Hey all! Great forum going here, glad I found it!

I need to charge a 12V battery with improvised means. The battery is lead-acid, sealed in gel, of the valve-regulated type. The official charger is (I think, from this ebay post) a 17V 1700 mA type. I need to charge it every now and then, and even a half-charge would be more than enough.

So my approach: I have an old transformer lying around from my electronics classes in college. I have tried to trickle charge it at 12V 300 mA but even after 12 hours I saw no improvement.


My transformer is rated at 5V 1000mA and 12V 300mA.



But I have found two leads that have a voltage difference of 18V. I dont know how much amperage will come off there...

My questions are:

1-- Is it safe for me to attempt to charge it at 18V? (5 minutes? 30 minutes? an hour?)

2-- How much amperage would I expect to be able to draw when using the leads that measure at 18V?

3-- Should I totally change my approach and just charge it off my car's battery, as if I was jumping another car?

I bought this electric mower for 20$, but it came with no charger. The previous owner had run leads to attach directly to the battery (the bolts coming through the top). See below:




Our yard is quite small so I dont need a super professional approach, just one that works enough. Thanks!!!! all help is appreciated!

Originally Posted by mneary

Your (+/-)12V should have 24V between them, capable of 300mA. To charge your battery, place this 24V in series with a 12V tail light bulb from the car (5 watt, not 27 watt).

Monitor the voltage and don't let it exceed 13.8V. 30-40 hours if it's a totally flat 12AH battery, at 300mA.

The lamp is NOT a charge monitor, it's just a resistor! It'll light, not at full brightness, and will stay about the same even after your battery is done.

I like the idea. Certainly in theme with my thread title

Follow up:

What is the typical resistance of the bulb you mentioned? I have some spare bits and pieces around - it's probably easier for me to pull some resistors off the shelf than remove a tail light bulb.

Also, Why should I use the 24V option? There is indeed a set of leads that reads 24V, but I decided not to mention it because the 18V option was already exceeding the 17V original charger.

Originally Posted by unclejed613

the bulb works as a ballast, because it's resistance is nonlinear. it's resistance rises with temperature. with 24V across the bulb and battery, the bulb will "soak up" the difference between the 24V and the battery voltage. in doing so, it will also act as a current limiter. as the filament temperature rises, the resistance rises, until it reaches an equilibrium point and levels off. then as the battery charges, the current drops, the temperature drops and so the resistance drops, and the bulb maintains equilibrium. with the 24V source, you will want to monitor the battery voltage often. i have found it more accurate to actually disconnect the battery from the charger, and then measure the battery voltage. batteries with weak cells will measure much higher with the charger connected, because of increased internal resistance in the battery. i have a 10A open frame power supply i use occasionally for charging gel cell batteries. the supply has foldback limiting, which means if a preset current is exceeded, the regulator will start dropping the output voltage until the current is at the preset level. i find that by setting it to about 5 amps or so, the power supply will source the charging current and at the same time the regulator output will be folded back to the present battery voltage. as the battery voltage rises, it can be monitored with a meter. there is one problem, however. if the battery has a weak cell, the charging current won't trigger the limiter, and so the battery's state of charge can't be accurately monitored with the supply attached. a small 300mA charger with a lamp is going to behave similarly to my foldback limited supply, but if there is a weak cell it will read higher than the actual battery voltage with the charger connected.

Thanks for taking the time to educate me about the bulb's role. Good stuff.

Originally Posted by mneary

I need to reiterate: If you're using a McGyver circuit that doesn't have charge monitoring, you need to monitor the voltage manually. And often.

The bulb could be replaced by a resistor of 30-35 ohms 20-25 watts. Although P=V*I says <10 watts, it would burn any thing or person that gets too close (150 deg C is common). I suggested the bulb because most people don't keep a good assortment of 25 watt resistors, and a hot lamp is a nice visible reminder not to touch.

I suggested 24V because I didn't really trust the "18V" that you happened to find. In retrospect, I could have guessed that you found 17V between the +5 and -12 lines. If you use the 17V, your resistor would be 10 to 12 ohms, 10 watts. Again, arithmetic might say <5 watts but a 5W resistor will be a burn hazard.

Thanks!! I dont know how I measured 18V... You were right it -- I double checked it and I got 17.2V, which is exactly what I need (except this charger does not have too much current output...).

I got these bulbs for 2$: two filaments, one @ 27Watts and the other at 8.3Watts. The inner lead's filament is like a straight thick wire, and measures about 2.2Ω I suspect this is the 27 watt filament because it is much thinker than the outer lead's filament - which is a thin coil and measures about .7Ω.




Please correct me if this assumption is wrong -- If I assume the output is limited to ~300mA, then :

P=V*I = 17.2 * .3 = 5.16W.

SO I could use either filament but just to be safe I will start by using the one which I guess is the 27 Watt filament.

Thanks again!

Also the cheapest battery car charger I could find in a store was like 40$. I like this cheap option better.

Seems to be working:

I have it hooked up to the 17V pins. There is a dull glow from the bulb.

-------------------------------------------
Time (min) ..........Volts measured on battery
-------------------------------------------
0 ................. 12.51
1 ................. 12.71
2 ................. 12.79
3 ................. 12.81
5 ................. 12.82
10 ................. 12.84
20 ................. 12.85
40 ................. 12.86

current was measured at 340 mA.
I dont really know how long to push it...