2 x 12V Battery switch for solar charge controller (not using relays)

Thread Starter

gkeep

Joined Oct 21, 2017
69
Hi Guys,

I am after some thoughts on my circuit design. I want to build a circuit board for switching between batteries for both charging and discharging some SLA batteries.

For simplicity I have omitted irrelevant parts of the circuit design for my question.

I was wondering if there is something simpler out there or if I'm going crazy going down a wrong path. I want to be able to charge and discharge batteries with a max of about 10-15 amps. The issue i have found driving the circuitry with MOSFETs are, the 2 mosfets with common source (or drain) have too much of a voltage drop across the body diode to handle that kind of current. While having the driving mosfets in parallel means battery can undesirably self-discharge or charge through the body diode.

I have come to a solution, but i was wondering if there is a better or easier way. I originally went with relays but I thought they consume over a watt running the coil. I know there are some mosfets that can operate in the mOhm rds (on) range, but im relatively inexperienced with BJTs.

I guess i just want to know if my idea is bad or not. I know IGBTs are meant to do what im trying to do here, in a single package, but i cant seem to find the right one.
 

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Ian0

Joined Aug 7, 2020
4,821
I'm not sure what you are trying to achieve. Why are you switching between batteries?
One thing is certain - IGBTs are not suitable - they are rarely used below 400V - MOSFETs would be much more efficient at 12V.
Relays might actually be more efficient than MOSFETs if you don't switch over very often.
 

michael8

Joined Jan 11, 2015
272
The "trick" is to realize that a MOSFET will conduct in either direction once the G-S voltage is correct to turn it on.
Thus it can be turned on to bypass the internal diode lowering the voltage drop. Two MOSFETs in series
in opposite directions when off can block either direction of current flow as the internal diodes opposed each other.

In the diagram, Q5 pmos feeds 12V power into the 5V supply. While this might work it might also cause
the 5V supply to rise. I wouldn't do it...
 

Thread Starter

gkeep

Joined Oct 21, 2017
69
The "trick" is to realize that a MOSFET will conduct in either direction once the G-S voltage is correct to turn it on.
Thus it can be turned on to bypass the internal diode lowering the voltage drop. Two MOSFETs in series
in opposite directions when off can block either direction of current flow as the internal diodes opposed each other.

In the diagram, Q5 pmos feeds 12V power into the 5V supply. While this might work it might also cause
the 5V supply to rise. I wouldn't do it...
Hi Michael,

Thanks for the feedback. I didnt realise if the mosfet was turned on it would conduct backwards "around" the diode, invalidating the forward voltage. Will this still take on the Rds on resistance?

I figured if current flowed backwards through a mosfet it would incur the voltage drop, on or off.

If what you are saying is correct that seems like an easy solve. back to back p-mosfets.
 

LowQCab

Joined Nov 6, 2012
1,913
"" Batteries aren't identicle. ""
Is it really worth it to go to all the trouble to keep them separated instead of buying a matching pair ?

By the time You get all this Circuitry figured out, and paid for,
You probably could have bought 2 identical new Batteries.
.
.
.
 

michael8

Joined Jan 11, 2015
272
I didnt realise if the mosfet was turned on it would conduct backwards "around" the diode, invalidating the forward voltage. Will this still take on the Rds on resistance?

search for: back to back mosfet

If what you are saying is correct that seems like an easy solve. back to back p-mosfets.

Yes. You still have to deal with all the "normal" mosfet considerations: don't exceed max gate voltage, heat,
and you have 2 mosfets worth of Rds. Remember Rds goes up with temperature and resistive losses
are related to the square of the current. Also don't exceed the max gate to source voltage (with only 12 volts
not a likely problem for you).
 

Ian0

Joined Aug 7, 2020
4,821
Is this true? How do you know that?
Whilst they don't have to be identical, it would be preferable if they were similar - the same type by the same manufacturer would be idea.
"Capacity" is just "plate size" - you can add more capacity by adding more plate area, you don't have to add the same amount every time. Impedance is inversely proportional to plate area, so the current will share (reasonably well) between similar batteries of different capacity.
Batteries of different capacity still require to be charged to the same voltage profile and that will happen if they are in parallel.
However, if they are of different ages they may behave differently. A well used battery takes more current on "float" than a new one.
So, different size batteries of the same type in parallel will probably be OK, but batteries of the same size and type in parallel would be preferable.
 

Thread Starter

gkeep

Joined Oct 21, 2017
69
The reason for all ofthis, is i charged 3 different flooded lead acid batteries to full then hooked them up in parallel to a solar charge controller. i left them hooked up for a week or two only to find that two of them died and were completely unfixable. they were all about 70Ah.
 

LowQCab

Joined Nov 6, 2012
1,913
"" .......... i charged 3 different flooded lead acid batteries to full
then hooked them up in parallel to a solar charge controller. ""


Well OK fine .....
How old were each of the Batteries ?
What conditions were each of the Batteries subjected to over their life-times ?
Were any of the Batteries ever run dead and then left in that condition for more than a couple of days ?
Were all of the Batteries kept on a "Float-Charger" ? ........ at what Voltage ?
And, there are many other conditions that can seriously affect the Life-Expectancy of each of the Batteries.

If the Solar-Charger was working properly, ( ie, supplying 13.6 to 13.8 Volts ),
and the Batteries still died,
they were already no-good when You first hooked them up.
.
.
.
 
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