The described circuit method of connecting batteries in parallel not only charges and discharges them uniformly across common sources, it also completely isolates them inhibiting any cross-discharging possibility. The idea was requested by Mr. Ron.

Technical Specifications

I found your site and it is most impressive.

I hope that you may have a solution to the following problem. I have two 12 volt batteries in my caravan and for charging or providing power to appliances, they are wired in parallel.

I have been told that if one battery was to drop a cell then the healthy one will attempt to charge the bad one due to variation in voltage.

The battery with the dropped cell is useless and in the scenario I gave, the healthy battery will not remain healthy for very long.

Is there a solution that keeps the batteries in parallel for charging and discharge in normal use, but separates them in a fault situation? Would appreciate any advice that you can give. Ron

Simulation and Working

The shown method of connecting batteries in parallel without cross discharge is very simple and involves the use a few diodes.

The diodes effectively block the inter links between the batteries preventing any possibility of cross discharge, yet allows them to charge from a common source and discharge uniformly across a common load.

Although the diodes provide an easy alternative for the above actions, it drops around 0.7V across itself.

The above drop might look insignificant, however during critical situations the issue could make a lot of difference.

In order to make the circuit more efficient a mosfet equivalent circuit could be wired up as shown, replacing each of the diodes.

The resistor could be anything between 50 and 470 ohms, the mosfets should be a P-channel type with voltage and current rating slightly above the max specified limits.

The mosfet option provides identical features like diodes with an exception that it won't drop anything critical.


Charging lead acid batteries in parallel with simple current control indicator feature:



Important Feedback and Questions from the Readers regarding how to connect batteries in parallel.

Dear Swag,

Thank you for this useful circuit>

Please, tell me if its suitable for 115 AH batteries or not



Dear Sayad,

Yes the concept is suitable for all batteries.

For 115ah battery, 20 amp diodes or mosfets would be needed.

Thank you for making this information available on the web.

I have two questions about your design.
1. Is the mosfet you specify an enhancement or depletion-type? Or will either work in this parallel battery charging concept?

2. It is unclear to me how a three element/lead mosfet it actually wired into the battery cable between the batteries. I understand the assignment of the leads (Gate, Source and Drain) though I am also unclear how to identify which of the outer is which).

I guess there is more than one question there. Anyway, I have had three batteries charging in parallel without protection several years ago, and one of them failed, and destroyed the other two. I now charge six batteries but with separate chargers, and would like to bring that down to using only the charge controller from my newly installed solar panels.

Thank you.


Thank you for asking this question!!

It is a P mosfet (depletion) which is supposed to be ON as long as its source voltage is higher than its gate voltage.

all the gates are wired with the common negative (-) of the batteries via individual resistors, the sources are wired with the respective battery positives, and the drains are connected together with the load positive.

I have assumed that this would effectively replicate a zero loss diode.

Another Feedback

Sir help me.
I have made an automatic battery charger circuit using opamp 741

Everything was done perfect. But the problem occurs during charging. I have used a battery monitor circuit using ICLM3914 indicating 10.5 V to 13.5 volt( fully charged ) by 10 LEDs.

I have caliberated the circuit to be cut off charging in 14.5 V and starts again in 11.5 V using a variable Dc supply.

When i am connecting the battery, no problem us there. Battery monitor working properly. But when i am switching on the dc power source ( which is a 15 V 5 A ac/dc adapter ) the monitor level changes and suddently indicating the higher battery voltage ( sometimes indicating battery is fully charged eventhough it is not ). ALSO THE RELAY ACTIVATES AND CHARGING IS STOPPED..... This problem was found constant by testing with a 11.5 V battery and 12.6 V battery. So will you please help me in solving this.


Hi Arun,

Your power supply current should be 1/10th of the battery Ah, please confirm this first.
Alternatively connect the power supply directly to the battery and check the voltage at the battery terminals, it should drop to the discharged level of the battery, confirm this too.

Another remedy could be to connect a 100uF cap across base/ground of the transistor.

The above suggestions should fix the problem

I am also having another problem that when i am connecting the parallel config. of the batteries (as in this blog ) to the charging station, the relay alternatively switched on and off making the all LEDs in the battery monitor blinking simultaneusly. But no such disturbance is seen while using a single battery. Then what i have to do? No mistake occured in connecting diodes. Everything perfect. Since this only includes battery charging, i have only employed the section of alternator in this blog

The above circuit is very basic, it only consists of diodes so that the battery which has the lowest charge begins charging first, then the next lower charged one and so on....the presence of diodes should not cause any interference to the charger according to me....the problem could be with your charger circuit....try using a high grade regulated power supply and then check the results.

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