12v Lead Acid Battery Charger aka I need to stop killing my batteries!

Hi Kevin. Lead acid batteries are to be charged with constant voltage. If you are using that constant current charger, they are being killed. A 7812 regulator with a couple of 1N4004 diodes in series to its ground reference pin will raise the regulation to about 13.3 V, good enough. Miguel

Greetings Kevin, Though I'm no great shakes in electronics I think I can answer your questions. First, since there is no filter on the charger it is delivering pulsed DC. When measured with your meter it will show the wrong voltage. Connecting to the battery puts a filter in the circuit and your voltage reading is now closer to correct. Second, use a resistor to limit the current. 15 volts should be OK for the battery. It's the high current that kills the battery. Does the battery maker have a website or a phone number? If so, maybe you can ask them about the voltage and current requirements to keep the battery charged without overcharging. Cheers, Eric

Hi, Thanks for the reply

I think you missunderstood my posting a little, it is not a 'constant current charger', it is a charger designed for 12v lead acid batteries, just that measuring the current flow from the charger during the charge cycle it is a consistant 400ma.

Sounds like a nice simple option to improve the wall charger that will also physically fit inside the wall charger.

Thanks Kev

Actually, after just doing a discharge test it barely has 10% of its original capacity, so alhtough this thread is relevant as I need to not kill the next one, I dont think I will be geting much use out of this one again unless I can rejuvinate it

Cheers Kev

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The current is pulsatile, but its average value is approximately constant under the conditions that exist while it is destroying your battery(s). See the post by Mr. Fields to get a better idea why that current is approximately constant.

It is a decent solution, but it will dissipate more power than the diodes and tranformer presently do, so you need to be sure that the circuit does not overheat in that location.

--- Yes but, unfortunately, if it's putting 400 mA into the battery when it's discharged and also putting 400 mA into the battery when the battery is fully charged then it is, in fact, a constant current supply!

Lead-acid batteries generally want to be charged using a constant-voltage supply because as the battery voltage rises when it's charging, the current being put into the battery tapers off with time because the difference between the battery voltage and the charger voltage gets to be smaller and smaller, so the current being forced into the battery gets lower and lower as the battery voltage goes higher and higher.

However, I suspect that what's happening in your case is that as the battery voltage increases when it charges, the diminished load on the transformer causes its output voltage to rise as well, forcing the voltage difference between the battery and the transformer to remain more or less constant, forcing the current to remain more or less constant. Also, I believe that as the battery charges its internal resistance decreases somewhat, which would aggravate the effect. With a regulated (or voltage-limited) supply, though, the change in the battery's internal resistance would be more than compensated for by the rise in battery voltage, so eventually the system would come to rest with only the trickle charge current flowing into the battery.

-- John Fields

Ahhh, ofcourse, RMS and all that.

If 12v = average, then peak = 12/0.637 = 18.8v, rms = 1.11 x 12 =

Not quite the 15v seen but then without putting a scope on it, it does explain it.

Yup, I have now realised that this battery is pretty much dead and have been experimenting with a battery that does have 50% of it's capacity left. Charging the second cell (12ah) started with a lower voltage and a current of around 600mah, the voltage slowly rose through the charge to 15v and then sat at 400mah current.

What part does the Rs play in that circuit? I appretiate it sets the output impedance, which has an effect on the current flowing. If I know the internal resistance in a battery limits the initial charging rate, and the voltage is only set at the appropriate float charging voltage, when the battery is fully charged the voltages will equalise and the current flow will become zero?

Cheers Kev

Kevin Walton formulated on Monday :

I keep an automatic motoecycle battery charger in my flite box. never killed a battery that way