Charging a battery with two charge controllers

[Mahmoud_yehia]

Hello,
I am working on a solar car project and i need to charge a 48 V Li-ion battery pack (18650 cells), but i have to use mini-sub arrays (~ 400W) and connect each with a MPPT charge controller.
what i would like to know is :
- Can i just connect the battery pack BMS (Battery Management System) charging leads in parallel to the charge controllers ?
- what battery management system should i use for this 48 V 100 Ah pack ?
- would you recommend a configuration where a use a buck or a boost charge controller ?
i would really appreciate your input.
thank you

[Seekonk]

You throw around a lot of fancy terms, do you understand the implications?  I would never let a MPPT controller anywhere near my system.  I do operate at power point which is easy enough to calculate for multiple arrays.  Two MPPT controllers would definitely fight each other.  You need a processor that has central control and not hack together a bunch of commercial crap.

[Mahmoud_yehia]

i am new to this, can you elaborate?
are you saying that i shouldn't use a mppt controller!
wouldn't i be losing energy ?, i know it may be a small percentage but in a solar car, you need every bit of energy.

[station240]

The issue with multiple charge controllers, is getting them to recognise when the batteries are charged. As they can finished charging and switch to float mode, then see the other charger's voltage still present on the battery assume it's a fault and disable themselves.

Another issue is limiting the charge current, do you set each charge controller to a fraction of what the batteries needs, then lose power if some cells output less ?

What you really need is either a master/slave capable charge controller, or some sort of bigger device with multiple inputs.
I have no idea as to what is available that is suitable.

To be honest, the extra weight of the big inductors required for MPPT could make it pointless in a solar car, especially with multiple inputs. As the extra weight could cancel the extra power gained.

I'd suggest having a talk with electrodacus
and look at his project.
https://www.eevblog.com/forum/oshw/solar-bms-a-solar-charge-controller-for-lifepo4/
It's not 48V though, but I'm sure he can give you some more expert advice.

[Mahmoud_yehia]

well, i will use a battery management system so it will be the one that cuts the feed to the batteries when they are full.
i imagine that i will have a power bus where i will connect the output of my charge controllers, battery charging and discharging leads-with a control system to decide which will be on- and the motor controller input.

i think there might be some interface power circuit that i should use but i don't know any.

i am already in contact with electrodacus.

[Seekonk]

You never stated what your battery chemistry is.  Electro is into lithium and doesn't believe in MPPT.  I track at power point, I just don't use IV searching. Solar industry has large MPPT in the rear view mirror.  They have gone to microinverters for each panel.  So why do you need two controllers?  That isn't a lot of power.  48V makes this seem like a high school project in which case this will be a catalog project wtihout research.

[Mahmoud_yehia]

i will be using a li-ion 18650 pack, i am a part of a team that will be competing in the world solar challenge, we are still in the survey phase, it is true that MPPT in stationary systems makes little sense but in my particular case, every bit of energy matters.
i need to use more than on charge controller because the size and the weight of the controllers matter and because not all the cells on the body are under the same conditions, so we try to group the ones expected to have similar exposure to the sun together for maximum efficiency.
 

[Seekonk]

Very true, I would be using multiple charge controllers for that reason.  If you look at the PV curves, the power point voltage only changes with temperature, not light level.  If you monitor the cell temperature for each small group the power point voltage can be calculated.  In my system I have several PV arrays (even different voltage strings) and they all feed into a common voltage buss which multiple devices draw off of.  That could never be done with traditional MPPT that uses IV hunting method.  Any buck or boost converter can be easily turned into a tracking regulator by hijacking the feedback loop.

[mtdoc]

You throw around a lot of fancy terms, do you understand the implications?  I would never let a MPPT controller anywhere near my system.  I do operate at power point which is easy enough to calculate for multiple arrays.  Two MPPT controllers would definitely fight each other.  You need a processor that has central control and not hack together a bunch of commercial crap.

To each his own - I'm sure your system works great for you but multiple MPPT controllers for multiple arrays is used by literally thousands of experienced off grid users and works well - especially with arrays at different orientation to the sun.  Of course proper system design and installation, etc, etc is key

To the OP:  MPPT controllers are a type of buck converter. I know nothing about your application but in stationary home PV systems they have several advantages to traditional PWM controllers beyond the modest MPPT gains. One is the ability to (efficiently) use PV string voltages that are much higher than battery bank charging points. Having that headroom will help across a range of temperatures and also during rapid changes in solar irradiance (passing clouds, etc).  Programmable MPPT controllers are being used commonly with Lithium battery chemistries now. Whether the extra expense, complexity and weight of MPPT controllers for your unique application makes sense is another question.

[Mahmoud_yehia]

Do you have any reading material that you would suggest ??

[Seekonk]

If you are just driving this to the food market, anything will work. To win a race will require
some serious thought and creativity.  Typical systems are battery centric, everything goes in
and out of the battery. I see everything fed from a capacitor bank power buss that is slgghtly
higher than the battery voltage. NOTE: Solar panels are normally self current limiting. Connect
a PWM BMS to a capacitor bank and you will get enormous surges. For regulators I like boost
converters. Those 600W ebay boost converter configurations are ideal for boosting a 36V string.

Assuming that the solar panels are capable of driving the vehicle alone and the battery has a
50% charge, there will be times when: 

All battery and panel power is sent to the motor.
Solar power is diverted to charging battery.
Only panel power is sent to the motor.
Only battery power is sent to the motor.
And variations of the above.

Some of these can be automated to a small extent but a drivers skill will be key. Study solar
panel characteristics as a function of temperature. Expect your panels will be quite hot and
obtaining anywhere near 50% of rated panel power will be difficult.  Best of luck with your project.

[Mahmoud_yehia]

Do you have an recommendations for a BMS that can handle high voltage battery packs ?

[mtdoc]

Do you have an recommendations for a BMS that can handle high voltage battery packs ?

If you're trying to shave off as much weight as possible, you may not want to use a BMS.    I'd recommend asking your LiPo BMS and battery specific questions on the Endless Sphere Forum.  There is a lot of collective knowledge there regarding care and feeding of LiPo batteries - especially for weight sensitive, maximum efficiency applications.

[Red Squirrel]

I'm far from an expert, I've actually been reading a lot on solar/MPPT myself as I have a little project I'll be starting on soon, but in your situation what I would look at doing is see if the controllers allow to set a voltage.  So set one controller voltage to 54 volts (float voltage for a 48v system) then let the other controller vary the voltage to do the smart charging stuff to make batteries charge faster.   Failing that just set both to float voltage or get other controllers that allow you to do that.