Charging Strategy for Small Loads

Inq720

Odysseus, expert on the Siren's call
I have a Lishen 272 Ah battery using a Overkill Solar BMS. This will be going into a boat. During the boating season, it will be used more heavily. My question is for the off-season. Say for four months, it will be sitting idle. It will likely have a small fan and maybe some electronics totaling about 5 watts. Here is what I've learned in my reading (mostly from this forum).
  1. Its not good on cells to keep them topped-off like lead-acid at 100% SOC with a trickle charger.
  2. The recommended long-term storage voltage is 3.28 volts/cell (about 40% SOC).
  3. I see that my Overkill Solar BMS is programmable.
I'm considering leaving it on a trickle charger... BUT I would adjust the "Cell over voltage" parameters.
Normally, the trigger value is 3.65 volts and the release value is 3.5 volts.

It is my thought if I,
  1. set the cell over voltage, trigger value to 3.30 volts (about 70% SOC) the BMS will disconnect the trickle charger.
  2. set the cell over voltage, release value to 3.28 volts (about 40% SOC) the BMS will the re-connect and let the trickle charger recharge it.
Does this seem logical? Is this a good strategy? Is there a better one?

Thanks.
 

snoobler

Solar Honey Badger
Moderator
1. yes
2. pretty much.
3. Yes.

1. Yes.
2. Yes.

I don't know if there is a lower limit for either value.

So this is unattended, but drawing power?

I can't say for sure that this will get the intended results, but it sounds like a logical strategy. However, BMS switches can wear out. Having it cycle repeatedly is not necessarily a wise approach. Could you just attach a trickle charger set to 3.28V?

If your off-season is cold, this works to your advantage. Temperature has a big influence on storage deterioration. Even higher states of charge can tolerate extended storage in cool/cold temps.
 

Inq720

Odysseus, expert on the Siren's call
1. yes
2. pretty much.
3. Yes.

1. Yes.
2. Yes.

I don't know if there is a lower limit for either value.
Oh! Didn't think of that. I have not actually tried adjusting them.
So this is unattended, but drawing power?
For years it'll be in my yard, but later it might be in a slip or stored on the hard at a boat yard. The main power usage is a fan to keep the air circulated inside and Arduino type devices to communicate via WiFi. I would easily see how the battery (and other things) will be doing real time.
I can't say for sure that this will get the intended results, but it sounds like a logical strategy. However, BMS switches can wear out. Having it cycle repeatedly is not necessarily a wise approach. Could you just attach a trickle charger set to 3.28V?
I thought BMS's use Mosfets. These are commonly used in PWM devices switching thousands of times a second. Can you point me to some of the failure points of a BMS? Besides, I'm thinking this strategy will fully charge to the high end, switch off and not switch back on till it reaches the bottom end. If I'm calculating correctly, (70% - 40%) * 272 Ah * 13 V / 5 W = 212 hr => ~9 days. So besides being in the sweet spot of SOC, I'd only be cycling it once every 9 days. If I assume this is for four months, I'd only be cycling it ~250 cycles/year.
If your off-season is cold, this works to your advantage. Temperature has a big influence on storage deterioration. Even higher states of charge can tolerate extended storage in cool/cold temps.
Thanks, I didn't even think about that problem (still thinking lead-acid). Now thinking of it, the Overkill has temperature sensor so it will keep it from charging when under 32F. Maybe, if I bump up the Cell under voltage, trigger/release values to something like 3.0/3.2 volts, I'll keep from going to 0% SOC and keep the things humming inside going for several days more.

Thanks for helping snoobler.
 

Johncfii

Solar Enthusiast
I think your idea to use the BMS to manage charging could work. However, I suggest using the BMS “Batt Over Voltage”, and “Batt Under Voltage” parameters to control charging, rather the “Cell“ over and under voltage parameters. This would allow you to keep the “Cell” parameters set and active for normal cell protection. You will not be able to easily pass charge and discharge current through the BMS with this scheme. You will need to wire the BMS to control relays, rather than to connect and disconnect the batteries from a main bus. It would be possible, but a bit complicated to use the BMS to simultaneously control a charger AND control battery connection to a main bus.

I can suggest two other possible approaches, just as something else to consider.
1. Connect an appropriate charger to a simple timer that would charge the battery back up with appropriate frequency, for appropriate duration. Simply wire the timer to switch on primary power to the charger; perhaps once a week, and then back off after a period of your choosing. You choose the appropriate cycle after observing the system for a while. This control scheme could pretty easily be integrated with BMS low temperature protection.
2. Promariner company makes a brilliant charger called their Pronautic line. They are a bit expensive, being available with various capacities, from 10 amps (for about $250), up to 60 amps. They have a preset configuration for Lithium batteries, or the settings are customizable. They have a feature that monitors the battery continuously, when connected to shore power, and automatically re-starts a charge cycle when battery voltage drops to a preset level. I have had two of these chargers in service for four years (though with lead-acid batteries, and not lithium) and they have performed perfectly to protect my batteries from freezing damage during winter.
 
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Inq720

Odysseus, expert on the Siren's call
I think your idea to use the BMS to manage charging could work. However, I suggest using the BMS “Batt Over Voltage”, and “Batt Under Voltage” parameters to control charging, rather the “Cell“ over and under voltage parameters. This would allow you to keep the “Cell” parameters set and active for normal cell protection.

I can suggest two other possible approaches, just as something else to consider.
1. Connect an appropriate charger to a simple timer that would charge the battery back up with appropriate frequency, for appropriate duration. Simply wire the timer to switch on primary power to the charger; perhaps once a week, and then back off after a period of your choosing. You choose the appropriate cycle after observing the system for a while. This control.scheme could pretty easily be integrated with BMS low temperature protection.
2. Promariner company makes a brilliant charger called their Pronautic line. They are a bit expensive, being available with various capacities, from 10 amps (for about $250), up to 60 amps. They have a preset configuration for Lithium batteries, or the settings are customizable. They have a feature that monitors the battery continuously, when connected to shore power, and automatically re-starts a charge cycle when battery voltage drops to a preset level. I have had two of these chargers in service for four years (though with lead-acid batteries, and not lithium) and they have performed perfectly to protect my batteries from freezing damage during winter.
Some good ideas. I think I understand the benefit of using battery parameters. I guess my thought was if they go out of balance, since they'll not be in the range of the BMS's balancing act. But... surely, they won't go that far out of balance in four months.

Also, way down on my list of things to do... is to take advantage of the Overkill's Arduino API and write my own web based GUI to monitor and control the batteries. I could have the UI make all those changes for a on-season versus off-season mode. Also, from another thread, I could have the same UI do the weather thing... In my off-season case, if a weather report predicts a long cold spell, could up the SOC on a one time basis before the cold hits.
 

Johncfii

Solar Enthusiast
Some good ideas. I think I understand the benefit of using battery parameters. I guess my thought was if they go out of balance, since they'll not be in the range of the BMS's balancing act. But... surely, they won't go that far out of balance in four months.

Also, way down on my list of things to do... is to take advantage of the Overkill's Arduino API and write my own web based GUI to monitor and control the batteries. I could have the UI make all those changes for a on-season versus off-season mode. Also, from another thread, I could have the same UI do the weather thing... In my off-season case, if a weather report predicts a long cold spell, could up the SOC on a one time basis before the cold hits.
Sure, BMS protection feature should normally never be called on to protect the battery. But .... we spend the money on a BMS for some reason, so I guess it must happen once in a while. Therefore, perhaps worth retaining to the greatest possible/easy degree.

Thanks for the mention about Ardunio/Overkill interface. I didn‘t know that such was available. Now you’ve probably given me many hours of interesting potentials to investigate on this wintery Sunday. So much for my “to-do” list for today.
 

Inq720

Odysseus, expert on the Siren's call
Sure, BMS protection feature should normally never be called on to protect the battery. But .... we spend the money on a BMS for some reason, so I guess it must happen once in a while. Therefore, perhaps worth retaining to the greatest possible/easy degree.

Thanks for the mention about Ardunio/Overkill interface. I didn‘t know that such was available. Now you’ve probably given me many hours of interesting potentials to investigate on this wintery Sunday. So much for my “to-do” list for today.
Some incredible abilities in that thread I mentioned above about monitoring weather sites and adjusting accordingly.
I figured most of it can be done with a simple ESP8266 board for a couple of bucks and some long winter nights of programming. I haven't had time to look at the Overkill programming API stuff, but know its available.
 
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