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Using Circuit to Charge Car Battery - What Should I Connect to ground? Answered

I took a charger that matches what I need to charge a car battery.  The charger uses a transformer and has 3 connections, as does the battery it's intended for (+/-/T).  When I plug the + and - alone with the original battery, the switch kicks in and the charger doesn't charge.  When I plug all three, it charges fine.

What can I do to charge the car battery? It only has a - and + and when I plug it in, I can hear the same switch kick in and it won't charge.

I need to do something with that earth conncetion. I thought of plugging it to an 'earth' in the wall but I don't want to inadvertently introduce high voltage to the circuit.

I sketched the circuit in my process to get here, here it is (hopefully correct :) )-


If your "car battery" is the standard 12V starter battery (two terminals +/-) it is a lead-acid battery. Lead-acid and LiPos are totally different beasts. A LiPo-charer CAN NOT charge a lead battery. If you try, you may end up with a bursted battery and some cooking sulfuric acid sprayed around.

Why can't it? It's a transformer based charger and doesn't seem to have anything in the circuitry which would prevent it from charging a lead-acid. I say this from a lot of reading, but I am not experienced with this so would be happy to hear any reasoning behind it.

Any source with a voltage higher than the batteries current voltage can be used to charge a battery. That is not the problem.

The trick is to regulate the voltage (and thereby the current or vice-versa) to give the battery only as much energy as it can store in that moment and (even more important) to know when to stop charging or switch to another strategy (e.g. trickle charging).

The reason for needing different strategies is the different chemistry of the batteries - as it already given in their names.

Check the web for battery charging strategies. A nice chart for lead-acid is here.

But even charging is not charging. Given your car battery, you may want to:

a) speed-charge it just enough to start your car with it. You may choose a voltage a bit higher than normal an stop charging after a few minutes. (The car's charger will do the rest of the charging)

b) normal charge it when the sun shines, following the chart given above. You would do that if the battery is used to power some light at night. Applications where you need full charge and long life time.

c) just trickle charge a battery of a motor cycle that got stored away for winter.

As your charger seems to have a transformer, you may use that and just add a lead-acid charger electronic. They aren't very complicated and lead acid is somewhat forgiving.

Thank you for that. I played with it and when shorting them both (t and -), it charged normally, but the current then went to high because of the difference. Any idea how I could lower the current? I also tested the transformer on it's own with a capacitor and rectifier and I see it can put out 12.4V or 25V. Which would you recommend I use? What would you use to regulate the votage? Is there one with a switch so I can change between 13.8V and 14.4?

I've Never seen a 12v car battery with a "T" connection

I would assume that the "T" connection is for battery feed back and would sugest either a Li-Po battery's Balance connector, Or maybe an Electric Vehicle battery?

I think you may be using the wrong charger / battery Combination. Can we see a pic?

Btw: A batteries ground is simply the negative terminal **Please post some pics before hooking it up though**

It is indeed a Li-Po battery and indeed the car does not have a T, that's my dilema :).

The charger was not intended for a car battery but it's the right Voltage and Current that I'm looking for with a sturdy Transformer so I think it should do the trick. I've been reading on such circuits online and I think this will do it (if I can just get it to charge).

Adding a few images:


AS IS, it simply won't work

you are trying to mix two standards, one that requires a line for feedback, and one that doesn't. if you really wanted to charge it connect the "T" to the batts ground and it should charge, and may possibly overcharge the batt and blow it up.... not too pleasant :(

**However you could rebuild a charging circuit using the transformer**

http://www.ti.com/lit/ds/symlink/lm117.pdf PG 25 look for 12v battery charger. the downside, the 317 will cause a voltage drop of 1.2V leaving you with a, still antiquate, 12.3v and it will be limited to 1.5A charging rate

You can also hook the transformer to the battery and charge it directly ! ( if you watch it; it will eventually overcharge, and do some no so nice things to whatever is nearby ! )

Forgot to mention... This is an AC transformer, so if you do rebuild it, look up "Bridge Rectifier" to chrange the AC to DC


Let me know if your interested in building it; I'll help you out where I can :)

I went for it, and when I shorted the - and T, it charged (but with too high of a current).

Thanks. I got a rectifier and hoping to put it together without going downtown for parts if I can - is there somthing else I could use instead of the lm117? Is there an alternative that doesn't lose so much Voltage?

I have some BDW94C transistors on a large heatsink, would that be of any use? Here is a pic of more parts I have that I can pull from the circuit :).

Say I use the 117 you mentioned above, how do I set the Voltage/Current?

I also sketched the circuit since asking this, so might as well add it here too :). You think it's good for lead-acid or I should redo?

I took the transformer out ofthe circuit and measured the 3 connections coming out of it through the rectifier. 1 is red, 2 and 3 are black.

1 and 2 = ~12V

2 and 3 = 25V

So this thing can actually put out 25V? 14.4V is ideal for charging my batt, could I somehow get that working? If you know how to do this and have the energy to explain, I would happily put it together. I've been obsessing over this project for over a week with little progress..


Correction - 13.8 V would be the goal if I use the 25V connection