Transformer Current

it's my guess your charger has a current limiter that has gone bad or out of adjustment.

Can you get at the details of the charging circuit?

You could always put a 50 watt incandescent bulb in series with the primary until your issues are solved. actually, I've seen many basic charging systems do just that as a current limiter and indication of charge! :)

Or it might mean they don't use bullet proof design, and allow the wall transformer to be overloaded during the initial charge. After all, they had a 5 amp fuse in there, which is over 3 times the 1.6 A rating you mentioned. If you intend to run 2 chargers from the transformer, buy a bigger one. To get one working, replace your 2 amp fuse with a 5 amp fuse. Monitor it and see if the current draw drops, and check how long it takes. The farther a battery is discharged, the greater the current it draws from the charger. As the battery charges, it draws less current. Monitoring it and watching the current drop over time may give you a sense of how much is being demanded of your 3 amp xformer. At least you know that the new transformer is rated at almost double the old transformer's rating.

Ed

On Thu, 5 Feb 2009 08:50:59 -0800 (PST), snipped-for-privacy@xemaps.com wrote:

:Nutshell: : :I have a lead acid 12V battery charging device that used a wall :transformer rated as 120 VAC 26W input, 12 VAC 1.6A out. The xformer :failed (primary opened) and I made a replacement (12.6 VAC 3A out) but :it is drawing 4.5A and blowing the secondary fuses I put in place. : :Synopsis: : :I have two "Basement Watchdog" battery operated sump pumps. The :batteries are 6 cell 12V lead acid batteries. The control panel on :each unit charges the batteries as well as giving status and alarming. :The controller accepts a dry contact (float) to trigger a pump cycle. :It has some led's, a piezo and a button to silence alarms/run pump. :The main load runs from the battery all the time. : :One of the devices gave me an AC failed indication. I checked the :transformer and was getting nothing from it. I removed it and went on :to other things. Several days passed and w/ rain in the forecast I :decided to top the charge of on the battery by moving the wall :transformer from the (still) good unit to the one that had failed. Not :too bright on my part because poof went the other transformer. Turns :out there is a problem in the one unit that caused the transformer to :fail. : :I pulled the failed control unit out and peeked inside and found a :shorted 6A bridge rectifier diode. I went to RS to get a replacement :and noticed they had a 12V 3A xformer so I decided to build a :replacement, one that would run both units. I made a simple box, wired :up the xformer (full voltage, not using the center tap), paralleled :one secondary leg to two fuses (one for each control unit). I :connected the wires to the control unit to one of the fuses and the :other secondary leg. Secondary was giving 14.6 VAC, no load. : :Power up and connect to the pump control unit and all seems ok but :then after about 20 seconds the fuse goes. Investigation shows that :the control unit is drawing about 4.5 A. Remember, this unit is the :one that had zero problems at all, the repaired one is still on the :bench. : :Can someone offer any ideas as to why the charger is drawing far more :than the original transformer was rated? I cut one open and could not :find any identifying marks on the transformer. The primary had failed :and the secondary was fused at 5A. The fuse was intact. Input was :labeled as 120 VAC 26W, looks like 0.2A roughly. Stepped down to 12V :that would be 2A. : :I am not certain why the high draw - maybe the initial charging cycle :draws much more than 2A, for some period of time, and then bleeds :down? That would mean the original transformer was rated for much :higher than what the wall transformer was labeled. It was fused at 5A :on the secondary. : :Thanks.

Those "wall transformers" (we call 'em plug-packs) will invariably have a thermal fuse built into the primary winding. If a prolonged overload occurs, which may not be sufficient to cause an external fuse to rupture, the thermal fuse blows in order to prevent a fire or other hazardhous situation. Unfortunately, this also means the transformer is now defunct and unrepairable.

It appears to me that whatever else is connected to the output of the transformer then it must have a low impedance or short circuit in it. When you replaced the 6A diode did you investigate to see what caused that to blow? If you simply replaced the rectifier without checking for other failed components then you may now have another dead rectifier as well.

it's either the charger or the battery. swap both until you locate the culprit.

more than a few minutes would be troubling.

Its possible the original charging circuit is so *dumb* it relies on the precise output voltage and internal resistance of the transformer to limit the charging current and also to taper the charging off at the float voltage. A transformer, measured at 10.7 V(rms) AC output off load (15.1 V peak), connected through a simple bridge rectifier will naturally float a 12V battery at about 14 V and if wound with thin enough wire, could be designed to limit the current into a battery discharged to 10 V to 1.5 A.

Check the original circuit to see if there is any active charge control. If there are no three terminal components between the brige +ve and the battery +ve they've designed it *DUMB*. Enter the wonderful world of Muntzed battery chargers.

If so there are a couple of approaches to fixing this, but it would be nice to confirm the problem first.

Its also possible a cell has failed in the battery in the good unit. If you have an adjustable current bench supply that can run on current limit indefinately, set it to 13.8 V out and to limit at less than C/20 A (where C is the capacity of the battery in AH) and charge on the bench till the voltage has been at 13.8 V for 4 hours or for a maximum of 24 hours (or use a dedicated SLA charger if you have one). Let the battery rest disconnected for 24 hours. If it measures about 12.5 V its OK. If its 2 or more volts lower, its dead. Also check it can run a C/20 load for 8 hours. Recharge fully immediately after testing. If the battery out of the good unit has failed, its probable the bad unit lost two cells which killed its charger. If you have more experience with SLA batteries, there are shorter testing procedures, but they can damage the battery or give misleading results so I'm not giving them here.

I don't know if this is the same sort of unit that my friend had in her basement, but it sounds similar. In her case, the battery would not stay charged and it failed the self-test as well as the routine that was supposed to cycle the battery and fix problems, so we got a new battery.

But I tried to test the system by pouring water into the sump where the pump was located, and it seemed like it would only work when the water level got very high, and then it only pumped out a small amount before shutting off. The level sensor seemed to be very poorly designed.

She also had a regular 120 VAC sump pump, which had been disconnected, but I saw from the nameplate that it was fairly new. I plugged it in, and it very quickly drained nearly all the water out of the sump, as would be expected with a 1/3 HP motor. The tiny pump in the original design was probably about the same as your 25 watt pump, which is about 1/30 HP, so it would take ten times as long to do the job, and apparently did not do a very thorough job at that. I think she fell victim to a fast talking salesman who sold her a "bill of goods" she really did not need, and he probably installed it poorly.

So, I bought a 1000 watt inverter for about $60, and hooked it up to the battery, which I left on the original charger, which seemed to work OK. I could have rigged up a transfer relay to detect loss of AC power and switch over to inverter power, but that would have taken a large power relay to switch the 12 VDC primary at 50 amps. So I instructed my friend to simply move the plug from the sump pump to the inverter and turn it on if power was lost. She is home most of the time and there is fairly good drainage anyway, so it's not as if it needs to run very often.

You could also use a heavy duty UPS for the regular pump, which I would trust more than the wimpy backup pump. It's a good idea to check the operation of your pump like I did, by simply pouring a 5 gallon bucket of water in the sump and seeing how it works.

Paul

Some quick points about the Battery Watchdog:

• The worst thing you can do is disconnect the battery with power on. A poor battery connect will do the same damage. This damages the regulator portion of the circuit, and it will attempt to charge the battery at a significantly higher voltage, leading to an overcurrent/ blown fuse condition, and/or burning out the battery. If you've done this, bad on you. RTFM.

• It's very possible you may have a bad battery, too. It's a standard marine-type deep-discharge battery, so you should be able to troubleshoot this yourself without the Battery Watchdog. Disconnect (please unplug the power first!), and top off the battery cells without overfilling with distilled water. Put the battery on a known good automotive battery trickle charger for a while. Use a DVM to read the voltage on the battery while it's charging. You should be seeing about 13.8-14.4V across the battery while it's charging. If it's getting a lot higher or lower than that, your battery may be unable to take a charge properly. Then remove the charger, wait a minute, and read the voltage again. You should be seeing somewhere around 12.6V. If it's lower than that, you may have a dead cell. The last test would be connecting an automotive headlight to the battery as a load, and then looking at the battery voltage. For a freshly charged battery, if it drops significantly below 12V, the battery is dead or dying. Replace it.

If you're still having problems, you can try email or calling the Battery Watchdog technical people. They're located in Illinois (Central Standard Time), and were reasonably helpful when I had a problem. You can get their contact information from their website.

Good luck Chris

Thanks to everyone for the advice so far. I have been in contact w/ the BW people and am waiting for clarifications. A comment from a support rep was "1.6 amp is the DC output; 5 amp is the AC input." He also mentioned that the 5A fuse was to protect the system if a bridge rectifier failed (as mine did but the thermal TCO in the transformer opened, not the fuse). The wall transformer had no rectifier in it, it is a pure transformer, although the secondary fuse was on a small PCB with component markings for an obvious bridge rectifier setup. I did pick the transformer apart and found an open thermal device (as Ross mentioned). It was rated at 130C. There were no marking on the internal transformer parts to indicate its actual rating.

I m guessing the natural draw on this device is close to 5A, especially on a battery that has had its charge depleted somewhat. I am thinking of simply dropping in a 10A xformer and fusing each device at 5.

ehsjr mentioned simply dropping in a large fuse and monitoring the current draw over time in an attempt to divulge its behavior. I was thinking this as well but the transformer in place at the moment is rated at 3A. If I am pulling 4.5 do I risk failing the transformer or are they capable of significant higher current draws than indicated?

Possible silly question: Can I take a 24V xformer and use the center tap for each of my 2 12V devices? Will the DC output from the bridge rectifier be ok or will there be serious sags every cycle (the rectifier feeds an LM7805 voltage regulator - I did some checks on the repaired, unloaded, control board using my homemade transformer and got 14.6 VAC (< 0.2 A) out, appx 16 VDC into the 7805 and a nice 5.00 VDC out of it)?

Thanks again

The transformer probably was current limited to the point that the output short circuit current was not enough to open the fuse (at least not very quickly), so it just overheated (which caused its output current to drop even more), and the TCO opened. Sounds like a rather poor design that was not properly tested.

Transformers generally can be overloaded on the basis of duty cycle up to as much as 5x or 10x continuous rating. The duty cycle goes by the inverse square of the overload, so a 1.4x overload is 50%, 2x overload is 25%, 3x overload is 10%, and 10x overload is 1%. The actual on and off times are greater at lower levels, so you might be able to do 15 minutes on and 15 minutes off at 1.4x, while for 3x you might only be able to do 1 minute on and 10 minutes off. It all depends on the temperature rise at the hottest point, and the rating of the insulation.

Probably best to use a FWCT with two diodes and dispense with the bridge rectifiers. You could use a FWBR on each 12 volt winding as long as there are no common connections in the DC output circuits.

Paul

On Mon, 9 Feb 2009 07:45:05 -0800 (PST), snipped-for-privacy@xemaps.com wrote:

:Chris wrote: : :> If you're still having problems, you can try email or calling the :> Battery Watchdog technical people. They're located in Illinois :> (Central Standard Time), and were reasonably helpful when I had a :> problem. You can get their contact information from their website. :>

:>

:> Good luck :> Chris : :Thanks to everyone for the advice so far. I have been in contact w/ :the BW people and am waiting for clarifications. A comment from a :support rep was "1.6 amp is the DC output; 5 amp is the AC input." He :also mentioned that the 5A fuse was to protect the system if a bridge :rectifier failed (as mine did but the thermal TCO in the transformer :opened, not the fuse). The wall transformer had no rectifier in it, it :is a pure transformer, although the secondary fuse was on a small PCB :with component markings for an obvious bridge rectifier setup. I did :pick the transformer apart and found an open thermal device (as Ross :mentioned). It was rated at 130C. There were no marking on the :internal transformer parts to indicate its actual rating. : :I m guessing the natural draw on this device is close to 5A, :especially on a battery that has had its charge depleted somewhat. I :am thinking of simply dropping in a 10A xformer and fusing each device :at 5. : :ehsjr mentioned simply dropping in a large fuse and monitoring the :current draw over time in an attempt to divulge its behavior. I was :thinking this as well but the transformer in place at the moment is :rated at 3A. If I am pulling 4.5 do I risk failing the transformer or :are they capable of significant higher current draws than indicated? : :Possible silly question: Can I take a 24V xformer and use the center :tap for each of my 2 12V devices? Will the DC output from the bridge :rectifier be ok or will there be serious sags every cycle (the :rectifier feeds an LM7805 voltage regulator - I did some checks on the :repaired, unloaded, control board using my homemade transformer and :got 14.6 VAC (< 0.2 A) out, appx 16 VDC into the 7805 and a nice 5.00 :VDC out of it)? : :Thanks again

And thank you for responding to the group - it is much appreciated. Some others who request help and get a string of replies fail to let the group even know that they are following the thread they started. They often don't have the courtesy to make any further comment following their initial request. In such cases it is no wonder that these posters might be accused of trolling.