Best way to improve lithium ion battery charging via solar panels

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ng.sa

Member
Joined
Apr 4, 2017
Messages
91
G'Day Guys

I have more than 10 portable power packs Tesla Model X 2170s - 24V 7s2p ( Max. 29.2V when charged via 240V mains initially ) No BMS that I have been testing to find optimum charging, mainly concerned with high and low marks, MTBF, Total Cycles, Virtual Cycles and a few other very obscure settings.

I'm now looking at charging via solar panels, these packs have powerwalls, so I can easily remove the 2170s and test re-charging via mains or cig. socket to charger in car on long trips. In Oz long trips are like 2K to WA from SA ( before .cn virus ) so bottom to top of country.

I have learnt via experience and reading on forums like these to stay well within high and low water marks, so usually < 28V > 23V so a 5V swing on discharge.

So I have really kid gloved these cells and they have never been left outside the house or vehicle so never very hot or cold, 2170s are new not recycled.

Some of the things I have tried / changed

- Quality wire
- Short cable run < 40cm from charge controller to LIBs
- Spraying water on rear of panels to remove heat
- Cleaning panels sometimes 2-3 times a day
- LIBs and charge controller always in shade
- Adjust angle of panel hourly
- Diff. panels, getting slowly bigger, most recently 350W, I have only used 1 panel because I want to mount on a vehicle

I cannot get these packs to 'fully' charge within a 'solar' day.

So I'm wondering, what can I do to improve charging performance via solar panels ?

TIA
 

jdeadman

Member
Joined
Mar 28, 2017
Messages
959
1 350W solar panel you should expect to get on a really good day no more than 2.5KWh Mow much power are you using from this pack? What is your goal in terms of power generation? how many KW (and do not say fully charge them as that is not an figure we can surmise by the information provided )
 

ng.sa

Member
Joined
Apr 4, 2017
Messages
91
I have laid this out as clearly as I can

"
what can I do to improve charging performance via a solar panel ?
"

This is not for a house this is a mobile panel and mobile power packs - notice 10+ for testing. To quote an IT term, I'm not yet in production, I'm not 'Gold' - I'm in UAT

So questions about capacity is like look at my big propeller on my hat :)

For ppl. that learn by example an appropriate response would be

- What charge controller are you using ?

This is my gut feeling that CC may need investigation, but I'm a newbie so have got there from 1st principals.

Pls. don't waste your time if you have never struggled and solved issues with low voltage systems.
 

Redpacket

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Joined
Feb 28, 2018
Messages
1,329
Gently with the tone there please, there's a lot of combined experience on this forum but we help out of good will.

Like FloydR asked, please advise how many 7s2p packs you are charging at once, it makes a lot of difference.

Re solar panels, larger panels & mobile use may lead to silicon cracking due to flexing - you might be better off with several smaller ones. A stray stone might also take out the only panel....
Yes, as you've found cooling them increases output but regular thermal shock from quick temp changes may also cause silicon cracking.
Sounds like you are adjusting angle already - panel at near right angles to sun is best, great - but reality is adjusting 3-4x a day should be enough to not make much difference.

Real issue is it sounds like you need a proper MPPT charger that runs the panel at it's optimum output point.
If you just connect the panel to the pack(s) or are using a PWM based charger (sounds like you have this type), you'll be loosing maybe 30 - 40% of the possible power production.
If you are having to use "short cable runs" that says the cable is too thin.
1x 350W panel should be producing about 10A at MPP. If you can measure the voltage drop on the cables, you want <1V max total round trip loss.
Cable run from controller > packs should be thicker (current is higher here, maybe ~14A?) than panels > controller (current is ~10A here)
I'd be using proper min 4sqmm (not typical "auto shop" stuff) panels to controller & maybe 6sqmm controller > packs.

So if you now have a system capable of ~14A into a 7s2p pack, that's a fairly high current charge approx 7A per cell, but 2170's are 4.8Ahrs cells (yours are these right?) so that's approx 1.5C charge rate, not that bad. You don't want cell heating though.
 
Last edited:

floydR

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Aug 23, 2017
Messages
1,328
So questions about capacity is like look at my big propeller on my hat :)

I was not calling attention to myself merely trying to figure out the total watt hours needed. 260wh vs 2.6KWh big difference as Redpacket stated

I am currently testing a low voltage system 36v LiFePO4 90 Ah charged with 3 "12"volt 100 watt panels in series<very old panels> SMK MPPT 60A charge controller. Also tested with a MPT-7210A boost controller with two panels in series.
Besides it isn't a propeller on my hat it is a directed air Cerebral cooling prototype. :)
Later floyd
 

Katarn_89

New member
Joined
Jun 10, 2018
Messages
9
Sounds like you need more solar capacity. 350w is only going to go so far. Getting more panels is going to far more for your output then cleaning them. If they're not in a dusty environment then the effect will be minimal. I hardly ever clean mine.
 
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