Charging 16s 100Ah (non-solar application)

rio

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Hi everyone,

I'm putting together a 16s 100Ah battery bank with an Overkill 16s BMS. This bank will power an electric ride-on lawnmower (basically doing an AGM to LiFePO4 conversion). I have a question about selecting a charger. I noticed that LiFePO chargers (such as this one, or this one) have a max charge voltage of typically 3.6V or 3.65V per cell. However, reading through some posts I've seen general recommendations to not charge LiFePO batteries routinely to 100% if not needed, but rather use ideally 80% of the range. Since I can't lower the max charge voltage in these chargers, is it common practice to just let the BMS cutoff the voltage by means of setting the overvoltage protection at a desired max voltage? If not, how should I be controlling the top charge level?

Thanks!
 

FilterGuy

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It is not great to constantly charge cells to a really high voltage, but the bigger problem is leaving it at that high voltage for long periods of time. If the "charger" charges up to the preset value and then sits there at the high voltage, it is really hard on the cells. I would consider it a must that the charger turns off charge after some period of time (Even if it is just a timer).

This one seems to have a turn-off at the end of the charge:



I am not aware of a 64V charger that will allow you to change the charge voltage. :( (But I have not really searched for one either).
 

rio

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Thanks @FilterGuy,

That's very helpful. So is it not a good idea to set the BMS overvoltage protection below the charger's charge voltage?

So for example the charger that you linked to has a charge voltage of 58.4V (3.65V per cell). This is pretty much the default charge voltage that I've seen among 48V LiFEPo4 chargers. Wouldn't my BMS inevitably hit overvoltage protection mode the moment the first cell reaches 3.65? To take that a step further, if this is considered normal, can't I just set my BMS overvoltage protection at, say, 3.55V per cell and 56.8V for the whole battery and call it good? It doesn't sound right but I'm trying to understand how the BMS fits in with the charger's voltage.
 

DLTooley

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Using the BMS as backup protection is generally preferred, but I don’t know of any thing specific beyond that redundancy.

I don’t know anything about available 48v chargers save for the adjustable chargers folks use to top balance. That might be something to explore, including in multiples.

Or get a solar charging setup!
 

FilterGuy

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So for example the charger that you linked to has a charge voltage of 58.4V (3.65V per cell).
That depends on the settings in the BMS. Many of the non-smart BMSs have the cell overvoltage set higher than 3.65V.
 

Ampster

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Ironically the link posted refers to the charger as intelligent. For that price I would expect some degree of programmability. At a minimum I would want some ability to vary voltage and ending Amps for the Constant Voltage phase.
 

rio

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Do you know of any 48V chargers that give that ability?
A couple of guys doing the same conversion as me purchased this one: https://www.amazon.com/58-4V-Charge...rds=48v+lifepo4+charger&qid=1632939264&sr=8-6, which is no longer available. They opened the housing and found a pot that adjusts the output voltage. Obviously not intended by the manufacturer, but it's working out well for them. But the build quality of the charger is rather poor and they're not all that thrilled with it.

That depends on the settings in the BMS. Many of the non-smart BMSs have the cell overvoltage set higher than 3.65V.

Right, I'll be using an Overkill BMS which allows me to change the overvoltage setting. My question is whether it is accepted practice to purposefully set this overcharge voltage lower than the charger voltage as a means to limit the charge highpoint. I don't think I need the full 100 Ah capacity for my application so I would like to limit my top charge to maybe 90%.

Also, I'm new to the LiFEPO4 world and trying to learn. So maybe I'm not asking the right questions!
 

FilterGuy

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Right, I'll be using an Overkill BMS which allows me to change the overvoltage setting. My question is whether it is accepted practice to purposefully set this overcharge voltage lower than the charger voltage as a means to limit the charge highpoint. I don't think I need the full 100 Ah capacity for my application so I would like to limit my top charge to maybe 90%.
It is not normal practice, but I don't see why you could not do this. The only downside is that you only have one level of protection.... but there is no law that says there must be two levels.

One thing to note: Once the BMS cuts charging, the cells will settle to a lower voltage. Make sure the recovery voltage is lower than the resting voltage after charge.
 

rio

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Hi @FilterGuy,
I've been following your Top-Balancing write up (so well written, by the way!). I've been doing it with a CV/CC PSU that unfortunately only reaches 3A max. I first had them in series with BMS and finished off in parallel as you suggested. I set my voltage to 3.6V and eventually the PSU switched from CC to CV mode and current started dropping. At this point due to my schedule I struggled to find a large enough amount of time where I could supervise it, and so I turned the supply on and off a few times in the course of a few days. As of last night, the tail current was down to just about 100mA, so I decided to call it. I let it sit overnight still in parallel and after disassembling it this morning all the cells measure 3.571V independently. My thinking is that this being above the 'knee' it is probably good enough and at at this point the BMS can handle any remaining top balance (if still needed). Plus I plan to set my charger no never charge above 3.5V. Would you agree with this assessment, or would you suggest to put them back up in parallel until current is zero?
 

FilterGuy

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Hi @FilterGuy,
I've been following your Top-Balancing write up (so well written, by the way!). I've been doing it with a CV/CC PSU that unfortunately only reaches 3A max. I first had them in series with BMS and finished off in parallel as you suggested. I set my voltage to 3.6V and eventually the PSU switched from CC to CV mode and current started dropping. At this point due to my schedule I struggled to find a large enough amount of time where I could supervise it, and so I turned the supply on and off a few times in the course of a few days. As of last night, the tail current was down to just about 100mA, so I decided to call it. I let it sit overnight still in parallel and after disassembling it this morning all the cells measure 3.571V independently. My thinking is that this being above the 'knee' it is probably good enough and at at this point the BMS can handle any remaining top balance (if still needed). Plus I plan to set my charger no never charge above 3.5V. Would you agree with this assessment, or would you suggest to put them back up in parallel until current is zero?
You are fine. Put it all together and enjoy your battery.
 
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