Total Beginner, Diy leisure battery charger current/voltage limited.

[Another Dave]

Hi there, I am Dave from the middle of the UK, wrong side of 40 

I stumbled across the EEVblog channel on youtube and it has got me tinkering again, i tried when
I was younger but had a short attention span so gave up quickly.
The radio shack 75 in 1 kit was mostly used to created smoke and pops. 

I fixed a couple of LCD TV's after i watched the Samsung repairs, I was chuffed to have
repaired them even though one had a broken screen. 

Our freebie non working TV is a 42" that cost £1.20 to repair thanks to Youtube.

Anyway, back to my reason for posting.

I have a caravan where the lights and water pump run from a 12v  100Ah leisure battery, The charger
is starting to act up with voltages varying between 11.9 and 14.5 volts. Not load dependant it just
fluctuates even when nothing extra is switched off or on.

I have a buck and boost converter with adjustable current and voltage settings and thought that
would be a good idea for the battery charger. Because its left on charge for maybe a couple of weeks
at a time i would rather have a slightly lower voltage than risk boiling the water away in the battery.

Several cheap sub £5 modules available on ebay but they are usually upto 3amp or 5amp with
additional cooling. I would prefer something closer to 10A with a stable voltage when pulling
fairly heavy loads for short periods also.

I want to run the parts well below their design limits, so in the case of the 3/5amp boards then
3 must be the maximum.

Any idea's or suggestions? 

Many Thanks
Dave.

[KL27x]

with voltages varying between 11.9 and 14.5 volts.

This sounds very familiar. It's possible that the charger just needs a tweak. Check the regulator heatsink/fan. Give it a thumb test while operating, to check the temp. It could have a bad thermal connection and need nothing more than to be desoldered, cleaned off, and re-installed with fresh thermal paste, if necessary. Check to make sure the fan is working. And/or add some additional cooling if you can.

The varying voltage (but not over 14.5V), I take as a clue that the chips internal fail-safe is having to kick in when it overheats. I have experienced same, fixed with a simple addition of a fan over the regulator.

That said, the battery, itself may be getting old, which can lead to internal discharge, which increases the demand on the charger, so check that out, too. The battery could have precipitated the failure. If you can increase the output of the charger, you might be fine for another several years, or it might be more expdient to replace the battery, too.
 

[Another Dave]

The battery is a new one taken from a car as i wondered if the leisure battery was to blame.

I had this battery running with a 100w bulb and a 600watt inverter for over 4 hours and it still held
12.4 volts.  I dont think the charger has a cooling fan. Its under a seat and never heard it but i will
have to pull it out and check it either way.

Not sure if its supposed to get as high as 14v, Always thought it topped out at 13.8 to prevent the
battery gassing and boiling its water away?

I have a CTEK 3600 charger and thought about using that, but unsure if it will overload if i draw
more current than it can put out. I did that once with a car charger.

Switched the electrics on and it blew the fuses in the charger.

Thanks.

[Seekonk]

That sounds like a ferroresonant transformer charger.  If high voltage cap opens up the voltage would drop drop to about that value.

[Another Dave]

OK, never heard of that term before, High voltage cap, So possibly just another capacitor like
the TV's?

Have to see if i can get up there at the weekend and take it out.  What else can go wrong with
that type of charger? 

Is my ideal of a current and voltage limiting charger going to cost more than buying another
charger that will probably work just as well?

Most of the cheap ready built units on ebay seem to be fairly low current devices. Can they be used
in parallel?

Thanks again.

[KL27x]

Not sure if its supposed to get as high as 14v, Always thought it topped out at 13.8 to prevent the
battery gassing and boiling its water away?

In my experience, it is common for the battery chargers in vehicles to regulate to 14.0-14.7V.

13.8V is the ideal voltage for a mains plug-in battery charger/tender, where you might leave the battery indefinitely.

Hence, the fluctuation between 11.X and 14.5 could easily just be the charger cutting in/out, with the lower value being the current float value of the battery and the higher value being the regulated output.

[Seekonk]

Ferroresonant or constant voltage transformers are used in some battery chargers, usually older or forklift type chargers.  they usually have about a 4-10uF 400V capacitor on the resonant winding.  Read the following for more detail   http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/voltage-regulation/

[Another Dave]

KL27x, Its a caravan that you tow behind your car, just a posh tent really. The 12v runs the lights and waterpump only.

The charger will be left on 24/7 when using the caravan, I can see what you mean about the voltage dropping when the charger sees a full battery and cuts off. But im measuring the voltage at the fuse panel for the lights.

The voltage there should not drop to below 12v with a good battery and very little power draw, should it?

I think the charger is actually draining the battery. Sometimes the lights dimmed and the voltage dropped to 12v when i only had 1 LED light on which was pulling less than 1 amp.

Didnt get a chance of going up to the storage and taking it out.

Thanks.

[KL27x]

I can see what you mean about the voltage dropping when the charger sees a full battery and cuts off.

No, it is probably never supposed to cut out. Just run at 14.5V. I think it could be overheating and cutting in/out due to internal protection.

The voltage there should not drop to below 12v with a good battery and very little power draw, should it?

Float voltage of a lead acid battery != load voltage. A 12V lead acid battery has a charge voltage of 13.8V, a float voltage of 12V, and a load voltage of 10.5V.

Charge a 12V battery to 13.8V, then disconnect it. The float votage will drop to 12.5V in a matter of minutes, doing nothing. And it will eventually settle at around 12V for a good while, until self discharge brings it lower.

Charge a 12V battery to 13.8V, then draw a significant load. The voltage will drop to AT LEAST something under 12V, immediately. Under a great enough load, voltage will drop to ~10.5V on a fully charged battery.

Unless your lights are all LED or flourescent, it is seeing a significant load. Jumping between 11.X and 14.X V is more or less what you would see if the regulator were cutting in/out on a fairly charged battery under load. Might jump down to 10V or so, if the battery is low or the load is high. This is what you'd expect, else I wouldn't have suggested it.

Voltage jumping between 11.9V and 14.5V, specifically, is more or less exactly what you would see with a good, fully charged battery, under a partularly light load, when the regulator cuts in/out. Which, oddly, enough, is exactly what you described as your situation.

I had similar symptoms with a vehicle. The battery was randomly dying. And so I measured the voltage like you did, and I found essentially the same results that you did. And then I hooked up a panel meter to the power rail and monitored it while operating said vehicle. And I found out the voltage would randomly switch between 14.0-14.7V and 10.0-11.5V, or so, the range of higher voltages dependent on current rpm. The range of lower voltages dependent on state of charge on the battery. Actually, this was not quite random. Usually, the regulator would cut out after the engine was hot and the vehicle was stopped and restarted. So drive to work and back, and you'd be fine. Stop for gas, and the vehicle would run off the battery the rest of the trip. If you made it home, the vehicle wouldn't start the next day. I googled and found out that said vehicle was known to have regulator failures, particularly in warm weather. I ordered a replacement regulator. In the interim, I placed a fan over the regulator that would run whenever the ignition was on. The panel meter showed 14.xV all the time, thereafter. I kept said vehicle for another 6 years, and it started every time. I never did change out the regulator. Still have it in my parts box.

[Another Dave]

Lighting was originally fluorescent tubes, but replaced a little while back with LEDs with the thought that we
maybe using more campsites without electric hookups to recharge the battery.

The lighting consists of a pair of 4.5 watt units and 4 x 7 watt units. Mostly one or two on at a time,
and only for say 4 hours at night.

The only other item that uses power is the 12v waterpump, That draws upto 5 amps but only used for short
burts, maybe 5 or 6 seconds to fill a kettle, and still under 30 seconds to fill a sink when washing.

With the minimal amount of power being drawn my battery should not drop below 12.5 with the mains
charger on all the time should it?

I tested the battery before we went away and it stayed above 12.2 (12.4)? volts after 4 hours with a 100w
tungsten bulb running off a 600w inverter which is not very efficient. Had no issues with holding
the voltage with that load. so a few LEDs should not make much of a dent should it?

I fully charge the battery before we go away and at first its fine, but then the red LED warning of a low
battery starts to flicker after a couple of days, initially when switching the waterpump on and then
after that switching the old fluorescent lights on.

A good battery should not be dropping below 12v in my situation should it?

Thanks for putting up with me

[Seekonk]

A good battery should not drop below 12.6V under lightly loaded conditions.  12.2 is near dead and should never be allowed. Even a light chatrg of 1A should quickly get it above 13.5V.  It could be  a shorted cell in the battery.  Hard to tell, your details are rather sketchy to make any decision with.

[Another Dave]

Its not the battery, I tested it just before we fitted it and it held its charge very well, I do
have some figures somewhere with the amperage of the 100w bulb running on a
12v - 240v inverter. But it was running for several hours and did not drop below 12.2
(possibly 12.4). Voltage tested with the load still applied so not a recovered voltage.

The problem only appears a few days after using the electrics. The battery is not recharging properly.

It happened to the previous battery also, I assumed the other battery was weak as it was approx
6 years old.

It was always my thoughts that the charger would keep the leisure battery between 13.5 and 13.8
at all times except when heavy loads are applied above the chargers rated output?

The only times we would be close to or possibly exceed the chargers output is when all the lights were
on and we operated the waterpump, but thats usually under 1 minute per hour.

During the day we were out so the charger had plenty of time to top the battery up.

I cannot see it being anything else. I really need to get the charger out.

i could plug my wattmeter in and see how much power all the lights and pump draws, but its
in storage now so i cannot power the charger.

Is there any other info i can give?

Thanks again.