Lead acid battery charging with wall wart?

[cobra111]

have a device that comes factory with a sealed AGM battery and a wall wart for a charger. My dad gave me the device because it was not charging. I determined the transformer for the charger was toast so I just grabbed another. Factory spec was 800ma. I only had a 500ma.

The charger was the same 15VDC output and I verified that. However, there is a small series device on the chord that I determined was more than just an LED, it seemed to be a current limiter or additional voltage reg that brought the voltage in at a nice trickle of 13V so I just left it plugged in for later checking. Came back to another toasted charger. Transformer was hot and no output.

I checked the current to the battery at only 200ma which I found odd considering the battery was nearly dead and the charger prob should have been taxed but I left it.

Curious what is going on here? Do these little wall warts current limit at all? The switchers maybe? I have one and plugged it into the battery and was charging at 1.5A and rated at 1.0A so I unplugged until I know more. I suspect that little LED device might have toasted the charger transformer?

 

[Ramussons]

have a device that comes factory with a sealed AGM battery and a wall wart for a charger. My dad gave me the device because it was not charging. I determined the transformer for the charger was toast so I just grabbed another. Factory spec was 800ma. I only had a 500ma.

The charger was the same 15VDC output and I verified that. However, there is a small series device on the chord that I determined was more than just an LED, it seemed to be a current limiter or additional voltage reg that brought the voltage in at a nice trickle of 13V so I just left it plugged in for later checking. Came back to another toasted charger. Transformer was hot and no output.

I checked the current to the battery at only 200ma which I found odd considering the battery was nearly dead and the charger prob should have been taxed but I left it.

Curious what is going on here? Do these little wall warts current limit at all? The switchers maybe? I have one and plugged it into the battery and was charging at 1.5A and rated at 1.0A so I unplugged until I know more. I suspect that little LED device might have toasted the charger transformer?

I determined the transformer for the charger was toast so I just grabbed another. Did you open the faulty charger? Was it a Switch Mode Charger? If yes, it normally would have had a current limiter.

The "other one" that you grabbed - was it also Switched Mode Charger? Or was it a simple transformer rectifier type? A simple transformer type has all the chance of burning out if overloaded. The discharged battery might have drawn more current than what the charger could handle.

 

[MikeMl]

Charging smallish 12V SLA requires that a charger meet two independent requirements:

1. to prevent damage to the battery if the charger is left on indefinitely, the charger output voltage must be limited to ~13.8V. In fancy three-state, microprocessor-controlled chargers, the charging voltage may go as high as 14.7V for several hours, but it will revert back to ~13.8V when the charger finally switches to "float" or "maintain" mode.

2. the initial charging current (when charging a mostly discharged battery) must be limited to a value less < = specified by the battery manufacturer (might be 1A for a 4AH battery, look it up). In inexpensive transformer-based chargers, the initial current limit is provided by core saturation in the transformer itself. In switchers, it is provided electronically.

If your new transformer-based wall-wart output current is different than the old one, it could be due to differences in the actual transformer used, how rectified (full vs half wave), internal filter capacitor or not, etc. Likely has nothing to do with the voltage limiter part.

To test any SLA charging system, just monitor charging current into the battery several hours after the battery voltage first reaches 13.8V. If after 24 to 48 hours, the battery is not sitting at ~13.8V with only a small leakage current (few mA) flowing into the battery, either the charger is junk or the battery is junk.

 

[Mosaic]

What was the battery's standing voltage before attempting a charge and how long did the battery sit without charge?
While AGM batts are starved electrolyte units and resist acid stratification related sulphation, they can still sulphate if left sitting in a discharged condition for weeks to months.
If this is so, then a taper charger won't be able to do much in recovering the battery. The 200mA current is indicative of sulphation induced, elevated internal resistance. A high 'charging' voltage > 14V with low current flow is almost a sure sign of sulphation or severe plate corrosion. The latter is unlikely in a young AGM battery.
You will need a smart charger with thermal sensors to recover the battery.

 

[Tony Stewart]

The quickest way to burn out a small transformer is have it charging a dead battery, which effectively shorts out the transformer bridge diodes and coils.

Next way is assume it has a current limiter only to find it is some delayed soft start circuit.

They also rely on convection cooling.

Was any of this relevant?