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Old 09-16-2022, 07:28 PM   #1
Bus Nut
 
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Setting custom charge voltage to hold batteries at 50%

I’m interested in extending the life of my lifepo4 batteries (two 12v 105ah batteries with a BMS built in to each unit). I understand that lithium batteries degrade less when they’re stored at 50% charge, as opposed to 100% (at least for my Tesla, they recommend to keep it between 20-90%). My bus sits in my driveway for the vast majority of the time with one 300w solar panel charging through an AIMS Power 60 amp MPPT that has an option to set a custom charge voltage. My question is: can I just set that custom charge voltage to, say, 25v to prevent the batteries from charging to 100%? I can also charge via my inverter, which I’ll plug in before any planned trips. Is this a dumb or smart idea?

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Old 09-16-2022, 09:52 PM   #2
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Is it possible to manually isolate the batteries at the desired charge?
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Old 09-27-2022, 09:09 PM   #3
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I can isolate each battery, and charge them individually… but since they’re 12v and my MPPT is 24, it wouldn’t make sense. Im gonna change it to 25 volts and see what happens.
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Old 09-28-2022, 09:50 AM   #4
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Not sure if the bms of each battery talk but if not then theoretical it is possible that one is completely charged and shuts of while the other one is discharged . Your charger / solar controller would not know the difference.

You would need a battery equalizer or individual charge controllers for each battery.

If you let them sit for a long time then disconnect the bms as well since it puts a small draw on the battery that add up over a couple of month.

The bms only does a good job when you charge the batteries into the knee curve .

Good luck.

Johan
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Old 09-28-2022, 07:48 PM   #5
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Quote:
Originally Posted by TheArgobus View Post
I’m interested in extending the life of my lifepo4 batteries (two 12v 105ah batteries with a BMS built in to each unit). I understand that lithium batteries degrade less when they’re stored at 50% charge, as opposed to 100% (at least for my Tesla, they recommend to keep it between 20-90%). My bus sits in my driveway for the vast majority of the time with one 300w solar panel charging through an AIMS Power 60 amp MPPT that has an option to set a custom charge voltage. My question is: can I just set that custom charge voltage to, say, 25v to prevent the batteries from charging to 100%? I can also charge via my inverter, which I’ll plug in before any planned trips. Is this a dumb or smart idea?
True dilemma, but important to solve.

As I'm sure you know, the flat part of the charge curve gives no valuable information in terms of state of charge. Depending on which chart you bring up on Google, any given voltage at 50% charge could indicate anywhere from 30% to 95% charged on some other chart. No help.

Mine is a 12 volt 280 Ah system, and here's what I did to find 50% SOC.
After fully charging the battery (beyond the shoulder of the curve, where there is some correlation to SOC) I discharged down to 50% based on the SmartShunt value, 140 amps used, where the voltage sat at about 13.1. Then I set my charge controller for that value 13.1 volts*, and let it charge/discharge for several months of minimal use (I'm still building the damned thing and haven't needed to charge it up full for a trip yet). Then I monitored the average discharge value (it's something tracked in the Victron SmartShunt) over several months.

Right now the shunt says it sits at an average discharge per cycle of -166Ah, or a theoretical SOC of 41%. Not a bad way to estimate, I think, if I can believe the various BMS and SmartShunt readings. Here's what I did:

1. Fully charge the battery.
2. Using a shunt, discharge the battery to half its capacity
3. Measure the voltage at the halfway point. If you have the patience, measure it as it approaches that point, note the point, and continue to measure it as it dips 10-20% below below half charged. It might give some indication, for your battery, of where the voltage point estimate is for 50% and whether there is any appreciable voltage change slightly below or above that mark.
4. Set your charge controller for that voltage and observe average amps discharged over time. Adjust voltage as necessary, or if you're a true geek, experiment with slightly different voltages to see if there is in fact an accurate reading associated with a specific SOC for your battery and system.


Here's a link to some voltage curves for 12, 24 and 48 volts, maybe this will help some:
https://footprinthero.com/lifepo4-ba...voltage-charts

* Not to confuse things but my Renogy 40A MPPT charge controller consistently reads .3 volts high, so it's set at 13.4, but direct measurement at the battery plus readings from the SmartShunt and BMS all confirm the actual voltage is at 13.1.**

**Except when the Daly BMS decides to act up and report one of the cells (random cell each time) as wildly high or low, seizing up the system, requiring me to unplug the BMS and wait a day for the thing to calm down and return to normal operation.***

***I told that BMS the next time it does it, if I'm on a trip, I'm gonna rip it out and put in the other one I bought for just this purpose.****

****The point of all this is to say that even if there is some science involved, it's also art, and then you just have to accept that none of these electronics are really ready for prime time, and the state of technology hasn't caught up with our consumer needs for an accurate measure of SOC.
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Old 09-28-2022, 08:32 PM   #6
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Quote:
Originally Posted by joeblack5 View Post
Not sure if the bms of each battery talk but if not then theoretical it is possible that one is completely charged and shuts of while the other one is discharged . Your charger / solar controller would not know the difference.

You would need a battery equalizer or individual charge controllers for each battery.

If you let them sit for a long time then disconnect the bms as well since it puts a small draw on the battery that add up over a couple of month.

The bms only does a good job when you charge the batteries into the knee curve .

Good luck.

Johan
I hadn't thought about this before, so thanks Johan for this tidbit.

Batteries in parallel form a complete circuit (so I just learned from a search and regurgitate here). That is to say, with the negative poles connected and the positive poles connected, each can operate as a 'charger to the other, thus, balancing. Two batteries connected in parallel will not normally get out of voltage synch.

Batteries in series do NOT form a complete circuit with respect to each other, ever, so if one is lower than the other it will always discharge to a lower value and charge up to a lower value than the other, all things equal.

But, if there is a voltage imbalance with two batteries in series I suspect the battery with the lower voltage tends to accumulate current (thus charge) at a higher rate than the battery with the higher voltage. I think it does, because of Ohm's Law.

In other words and using 12.5 volts and 13 volts for the lower charged and higher charged battery, with a 20 amp charger running at 14.6 volts the voltage difference for the first battery is greater than the second, 2.1v versus 1.6 volts, so according to all the electrical rules wouldn't you be dumping more current into the first (less-charged) battery, and therefore over time achieve balance?
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Old 10-06-2022, 05:58 PM   #7
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Do not let the bank sit at high SoC anymore than needed.

But if you are going off grid and want full capacity

charge for longevity at 3.45Vpc

only go higher for occasional maintenance, if your stupid balancer requires that.

When in storage mode, 3.1 - 3.2V is a good range

But do not let voltage drop anywhere near 3.0Vpc

cells 100% isolated is best, especially from the BMS
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