Battery charger with no batteries: crazy or what?

[MapisM]

A bit of background:
I've got 4 pairs of 105A batteries for 24V DC domestic services onboard. Which might seem a lot of amps, and it is, IF and WHEN the batteries are good.
But at the moment, I can run my DC stuff (fridges, WC, pumps, winches), only as long as either the battery charger (i.e. when docked, or anchored with the genset on) or the alternator (i.e. while cruising) are running, because the batteries go below 24V in a matter of minutes when left uncharged.
The reason why I postponed the batteries replacement, which was actually already due at the beginning of this season, is that I have in mind a small winter project, replacing the existing battery charger with a Victron Multiplus 3kW (as per BartW suggestion/proposal, thanks again B!), and reviewing the connections to use also the Victron as an inverter, thus having AC onboard at all time - for small(ish) AC stuff of course, not airco or cooktop.

Now, the problem I'm facing is that the batteries, which weren't in good health at the beginning of the season, didn't get any better by further using them.
This means that I need to keep the charger on at all time while docked, to keep a decent voltage level.
But since the charger is pretty old and doesn't have a temperature sensor, the charge it keeps sending to the batteries (even when nothing is being used onboard) is enough to overheat them in the last months.
It got worse lately, and now if I keep the charger constantly on, the batteries get hot enough to hardly be able to keep a hand on them for more than a few seconds.

Hence the crazy idea: since the battery charger is man enough in term of amps to drive pretty much all my essential DC stuff, would it make sense to disconnect the domestic batteries completely and keep the charger on?
By the same token, while cruising and with no genset running, the alternator would feed directly the 24V circuit, with no batteries in between.

Obviously, I'm not considering this as a permanent solution, and it would only be meant to last for another few weeks, before lifting the boat, replace the charger with the Victron, and eventually install new batteries.
But if on one hand it would be nice not to be woken up in the middle of the night by the explosion of overheated batteries, I fear that I could fry the charger or the alternator (or both) by disconnecting the battery...
...but I'm not positive on whether this is a realistic risk or not, hence this post.

All suggestions will be more than welcome, thanks in advance!

 

[vas]

not quite a reply, but my victron (charger only) works fine with no batteries connected to it feeding various devices (fridge included)

I wonder, how big a project would it be to just remove the old charger and fit the Victron NOW, throw the batteries away NOW (or simply not connect them!) and fit new domestics plus do all the bits to feed some sockets with 220V from the inverter during the winter?

and to answer your Q, no I don't like the idea of old style charger alone running all the el. devices onboard. Don't think it's going to set things alight, but could possibly burn a few of your 24V h/w. Worth the risk?

cheers

V.

 

[jfm]

Not such a crazy idea. At basic level a batt charger is a 230ac - 28dc transformer/rectifier. On my boat somewhere there are a couple of catalogue small victron batt chargers installed as transformers/ rectifiers with no batteries, to provide back up 24 dc power to some of the big 230v AC relays that have 24 dc coils iirc

It is possible the batt charger will sense that absence of batteries and not function, but maybe yours isn't computerised enough and this will not be a problem. You could just select the two least hot batts in your current installation and connect those to overcome this problem if needed

As you know your charger might output 28.8v but I doubt you care. Your DC loads will generally not care about 24-28 volts and if they do they will already have protectors/ regulators

The engine alternators will not work unless they have an excitation voltage. That might come from the domestic batts via a splitter device ( to share with the engine starter batts) and you will need to hotwire that somehow if you disconnect the domestic batts. Again the idea of keeping the 2 least hot old batts would do this, or cheat/ Hotwire to thr engine starter batts. Remember the function of the excitation voltage is to tell thr alternator what output voltage to create, to correctly charge the batts. But as you are using the alts to power domestic loads it doesn't matter so any excitation voltage near to 24v will do

This is " off top of my head". Would be good to get other ideas because I prolly missed something. But you basic idea flies imho

 

[BartW]

old fashion battery chargers could be just rectifiers, and a resistor, without any stabilisation, (iirc)
worse case, giving a rectified sinewave, peak voltage is 1,4 x nominal voltage
the battery serves as a capacitor, to smooth the sinewave
I'm not sure how your electronics would react to such a supply
I'm also not sure what charger you have, / that old fashion

 

[nimbusgb]

Your batteries are getting hot under charge because they are shot, probably suffering from sulphation, plates shorting out across the bases from deposited lead sulphate. A healthy battery won't take on too much current from a reasonable charger.

Disconnect them from the charger. Why, because if they get hot enough they may distort and break a case and then leak acid into the boat or even worse catch fire.

Put in a couple of cheap, £60 quid batteries, to smooth the output of the charger, allow you to run the engines and alternators and absorb voltage spikes, until you are ready to change the bank completely.

 

[BartW]

Your batteries are getting hot under charge because they are shot,
....
Put in a couple of cheap, £60 quid batteries, to smooth the output of the charger, allow you to run the engines and alternators and absorb voltage spikes, until you are ready to change the bank completely.

would also be my proposition

 

[Beamishken]

Your batteries are getting hot under charge because they are shot, probably suffering from sulphation, plates shorting out across the bases from deposited lead sulphate. A healthy battery won't take on too much current from a reasonable charger.

Disconnect them from the charger. Why, because if they get hot enough they may distort and break a case and then leak acid into the boat or even worse catch fire.

Put in a couple of cheap, £60 quid batteries, to smooth the output of the charger, allow you to run the engines and alternators and absorb voltage spikes, until you are ready to change the bank completely.

My thoughts too!

 

[MapisM]

Many thanks folks, I appreciate your responsiveness.
Swmbo is in fact beginning to scream, 'cause I'm restricting her the availability of onboard devices....

Back to the point: actually, I already thought of buying a couple of cheap batteries just meant to work as stabilizers/absorbers, so to speak.
But aside from the fact that in principle I hate wasting even small amounts of money (not having any other use of such batteries afterwards), the cabling is all arranged for batteries with bolt terminals - a type of batteries which afaik are never really cheap.
And obviously, it wouldn't make sense to buy a couple of proper batteries now, and then put another six of them in parallel afterwards, before the next season.

Otoh, I see your point, of course.
Maybe I'll try first to leave only the "colder" couple of batteries connected first, and possibly buy two later, if that doesn't work.
In fact, even if I didn't mention it (and I wonder how jfm guessed that!), there is one couple of batteries which doesn't seem to overheat as much as the others, for reasons only known to God. Oh, and jfm, apparently!
If after tomorrow you won't see any further posts from myself, and you'll read of an exploded wooden boat in Sardinia, you will know what happened!

Last but not least, just in case someone might understand better than myself how my charger works, I'm attaching a pic below.
Unfortunately, I don't have any manual, and the company doesn't exist anymore, so all I know is what is shown in the red label.
Which btw shows the following values:
Model: B250/3 NC K
Input: 220V 50Hz 11A
Nominal output: 24Vcc 50A

 

[BartW]

how many battery's is the engine start bank ?
usually they are a bit oversized,

perhaps you can take a pair from there, for a short period of time ?

 

[MapisM]

Good idea, but nope, I've only got one pair of starting batteries.
I mean, not even one pair/engine, just one which starts both.
Which I think is ok, because I can also put the domestic bank temporarily in parallel, by mean of a button at the helm, in case of any problem with the starting batteries. So, a decent degree of redundancy is available anyway.
Ahem, at least as long as the domestic batteries are not shot, that is...

Anyway, in the meantime I have an update.
Broadcast programming is awful tonight, so I thought that I could as well start to re-wire the batteries connections, leaving online just the "colder" battery pair.
Then again, while I was at that, I thought to give the "zero batteries" option a try.
Now, to jump in conclusion, the good news is - you guessed it, since I'm still posting - that the boat didn't explode.
But there's more: the system actually seems to work just fine!

This is the sequence I followed:
1) turn off all onboard DC stuff
2) turn off the charger
3) disconnect the batteries
4) turn the charger back on

At this point, with zero load, the charger output was 27.5V.
As I began to turn on some DC equipment, the voltage slowly dropped, reaching around 25.8/26.0 with all the essential stuff running, and an absorption in the 20 to 25A region.
Of course I don't have the faintest idea of how smooth (or not) the waveform is, but all onboard equipment seem to work just fine.

The only perceivable problem - which is not so annoying in practice, but I'm mentioning it because I would be interested to hear the panel views on potential risks, if any - is that upon start of some equipment requiring a relatively high ampage (like the fresh water pump, the passerelle, the bigger fridge), it looks like the charger isn't fast enough in increasing the current output, so the voltage suddenly drops, even to less than 20V.
It's just a matter of a second or two though, because as soon as the charger senses the higher current demand and increases its amps output, the voltage immediately returns to 25+.

Of course, I didn't try to stress the charger above its 50A capacity.
Which is possible in theory, by running all onboard pumps (fresh water and WCs), winches, windlass, instruments, etc.
But that's not the point of course, and I don't mind being a bit careful and using the more demanding stuff only one at a time.

Bottom line, unless anyone can think of some potential problem which I'm missing, I wouldn't even try to leave one pair of batteries attached, and I will live with the "batteryless" solution for the few last weeks of this season...

 

[MapisM]

Forgot to mention:
I've got quite a few cables connected to the battery bank, therefore I thought that rather than disconnecting everything, it was easier to just detach the short cables used for coupling each pair of 12V batteries, to get a 24V bank (see pic below).
I assume that in this way, even if each battery still has one terminal (either positive or negative, but just one anyway) attached, the current flow shouldn't affect/overheat them in any manner, correct?
Again, thanks in advance for advising if I'm missing something!

 

[elton]

This thread got me thinking about my own batteries. I normally connect a Halfords "intelligent" charger to the house battery when I'm on the pontoon and power everything off that battery. After 4 or 5 years, it's now clapped out, whereas the engine start battery, which is considerably older, is still healthy.

I now plan to fit a relay (£5), to switch automatically from the battery, to a mains powered 12V supply, when a mains power is present.

 

[Monique]

Your batteries are getting hot under charge because they are shot, probably suffering from sulphation, plates shorting out across the bases from deposited lead sulphate. A healthy battery won't take on too much current from a reasonable charger.

Disconnect them from the charger. Why, because if they get hot enough they may distort and break a case and then leak acid into the boat or even worse catch fire.

Put in a couple of cheap, £60 quid batteries, to smooth the output of the charger, allow you to run the engines and alternators and absorb voltage spikes, until you are ready to change the bank completely.

IMO, you are wasting your dosh by doing this.

Bite the bullet and do it right... ONCE

 

[newWave]

You might have done this already, but put some isolation tape on the cable ends that you removed, just to prevent shorts when the cables jump around while under way.

 

[rich]

to prevent shorts when the cables jump around while under way.

take the other end off as well.

 

[jfm]

All sounds good. You only have to undo one battery terminal mapism; disconnecting the second terminal as well is meaningless. Therefore the two disconnections you have made in that picture serve perfectly well to disconnect the whole battery bank. Sure tape them up if needed but your boat is stable and it's only 12 v so I'm not expecting an explosion for that reason!

If the output voltage from charger looks like 25v but is in fact a waveform you have no problem for passérelle, anchor winch and incandescent lights. Your nav electronics might object. So you could connect the two cold batteries, or temp hotwire the feed to the dash electronic to the engine start batts. Maybe just the radar because chart plotting can be done by iPad or mk1 eyeball

 

[Deleted User YDKXO]

Stupid question, Mapism but why don't you just bite the bullet and replace the batteries now rather than over winter? You're going to have to spend the money anyway and I doubt that batteries are going to get any cheaper! And why potentially risk your 12V circuits by doing this just to delay the inevitable? Btw and as you might remember from a few months ago, I had a case of overheating batteries accompanied by a strong acid smell throughout the boat. It was a bit scary at the time and I'm glad that I replaced all the batteries immediately (especially as I haven't got the bill from SL yet!)

 

[Pinnacle]

P

Love the pipes in the top right hand corner of the pic!

 

[jfm]

The "pipes" are the battery cables I think Pinnacle, in nice protective conduit

 

[Pinnacle]

The "pipes" are the battery cables I think Pinnacle, in nice protective conduit

You think????

I can see one +ve and one -ve cable using them.....but why so many? Isnt there a single master switch for the house batts?

This is a pretty pointless post anyway, since I am sure MapisM will be along shortly to clarify the position ( and confirm you are correct! ).