Charging Batteries via multiple sources.

Submitted: Monday, May 06, 2013 at 13:27
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Hi everyone,
I know this has probably been asked before but I'm looking for some help.

So we want to be able to charge the battery in our caravan via 240v mains when at powered sites and with solar which will probably be at first a single 80w solar panel as we are short of funds and we can add more later. The problem is I want to be able to charge from both at the same time, why, well simply because when we use our caravan we going on a holiday and I don't want to have to be switching isolators to switch between solar and our gene. Later on I'd also like to draw of the alternator but I'm not worried about that for now. We probably will run a 240 fridge, 12v lighting and other od bod stuff like a phone charger. One more thing, what would you suggest for a battery type, I was thinking just a deep cycle battery but I'm not really sure. I think we want around a 100ah battery.

Thanks guys and I hope you can help me out with this problem.
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Reply By: Cravenhaven - Monday, May 06, 2013 at 17:02

Monday, May 06, 2013 at 17:02
Its no problem at all, just connect both your AC charger and your solar panel regulator to the battery and they'll both do their best to charge it.
Make sure that your AC charger is a modern multistage type so that you dont overcharge your battery.
Alternatively you could buy the BMS unit from Redarc, it has a solar and an AC charger built in to the same unit but its hideously expensive.

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Follow Up By: Mick O - Monday, May 06, 2013 at 20:40

Monday, May 06, 2013 at 20:40
Concur with Cravenhaven. Redarc have some great products that would be perfectly suited to your needs. The other option is the Redarc BCDC which will cater for your DC-DC charging and also solar. You'll have to do the 240V AC with a charger of some description. The BMS will choose the best power source at that particular time and use it (ie AC over everything else, DC when the cars going and solar if not moving).

Yep expensive but Aussie made, reliable and excellent back-up.

A bit of a review on my experiences with the Redarc products here.

Redarc BMS and BCDC 12-20 review

Cheers Mick

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Follow Up By: Jason – Perth - Monday, May 06, 2013 at 21:20

Monday, May 06, 2013 at 21:20

As Mick has said, the Redarc will do it for sure. I have exactly the setup you are seeking, with the Mains power supply (acting as a solar panel) as well. My system auto switches between solar, mains and alternator.
The Redarc units will require a 24V mains supply to get around the low volt cutouts.
Works a treat! The supply is a 10Amp supply which takes about 5.5 hours to recharge my 105Ah battery from "flat". 10A is a bit small, I would have preferred a 15-20A supply but they weren't available.
An 80W panel is a bit small but will depend on your load, where you are going and when.

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Follow Up By: Member - Syd R (QLD) - Sunday, May 12, 2013 at 17:45

Sunday, May 12, 2013 at 17:45
I concur the BMS from Redarc is great and I have no problems and would recommend it and worth it.
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Reply By: Member - John and Val - Tuesday, May 07, 2013 at 08:43

Tuesday, May 07, 2013 at 08:43

First thing is to decide just how much power you need. Suggest you might find Electricity for Camping a useful read. Depending on your usage, you may need more than a 100Ah battery. (Our own rig carries 200 Ah in the Troopy and another 40 Ah in the trailer, with 150W of solar capacity plus a 30A dc-dc charger, and suits our usage patterns pretty well.)

As others have said you can charge from solar and from the mains simultaneously, but personally I wouldn't, unless you plan on using a generator to run the charger. A decent mains charger will deliver 10 to 20 amps, more for the bigger ones. The most you will get from an 80W panel is about 5 amps. The panel doesn't help a lot when the mains charger is running. Of course if you are using a generator, the solar panel may reduce the charging time by 20% - on mains though time isn't really a concern.

It will cost a bit, but suggest it's worth working towards probably two 80W panels with an MPPT controller, and a dc-dc charger to provide good charging from the vehicle (ours does most of the battery charging). Dc-dc chargers that include battery management and an MPPT controller for solar are available and would be a good choice. This is very dependent on your usage patterns and total demand, but unless you are very frugal, I doubt that 80W and a 100 Ah battery will meet your needs.

Some kind of mulltistage mains charger is near essential. Though not as simple as buying a charger off he shelf, Jason has suggested a good option - using a 24V mains power supply to feed into the solar controller, which then supplies the multistage smarts to manage charging. We do something similar, using a 13.8V power supply to feed our multistage dc-dc charger. Works for those familiar with this stuff, but maybe not for everyone.




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Follow Up By: Joel T2 - Tuesday, May 07, 2013 at 09:04

Tuesday, May 07, 2013 at 09:04
Hi John and Val,

I think I may have got on the forums a little early, I was up all night last night and I agree with what you stated. So with some math I figured 2x120w panels should give me a good amount to charge a 100Ah battery in about 4-5H. We have a mppt charger in mind through jaycar (MP-3735) but that's not set in stone. Also Ever Battery have a charger I think will work well it's a "Pojector IC1500", our biggest draw will be the fridge which is 1ah(288w/h) after that is the tv 240w/h, which leaves us to my estimate around 300w/h for everything else we get together.Better yet the price tag for the essentials is only just over 1k. Sorry that's a bit long but what do you think? I think the way it's looking after we check out the alternator to see if we can fit a dc-dc charger we might not even use the gene that much. Thanks
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Follow Up By: KenInPerth - Tuesday, May 07, 2013 at 11:05

Tuesday, May 07, 2013 at 11:05
John and Val

If you pick this up, I am curious as to what your 13.8V power supply is - something off the shelf or something you buillt ??

I use a Ctek D250S Dual and am looking for a good option to feed into the solar input on that, but it seems the Ctek likes something a bit higher (like around 18V) although I have not done a lot of testing on this yet ...

Also as a note for this thread - when I tried putting a Smart Charger directly on the batteries with the Ctek output also connected the Smart Charger seemed to get very confused - hence I am looking now to feed a 240V power supply in through the solar input of the Ctek as a secondary charge source.

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Follow Up By: Joel T2 - Tuesday, May 07, 2013 at 11:42

Tuesday, May 07, 2013 at 11:42
Ahh I get it know, you just use a plain old AC-DC Power supply into the solar charger which treats it as a solar panel, basically cutting a charger out of the equation! Nice Idea
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Follow Up By: KenInPerth - Tuesday, May 07, 2013 at 12:11

Tuesday, May 07, 2013 at 12:11
That seems to be a good simple option bearing in mind not to exceed the max allowed votage at the solar input for the unit you are using, and it may have a minimum cut in voltage.

There are 18V 20A power supplies available for around $60 on eBay I am considering getting one to play with ....

My conundrum at this point is that if I add a 3rd source to the equation (ie. solar panels) how to "share" the solar input with the power supply and the solar panels - by diodes, or auto switching relay, or what (keeping it as simple and reliable as possible).

I am sure someone out there will ahve some good ideas on that.
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Follow Up By: Joel T2 - Tuesday, May 07, 2013 at 12:30

Tuesday, May 07, 2013 at 12:30
Yeah I'm guessing you would need some diodes on the solar side to stop power leaking back through the cells but apart from that shouldn't they both just feed into the charger and work fine.

Not to sure...
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Follow Up By: KenInPerth - Tuesday, May 07, 2013 at 12:45

Tuesday, May 07, 2013 at 12:45

Because you never really know the design of the output of any device I would always opt for full isolation of one source from the other by some means.

Just isolating the solar panels as you suggest would not stop the solar panels back feeding into the AC / DC power supply which would waste solar power and maybe damage the power supply depending on it's design.

So for me full isolation is aways the best option - and diodes also rob you of at least 0.6V which can be a significant loss in some situations.

Anyway this is a whole new discussion point.
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Follow Up By: Joel T2 - Tuesday, May 07, 2013 at 13:08

Tuesday, May 07, 2013 at 13:08
Good point Keith, so either 2 manual switches, diodes on both sides which as you mentioned rob voltage, or some sort of microcontroller switch which picks the highest amperage. I think I have a solution, I do have some "picaxe" stuff I could make something up some replays, only problem is how to measure the amperage without damaging microcontroller...... Not entirely sure but this could defiantly work
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Follow Up By: Member - John and Val - Tuesday, May 07, 2013 at 15:46

Tuesday, May 07, 2013 at 15:46

re battery sizing - your figuring looks right, BUT appears to assume that your 240W panels will deliver 240W and your 100 Ah battery will deliver 100 Ah. Unfortunately, those critical assumptions are wrong.

240W of panels will not deliver 240W to your batteries, which is what you'd need to get 100 Ah in 4-5 hours.

1)Panels are rated at their maximum capacity (usually at 25 deg C) under full sunlight (actually 1kW/square metre) at normal incidence, and when delivering current at their optimum voltage. These conditions are rarely achieved due to sub-optimal temperature and orientation, shading, atmospheric dust, clouds, latitude........ Averaged over a year 240W (nominal) will probably supply the equivalent of about 80 Ah per day. Depending on the controller, 5 to 25% of this will be lost before it reaches the battery. Depending on usage patterns, the battery itself may be 85-95% efficient.

2) One should avoid running batteries down much past their 30% charge level, as going down much further will seriously reduce their life-span. 2/3 of rated capacity is about as much as it's wise to draw from a battery, and it should be recharged asap to minimise deterioration.

"our biggest draw will be the fridge which is 1ah(288w/h) after that is the tv 240w/h, which leaves us to my estimate around 300w/h for everything else we get together" mmmmm....!!

Joel - it's important to get the units right here (I'm not just being pedantic or an academic 'expert' !)

The fridge - first - If it's a 3 way fridge, forget running it from batteries as it will draw far more than they can sensibly supply. Assuming it's a compressor type I'd expect the fridge to draw an average of at least 1 amp (NOT 1 Ah!!), probably nearer 1.5 amps. If we go with an average of 1 amp at 12V then it requires 24 amp hours (Ah) per 24 hours. If we assume our battery delivers 12 volts (which it doesn't!) then that comes to 12x24= 288 watt hours (NOT watts per hour or w/h). (BTW best to think in terms of amphours rather than watthours with this stuff. )

TV - depending on size, I'd expect this to draw perhaps 2 or 3 amps for say 2 or 3 (?) hours per day, so that's say 5 to 10 Ah. Charging phones, laptops, torches,..... maybe another 10Ah to give a total requirement of about 45-50 Ah per day. (This sounds about right - Our rig uses about 35-40 Ah per day.)

With this level of demand, I'd aim for 200 Ah of battery storage. Depending on your likely traveling time per day I'd seriously consider a dc-dc charger rather than aiming for lots of solar. A 30 amp dc-dc charger will push close to 30A into the battery/s while the engine is running at more than a fast idle, and meet your daily electrical demand in less than a couple of hours. With 200 Ah of battery you will get by for 2 or 3 days without any charging. If you are stationary for more than a day or two, 160-200W of solar panels should handle your requirements provided the sky is clear. (I note that you are aware of the need to consider our alternator's capacity if you go for a dc-dc charger.) There are several quality dc-dc chargers available, some such as the Ctek, Redarc, Projector include an MPPT solar controller.

In regard to some of the other contributions - There's been interest in using the solar input of an MPPT controller to accept input from a dc power supply so as to simulate a multistage mains charger. The solar input needs to be at 18 volts or more. To pretend to be a solar panel, the power supply needs to be at least 18V, which is an inconvenient voltage and not much use for anything else. Not too sure how the controller will respond to a steady voltage supply, as it's expecting a solar panel which is pretty close to a current source, rather than a voltage source. (These controllers monitor their source voltage/current while periodically loading and unloading the source (panel) to see how it's travelling.) Certainly, to avoid confusing the controller, the power supply and the solar panels should not be connected simultaneously to its input unless diodes (preferably Schottky diodes so as to minimise losses) are included to isolate the two sources from each other. (Current flow from one to the other will be negligible in any case, but isolation is necessary to prevent the filtering in the power supply from upsetting the operation of the controller.) Better in my view would be to employ a dc-dc charger and feed it (and NOT the solar input) with a 12-15V power supply that can be set to say 13.8V. This will act as a multistage charger, with the further advantage that the power supply can power the whole rig from the mains if need arises.


asks about my 13.8V supply. I picked one up cheap years ago on the grounds it "might be useful". Variable from 5 to 20V and capable of 40 amps. It has turned out to be very useful! 13.8V supplies are readily available as a standard voltage for "12V" battery systems - see ebay for lots.

Your 250S is already a smart charger, and if you add another smart charger, both will become confused. That's one reason I prefer to simply add a mains power supply to the front end of my multistage dc-dc charger. Better in my opinion to avoid interfering with the solar input, which has it's own higher voltage requirements.

Guys,..... please have a look at Electricity for Camping. It's the most popular blog on ExplorOz. I wrote it to answer most of the questions that come up here, and it saves a lot of typing!!!!!


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Reply By: Joel T2 - Tuesday, May 07, 2013 at 11:58

Tuesday, May 07, 2013 at 11:58
Hi again all,

So just to confirm I can instead of buying a hugely expensive ac battery charger I can simply use a switch mode power supply running into the solar regulator/battery charger and all will be fine. Is this going to make any issues when the pair are feeding in together or will all just work normally.

Thanks Guys
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Follow Up By: KenInPerth - Tuesday, May 07, 2013 at 12:33

Tuesday, May 07, 2013 at 12:33

The simple answer is "yes the Regulator/Charger should sort itself out in that regard". I have assumed you are running a dual input charger of some description with one input being the Alternator and the other available for "solar" - but the general assumption would be if both DC inputs are supplied by any source within spec it should work as designed and will take the best source available at the time for charging, but you need to be sure of the specs for each individual input in terms of voltage limits.

We can theorise all we like but some devices will not behave the way we expect when we are "making it up as we go along" and not dealing with stuff designed to fit together properly like the Redarc gear.

No one answered your question about the battery dd they ?? - you really need Deep Cycle as opposed to a Cranking battery - and that is a whole new discussion and has been bashed around well and truly.

Bottom line for me is that you will get what you pay for - you might get away wth a "low cost" flooded battery (around $120 for 100AH) or may need a higher cost AGM or other type depending on how the battery is used and re-charged.

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Follow Up By: Jason – Perth - Wednesday, May 08, 2013 at 00:34

Wednesday, May 08, 2013 at 00:34
Hi Joel,

To clarify my earlier post, a standard mains power supply won't work when connected to the solar input of a dc-dc converter like the Redarc. This is because they do not emulate a solar panel. As John said, as solar panel is a current source and as such will continue to deliver maximum current even when shorted out. Most normal power supplies will go immediately into protection mode and fold back the volts or current when shorted out, so will confuse the MPPT regulator no end.
A power supply that delivers continuous maximum current, even when shorted will work with an MPPT regulator no problem. The controller will sample the input volts (as per John's post), but will continue to track the maximum power point. In my system, the maximum power point sits at 240W (+24V and 10 Amps) and as there is no variability in the output, (no cloud or shadows!) the MPPT Regulator will sit there happily.

I also agree with John in that it is a bad idea to connect the mains supply and solar panel at the same time. I have a change over relay that detects which source I am using and switches either one OR the other into circuit as required.

The Redarc BCDC1240 Manual has the circuit diagrams you need for the change over relay. I just wired the power supply as a solar panel.

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Follow Up By: KenInPerth - Wednesday, May 08, 2013 at 10:52

Wednesday, May 08, 2013 at 10:52

The other (better?) solution to this (which I think is what John was referring to) is to switch the sources between the alternator input of the charger rather than the solar input and keep that just for Solar panels (there would not be many situations where you would be running the car engine and have mains available at the same time). You would get one of the 13.8V power supplies John referred to for that and the change over wiring diagram of the Redarc Jason refers to (or many variations) would still apply.

However if you don't have a solar source (currently my case) I have been using with good results (in a test situation) the solar input fed from mains power supply (which is a smart charger I modified for purpose) rather than use additional circuitry such as the change over relay.

For all to correct me if I am wrong here ....

Sorry guys - don't want to turn this into a technical discussion and am trying to keep it simple and will not bang on any more after this as I think it is done and lots of good information has been provided.

For the sake of clarity and not to give Joel or the thread information that is perhaps not as accurate as it could be, I would suggest the MPPT is not going to "short out" and shut down the mains power supply. That would be a fault condition and not normal operation. For a Solar panel, the MPPT may present close to a short circuit on the solar input, but for other power sources will behave differently as discussed below if anyone wants to read on.

I would suggest the situation Jason presents with a change over relay feeding the solar input is the same thing - one of the sources (the alternator shown as the Battery via a 50A fuse) being a protected source that will "shut down" (because the fuse blows) when there is a fault condition that "shorts out" the solar input. It will not continue to provide "maximum current' if shorted out.

Substitute a mains power supply for the alternator and you have exaclty the same situation. The fact that some power supplies will continue to supply current in fault conditions (which is either a non-fused or a current limited supply and are usually laboratory and test bench units) is a design feature of the power supply itself - but somethign else is usually going to burn up under those conditions (I can attest to that from being on this earth a few years) and most power supplies will always have a fuse somewhere in them.

The way an MPPT regulator works is a rather technical discussion but I suggest it does not actually "short out" the input to derive maximum current, but it will vary the load it presents to the solar input to derive best power from the source under present conditions. For Solar panels that may approach a short circuit depending on conditions and the specific panel's characteristics. When the source is a power supply rather than solar it will behave differently as it will not have to present such a low a load to derive the same efficiency - what load is presented will depend on the characeristis of the power supply.

Solar panels, mains power supplies, and alternators are all power sources but with different characteristics and the MPPT will present whatever load it needs to derive best efficiency (power) based on the Voltage vs Current characteristics of the source.

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Follow Up By: Jason – Perth - Wednesday, May 08, 2013 at 11:04

Wednesday, May 08, 2013 at 11:04
Apologies all if I implied an MPPT regulator "shorts out" the input to derive max current. That isn't what I said or meant.
I won't ramble on any more unless Joel specifically would like more information. Happy to respond to any questions you may have Joel.


Ken, did you ever work for a comms company in Osborne Park?
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Follow Up By: KenInPerth - Wednesday, May 08, 2013 at 11:19

Wednesday, May 08, 2013 at 11:19

Not for a comms company - I had a short stint at an IT company in Osborne Park who sold Toshiba Laptops mainly and an accounting system.

My era starts in the PMG for those who remember those days and I left there when it was Telecom and before it became Telstra (and has gone downhill ever since - but probably not because I left them).

Once a tech head always a tech head ????

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Reply By: Joel T2 - Wednesday, May 08, 2013 at 20:45

Wednesday, May 08, 2013 at 20:45
Hi All,

Thanks everyone for your help, expertise and recommendations.
You've all helped me no bound. So I think I understand what's going on now.
I'm going to get 2x120Ah Batteries so that I can have some extra stuff without dropping the battery's much below 33%, Will be 2-3 120w Panels, I will get the Mppt solar charger capable of the three panels, but will only get two for now. As for the Alternator that's, well not going to happen in our commodore but later on we might add it to the Mitsubishi canter(Yes guys I understand it's so much easier, faster and cheaper to use the alternator, but just not gunna happen yet :). And the Stuff about the power supply running through the solar panels, I'm not sure.. I have a fair understanding of electricity(been doing micro controllers and amp circuits for awhile now.) but when there's a fair amount of money in the process and there seems to be a easier way (Dc-Dc charger fed by Ac power supply) I think this will be better suited to us. Again thankyou guys so much, great forums and best of all its Aussi ! Two more Question though, 1) I need an easy way to check my battery level, and yes I understand that simply reading the voltage doesn't actually tell me my battery's charge but do you guys think that the "Digital Dc Power Meters" (Jaycar #MS6172) would be good enough or have you something better in mind. 2) what's a good mppt solar charger rated to 40amps, I guess I'm moreso asking what a good brand is. Something that's not rubbish but isn't going to break the bank.

Thanks Guys,

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Follow Up By: sam h6 - Thursday, May 09, 2013 at 14:05

Thursday, May 09, 2013 at 14:05
Hey mate we have just set off round aus and having the same issues. We have ditched the cheap ac charger the caravan ppl installed and fitted a ctec multi 25000. I have also fitted a new solar control with settings for calcium batteries which we have and two 120 watt panels. I have got but not installed yet a redarc bcdc 40 watt charger looks like a great unit. I will install it at some stage when I get the urge.
We hade massive issues charging our batteries but now I have the right chargers for calcium batteries it's all coming together.

Good luck with it all it can be very confusing with so many options and different opinions.
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