Very Simple DC-DC Float Charger Circuit - 12v lead acid

Question

Forgive my ignorance of electronics. I am looking for a very simple float charge circuit with a variable DC input.

The situation is as follows: I have a off grid solar system on a boat with 4x 6v deep cycle golf cart batteries in series-parallel, forming a 12v battery bank, charged by an mppt solar charge controller. I want to use this same charge controller to top off the starter battery (standard 12v lead acid) without making it part of the deep cycle bank. It only needs to run when there is sun and voltage is above 12.7 vdc.

The problem: almost all commercially available float chargers are for AC input and include a transformer. DC-DC chargers are quite expensive, and very overkill.

The goal is to make as simple a circuit as possible for $10 or less in parts. Over and under-voltage protection are already handled by the primary MPPT charge controller.

Solution? Can I simply put a current limiting resistor and a diode from the deep cycle battery bank that will allow 200ma of current flow into the starter battery when above 12.7 vdc? Something like this:

As you are rolling your eyes at this, remember how grateful you are to have a basic education in electronics so simple circuit do not baffle you!

Many thanks!

Answer 1

The output voltage of the LM317 circuit you suggest will be at least three volts below the input voltage, so won't charge your start battery.

There are Automatic Charge Relays available that will automatically connect the Start and House batteries when either battery is being charged.

I have used a manual switch to connect the Start and House batteries when the boat is not in use - but you must remember to turn it off when at anchor and using the House bank, so you don't discharge the Start battery.

If all batteries are flooded Lead-Acid, there is no problem connecting them in parallel for charging.

Answer 2

Here's my quick 'n dirty ACR design for $10 in parts or less

Not very simple but works. Rev B has indicators for Charging (Red) and Charged (Green) , however I raised the float voltage. (perhaps I shouldn't have)

My Specs:

OCP <= 1.1A with Max heat loss of 2W in power driver
OVP >= 13.2V cutoff
UVP <= 13 to 13.5V cutoff from lack of sun after charger

Design Result meets Spec

20ks /div is about 5 days.

Preset Reset . Step Trigger set to = 12.7V on Start Bat, then open switch and press Stop to toggle and resume. Reset at any time will initialize Start Bat to 12V which is modeled by ESR*C with leakage Rp and Level 1 model.

After trigger to stop trace, Open SWITCH and resume using STOP. 12.5V zeners @ 1mA used for voltage detection may be done many ways.

Rev A

Rev B

To control simulation; Slow down slider for analysis, speed up if impatient 5 days= 20k seconds / division

Slider for MPT Battery to simulate Diurnal Solar Power and outage.

Reset puts Start Battery at 12V ( simulated by ESR for 800 CCA and 50 Ah)

Change anything for the heck of it.

Recommendation :

• your simple circuit with the LM317 and I limit = 1.25V/R won't work at 200mA due to input output differential

But at 0'C LM317 @ 200 mA has VIO differential of 1.75V so Vout >= 12.7 when Vin >=12.7+1.75V = 14.45 which exceeds MPPT charger Voltage.

However there are FET based LDO's that will work with lower VIO and same design. Use that with a case and insulated small 5W heatsink .

Cost Reduced Version Rev C

500m ~ 1.2A , 5 Ohms* 120 mA = 650 mV for Ic1= 13mA (1k)

Q2 will dissipate 2W at 1.5A until Bat charges up and current reduces. then 200 mA per day briefly from overnight decay . 14.2 to 12.8

e.g. http://ww1.microchip.com/downloads/en/DeviceDoc/MIC2915x-30x-50x-75x-High-Current-Low-Dropout-Regulators-DS20005685B.pdf

Rev D Cost reduction

^{simulate this circuit – Schematic created using CircuitLab}