hour avatar image
hour asked

Lithium longevity settings with SmartSolar, power throttling

I have a 90ah DIY (4x3.2v) lifepo4 battery that I've assembled. It's comprised of used cells that are somewhat old, so I've done a lot of testing indoors with charging and discharging to see what I can comfortably utilize and calculate my needs. Running 712 BMV, 65a battery protect, 75/15 SmartSolar MPPT.

I'm basically set on treating this battery as having 60ah usable capacity and have done several charge/discharges like this, recharging with an RC hobby charger. If I charge to RESTING 13.34v at terminals (or 13.29 on BMV) I can consistently pull 60ah out of the pack @ 75w continuous draw. With the BMV set to 60ah capacity, I reach zero percent and about 10 minutes later my Battery Protect cuts off (set to 12.55 cut off, rebounds after 75w load is cut to 12.8 or there about). This is perfect for my needs and then some.

So today I decided to do a real world solar test, using the 75/15 MPPT (receiving temp and voltage info from BMV via network). My controller settings are Absorption Voltage: 13.29v (measured at terminals its 13.4). My float voltage is set to 13.25 (measured at terminals 13.28-ish). My charge current is set to 15A to let the max of my 200w series'd panels do their thing.

What I'm observing, is that even though my battery is at 20% capacity as measured by the BMV, the MPPT is only allowing in 45w and is floating - no load. Panels @ 40.78v. It's peak sun right now and not a cloud in the sky, and if I adjust the absorption and float way up, it'll go from 45w to 150+w.

So the charge controller is throttling charging to the battery because the voltage it's reading while charging, and my settings for float/absorption voltage. But the battery only has 20% of my configured capacity in it.

I'd like to charge at a much higher rate in cases like this, since I'm at 20% of my capacity and in the real-real world, throttling my charge current when I'm at a low SOC will surely bite me in the butt if it gets cloudy the second half of the afternoon. What value(s) can I change to achieve this while ensuring that I never exceed a resting voltage of 13.29 @ BMV (13.4 @ terminals). I'm OK if the voltage goes up to 13.7v when charging at 10+a, this was observed when charging with a hobby charger nearing full (and settled off-charge to 13.29@BMV)- and is right about the point when my 6a active balancer board starts to lose control of one cell in particular. But I don't ever want the battery to get charged to 13.7v resting.

Thanks, sorry for wall of text - it probably could have been condensed in to something much shorter but I didn't want to skimp on detail!

MPPT ControllersBMV Battery Monitor
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3 Answers
boekel avatar image
boekel answered ·

charge to 3,45V per cell, this doesn't do any damage to the cells. at lower voltage, as you've found out, you won't fully charge the batteries.
on discharge try to stay above 3V per cell

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hour avatar image hour commented ·

Thanks for your response. I'm definitely good on discharge. I have found that if I charge @ 150w from a hobby charger, my one oddball cell begins to drift more than the 6a balancer board can control when the pack is reading 13.7v across the terminals. Nothing crazy, that's just where it begins. So if the pack sees 13.7v or so, I'd want it to be brief and only as a result of 200w solar on a bright day. If it saw 13.7v from a 40w output cloudy day, wouldn't that make that reading (way above what I ever want to keep pack at) more..accurate? Not a superficial 13.7v from tons of solar, but a slowly achieved 13.7v? I struggle with a few concepts of all of this, have for years..

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wkirby avatar image
wkirby answered ·

I run an old pack which was in a motorcycle for 8 years before I recovered it.
Cells are not very good, they can drift quite far apart from one another at the top and bottom. It should be 7KWh, but I get about 4KWh from it.
I can pretty much go to 3.5V per cell before it gets silly, so I stick to about 3.45V, like Boekel says. The bottom end for me is about 3.1V and then the difference between cells starts getting a bit too far.
Sure, if you only go to 13.29V you are really not going to be able to push much energy in.
If you are really struggling to keep the cell Voltages close to one another then the pack is possibly too far gone.

Try to balance your cells at the top then set a discharge limit at the bottom before the cell Voltages go apart.

See if you can fix the Voltage reading discrepancy of the BMV.
Take the fuse out of the inline fuse holder along the red wire. It's probably a 100mA or 0.1A one. The Voltage drop over this fuse will cause the readings to be off by about the amount that you are seeing. Put a 1A fuse in there and that should improve the discrepancy. A 1A fuse is still fine to protect that red wire.

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hour avatar image hour commented ·

Thanks, I saw another post by you about the 1A fuse. I'll pick one of those up so I don't have to play the multimeter + best guessing game.

I had previously bottom balanced the cells at like 2.8v or something extremely low when I first built the pack a couple of weeks ago. This was great, they got a little wonky at the top end but I never wanted or needed to charge that high, and they discharged in perfect harmony way down to just above 3v. That has changed since the addition of the always-balancing active balancer, and they don't ride down together as well as they previously did. When I'm reaching the under-load voltage cutoff for the batteryprotect (12.55 @ 75w draw) my min-max has gone from like 0.005v to 0.100v. I think that's about what it starts to look like when charging along at 150w and the voltage at terminals is read as 3.7v, too. Just one dang cell.

I don't know where to draw the line, but when I notice that jump - which seems to happen all of a sudden, I call it quits. That's all fine and good since it was part of my 60ah usable capacity calculation.

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ben avatar image
ben answered ·

You’ve gotten great responses (I converted them to answers, since that’s what they were).

One additional thing I noticed in your very detailed write-up is that your charger voltage is lower than your terminal voltage. That can’t be true in real life, so you must be seeing meter discrepancy. It’s best to measure everything with a single meter, preferably one whose voltage you trust to 1000 counts or more. But at least with a single meter, the deltas will usually be correct, and sometimes those are the important thing.

You’ve probably figured out that you’re going to have to tweak settings to try to squeeze some life out of these cells, and your “performance envelope” is going to be squeezed at both ends because of cell misbehavior. It’s still a fun project to try to get what you can out of them, though, and one day if you upgrade to fresh cells, it will seem so easy in comparison. :-)

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