Connecting Batteries in Parallel without Cross Discharge

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

Thanks

Reply:

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.

Reply:

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.

Solution:

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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Comments

Derwent River said…
Thank you for your prompt,very clear and simple explanation.

Ron
Swagatam said…
The pleasure is mine:)
J-boy said…
please sir, is it possible to produce 7.5v and 4.5A when 4x1.5v are in series, and 2x1.5 are in parallel. Can the 6 of them produce the above mentioned voltage and current if the two in parallel is combined with the four in series?
Swagatam said…
sorry, you cannot 4.5 amps from the mentioned combination, not even 2 amps.
achilles hector said…
what's the use of the mosfet?
how will I replace the diode with mofet?
where will I connect the MOSFET?
J-boy said…
Sir i am producing a mobile charger that can produce a 7v output and a constant 800ma when powered with a 6x1.5v duracell rechargeable batteries. But i dont knw how long it can last and the number of times i can use it to charge my NokiaN70 before recharging the duracell again. And what will be the apropriate Zener voltage to use and am using a the transistor D882 in common base? Thanks
Swagatam said…
mosfet will allow more current.
replace its S and D with diodes anode and cathode.
Swagatam said…
You can include a low voltage indicator circuit with it to detect the under voltage situation of the pack. This will enable you to monitor and switch OFF the connection when the LED illuminates.
wilfrik2003 said…
I must say thank you so much for this hard work. However, I have for four(4) batteries of 200AH each and and an inverter connected to it.

Can you please highlight the schematic diagram and the the value of the mosfet to use
Swagatam said…
You are welcome.

You can use any P-channel 50 amp mosfet such as MTP50P03HDL and replace them for the diodes, the source of the mosfets will replace with the diode anodes while the drain with the cathodes. The gates must be terminated to ground via 100 ohm 1/4 watt resistors
harikrishnan said…
What will be the specification of MOSFET to be used for three 12V 20 Ah batteries connected in parallel ( constituting an overall 12V 60 Ah combination )??????
Swagatam said…
any mosfet having voltage and current rating twice of the battery charging rate will do.
Samuel johns said…
SIr when my 12V lead accid battery banks( which are four no.s of sealed lead accid batteries taken from my old electric scooty ) are charged to full (approx 14.5V using an automatic charger made by me ) they shows the full voltage at the moment when chaarging has accomplished, but when time goes on and testing after few more minutes i get only 12.8 V constant voltage in my all batteries??? Can you please tell me how it has been occured???

I also want to know that what much power a 48V 20Ah battery will draw if i am gonna use it for an inverter????

SORRY 4 the BAD English
Swagatam said…
Samuel, as long as a charging input is connected with the battery it will show a 14+ volts after it fully charged, however once it's disconnected from the charger it's voltage will fall and show around 12.8V as in your case which also means that your battery is 95% fully charged....it's OK
Swagatam said…
inverter power and backup depends on the load connected at its output....
Gautham said…
Sir do you have any idea of taking parallel connection from a series connected batteries without using any damages to the battery? Do you think it is possible by employing several diodes or a relay ?
Swagatam said…
Gautham, yes it is possible using a few relays.
betakorbo said…
Hi,
as far as I know when the FET channel is open and the blocking diode is bypassed, the mosfet becomes a "wire" not blocking anymore cross currents between batteries. For this reason we would need a control circuit to detects whether the diode is forward or reverse biassed and controls the FET gate voltage accordingly

Am I correct?

Thanks
Benedetto
Swagatam said…
Hi, yes you are correct, I forgot about the internal diode with the modern mosfets.

In that case we can employ BJTs in the same manner as the mosfet.
Swagatam said…
only series parallel connection would be enough....

connect 2 x 2 in series to yield 24V, and then connect the 2 x 2 in parallel to get more 2 x AH
Surya said…
Hi. Can we use the above method for cellphone batteries also? I had two identical phone batteries of same age ie bought at same time. I am planning to connect them in parallel for diy purpose. Please reply.
Swagatam said…
yes you can according to me.
Aron Randhawa said…
Hi,
I am experiencing cross-talk in my charging circuit due to current rolling back from the drain and out of the source pins of my p-channel mosfets. They successfully work as switches but not as diodes. These are the type of mosfets I purchased:
cpc.farnell.com/infineon/irfu9024npbf/mosfet-p-55v-11a-i-pak/dp/SC11062
Is there something I am missing?
Thank you very much for your time and for writing such a helpful post.

Aron
Swagatam said…
Hi, sorry I am having difficulty simulating how current from drain can leak towards source, especially across two isolated mosfets,

if it's true, then I think you should try using N mosfets in emitter-follower mode, I am sure this will solve the issue and cause no cross talks across the batteries while charging them.

Aron Randhawa said…
Thank you for the very quick response! Okay great, I will give that a go.
Pavan Shetty said…
Hello Swagatam, The circuit is exactly what I was looking for in order to prevent backpowering. I am connecting 2 6S 5000mAh LiPos in parllel for my quadcopter. Will the above circuit with only diodes be sufficient or do I need to make any modifications? Thanks
Swagatam said…
Hi Pavan, you can get the intended results using diodes, but make sure the diodes are Schottky type in order to ensure minimum drop for the load...

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