Mppt charge controller schematic assistance

Hi Mike,

i think Al's point to protect this battery is suites more for what i am looking for with his suggested LM317 circuite than a zener diode, i just wait if he can attched any scheme concerning this 6 cruisal points as i see them also like that.

barg said:

Hi Mike,

i think Al's point to protect this battery is suites more for what i am looking for with his suggested LM317 circuite than a zener diode, ...

Click to expand...

You missed the point. I do not suggest that a Zener be the shunt regulator of choice for the final circuit. I used the Zener in the simulation only to make the point that all of the panel current initially goes directly into the battery with no losses whatsoever. There are losses only after the battery is very nearly fully charged, when it doesn't matter.

For a practical circuit I would use an active regulator consisting of a precision voltage reference like a TL431 with a PMOS current booster.

For example, look at this one:



If anything, its knee is too sharp. It might be nicer if the fall-off of the current into the battery wasn't quite so steep. It is easy to modify this circuit to prevent that: just put a 0.1Ω resistor in its positive lead. Note that this circuit is adjustable by just changing the voltage divider ratio of R1 to R2.

So the next step is a means of measuring the current into the battery. I have ways...

barg said:

Hi Mr Al,


Well, thanks for your detailed infightings notes.


I think I might use a pwm ready circuit I found in the net that principally works like that: it charges the battery freely up to 14.4v and than stays there for a while and then starts to modulate until it gates down to 13.5v level and then stays there until charge is stops=dusk or with no time limit. That’s covers points 1-2 as you noted.


As for point 4 timer, I am not sure this circuit has internal clock even it looks a neat solution and I don’t know how to add one i'l be glad to learn from you… basically I am looking for a simple addition if you know any?


As for temp sensing, I will check this option whether it is within the second bttr ptc. and for balancing, I prefer pack instead of balancing, great that you floated this option also.


As for point 3, low current cutoff point, please advice how it can simply done? Can you show any relevant scheme for this one?


As your lm317 solution, my concern is not to waste a power in linear charging but to use switch pwm as I noted, do you think it is a right point in a matter of efficiency with my spec as above?

Click to expand...

Hi,

Are you talking about the low turn on point or the low cutout point?
The low turn on point would be when the charger is allowed to charge the battery again.
The low cutout point would be to disconnect the battery from the load in the event that the load drew the battery voltage down too low.

Either way, a comparator circuit would detect each condition and take the appropriate action: either turn on the charger or turn off the load.

MikeMl said:

No power is lost with the scheme I showed in post #17. Only after the battery reaches its maximum-allowed-during-charge-Voltage is any available current diverted away from charging the battery. Prior to the battery reaching that voltage, 100% of the available solar current goes into the battery.

Click to expand...

Hello there Mike,

Are you sure about that? I ask because that is not the usual way of thinking about the use of solar panels. Normally we think of power in vs power out not just in terms of current.
To make this point more clear, consider an extreme case. The panel puts out a nearly constant current over voltage until it gets near to the max power point, in which case the current drops a little. But before that, the current is roughly the same for say 5v, 10v, 15v, and 20v. Now if the current is 1 amp, at 5v that means we would be charging a battery at 1 amp, and at battery voltage of 10v that means still 1amp, and at 15v that means still 1 amp charge current, and same for 20v. If the battery is a 5AHr unit, it charges in 5 or so hours.
Now if we convert the power of the array we get 5w, 10w, 15w, and 20w. When the battery voltage is 5v we'd charge at 1amp, 10v 1 amp, but when we get to 15w we would be able to charge at somewhat more than 1 amp, and at 20v we'd be able to charge a 14v battery at:
20*1=i*14
i=20/14=1.43 amps.

So if the panel was able to put out more power than 14 watts we'd be able to charge some 40 percent faster even close to the full charge level. BTW the array power output is roughly proportional to the radiant sun energy input, so the brighter the sun and more direct it shines on the panel the more power is available. In bright sun we'd see the most power available and thus the most power lost if we charge directly from the panel. To find out the practical advantage though we'd have to know the geographical location and how the panel was oriented toward the sun and if it had an automatic direction finder built in and how many axises that had.

But feel free to elaborate how you came to your conclusion if i did not understand you correctly.

MrAl said:

Hello there Mike,

Are you sure about that? ....

Click to expand...

I though I had already covered the MPPT issue in post #6. If I understand the OP, his panel has an open-circuit voltage of 18V, so its MPPT voltage will be about 15V, which is getting close to the voltage at the end-of-charge for his battery.

The additional amount of panel power you could harvest by putting a MPPT controller between the panel and the battery would accrue only at the beginning-of-charge when the battery voltage starts out at 10V. By the time you factor in the 85% efficiency of the MPPT controller, I don't think you will get any net reduction in charging time, maybe 5% at best, so not enough to make it worth dicking around with the MPPT controller.

Just connect the panel to battery, and be done with it.

MikeMl said:

I though I had already covered the MPPT issue in post #6. If I understand the OP, his panel has an open-circuit voltage of 18V, so its MPPT voltage will be about 15V, which is getting close to the voltage at the end-of-charge for his battery.

The additional amount of panel power you could harvest by putting a MPPT controller between the panel and the battery would accrue only at the beginning-of-charge when the battery voltage starts out at 10V. By the time you factor in the 85% efficiency of the MPPT controller, I don't think you will get any net reduction in charging time, maybe 5% at best, so not enough to make it worth dicking around with the MPPT controller.

Just connect the panel to battery, and be done with it.

Click to expand...

Hi Mike,

Oh only 18 volts? Sounds low for charging a 14v battery, but if that's what it is then that's what it is
Mine is something like 23v open circuit but i have not got around to using it yet. Hopefully soon, for a 12v battery and a couple lower voltage batteries.

Hi Mike,

I see no point in mppt also as noted.

i still dont get it for the low turn on point and low cutout point, is the circuite scheme with the TL431 as you attched before deals with these two issues by "burning" the small currents?

Hi Al,

yes, it is a nominal 18v to charge a 14.4v, do you think the gap is too small?

MrAl said:

Hi,

The low cutout point would be to disconnect the battery from the load in the event that the load drew the battery voltage down too low.

Either way, a comparator circuit would detect each condition and take the appropriate action: either turn on the charger or turn off the load.

Click to expand...

Hi Al,

Apreciate if you can suggest a schematinc for a low current cutout point using a comperator.

Thanks,

Barg:

I dont know much about the specifics of charging LiFePo batteries, but I do know how to make a current sensor.

R1 is the shunt. It will go in the minus lead of the battery.
Circuit is powered from the 14V shunt regulator I showed earlier.
Circuit has a moderate amount of hysteresis.
Output is high while battery current >30mA
Output is low as current drops below 15mA
Circuit will work with the LM33x comparators, or a LM358 opamp...

Hi Mike,

Is it possible for you to explane how this circuite works and how it cuts out the low current in my case?

Is this a solely circuit for low current cuttoff? or need to add also TL431 from the previous you noted before?

Thanks,

MikeMl said:

Barg:

I dont know much about the specifics of charging LiFePo batteries, but I do know how to make a current sensor.

R1 is the shunt. It will go in the minus lead of the battery.
Circuit is powered from the 14V shunt regulator I showed earlier.
Circuit has a moderate amount of hysteresis.
Output is high while battery current >30mA
Output is low as current drops below 15mA
Circuit will work with the LM33x comparators, or a LM358 opamp...

View attachment 96386

Click to expand...

Hello again Mike,

Thanks for drawing up a circuit, just a little point here.

You dont really expect to sense sub millivolt signals with an LM358 or LM339 do you?
I could be wrong here, but i see a 10mOhm resistor for the sense resistor and that only develops 10mv at 1 amp, and the cutout point needs to be much lower than that, perhaps 50ma.
10mOhm and 1 amp develops 10mv, and at 0.1 amp we get 1mv, and at 0.05 amp we get 0.5mv which is only 500ua.
I would think it would be better to use a precision op amp here that has very very very low input offset voltage.
The input offset for the LM358 is already around 2mv, which could be 4 times the signal to be measured.
I am assuming the normal charge current is 1 amp, but if it is higher than that's different.

Just for reference for any design:
[1] The low battery charge current cutout point protects the battery from continuous low current charging during the charge process which is considered bad for the battery if it goes on for too long (and time is accumulated over charge cycles). The threshold action is to turn off the charge completely and dont turn back on until some low voltage point has been reached.
The low voltage point here can be something like 3.3 volts with a cell that charges to a max of 3.6v, but this can be varied.
Note this is when the battery is charging, and should not be too high nor too low.
[2] The second low voltage battery cutout point protects the battery from discharging to a level considered to be too low for the particular chemistry. For Li-ion 3v might be typical, but i have seen as low as 2.5v. You'll have to check this spec for your particular battery chemistry. Note this is when the battery is discharging.

You can increase the shunt from 10mΩ to 100mΩ and make it work. The Linear LT1018 has a lower input offset (1mV instead of 2mV). I would just trim it for a one-of application.



At the TS's level of experience, he should really buy a ready-made solution, or at least go with an Arduino and some ready-made sensors, like this one.

Hi Mike,

Yes 100mOhm would help some, but it may still be a little close. Some testing maybe.

I have to agree that he would probably be happier with a ready made solution, as long as it was a quality unit.

Thanks guys, I will look into it more carefully.