Caravan PSU / Battery Charger issue

[Another Dave]

I posted about this last year and have only just been able to remove it to investigate further.

My caravan has a built in PSU/Charger for the 12v 100aH leisure battery. Whilst on the mains supply
it should keep the battery topped up but i was experiencing the battery voltage fluctuating between
11 and 14 volts.

This has happened to several batteries so i know its the charger and not the batteries, in my previous
someone mentioned a " ferroresonant transformer charger ". Never heard of this and had to look it up.

I was expecting there to be something in it other than this....

[Another Dave]

The red/yellow wires are both 12v positive outputs, the earth is through the case and it uses
a little bracket to give multiple earths on the connector block.  Forward thinking from the designers
i guess.  Turn the earth to a + by snipping the earthing pin.

In use the battery voltage was dropping well below 12v even with a known good battery and very
little current draw, lighting is mostly LED's and the only other item powered is a small water
pump used for a few seconds at a time. Everything else is mains powered.

I have tried it briefly here for a few hours and the voltage varied between 14.5 and 15v open circuit. It
did drop and climb over a few hours. Putting it under load may aggravate that?

Quick look at the parts because im not touching that cap after having it powered,  it looks like it may hurt.    
One bridge rectifier and one 33000uF capacitor.

There is something attached to the side of the transformer, i initially thought a temperature sensor but
after reading the old thread it maybe a capacitor? 

(Seekonk..    they usually have about a 4-10uF 400V capacitor on the resonant winding )

Possibly that?



My numpty know nothing status still applies so please be gentle 

Thanks.

[richard.cs]

I don't think it's a ferroresonant design, it's just a crude charger with no voltage regulation at all. Still,  if it delivers 15V under no load I'd expect it to charge a battery OK (with risk of overcharging if used long term). It can only be that either you're drawing more current than you think or there's a dodgy connection inside that means the voltage collapses with even small loads. The thing on the side of the transformer will be a thermal switch that disconnects the power when the transformer gets too hot due to overload (as will probably happen if you connect a dead flat battery).

Measure the output voltage with a couple of 12V car bulbs connected (ideally try a few different loads) and let us know what it is. It's surely possible to get it working but any case it's a bit of a crap design.

[SeanB]

Very likely the bridge rectifier is open circuit or high resistance on at least 1 diode, so that would make it a half wave rectifier. Does anything get hot when under no load ( the toriod should stay cool, if not it is toast) and when you connect the battery. Check the wires to the scew terminals, I often have seen that they either beeak there inside the plastic or the small holding screw is loose or stripped, or the thin brass tube cracks.

[tautech]

In use the battery voltage was dropping well below 12v even with a known good battery and very
little current draw, lighting is mostly LED's and the only other item powered is a small water
pump used for a few seconds at a time. Everything else is mains powered.

That's not right and as Sean points out a faulty bridge is the most likely suspect.
From what I see in your pic this is the schematic:



Simple as it is there are 5 things to check:

Fuses
Transformer primary side resistance (not OC)
Transformer secondary side resistance (not OC)
Transformer secondary no load voltage
Diode bridge (DMM diode test)

Working correctly and connected to a charged battery I'd want to see ~ 14.5V max @ the battery.

[GEuser]

Personally speaking , now that you have that out either chuck it in the bin or use it for something else but anything not to do with battery's .

If battery's like the 100ah you mentioned are cheap over there then ignore the above , as fairly inexpensive Float chargers or trickle/float chargers are around and at a decent amperage out amount i'd go for one of those , look up Float chargers ..imo

[Martini]

Working correctly and connected to a charged battery I'd want to see ~ 14.5V max @ the battery.

If it's meant to be constantly on, you end a up the trickle charging zone and I believe it should be under 14 V (I'd say 13.8 V but it might varies with ambient temperature).


If I were the OP, I'd probably consider changing the charger for a smarter one to preserve battery life. If current consumption is low, a charger will be considerably cheaper than one 100 Ah battery, so...

[GEuser]

The old Farmer next door (5K's out rural here) had a charger on his kitchen table just the other week , he had one end pulled off trying to see why it would not work .

Anyway i brought it back here and pulled it to bits , all that happened was a bug had decided to walk across the 350Vdc tracks (a switch-mode) and got vaporized and blew the solder on pcb fuse .

Anyway again , i was really surprised with that charger , i forget the brand but a 30amp out and did everything! , it checked the battery whether gel/calcium/acid and another i forget , did a load test , de-sulfates and then reconditions (all if needs to happen) automatically , then sits on Float after it does its thing , actually i wanted it but he is a good neighbor and i was not prepared to hit him with a cricket bat  , i think it might have been a Repco brand (a auto store) but i'm not sure ... it was really wizz bang .

edited>duh  i forgot to mention the reason i was posting , 15 or 16v to de-sulfate for around 5 minutes , a test load of 6 or so amp , the recondition cycle never started but i think it loaded the battery down to a predefined voltage then started all over again , Float was a pulsing voltage hanging around 13.8v +.5v .

[madires]

I'd also recycle the old charger for something else and go for a proper charger, i.e. a charger with a controller monitoring the voltage and switching to float/maintenance mode. A sealed switch-mode based smart charger with 5A output costs about 50 bucks. And add a switch between charger and battery to prevent the charger draining the battery when not connected to mains, in case it's necessary.

[Martini]

Forgot to say but my inverter does all that and takes care of the switching between mains and battery power.

[Another Dave]

Thanks guys, it should be putting out a stable 13.8 volts to prevent the battery gassing off too much and
boiling the battery dry.

Its a 6Amp continuous charger with an 8A max output.   Drawing way less than the 6amps continuously
and possibly only exceeding that very briefly when the waterpump activates which is usually only
a few seconds whilst filling a kettle or the small hand basin.

Not tested the charger attached to a battery, i will do that next and post back my findings.  Not a fuse
both the mains and 12v fuses are fine.  Open circuit load = 15v which is rather high i think.

Bridge rectifier will be cheap to replace, but the 33,000uF capacitor seems to be quite expensive
and i prefer the idea of replacing the unit with a better one rather than waste £15.

If i can repair this cheaply then i would like to though.  I will see if i can find more info on the charger
but i think its just a basic one that should give 13.8v. I know it can be used as a PSU with no
battery connected also.

Anyone from the UK bought a charger recently that may suit my needs?

Thanks..

[Martini]

Supplying 13.8V to a discharged battery is too much anyway to maximize its life expectancy (too high a current).
I reckon you're better off with a smart modern charger. You could even use a relay to bypass the inverter when plugged to the main, thus you could draw as much power as you'd like rather than being limited by the charging rate.

[Another Dave]

The battery should never be discharged though. Its always kept at home and topped up monthly,
occasionally i use it to power something to drop the voltage a little bit and then leave it on
charge a small CTEK charger 3.6amp max? Thats designed to be left on 24/7.

In the caravan the battery is only there to stabilise the voltage and a backup if the main
power fails. Previously the battery was used to power the electric motors to move the
caravan, so a small discharge before actually using it.

But we swapped storage yards and can now reverse upto the caravan.

It starts with a fully charged battery and during the day we are mostly out so no 12v draw at all
and just the lights and waterpump later on and in the morning.  Nowhere near enough power
to drain the battery. It does take a day or so for the voltage to drop.

I had the same battery attached to an inverter and running a 100w bulb for over 4 hours and
the voltage was still at 12.2v.  So the battery is good and we do not draw that amount
of power in a 24hr period.

I think the charger maybe actually draining the battery also.

I have several chargers that can be used in its place, either wired in 24/7 or used on a plug as
and when needed.

Going to wire it upto a battery now and see if it draws power.

Thanks. 

PS.. even if i dont re-use it i would like to know what to look for and have a play with it. Eager to learn

[tautech]

Thanks guys, it should be putting out a stable 13.8 volts to prevent the battery gassing off too much and
boiling the battery dry.

You're a bit light there^^^ old style auto regulators were set in the 13.8-14.2V range.
Anything ~15+ V does gass lead acid batteries, however this was sometimes used as a maintenance charge to combat sulfation.

But this overlooks new LA auto battery technologies, some of which can be floated as high as 14.5V.
Deep cycle storage is the type best used for caravans.

Anyone from the UK bought a charger recently that may suit my needs?

The battery should never be discharged though. Its always kept at home and topped up monthly,
occasionally i use it to power something to drop the voltage a little bit and then leave it on
charge a small CTEK charger 3.6amp max? Thats designed to be left on 24/7.

A bigger one of those^^^^^ 

[Another Dave]

Connected upto my 100Ah leisure battery, Ive had a small sub 2amp load on it for a couple of hours.
Voltage at 12.64.  Connected the charger up and it jumped to 12.9 volts at 0.7amps charge.

After a few minutes the voltage increased to 13.2 and the charge rate dropped to 0.5amps.

I will give it a few hours and see what happens to the voltage.  Currently no load on the battery which
is how it would be during the daytime whilst we are out and about. No draw from teh battery.

No diode between the 33,000 uF capacitor and the battery so the battery charges the capacitor.

[Martini]

The inverter is off during the day as well?

[Another Dave]

No inverter in use in the caravan, i only used that at home to test the batteries and they held up
well over 4 hours with a 100Watt bulb on an inverter which i think drew closer to 140watts from the
battery.  I found the 600watt inverter used more power than the 300watt one for the same load.

So no inverter.  In the caravan its just the battery with the charger from a mains lead and LED lighting
and the waterpump.  Nothing else draws any load.


I connected the battery last night and the results..

22:00   13.5v    0.35A
23:00   13.7v    0.36A
00:00   13.8v    0.36A
02:30   14.0v    0.34A
13:00   14.2v    0.35A    All good it seems...

Until i add some load.  I put a 12v LED wand that draws approx 0.75A, Voltage dropped down to
13.7v and the chargers output went upto 0.6A.  Voltage dropped to 13.6v so i added more load.

I added the 600w inverter with a 10w small radio/cassette player. This showed a 0.75A draw from the
inverter cable.  Voltage dropped to 13.4.
I have now added 2 x 15w bulbs onto the inverter also and the voltage dropped to 12.7v.

Yet the chargers amperage has not yet gone above 0.6A.

Currently still on 12.7v and pulling approx 4amps but the charger is at 0.6A still. Because the battery
is good i expect it to take a while to drop the voltage further. I did expect the amperage to increase by
now. Maybe the voltage is not low enough yet?

12.7volts is not right though is it?  Dumb charger should be putting out 13.8v (ish) until it maxes out
at its 6amp output?

No seeing the voltage spikes that i was getting last time we used it, maybe the 0.6 amp is all its going to
output?

[tautech]

Yeah, it doesn't look right. 

Which charger are we looking at ATM ? 

Can you trust your DMM ?

[jitter]

I don't really trust that charge curve.
Normally, a lead acid battery needs a constant voltage charge and the current drawn mainly depends on the state of charge of the battery.
A really flat battery will try to charge at a higher current than the charger can deliver, so voltage will be lower than the nominal charge voltage until the current tapers off.

I have more experience with SLA and SLI batteries than deep cycle batteries, but maybe you need to discharge that battery somewhat deeper and see if the charge current increases. If it charges all the time at ~350 mA, even with a flat battery and a charger rated at 6 A, I think it's pretty sure it's not working like it should.

[Delta]

That is NOT a charger, it's an UNREGULATED power supply!  The voltage will wander all over the shop depending on load.

My guess would be that the big electrolytic cap has gone bad and has rather high leakage, draining your battery when connected.

Connect a battery to the "charger" without the mains connected and measure any current drawn from the battery.

You should be able to remove the cap entirely and that thing will still charge a battery.

[tautech]

You should be able to remove the cap entirely and that thing will still charge a battery.

Quite true 
That's how old dumb chargers were made, NO smoothing cap.

[jitter]

That is NOT a charger, it's an UNREGULATED power supply!  The voltage will wander all over the shop depending on load.

You would be inclined to think so, but actually, no, it is mainly determined by state of charge of the connected battery, it is by far the strongest of the two (read: lowest internal impedance). And since the output is a mere 5% of the input a few V mains wandering doesn't really do that much on the secondary side.
And yes, a flat battery will pull the voltage of the charger way down at first, but that happens to regulated chargers a well.

My guess would be that the big electrolytic cap has gone bad and has rather high leakage, draining your battery when connected.

Connect a battery to the "charger" without the mains connected and measure any current drawn from the battery.

Good tip.

You should be able to remove the cap entirely and that thing will still charge a battery.

Indeed. I have a 12 V, 6 A charger from Absaar which doesn't even have a cap on the output, and car batteries don't mind this at all. I know it wouldn't be too wise on SLAs which seem a bit more sensitive to ripple. Don't know for sure about deep cycle batteries, though.

I think I would prefer a more modern uP controlled charger that can keep the battery charged without risk of overcharging.

[Another Dave]

OK whats the punishment for making a STUPID!!! mistake?   

Test meter is one of those cheapo yellow things, running across the 10Amp unfused side. I expected it
to go bad so didnt risk my better meter which you may think is crap also 

I used some test leads with small croc clips on them, and when connecting 2 other old style chargers the
amperage never rose above 0.75A.
Silly me for using CRAP!!!  leads....

I even ran the battery down to 11v and the inverter started to beep with no increase in current from the
chargers. Connected with the proper leads and 4amps + was seen. Guessing the leads for that are also
not upto the job.

I knew i should have used my 60A wattmeter but too lazy to go and find the correct terminals i fitted to it.

Battery on charge again, connected directly and start again tomorrow or friday.

Whats my punishment???   



(Sheepish)   Thanks.

[tautech]

A good whipping with those shitty DMM leads. 

Most of us have been caught out like that. 

[jitter]

OK whats the punishment for making a STUPID!!! mistake?   

Test meter is one of those cheapo yellow things, running across the 10Amp unfused side. I expected it
to go bad so didnt risk my better meter which you may think is crap also 

I used some test leads with small croc clips on them, and when connecting 2 other old style chargers the
amperage never rose above 0.75A.
Silly me for using CRAP!!!  leads....

I even ran the battery down to 11v and the inverter started to beep with no increase in current from the
chargers. Connected with the proper leads and 4amps + was seen. Guessing the leads for that are also
not upto the job.

I knew i should have used my 60A wattmeter but too lazy to go and find the correct terminals i fitted to it.

Battery on charge again, connected directly and start again tomorrow or friday.

Whats my punishment???   



(Sheepish)   Thanks.

Well, seems like you learned this lesson the hard way:



At least you had the sense to try a different meter before condemning a possibly correctly working charger.

When you know a thing or two more tomorrow, I'm interested to know what the voltage is that this charger tops out on when the battery is full. Also if it's sized properly to maintain a trickle charge, or if this is one of those things - like my Absaar - that you just don't leave connected indefinitely.

Does your battery come with charging data (possibly printed on the side)?