summerhose
New Member
So I have a off grid summer house with:
The lead acid batteries died after only a few years and are barely used. They have been cycled maybe 70 times and mostly just stod topped up with float charging from the MPPT.
We only use the summer house like a month or so each year. So the rest of the time things just stand there doing nothing. And even when we are at the summer house, the battery tops up in a few hours from empty. On top of this most days we only use like 15% of the batterys capacity. We mostly just use some lights and computer and cellphone charging. The only time when we really need to true capacity the battery and the system is when we use power tools, the water pump or the vacuum.
So this brings me to what I want to solve. I want the new lifepo4 battery to last as long as possible and not die prematurely as the lead acid batteries did. I don't think the battery will reach the cycle life number, so that is not an issue. However..
The battery will stand unused for months on end. Even when we use it, we barely make a dent in the SOC it will stand at 80% in the morning before the sun comes up.
The issue that I am seeing with this is:
I see a few solutions with various benefits and drawbacks, but I don’t love any one of them.
1.
Manually discharge the battery down past 80% before we leave the cabin. By disconnecting solar charge controller and turning on the vacuum for 30 min. Ignore that the battery will stand almost full most of the time when we use it.
Benefits: I don’t have to find/create a technical solution
Drawbacks: Only solves half the issue. Requires one to have to think of the batteries more than I would like. Would be hard to educate other people of the require steps like looking at SOC values and things like that and people forget.
2.
Buy/Program some type of device that fetches the SOC value from the BMS in the LiTime battery then configures the BMS to stop accepting charge as the SOC reaches 80% enable charge when SOC is 20%. Program so that once every two weeks it allows the battery to reach 100% so the BMS can balance the cells.
Benefits: Easy to program using LiTime Bluetooth API.
Drawbacks: If something goes wrong with the BMS the battery might die even sooner. Due to something like programming in a way that possibly enables “charge” when “charge” was off for a reason (like low temp cutoff or something like that).
3.
Similar as option 2 but instead of letting the BMS do the job, program the device to disconnect the MPPT charger as the battery reaches 80%, reconnect the MPPT as the battery SOC reaches 20%. Allow the battery to go to 100% to balance every 20 cycles.
Benefits: Does not have the risk of BMS causing issues.
Drawbacks: Either I would have to do extensive research on victron bluetooth protocol (as it is not fully documented) or I would have to buy large relays to cut the power.
Maybe there is another charger or product that can fix the issue? Is there any solar charge controllers that use a SOC value in its control mechanism? Seams strange that my victron controller only allows you to set various voltages like float, bulk, and absorption to control the charging, maybe a newer device might have more options for configuration.
Does anyone have any other ideas? Or any feedback to the solutions that I have thought of. Maybe something using venus OS.
- solar on the roof - about 1000w (I get maybe 650w on a normal day due to location )
- Victron SmartSolar MPPT 100/50
- 24v 1200VA phoenix inverter
- A new LiTime 24v 100Ah 2.5kW lifepo4 battery
The lead acid batteries died after only a few years and are barely used. They have been cycled maybe 70 times and mostly just stod topped up with float charging from the MPPT.
We only use the summer house like a month or so each year. So the rest of the time things just stand there doing nothing. And even when we are at the summer house, the battery tops up in a few hours from empty. On top of this most days we only use like 15% of the batterys capacity. We mostly just use some lights and computer and cellphone charging. The only time when we really need to true capacity the battery and the system is when we use power tools, the water pump or the vacuum.
So this brings me to what I want to solve. I want the new lifepo4 battery to last as long as possible and not die prematurely as the lead acid batteries did. I don't think the battery will reach the cycle life number, so that is not an issue. However..
The battery will stand unused for months on end. Even when we use it, we barely make a dent in the SOC it will stand at 80% in the morning before the sun comes up.
The issue that I am seeing with this is:
- It is not good for the battery to be stored with SOC at 100%.
- The MPPT charger will mostly just be fighting to keep the battery above 96-99% and it can’t be good for the battery to always be in its end state.
I see a few solutions with various benefits and drawbacks, but I don’t love any one of them.
1.
Manually discharge the battery down past 80% before we leave the cabin. By disconnecting solar charge controller and turning on the vacuum for 30 min. Ignore that the battery will stand almost full most of the time when we use it.
Benefits: I don’t have to find/create a technical solution
Drawbacks: Only solves half the issue. Requires one to have to think of the batteries more than I would like. Would be hard to educate other people of the require steps like looking at SOC values and things like that and people forget.
2.
Buy/Program some type of device that fetches the SOC value from the BMS in the LiTime battery then configures the BMS to stop accepting charge as the SOC reaches 80% enable charge when SOC is 20%. Program so that once every two weeks it allows the battery to reach 100% so the BMS can balance the cells.
Benefits: Easy to program using LiTime Bluetooth API.
Drawbacks: If something goes wrong with the BMS the battery might die even sooner. Due to something like programming in a way that possibly enables “charge” when “charge” was off for a reason (like low temp cutoff or something like that).
3.
Similar as option 2 but instead of letting the BMS do the job, program the device to disconnect the MPPT charger as the battery reaches 80%, reconnect the MPPT as the battery SOC reaches 20%. Allow the battery to go to 100% to balance every 20 cycles.
Benefits: Does not have the risk of BMS causing issues.
Drawbacks: Either I would have to do extensive research on victron bluetooth protocol (as it is not fully documented) or I would have to buy large relays to cut the power.
Maybe there is another charger or product that can fix the issue? Is there any solar charge controllers that use a SOC value in its control mechanism? Seams strange that my victron controller only allows you to set various voltages like float, bulk, and absorption to control the charging, maybe a newer device might have more options for configuration.
Does anyone have any other ideas? Or any feedback to the solutions that I have thought of. Maybe something using venus OS.
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