Charging 96v Lithium Pack with Direct Solar

I think I just answered my own question:

The Midnite Solar Classic:
http://www.midnitesolar.com/pdfs/classicSL_SpecSheet_back.pdf

Looks really good. Up to 100v. In fact they even have a model that goes to 120v nominal battery pack. Awesome.

If I can just find one I can afford...

Cheers!
-Ocean

You cannot charge lithium-ion batteries using an MPPT charge controller (unless it is specifically rated for lithium-ion packs, and you can set it to 24s). Otherwise you will damage your pack, or worse cause a fire. If you set the MPPT output to 96V, you will only be charging your pack to 80% of rated capacity. And that is if the MPPT controller is exactly at 96V and not nominal 96V. If it is designed for charging SLA, it will go up to 114V which will exceed the Li-ion limit of 100.8V and cause a fire.

You will need a proper CCCV charger for 24s, in order to charge the pack. There is no way to get around this. To be ultra-safe, you also need a BMS, but that is another topic, and some folks with experience with Chevy packs do not think a BMS is necessary.

So you need an MPPT controller that supplies power to a step-up CCCV lithium-ion charger set to 24s. I recommend experimenting with a Chinese charger and see how that goes. They are cheap.

If the Voc is 44V, then the Vmp is about 36V, and the MPPT is set to nominal 48V out. The CCCV charger will step the 48V MPPT output up to 24s nominal (86.4V), which is 24x4.2 = 100.8V upon full charge.

The following MPPT controller ($132) is 40A at 48V, which is 2kW. If you need 3kW, you may be able to connect two of these in parallel.
https://www.ebay.com/itm/40A-MPPT-S...-24V-36V-48V-Max-PV-Input-130VDC/112515341337

For the CCCV charger, you can use this 900W digitally set controller ($20). I have not used this, but I believe you can have multiple in parallel to reach 3 kW. You set the voltage out to 100.8V and the current out is set to 9A. This charger will assure that you never feed more than 100.8V to the battery pack, even when the MPPT deviates from 48V.
https://www.ebay.com/itm/900W-Digit...ower-Module-Boost-Converter-New-/172597536678

For battery pack longevity, do not charge to 4.2V but to 4.1V or 4.15V. Set the charger to 24*4.1 = 98.4V.

Neither of the two controllers you have linked are suitable for lithium-ion charging, unless the power is first supplied to a CCCV charger.

jimmyaz said:

First off, I want to say I am no expert. But why couldn't you use a MPPT charge controller to charge lithium?

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Neither have I used an MPPT controller to charge lithium-ion. But will I leave my home to the chance of fire if the unrated MPPT drifts?

1- MPPT controllers are not obliged to retain a voltage. Most appliances can work with a range of voltages and are nowhere as sensitive to the voltage level as a lithium ion pack is. And there will be lots of other loads on the charging line - it is not that the house will go without power when the battery is being charged. The voltage will fluctuate - and largely so. And then it can drift.

2- Depending on the power of the solar panel and the MPPT controller, and the size of the powerwall, it is possible that the MPPT controller will not behave as a constant current charger but rather a a constant voltage charger for the entire cycle. So it is likely that the pack when low in voltage may be charged at very high rates - 2C or 3C, when the max charging rate of the pack can be as low as the usual 0.5C.

3- MPPT controller manufacturers are not rating their products for lithium ion. Obviously they wish no liability when something may go wrong. And things can go wrong in many ways which they have yet to cover in the MPPT charger. I am pretty certain that the Tesla powerwall has a li-ion charger built into it, and does not rely on the solar controller which has been designed for a lead-acid battery pack.

Solarsail said:

Neither have I used an MPPT controller to charge lithium-ion. But will I leave my home to the chance of fire if the unrated MPPT drifts?

1- MPPT controllers are not obliged to retain a voltage. Most appliances can work with a range of voltages and are nowhere as sensitive to the voltage level as a lithium ion pack is. And there will be lots of other loads on the charging line - it is not that the house will go without power when the battery is being charged. The voltage will fluctuate - and largely so. And then it can drift.

2- Depending on the power of the solar panel and the MPPT controller, and the size of the powerwall, it is possible that the MPPT controller will not behave as a constant current charger but rather a a constant voltage charger for the entire cycle. So it is likely that the pack when low in voltage may be charged at very high rates - 2C or 3C, when the max charging rate of the pack can be as low as the usual 0.5C.

3- MPPT controller manufacturers are not rating their products for lithium ion. Obviously they wish no liability when something may go wrong. And things can go wrong in many ways which they have yet to cover in the MPPT charger. I am pretty certain that the Tesla powerwall has a li-ion charger built into it, and does not rely on the solar controller which has been designed for a lead-acid battery pack.

Click to expand...

I guess anyone can give out advise/infos even if they don't have it or use certain products. That's the freedom of internet and forums.

I am not saying charging lithium is 100% safe... it doesn't matter what charger you use or what you do.. .there's always a risk... How much that risk is depend on what the user do to minimize the risk. But to say Solar MPPT charge controller can't charge lithium is just outright nonsense.



"2- Depending on the power of the solar panel and the MPPT controller, and the size of the power wall, it is possible that the MPPT controller will not behave as a constant current charger but rather a a constant voltage charger for the entire cycle. So it is likely that the pack when low in voltage may be charged at very high rates - 2C or 3C, when the max charging rate of the pack can be as low as the usual 0.5C."

Obviously you do not know much about Solar charge controller or what they can do... For example, with the Midnite Classic, you can set the maximum output regardless of how much solar power you got coming in... You set it at 10amps... it will not exceed 10amps charging... regardless if you got 50amps coming in from solar.

Oh... and about the Drift thing you talk about... Unfortunately I'm too stupid to understand what that is.

I hope the OP would do more of his own research and can really determine what's REAL/TRUE infos.

Wow I didn't realize this discussion had gone this far (somehow missed my inbox)... but thanks for all the feedback.

I will say that I do have direct experience charging lithium packs with solar mppt chargers... so the concept is ok for me... I was just looking for something that would output up to 100v.

I am off grid. My own house batteries are 7S L-Ion by Tesla (Mercedes B-Class hybrid batts). These are a perfect drop-in for a 24v system. I have an older Outback MPPT150 - 60 amp controller. I have it set to bulk the batteries up to 29.2 (/7 = 4.17 per cell) then it floats them at 28.8 (4.11 per cell). There is NO temp sensor connected and therefore there is no compensation / automatic adjustment for temp of battery. I've never seen my batts go higher than this EXCEPT for an instant in the case of pulling a heavy load (like a compressor) -
then when dropping the load the voltage spikes because the controller was trying to keep up with it. But that spike only lasts for as long as it takes for the controller to drop the output so the batts read below 29.2

And they have been great. Very little voltage sag even under heavy load. I have three of those 7S Tesla's (in parallel) for about 9kWh and I've been running them this way for a couple years now... so far so good. My solar array is only 1500 watts on the very best day so they rarely see more than 50 amps charging... it's actually a great setup. The only thing missing is a 7S BMS for each module... but they do seem pretty consistent.

Anyways I just wanted to do something similar for the Bradley - except running at 100 volts max (charging voltage). The specs on that Midnight Solar 250 seemed like they would do that. And yes, as was mentioned, they are highly programmable so limits are pretty easy to set. But I will take a look at the other links that were provided. Thanks for that!

Cheers! And Merry Christmas!!!
-Ocean

Hey Thanks Electro,

Those are interesting... but I have to admit I'm skeptical. The specs aren't clear enough to really know if I can limit the output voltage to exactly 100 volts. Plus it's a PWM, which limits the type of array that can be used (although a common array of 3 Series 44v (open circuit) panels would work fine)

But I feel more confident with these:
http://www.midnitesolar.com/product...ctCatName=&productCat_ID=21&sortOrder=1&act=p

Looking through the documents (which for some reason wouldn't come up for me just now... but I have looked at them before)... I know that these controllers have a maximum output of 100v (72v nominal system)... exactly what I need. Plus it's a MPPT controller which means I can feed it with an array of up to 200 volts and it will find the max power point of the array and convert it to the batt charging voltage at up to 79 amps.. which is pretty darn good. Conversion Efficiency of like 98+%

And I've found them on Ebay for about $550.... not too bad.

Midnight Solar is an offshoot of Outback which is an offshoot of Trace / Xantrex... these guys are the original hard core badass developers of solid, long lasting, high performing off grid solar energy systems.

Cheers! And Merry Christmas!
-Ocean