Decided to build a couple of battery chargers for my electric golf carts.
http://www.electroschematics.com/7687/24-to-36v-battery-charger-circuit/

Would really be nice if it had some sort of LED indicator to indicate when it's charging and when it switches to float. A single LED that is lit when charging and off when it switches would be OK. Any ideas on the simplest way to add some sort of charge indicator?

Schematic from the above article.

 

It's early in the morning and I'm not entirely awake yet, but I don't think this circuit "switches to float." Q4 is acting as a voltage comparator with a soft transition region. For low battery terminal voltages the output is current limited to 1 A, so the output voltage rises as the battery charges. When the output current drops below 1 A the regulator shifts to constant voltage mode and the output current decreases as the battery terminal voltage continues to increase. This will reach some kind of equilibrium based on the exact adjustment of the output voltage level and the characteristics of each battery, and that can be set such that the long term output current is correct for a float condition, but that is not the same as a circuit that still is in constant current mode but restricted to a float level current.

And, I'm not sure what the Q3 constant current source is all about. This type of regulator usually has nothing but a resistor supplying drive current to the pass transistor. If I wanted to get more bang out of that current source, I'd have it bias the zener for a more tightly controlled output with less ripple.

But about your question - usually there is an LED in the output constant current regulator that comes on in CC mode and goes out in CV mode, the opposite of what you want. But that doesn't tell you when the regulator has topped out in CV mode. The "right" way is a current shunt and comparator to indicate when the output current falls below a certain level (the float current value) regardless of the output voltage level, but that's probably the most complex. I don't see anywhere in the circuit where a current changes from something below 5 mA (something we could run through an LED) in charging mode to greater than 5 mA in float mode. So the easiest thing might be a voltage comparator like 1/2 of an LM393 looking at the wiper of the adjustment pot to see when the output voltage is above the "float" value.

ak

 

At 1A max, this charger would take ~4days to charge my 36V EzGo...

It is a very complicated float-charger, which is useful only if you leave your golf cart unused for >3months...

I say complicated, because I can do the equivalent with about 1/3 of the parts in the linked schematic...

 

Linked?

 

Thanks for the replies,
I'm just trying to build something that I can leave connected over the winter and won't overcharge the batteries. When they are in use over the summer I use 30 amp chargers but they only drop to about 8 amps when the batteries are at full charge.

I already have most of the parts in my stash to build a couple of these chargers and it looks pretty safe. The indicator isn't a deal breaker, just thought it would be nice to be able to tell when it reaches the maintenance level.

 

These are lead acid batteries, right? All I do for my boat's battery tender is a constant voltage supply (using an LM317) with a current limit to protect the charger circuit when the battery is less than fully charged. Many folks just use the thermal/over-current cutout properties of the LM317 itself for the latter. I can't say it doesn't work - it does, but it's a poor design practice in my opinion.

You can leave a constant voltage trickler attached all winter long. The current will drop to just a few milliamps.

 

a constant voltage supply (using an LM317) with a current limit

I do that with a lawn mower. LM317L for a 100 ma limit and whatever voltage I decided on some 3 years ago. Still going like the Energizer Bunny.

 

I'm more of a technician/assembler than engineer and found this circuit on the net. It uses a 741 opamp as a comparator and looks like it may do what I am asking for. Thinking if I can set a reference voltage with a resister divider from a regulated 9 volt supply (7809) and tie the inverting input to the base of Q4 in the above circuit it just may give me the indication I am looking for.

The voltage on the base of Q4 should change depending on the batteries state of charge. Not sure what the 6.8V reference is related to as far as 24 or 36 volt batteries or the state of charge. Thinking I can set the comparator reference with a 10 turn pot and assume it will need to be something around 6.8 volts.

Guess my questions are..
1. Does it look like I am in the ball park.
2. What values to use for R1 and the pot, thinking R1 = 4.7K and R2(pot) = 10K should give me a voltage range between 2.8 and 9 volts.

 

Get a $2 LED 3-wire 0-100Vdc LED Voltmeter like this. It can be powered with 5V to 30V. It will tell you much, much more about your battery's state-of-charge than a go-nogo LED that just reports one voltage level...

 

That's a pretty durn good idea and will cost less than the comparator parts.
Thanks...

 

I have a voltmeter like the one I linked in my EzGo... It shows the voltage during charge, as well as the voltage under load while I am driving the cart.