Sunny Day
Member since: 1985 Post Count: 3
I have my offgrid system set up to charge my main battery bank, which then distributes DC power to remote loads via (fuse protected) cabling around my home, as well as via offgrid inverters connected close to the battery for mains powered loads. Where possible I run loads on DC and seek to eliminate the inefficiency of converting up to AC mains via inverter then back down to DC with a wall wart power supply. Hence all my small devices like broadband modem, wifi router, iPhone and iPad chargers, etc all run on DC with efficient Buck boost converters where needed and I’ve binned their mains power supplies.
Now I am pondering the possibilities of adding a second smaller battery on the far side of the house, to act as a buffer to give a more stabilised DC level to the load devices there, as I sometimes notice voltage dips when the DC load current being distributed from one side of the house to the other is peaking.
I can think of two alternative approaches this and thought I’d ask for opinions and experiences of others, to see what I can learn:
1) The secondary battery acts only as a buffer, absorbing the load current peaks and providing the local loads with higher current when needed. Then it recharges slowly from the main battery when local loads are not connected. In this case the secondary battery will be small in capacity (I’m thinking 5Ah in comparison to my 560Ah main battery) and can recharge from the main battery using a current limited constant output Buck boost converter which can handle an input voltage range below or above the output voltage. This kind of thing would do it.
2) Alternatively, the secondary battery could be larger, and be used as a meaningful part of the overall storage capacity within the system. Then it needs to do more than just run the local loads, it needs to be able to recharge the bigger battery when that is run down to a level below the voltage of the secondary battery. So then a bi-directional DC DC converter is needed, which can charge or discharge the secondary battery, depending on the voltage levels at input or output. That is a class of device I have not seen on AliExpress, but maybe I’m searching for the wrong thing, so what would you use to control power distribution between two batteries in this case?
There’s even an option 3 which is just add the secondary battery at the main battery bank, in parallel to boost that capacity then deal with the transmission line voltage drop issue by using a step up boost converter to send a higher DC voltage around the house, then use step down converters local to the loads. I’m not keen on this approach as all that converting up and down is inefficient and a little expensive, especially if using devices which can handle the peak current of 4 or 5 smartphones and tablets all fast charging on USB-C simultaneously. I suppose option 4 is to install more and thicker DC cables from one side of house to the other…again expensive, difficult, I’m not doing that
The secondary battery would have its own small BMS in all cases.
Now I am pondering the possibilities of adding a second smaller battery on the far side of the house, to act as a buffer to give a more stabilised DC level to the load devices there, as I sometimes notice voltage dips when the DC load current being distributed from one side of the house to the other is peaking.
I can think of two alternative approaches this and thought I’d ask for opinions and experiences of others, to see what I can learn:
1) The secondary battery acts only as a buffer, absorbing the load current peaks and providing the local loads with higher current when needed. Then it recharges slowly from the main battery when local loads are not connected. In this case the secondary battery will be small in capacity (I’m thinking 5Ah in comparison to my 560Ah main battery) and can recharge from the main battery using a current limited constant output Buck boost converter which can handle an input voltage range below or above the output voltage. This kind of thing would do it.
2) Alternatively, the secondary battery could be larger, and be used as a meaningful part of the overall storage capacity within the system. Then it needs to do more than just run the local loads, it needs to be able to recharge the bigger battery when that is run down to a level below the voltage of the secondary battery. So then a bi-directional DC DC converter is needed, which can charge or discharge the secondary battery, depending on the voltage levels at input or output. That is a class of device I have not seen on AliExpress, but maybe I’m searching for the wrong thing, so what would you use to control power distribution between two batteries in this case?
There’s even an option 3 which is just add the secondary battery at the main battery bank, in parallel to boost that capacity then deal with the transmission line voltage drop issue by using a step up boost converter to send a higher DC voltage around the house, then use step down converters local to the loads. I’m not keen on this approach as all that converting up and down is inefficient and a little expensive, especially if using devices which can handle the peak current of 4 or 5 smartphones and tablets all fast charging on USB-C simultaneously. I suppose option 4 is to install more and thicker DC cables from one side of house to the other…again expensive, difficult, I’m not doing that
The secondary battery would have its own small BMS in all cases.
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