Charge LIPO batteries with solar panels .. #2

Thread Starter

acsus

Joined May 8, 2009
7
I have 101V x 25A PV + 100.8V x 30A (Max) 24S Li-Po + 5kW BLDC.
Voltages are very close, using an MPPT would create loss. Therefore, I am planning to directly connect PV to Li-Po.
Any comments?

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Ian0

Joined Aug 7, 2020
5,527
If you have 101V at 25°C according to your solar panel datasheet, then you have 111V at -10°C and 85V at 75°C.
Your battery will be 100.8V at full charge but probably 84V at full discharge.
You are much more likely to have 111V from your solar first thing in the morning when your batteries are at 84V, and 85V from your PV when your batteries are almost fully charged at 100.8V later in the afternoon.
Get a MMPT!
 

Ya’akov

Joined Jan 27, 2019
6,033
If you actually have a problem with insufficient output from the PV panels, the real solution is to increase that, not to remove the charge management hardware. The MPPT is not only something that optimizes charging, it is also a safety feature.

In any case, making the design even worse to compensate for a bad design is a recipe for tears.
 

Ian0

Joined Aug 7, 2020
5,527
If 101V is three panels in series, then you really need four panels in series and an MPPT as most MPPTs are buck regulators and step down.
If you are making you own MPPT then you could possibly use a SEPIC, but SEPICs are not the best for efficiency.
You could have two panels in series and a step-up MPPT or four in series and a step-down MPPT. The advantages of step-up is that you can never over-charge your battery, and that the current from the panel is continuous.
The disadvantage is that the output current is higher than the battery current so the cable losses are higher.
 

bassbindevil

Joined Jan 23, 2014
461
If that's 101V from the panels with no load, you might have 85 volts under load. So, you'd need more solar panels or a lower battery voltage.
And you should have a charge controller that can be configured for charging lithium batteries.
 

Thread Starter

acsus

Joined May 8, 2009
7
Ian0,

- I am in San Diego, CA. It is never -10°C here. In Dec-Jan-Feb, it is about 10 to 14°C in the morning. Of course, the temp goes down 1°C every 1000 ft you climb, but my elevation is between 500 - 1500 ft.
- The Min limit for Li-Po is 3.0V/Cell x 24 = 72V for my pack. Its Nominal Charge is 3.7 x 24 = 88.8V
- MPPT needs 33% HeadRoom (loss?) to operate, bringing the charge voltage down dramatically. I think it would rather be useful to charge a battery of 30A from a PV of 25A, if excess voltage is available, as you calculated.

So, I thought, PV being very close to battery max cut-off, and the motor consuming some power, it'd never reach 100.8V.

Yet, I considered getting a simple On/Off Controller, in case the motor is stopped, but I couldn't find one for ~100V. In the worst case, I thought I'd simply turn the battery switch Off manually to prevent damage (or, put a resistor in series for saturation at 100.8V(float)?).
 

Thread Starter

acsus

Joined May 8, 2009
7
bassbindevil,

PV output is 101 Vmp.
Voc is 118.2V. - Sorry, I should have mentioned these at the beginning.
Imp = 25A
Isc = 27.2A

Battery: 24S Li-Po
BLDC Motor: 40KV, 20kW max (5-6kW Nominal)
 

Thread Starter

acsus

Joined May 8, 2009
7
If 101V is three panels in series, then you really need four panels in series and an MPPT as most MPPTs are buck regulators and step down.
. . . . .
There are 6 groups of panels (Vmp=101V, Voc=118.2V, Imp=4.18A, Isc=4.53A) connected in parallel to get a total of Imp=25A

No room to put more panels!
 

Ian0

Joined Aug 7, 2020
5,527
- MPPT needs 33% HeadRoom (loss?) to operate,
No - it's not loss. It's a buck regulator and probably about 95% efficient. The headroom is to accommodate the variations in battery and panel voltage as the temperature and state-of-charge vary.
 

Thread Starter

acsus

Joined May 8, 2009
7
No - it's not loss. It's a buck regulator and probably about 95% efficient. The headroom is to accommodate the variations in battery and panel voltage as the temperature and state-of-charge vary.
The MPPT manufacturer told me that I needed 134V from the panels, in order to get 100V out of the MPPT to the battery pack.
So, I am looking to find a way to avoid the MPPT.
 

Ian0

Joined Aug 7, 2020
5,527
Think for a moment why people use Maximum Power Point Tracking - would it be to operate the PV at its maximum power point? Without MPPT it won't operate at its maximum power point and you will get less power.
So - you have four choices.
1. Operate the system without the MPPT and accept that you will get less power. If you get a PV panel power curve and plot your actual battery voltage on it as it changes from discharged to charged, you will see you much power you are missing.
2. Rewire your panels in a different series-parallel combination to get the required output voltage
3. Find an MPPT that suits your system voltage
4. Design your own MPPT to suit your battery and panel voltage. Even a crude voltage-based one would be better than no MPPT at all.
 

Thread Starter

acsus

Joined May 8, 2009
7
Thank you everybody.
In the light of your comments, I tried to re-arrange the PV arrays as 2S3P to get 204.2 Vmp x 12.54 Imp = 2560.2 W, so that I could use an MPPT Controller.
The LiPo battery bank stays the same, 24S x 30A.

However, now, I cannot find an MPPT for this setup. Thus, as Ian0 suggested, I am considering to build my own.
Being a Mech.Eng. with limited Electronics knowledge, I want to ask you if I can find all the electrical components to build one with 250V Input from PV and 120V Output to Battery+Motor(works w/98V).
Thanks
 
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