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diy solar

36v setup with alternator charging

nktio

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Apr 9, 2020
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Hey ya'll,
So I'm helping a friend build a budget solar system for his camper van. It will be using a 2000wh 36v li-ion (NMC) battery bank.
Why 36v??? The best cheap batteries I could find at the moment for this sort of thing are 5s EV batteries, so stringing them into a 10s makes a perfect 36v battery (and only requires one BMS).
However, he wants to be able to charge with the alternator. Through my own tinkering I've found that the cheap MPPT boost modules can safely accept input from a wall charger (at lower voltage than the batteries) or other sources-- I built a pedal powered charger using one. I've been using the MingHe kind but I ordered one of these:
If it doesn't work the same I'll order him a MingHe one (just have to wait for shipping from China).
I haven't tested with an alternator, but I think it should be fine as long as he keeps it disconnected when starting the vehicle and current is limited at 20a.

However, the amount of power these MPPT units can pull is largely dependent on the voltage difference between input and output. I worry that the jump from 13-14v up to 30-42v will not yield much. SO I was thinking adding another boost module inline before the boost MPPT to bump input voltage up to 24-30v might help ease the MPPTs struggles. I got one of these:

I see someone else tried something similar for a 12v system, and it worked (for a week!). Hopefully the boost module I got is higher quality (and actually has working current limiting). Probably not though...

I'm just wondering if anyone has any alternate suggestions, or additions to this (add some diodes for extra protection? a 30a current limiter in case the other stuff doesn't do its job?)? As far as I know they don't make anything designed to charge 36v li-ion NMC off an alternator, so I cant just get a commercial product. If this was a 24v system I'd just get a ISDT Q8 hobby charger to use as the intermediary, but those only work up to 34v (and I cant find a hobby charger with high amps made for anything more than that). 36v AC>DC e-bike chargers are typically limited to 2a, so getting a 12v inverter and plugging that in would be worthless.
I'll be happy if I can get the alternator to charge 150-250w-- that's all I feel safe pulling to not strain the alternator anyway, and the MPPT is limited to 300w too. There is a separate MPPT charge controller for solar so he'll be able to charge with both alternator and solar at once-- I only expect the alternator to be supplemental or useful in a pinch.
 
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First, if he's actually a friend, then you shouldn't be steering him towards 36V simply based on "best cheap batteries."

You wouldn't design him a system that requires that he keep it disconnected during starting - probably gonna forget some day.

Alternate suggestion:

Don't create some Rube Goldberg overly-complicated design for something that should be simple and easy. The increased cost of 36V equipment will likely offset any perceived advantage in pricing. Probably not the best act of a friend to suggest installation of highly flammable batteries to someone who has no clue.

I'd wish you luck, but that implies I want you to succeed along the planned path.

Instead, I wish you wisdom to see a better path.
 
I appreciate the concern! I've been working with EV grade NMC battery systems for a while now and will never be charging them up to 100% or depleting beyond 3v per cell, which is where the bad stuff tends to happen. These aren't used 18650 laptop cells or anything like that. There will be an adequate smart BMS with temperature sensing and fuses, and he will not be pulling any high amp loads (the cells themselves can handle over 150a continuous). NMC is used in more and more mobile power systems (most of the all-in-one portable power banks), most consumer devices, electric scooters and bikes, and most electric cars. All batteries are flammable and dangerous, especially if not properly managed, and it will be installed in a module where it could be quicky disconnected and discharged from the vehicle in case of fire. It seems to be a lot of superstition around NMC, and 99% of the time there's mishaps or failures its due to using equipment designed for lead acid or LiFePO4 (or forgoing a BMS and charging up to 4.2v per cell or higher, or using super degraded low quality cells).

The cost of the 36v components isn't any more than it would be for a 24v system, in fact it's almost the same equipment except for the inverter-- he'll just need at least 3 12v solar panels for the MPPT controller (which is what we planned anyway). This alternator charging is the only tricky rube goldberg part, and if we cant get it to work without a headache then so be it. I think it's worth a shot though!
 
oh yeah that could be handy! Yeah I could
use that to charge up to 36v then let the solar handle the rest. Its better to do CC instead of CV charging with these batteries but CV is how the scooter chargers work and they’re the same basic chemistry.
 
oh yeah that could be handy! Yeah I could
use that to charge up to 36v then let the solar handle the rest. Its better to do CC instead of CV charging with these batteries but CV is how the scooter chargers work and they’re the same basic chemistry.

What's the SoC of NMC at 3.6V/cell?
 
3.6 is the nominal voltage—so a pretty safe spot to aim for. And NMC doesnt have the flat voltage curve like LiFePO4 so its much easier to gauge SoC from the voltage. I was thinking of limiting the the alternator charging to something lower than 40v anyway cuz i dont trust the boost MPPT module to handle the charging on the high end. The MakeSkyBlue MPPT that the solar panels are hooked into will be much better for that.

The only thing is I wonder of any issues could arise from charging in parallel with a CC (the MakeSkyBlue) and CV at the same time. Might at least confuse the CC as to what the battery voltage is below 36v. I’m not sure that really matters tho— it would just make it CC/CV under 36v i think.
 
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Not sure if all NMC are the same, but our NMC (LG ESS) are 3.0 - 4.2 VPC (3.67 nominal).
 
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Not sure if all NMC are the same, but our NMC (LG ESS) are 3.0 - 4.2 VPC (3.67 nominal).
Not all the same, but the vast majority fit this. Technically 2.8 is the low cutoff and 4.25 for high (thats the range preset in most BMSs) but keeping it within 3 and 4.2 will make your battery last a lot longer. Keeping it between 3.3 and 4.1 even better. The voltage discharge curve of different NMC batteries can vary a lot though
 
Thanks for asking that question. It may play into a set up that I have in mind.
 
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initial testing with a 14v power supply is a success! if i tune the boost module up to 28v i can reliably pull 230-250 watts.

Apparently my friend’s alternator is capable of outputting 3500 watts so I doubly we’ll have any issues.... the big question is how long the boost module will last. with open air flow the heatsink is only warm to the touch after a few hours of 230w continuous and the fan on it is either broken or only turns on when a temp threshold is reached. So hopefully it’ll be fine, doesnt seem to be excessively stressed (and the module is -supposedly- capable of up to 1500w output).

I havent yet tested with the other boost MPPT controller but hopefully i can rig it up tomorrow.
 
working great with the other boost MPPT module too. the current readout on that thing seems to be pretty inaccurate, showing around 14.8a but my amp clamp is reading around 6a going into the battery (around 270w), and 10a going into the MPPT....so not sure which side its supposed to be monitoring (the manual is in Chinese) but it doesnt matter since the smart BMS has charge monitoring.

The boost module does seem to get hot faster with this one... i’m gonna mount them so case of the MPPT and the heat sink are touching and screwed flush into an aluminum base for extra heat dissipation. the fan on the boost module does work, i noticed it kick on this time.
 

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