Unregulated lead-acid battery charging, floating...

Thread Starter

Hamlet

Joined Jun 10, 2015
339
I want to create a standby 12v dc power source for my shop. (Jump-starting, testing winches, etc.)

I want it to be very simple, & rugged, with as little silicone if I can manage it. It doesn't have to be
fancy or the ultimate in efficiency or sophistication. I have a large transformer out of a UPS
with a 30A rating, a 50A full bridge rectifier, heatsink, and two 6v deep cycle golf cart
batteries. I do not anticipate any smoothing with capacitors.

Under 20A of load, the transformer drops to 12.95v DC. With a .25A load, it's voltage is 13.5V .
No load voltage is around 18v, and drifts around. I would stay this transformer is very "stiff".

If the float-voltage for a 12v lead acid battery is 13.6v, it would seem I could permanently leave
the transformer/rectifier hooked-up to keep the battery bank float charged without boiling the electrolyte.

Yes/no?

Second concern, which I believe is negligible, but maybe someone could advise, is if I were use a large
load for an extended period of time, in excess of the maximum charging current, so that the battery
was discharged to a condition of 10.5v, would the resulting bulk charge-rate exceed or overload the transformer?
This would only occur rarely.
 

bwilliams60

Joined Nov 18, 2012
1,350
Vehicle batteries depending on the makeup, SLA, GEL CEL, AGM, Deep Cycle etc all have ediffernt charging recommendations. I am assuming that because they are out of a golf cart, they are deep cycle batteries which is a good start. If they are SLA batteries you can charge them up to around 14.4 volts but once they have reached full capacity, I would think you would want to be able to shut the charging system down so that there is not excessive gassing and heat produced. Left unattended may result in battery damage or other consequences which would be even less desirable.
As for connecting a load which will over-extend the charging system, I would say it would be better to disconnect the charging system, do your testing and then allow the charger to bring the batteries back up slowly. Maybe add some batteries in for more capacity and longer testing capabilty.
If these batteries are not as I suspect, the game changes again.
 

KeithWalker

Joined Jul 10, 2017
1,299
The transformer appears to be OK for charging your battery, but you need to limit the current or you run the risk of damaging both the battery and the transformer if you use it to charge a flat battery.
Inexpensive mains powered battery chargers use a transformer with a secondary winding that has enough resistance to limit the current to a safe level and a voltage output that does not exceed the fully charged voltage of a battery.
You are going to have to use a series resistor to limit the current. You must calculate what series resistance you need to limit the current to a safe level for both the transformer and battery when you connect the charger to an effective short circuit. Once you have decided on a value, you must calculate its power rating to handle at maximum current at the full charger output voltage.
You will also need some way of either shutting off the charger or switching to a trickle charge when the battery voltage reaches about 14.4V or you will damage the battery.
Regards,
Keith
 

Thread Starter

Hamlet

Joined Jun 10, 2015
339
Vehicle batteries depending on the makeup, SLA, GEL CEL, AGM, Deep Cycle etc all have ediffernt charging recommendations. I am assuming that because they are out of a golf cart, they are deep cycle batteries which is a good start. If they are SLA batteries you can charge them up to around 14.4 volts but once they have reached full capacity, I would think you would want to be able to shut the charging system down so that there is not excessive gassing and heat produced. Left unattended may result in battery damage or other consequences which would be even less desirable.
As for connecting a load which will over-extend the charging system, I would say it would be better to disconnect the charging system, do your testing and then allow the charger to bring the batteries back up slowly. Maybe add some batteries in for more capacity and longer testing capabilty.
If these batteries are not as I suspect, the game changes again.
Yes, flooded lead acid, 225AH capacity. I have four identical UPS transformers. I could use one, or all four.

If I somehow depleted the charge, say 10.5 volts, and my transformer was not over-taxed, I image that letting the bulk charge self-taper into a float condition shouldn't be harmful, but it might take a long time (12hours? days?) to reach a steady float condition again. That might not be best for battery life, but I ain't going to be seeing such a situation very often. I just wanted to make sure simple as also safe. Everything will be fused.
 

nsaspook

Joined Aug 27, 2009
7,739
I use golf cart batteries (6V 225 Ah) for one of my solar energy banks.
https://forum.allaboutcircuits.com/...c-controlled-battery-array.32879/post-1438996

You can abuse the hell out of them but you really need to watch the water levels per cell if you do. I limit the charge current to below 20C which is usually in the 35-40A range with full battery charging power. You really need a supply that can give close to max amps at 15.5 volts occasionally because you will need to equalize the cells in the string if you cycle the batteries frequently. The higher voltage also will allow for a proper change controller to be installed that can 3-stage charge for you automatically.

One of the easiest ways to control your DC voltage with a simple transformer rectifier battery charger is to use a high power variac. You can also rig a separate series transformer secondary AC voltage buck/boost for the primary charging transformers.
http://www.bristolwatch.com/sr/buck_boost.htm

This will allow you to increase the DC voltage in a reasonable manner when you need it and enable manual charge control with the proper DC volts/amps metering.
 
Last edited:

Thread Starter

Hamlet

Joined Jun 10, 2015
339
I use golf cart batteries (6V 225 Ah) for one of my solar energy banks.
https://forum.allaboutcircuits.com/...c-controlled-battery-array.32879/post-1438996

You can abuse the hell out of them but you really need to watch the water levels per cell if you do. I limit the charge current to below 20C which is usually in the 35-40A range with full battery charging power. You really need a supply that can give close to max amps at 15.5 volts occasionally because you will need to equalize the cells in the string if you cycle the batteries frequently. The higher voltage also will allow for a proper change controller to be installed that can 3-stage charge for you automatically.

One of the easiest ways to control your DC voltage with a simple transformer rectifier battery charger is to use a high power variac. This will allow you to increase the DC voltage in a reasonable manner when you need it and enable manual charge control with the proper DC volts/amps metering.
Do deep cycle batteries need an equalizing charge when used lightly? (My depth of discharge is not likely to be very much, or often.)

I could use a relay with a timer to switch-in a capacitor, lifting my charge voltage once in awhile. That would be easy.
 

nsaspook

Joined Aug 27, 2009
7,739
Do deep cycle batteries need an equalizing charge when used lightly? (My depth of discharge is not likely to be very much, or often.)

I could use a relay with a timer to switch-in a capacitor, lifting my charge voltage once in awhile. That would be easy.
They need it less often but it's still required as a workout to prevent sulfation if you don't have full discharge/recharge cycles. My experience with golf cart batteries is that they like to be moderately cycled frequently instead of shallow float charging because that's what they were designed for. The self-discharge rate is higher than UPS type batteries because of the alloys used for thicker traction battery plates, usually higher SG and more rugged separators. This ruggedness also comes with the price of lower charge efficiency and increased water usage.
 

nsaspook

Joined Aug 27, 2009
7,739
I could use a relay with a timer to switch-in a capacitor, lifting my charge voltage once in awhile. That would be easy.
That might be a less effective than you think at high current levels because the batteries already filter the DC power. Capacitors are energy storage devices. The extra energy to boost voltage at high current has to come from somewhere and that somewhere is very high current pulses from your transformer thru the diodes. This causes harmonics, higher I/R losses than with more sinusoidal currents and increased core losses that can smoke both the transformer and rectifiers from the added current pulses.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
339
They need it less often but it's still required as a workout to prevent sulfation if you don't have full discharge/recharge cycles. My experience with golf cart batteries is that they like to be moderately cycled frequently instead of shallow float charging because that's what they were designed for. The self-discharge rate is higher than UPS type batteries because of the alloys used for thicker traction battery plates, usually higher SG and more rugged separators. This ruggedness also comes with the price of lower charge efficiency and increased water usage.
In my junk pile, I have some large filter capacitors (500,000uF), and a NOS Trace C40 solar charge controler, which has an automatic equalizing feature. Maybe placing this after the transformer will take the guesswork out. My only concern is that now I am complicating things, and I image that the C40 wants very clean DC, hence the caps.
 

nsaspook

Joined Aug 27, 2009
7,739
In my junk pile, I have some large filter capacitors (500,000uF), and a NOS Trace C40 solar charge controler, which has an automatic equalizing feature. Maybe placing this after the transformer will take the guesswork out. My only concern is that now I am complicating things, and I image that the C40 wants very clean DC, hence the caps.
I still use a old trace (xantrex) C40 on my original system.

Using it will take the guesswork out and it has adjustable absorption and float voltage setting. For it to work correctly you will need at least 15 volts on DC input to account for PWM losses in the controller.
 

nsaspook

Joined Aug 27, 2009
7,739
Nice set up! Can I trade a little current for voltage by using a capacitor?
No much really with large batteries attached at typical charging current levels. The electrochemical energy and voltage limiting factors of redox reactions in a large car battery is already equivalent to a several thousand Farad capacitor filter.

http://www.phy6.org/Electric/-E17-ElectEnerg.htm

Remember: We can 'charge' a capacitor but it doesn't actually store electric Charge, it stores energy.
 
Last edited:

Thread Starter

Hamlet

Joined Jun 10, 2015
339
No much really with large batteries attached at typical charging current levels. The electrochemical energy and voltage limiting factors of redox reactions in a large car battery is already equivalent to a several thousand Farad capacitor filter.

http://www.phy6.org/Electric/-E17-ElectEnerg.htm

Remember: We can 'charge' a capacitor but it doesn't actually store electric Charge, it stores energy.
Got it. Thanks for the link, pretty cool stuff.

Will a C40 accept raw unfiltered dc current without blowing?
 

nsaspook

Joined Aug 27, 2009
7,739
Got it. Thanks for the link, pretty cool stuff.

Will a C40 accept raw unfiltered dc current without blowing?
I've never run mine with a pulsed DC input but they are designed to handle wind turbine generated rectified DC inputs. The battery still acts as a filter during charge and I'm pretty sure the C40 internal PIC controller DC supply has its own DC filter for the control circuits.
 
Top