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:

Simple current limiter enter image description here

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!

  • \$\begingroup\$ Yes something better than can be done but first please define your Battery bank voltage range or limit . Is it 14.2V? \$\endgroup\$ Oct 26, 2021 at 16:16
  • \$\begingroup\$ How many hours do you expect it to drain or charge? I assume it is a std car battery 50 Ah \$\endgroup\$ Oct 26, 2021 at 17:09
  • \$\begingroup\$ I believe the charge controller does cut out at 14.2 volts. And yes, a standard 50 ah car battery \$\endgroup\$
    – ztrain727
    Oct 26, 2021 at 17:41
  • \$\begingroup\$ To be clear, the alternator will charge the battery when the boat is in use, but when in storage many months of the year is when a dc-dc float charge system would be golden. It only needs to charge during daylight hours. \$\endgroup\$
    – ztrain727
    Oct 26, 2021 at 17:45
  • \$\begingroup\$ @ztrain727 A quick goggle does find 24 V to 12 V DC-DC floaded battery chargers. For example, here. Not cheap. Probably because they aren't "float" but actually have a full-up DC-DC circuit for efficiency. Frankly, I think your comments added to Peter's answer as well as any other research you've done and then excluded (for reasons) should be added to your question so that others don't keep offering ideas you've already examined and removed from the table. From one of your comments I see that you want this on a small budget, for example. \$\endgroup\$
    – jonk
    Oct 26, 2021 at 18:33

2 Answers 2


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.

  • \$\begingroup\$ I was under the impression that putting a 50ah lead acid battery in parallel with 100ah batteries would result in overcharging the 50ah battery since it's voltage would come up more quickly, not to mention it's not being drained at the same rate as the other bank. \$\endgroup\$
    – ztrain727
    Oct 26, 2021 at 17:42
  • \$\begingroup\$ If I understand correctly, an automatic charging relay would not solve this issue \$\endgroup\$
    – ztrain727
    Oct 26, 2021 at 17:44
  • \$\begingroup\$ ACR's also cost $50-150, whereas I can get an AC powered float charger for $10, hence my interest in building my own circuit if simple enough to accomplish. That said, I really do appreciate the help! \$\endgroup\$
    – ztrain727
    Oct 26, 2021 at 17:53
  • \$\begingroup\$ @ztrain727 what you have understood about parallel batteries cannot be correct. When in parallel, the voltages will be identical (modulo any voltage drop on the connecting wires), and so they will simply not be aware of each other. However, if in parallel, then you don't have a starter pack and house pack, they will function as one overall pack. BTW, if you put them in series, then it's bad to mismatch cells. \$\endgroup\$ Sep 15 at 9:22

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

enter image description here

Rev B

enter image description here

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

enter image description here

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

  • 1
    \$\begingroup\$ Thank you so much for this! You say with a MIC2915x or same line, I could make my very simple circuit work? I am not flat out opposed to other functionality, but I really don't need led indicators or switches, happy to check the output occasionally with a multimeter. Could I use your circuit and remove some features and still have it work? \$\endgroup\$
    – ztrain727
    Oct 27, 2021 at 3:57
  • \$\begingroup\$ I bet you could.. Although I could (not) resist a boost switch as 200 mA takes a long time to charge a 50% SoC 50Ah battery But as @jonk said, a momentary ( 1us) short is good for desulfation, (when done a million times) so I made it float charge to MPPT Battery voltage instead. A long momentary short is good for warping the lead plates. (lol) \$\endgroup\$ Oct 27, 2021 at 4:48

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