# diy sla battery charger help

#### surfah

Joined Mar 21, 2012
2
Hi, I'm new here and know very little about electronics. I want to build a 36v battery charger in the future.

One basic question. I recently wrapped the secondary part of a transformer to put out a voltage near 40volts. After the bridge rectifier, the voltage is about 39 volts. I have 3 12 volt batteries wired in series, fully charged and showing just below 40 volts. Why is it when I connect the power to the battery that I hear electrolysis start to happen? I thought that if the output voltage is lower than the input voltage of the battery pack then the battery shouldn't accept any power.

That's my question for now. Later I want to know about the regulator part of the charger. Any good tutorials out there? thanks!

oh, Is phasing the primary to secondary important on the tranformer?

#### debjit625

Joined Apr 17, 2010
790
You will find a lot about batteries here
http://batteryuniversity.com/

surfah said:
I recently wrapped the secondary part of a transformer to put out a voltage near 40volts. After the bridge rectifier, the voltage is about 39 volts.
I didn't understood that.Normally we measure AC in RMS and after rectification the DC voltage will be greater than the AC RMS figure.Normally a 40 VAC RMS will not give you 39 VDC it will be more than that ,a 40 VAC RMS is about 57 VAC peak and after rectification you will get approx 55.6 VDC (taking into account 1.4 volt drop for two diodes in bridge rectifier section)

surfah said:
I thought that if the output voltage is lower than the input voltage of the battery pack then the battery shouldn't accept any power.
You can charge batteries with greater voltage,of course their is a limit ,for more info use the link I provided.

Is phasing the primary to secondary important on the tranformer?
It depends on the application...

Good Luck

#### #12

Joined Nov 30, 2010
18,076
My suspicion is that surfah that does not know the peak voltage of a rectified sine wave and is using a meter set on DC to measure the unfiltered pulses. He thinks he has 39 DC volts but he really has 55 volt pulses.

#### surfah

Joined Mar 21, 2012
2
debjit625 great website! Thanks!
debjit625 and #12 thanks for the info on the sine wave. I only have a basic meter.

Do you know what voltage regulator I should look into? something like the lm317 would work? How would I get the amps higher?

Below might be interesting for some other beginners
#1 38.5V only the battery voltage
#2 42V transformer turned on and connected to the battery
#3 39.8V only the transformer and bridge rectifier (not connected to the battery)

#### debjit625

Joined Apr 17, 2010
790
surfah said:
I only have a basic meter

It seems from your picture, its a digital multimeter ,it should have an AC RMS measurement option ,may be its not true RMS meter but that doesn’t matter much for this application.

I am not sure but the battery looks like 12VDC 7AH type....

Charging SLA battery takes a very long time, lead acid needs the longest time to recharge around 16 to 14 hours(taking into account if the battery is 80% discharged).You can use float charge, typically for a 12V battery you apply around 13.5 volts (2.25 volts per cell),at first a large current flows then slowly the current decrease. Many says that you can keep lead acid battery in float charge forever, but I think it should be disconnected to avoid grid corrosion.

Their is another fast charging system for lead acid battery ,but its complex(in sense of circuit).In this system we apply constant current of about 0.4C or higher(not to exceed the manufacturers max C rate for charging) to the battery until the cell voltage reaches to 2.45 volts (14.7 V for 12V battery),then we keep the battery in that constant voltage 2.45 volts per cell, until the current drops to a significant level. After that we terminate the voltage to a floating voltage of 2.25 volts per cell (13.5 V for 12V battery).

LM317 is not good choice for charging lead acid battery as its a linear regulator and lead acid battery takes so long to charge ,it will waste a lot of energy in form of heat. In your case it is very bad choice, if you go with float charge then you will need at least 40.5 volts for three 12V batteries connected in series, LM317 will not output more than 40 Volt DC.In case you go with fast charging method then LM317 can source max 1.5A and assuming you have a 12V 7AH battery a 0.4C of that is 2.8Amp which the regulator cant provide and even not the fast charging voltage.

I have never charged a system like this, mostly I have charged 12V and 6V,so I don't know which device will do it (may google help), but I am sure that LM317 is not going to do that.

Good Luck

#### wayneh

Joined Sep 9, 2010
16,102
It would be a far better strategy to charge each battery separately, not in series. While they're new and behave the same, you may get away with it. But as they age and behave differently, series charging will accelerate destruction of the weaker battery.

#### dwdw

Joined Sep 3, 2009
1
I wish I would have seen this post a month ago. You have a DIY wound trans former. Looks like MOT. Why not remove a couple turns at a time until you find the desired final charged voltage. At that time call it the equilibrium of the two voltages then it will self regulate as it charges.
Just the way I do-it
dwdw