Mobitronic Battery Charger

[Zhivili]

I have just purchased a Mobitronic 945-012-A battery charger for my Beneteau 32s5. I would be grateful for advise on how to wire it. The model is 12 volt, 45 amp. On my boat I have two batteries, both 100 amp. I believe I am correct that they are wired in paralell. Marine Super Store, from whom I purchased the charger tell me the charger is suitable for the two batteries. The Mobitronic only has one positive terminal out and one negitive terminal out. The boat batteries both have separate positives but a common negitive. How do I wire the charger to the batteries. Do I take a separate positive to each positive terminal on each battery and then bring back a single common negitive. Or do I bring a single positive to one battery and then a single negitive to the other battery. I would be grateful for advise.

 

[pvb]

If your two batteries have separate positive leads, it sounds as if the charger isn't suitable. Perhaps you could take it back to Marine SuperStore and get a charger with 2 separate outputs.

 

[richardabeattie]

Don't take this the wrong way but if you "believe your batteries are wired in parallel" you might want to get somebody with electrical experience to help you before you spend any more hard earned.

 

[VicS]

Your description of separate positves and a common negative suggests that you have two independent batteries. Do you have a 1, 2, both, off type of isolating switch or do you have separate isolating switches for each battery? If the later do you have one battery for "domestics" and one for engine starting or do you not have separate systems?

If you changed the charger for one with two outputs, as pvb suggests, you will be able to charge both batteries simultaneously but with a single output you will only be able charge them separately. That's no good if you want to wire it in permanentltly. If you attempt to wire both batteries to the charger you will be "paralleling" them via the wiring to the charger.

You will find the range of Mobitronic chargers here http://www.waeco.com/en/331.php but it does not include the one you have! I have found it on another website though. It is large enough to charge batteries up to 500ah, rather larger than you need.

It looks as though you have been given poor advice from Marine Superstore.

If by any chance your batteries are permanently connected in parallel then the chager you have will be suitable although still larger (and more expensive) than necessary BUT that would be a very silly arrangement and the best advice would be to split them using a 1, 2, both, off switch ( although others will suggest separate isolating switches) and then get a charger with two outputs, such as the 925 012 TB

I would go along with richardabeattie's advice and get someone with a bit more knowledge of "things electrical" to help you out.

 

[William_H]

All the advice above is good. However if you are stuck with the charger you have.... assuming they are not really connected permanently in paralell. Connect the charger to the battery used most for domestics. To acheive charging of the other battery you can fit a Voltage sensing Relay between the two poositives this will automatically connect the other battery when charging and disconnect when not. (this provides isolation)
Or you can turn your 1,2,both switch to both this leaves power on all systems when charging which may not be safe if you leave the boat and of course must be returned to normal when not charging.
Or you could connect a switch with wiring from one positive to the next so that when charging you connect both batteries in paralell. You must remember to open this switch when not charginng agian to provide isolation.
It is usual to have 2 batteries so that one can be discharged with out concern while the other one is isolated and retained charged for engine starting. If you don't maintain this isolation you will eventually not be able to start the engine. regards olewiill

 

[VicS]

Hi Will,
It's nice when someone agrees. Very reassuring.

I think Zhivili would have good grounds for returning his charger to Marine Superstore if it is not suitable as he bought it in good faith on their advice. Provided it is unused, in its original packaging etc and that he has proof of purchase of course. Marine Superstore incidentally is a large chandlery at Port Solent, the marina where he is based.

A VSR is indeed an option but I seem to remember someone on here advising caution. Something about the possibility of the wiring to it being subject to starter current in some circumstances, IIRC.

I don't think switching to "both" is really an option because that would mean the power is on to all circuits. I imagine the object of buying a sophisticated and expensive charger is that it can be left connected and switched on while the boat is unattended, just when you would not want to leave the main isolator switched on as you rightly point out.
Similarly leaving both isolators on if there are separate ones for each battery is not acceptable.

If a switch is used to connect the two batteries then it and its wiring must either be able to carry starter currents, as sooner or later, it'll be left closed and the engine started, or alternatively a fuse could be included in the circuit which is rated to carry the charging current but blow if subjected to anything larger. Apart from this there is a small chance that one day one of the batteries will fail and not only take all the charger output but run the other one flat as well, resulting in two flat batteries rather than at least one fully charged.

If Zhivili tells us about his switching it will give us a bit more insight into his battery arrangements.

 

[William_H]

Absolutely correct Vic. or shoulod I say "Abfab" olewill

 

[Zhivili]

Thank you for all of your advice, re the Charger I have purchased. My boat, a Beneteau 32s5 has a battery switch for battery one, two and both. You mention that the charger I have purchased is excessive. Would I be better to return it and change it for another. I think Marine Super Store have a Sterling Charger 10 amp costing about £60 more - I think this has two positve outlets - would that be better. Thank you for your help again.

 

[pvb]

Yes, get one with 2 outlets...

Sounds like you got the Mobitronic cheaply, possibly because it's a single outlet model. The 12v 10A Sterling at around £110 would be fine - but check with Marine SuperStore that it has separate outlets (it should have 3). You can then wire one outlet direct to one battery, and the other direct to the other battery, and your charger will work without having to have your battery switch on. The charger outlets are isolated, so one battery can't discharge into the other.

 

[VicS]

That confirms what I expected your battery arrangements to be. You can use either battery for all functions or both in parallel in an emergency. Its up to you whether you reserve one battery for engine starting, but that would be wise, and which one that is to be.

The Sterling charger sounds fine provided that is 10amps from each output. A rule of thumb is to have 10amps available per 100ah of battery capacity. As pvb says it has 3 outputs so you will have a spare one. Make sure, as pvb says, that is the one they are offering and not the 10A single output one.

It's a pity they don't have exactly what you need. It would be nice to have one a little bigger but if they say 10A then really that is enough. There is a 20 amp one but that is twice the size and is £60 more. A pity the spare output can't be shared between the other two if you see what I mean. Two outputs at 15 amps would have been lovely.

Have a good look at what is available elsewhere. I see no reason why you should not simply ask for your money back, in fact in law, that is probably what you are entitled to anyway as what you were sold is not suitable for your needs which you explained to them.

Not what you were asking about but when you switch from one battery to the other go through the "both" position if the engine is running or you'll damage the alternator. If the engine is not running then usual practice is to go via the "off position, but that is not so important.

Sterling website is http://www.sterling-power.com/index-l.htm# but I have seen better ones!

 

[Zhivili]

Re: Yes, get one with 2 outlets...

Thank you for all of your advice. I have spoken to Marine Super Store and have arranged without any quibble to exchange the charger for a Sterling 20 amp three outlet version. It costs another £60 but at least, thanks to your advice, its going to be the right item. I think Marine Super Store were advising me o.k. - its just that I probably mislead them as to whether the batteries were in par or series. Still they are changing it without question. Thanks again for your help.