charging circuit. When the leisure battery reaches the same or greater voltage as the vehicle battery I'm going to turn it off.

As I see it there will be two connections to the vehicle. These will be the common earth and that to the voltage divider. I think I should put the transient suppression on the connection to the voltage divider.

While the vehicle is running its alternator also powers the 3 way fridge I think that's 16 amps, It uses a seperate relay which also needs switching, it also good to have another relay which makes sure that all internal lights are turned off when the vehicle is running.

All this is the idea behind modern motor caravan split charge circuitry. These sometimes use the ignition light to test if the engine is running and to switch the relays. I don't know how to do this on my new Renault so I thought I would test the charging voltage instead. I want the engine battery to recover from starting the engine before I connect in my leisure battery. This is to prevent the leisure battery helping to recharge the vehicle battery. I realise I could up the charging voltage to the leisure battery using various devices but this is an unecessary complication at this stage.


Do I need to limit the current and how?

 

[BeanieBots]

You will need some sort of current limit. Even if it's not much more than the cable resistance.
I don't know a great deal about alternators but it should be possible to control the current there.
A fully charged vehical battery connected to a flat leisure battery will be quite a large volt difference. With no current limit, the battery terminal voltages will become the same almost instantly with only the batteries limiting the current. This would not be very good for their health. Also, as they are both at the same voltage, which one is the full one and which is the flat one? So, yes, some sort of current limit is required.
How much current can the cable cope with? (please use fuses!)
What is its resistance?

For the same reasons as above, you will need true differential voltage sensing AT THE BATTERY TERMINALS. You will not be able to use the chassis ground as a 0v reference as there may well be several volts developed across it. That's where the op-amps come in.
As you need op-amps anyway, I'd suggest adding in an offset so that your PICAXE ADC sees 0-5v for 10-15v at the battery. That will give much better resolution for the voltage measurement.

 

[manuka]

I've measured the current flows out of the leisure battery 2 amps for the CD player, 0.75 for the heating system 0.5 amps for the LED lighting system and 5 amps for the water pump. So the normal useage is 3.25 Ah with short bursts up to 8.25 amps. This means the battery gets pretty flat over the weekend.

I'd like to put a solar panel on the roof but it would have to be quite a large one, say 50 W, to recharge that much useage (estimate 4 amps * 12 volts * 4 hours) on the dull winter days we get here in the U.K.

That's a pretty trivial heating system- is it just a hand warmer?! Brr-winter -is this the time of the year that you're usually out & about? Solar PVs have been THE automatic choice for UK motor homes (yes-even in UK!) for years, & even an easily stored/erected 20W one may work wonders & save your bacon. See the calculator at => http://www.phly.co.uk/motorhome_solar_panel.php I've done a lot of field work with CIS/CIGS PVs recently & have been astounded at their low light/overcast/haze/ shadowed & off sun angle performance. The Shell/Siemens CIS ST-10,20 etc range look particularly appealing in the UK gloom, & rumours abound that prices are pretty keen in spite of currency woes. Hence ST-20 ~US$160.

Even with PICAXE/Op Amp monitoring you'll still NOT HAVE ANY ENERGY ENHANCEMENT unless you use a PV (or small wind gene), or start the motor. If the present deep cycle battery runs low then more storage (or reduced demand!) is needed,& that 150A alternator looks capable of serious en route charging duty. At rest sunbeams are cheaper than fossil fuels of course.

THOUGHT: I've recommended this to skinflints before -grab some 12V 7 Ah SLAs from a security firm. Most have huge stashes of near new SLAs removed from critical standby power supplies, & with the price of lead now low again you'll no doubt be doing them a favour. I regularly call by a firm here in Welington (NZ) & the car boot can become so loaded that road grip issues arise on the front wheels!

The batteries get passed on to local high schools,or remote users such as campers/weekenders. The (free) 5 x SLA shown below store 35 Ah, & when used with the nifty portable power pak/inverter (costing only an amazing US$25 here in NZ) nicely suit simple electrical energy needs for an off-grid weekender. A 20W PV can recharge them all nicely over the week (1½ A x 4 hrs x 5 days= 30Ah)

 

[rgooge]

I've searched and there are quite a few threads on battery charging. But I haven't been able to find one that gives me all the help I need. My plan is to build an intelligent leisure battery charger for my self build motor caravan .
I want to arrive at my destinations with my battery as fully charged as possible.

Can you help?

Here's the brief.

1. To connect to the alternator when the engine is running and the vehicle battery has been recharged.

2. To control the voltage and the current to optimise the charging operation of the leisure battery.

3. To seperate the leisure battery from the alternator when the engine is off or the battery is fully charged.

4. I also want to be able to determine how much charge is left in the leisure battery.

Have a look at:

http://secure.oatleyelectronics.com//product_info.php?products_id=270&osCsid=a709c3b01dd454111ea8ec83fc02f920

It does 1 and 3 from your list, 2 may not be really needed and 4 is a job for a simple Picaxe circuit

I'm using one of these to charge a 12V/110AH auxiliary wet cell battery in a camper trailer and it works quite well. Not all the bells and whistles you're after, but it's a start. Most car type alternator/battery combinations are designed to charge lead acid batteries, the battery is almost self regulating and disconnection on full charge should not be needed.

I generally measure the "at rest" voltage of the battery to determine it's state of charge, its one of thos 'going to...' projects for the Picaxe.

- Rob.

 

[VanMan]

Thanks guys I'm starting to see my way around this now,

Really, Manuka , the 0.75 Amps powers the igniters and other electrics for the the gas supply!

It sounds great down there in NZ with that lovely sunshine I suppose you just plug into the nearest geyser for a bit of geothermal