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A second, buffer battery remote from main battery bank for servicing DC loads

Sverige

A Brit in Sweden
Joined
Oct 8, 2020
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772
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59.5N, 15.5E
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.
 
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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.
And all of these buck boost, extra battery, heavier cable is better than just converting to AC for transportation? Into existing infrastructure?
How?
 
And all of these buck boost, extra battery, heavier cable is better than just converting to AC for transportation? Into existing infrastructure?
How?
It’s better because it’s more efficient. And efficiency is important to me, as I’m not living somewhere where I have sufficient solar to run on solar year round. If I were, I could shed watts without worrying about it, but when I run 4 months a year with my house fully back on utility mains because the sun is nowhere to be seen, I prefer to install a solution with the lowest quiescent consumption to minimise my overall annual energy consumption.

The extra battery is existing equipment I don’t have to buy, and you’ll note i specifically excluded the options of heavier cable or lots of voltage converters.
 
I have done similar things in smaller places.
Additional battery at rear of truck to cover voltage sag from surges from equipment at the rear of the truck.
I didn't do anything fancy or expensive. Just ran a large enough conductor with a fuse between them. Just had to figure out what size conductor worked, through trial and error. Or go extra large, from the beginning.
 
I know it’s non-trivial, but raising the voltage would cut the current significantly.
Yes it’s an option. I could reconfigure my main battery bank to 8S, but then I have to replace my 12V inverters or I could use a suitably sized Buck boost module to just raise the transmission line voltage then convert down again, local to my loads, but all these converters have a quiescent consumption which lowers the overall efficiency.

Right now I still prefer the option of deploying 4S LiFePO4 packs which I already own close to the load end of the system and will get around to trying this soon.
 
I have been thinking of the same as a buffer for a diesel heater. But I have agm batteries and would have to buy a small battery. Anything I should consider before I decide to do this?
Would any type of lead acid be ok?
Is it going to affect/drain the main bank even when not used?
 
Would any type of lead acid be ok?
Lead-acid batteries (not sure about Lithium) will drain each other if there's much of a temperature differential between them, as the colder battery tries to prop up the warmer one. Very slowly for small temperature differentials, but they will self-discharge faster sitting on a concrete floor due to the temperature differential between the top and bottom of the battery...
 

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