Battery AH
- Thread starter Swamprat
- Start date
Normally it's easier to have one big battery than two or more smaller batteries in parallel. But there are times when you must have multiple batteries in parallel. It depends on the rated max continuous discharge current of the batteries. If the max continuous discharge current of the one big battery is lower than the discharge current that you need (say for a large inverter) then you can typically get a higher max continuous discharge current by using two smaller batteries in parallel.
One other possible benefit of two smaller batteries over one large battery is redundancy. If you have one battery and it goes bad, your whole system is down. If you have two batteries and one goes bad then you can still run on one battery while you get a replacement for the bad one.
One other possible benefit of two smaller batteries over one large battery is redundancy. If you have one battery and it goes bad, your whole system is down. If you have two batteries and one goes bad then you can still run on one battery while you get a replacement for the bad one.
Hedges
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400,000 Wh/month divide by
30 days/month divide by
48V
= 278 Ah/day.
How many days of power do you want to save (for a rainy day)?
Or, will you use most power as it is generated from PV?
In that case, you can get night time consumption under 100 Ah.
Are you on-grid or off grid?
Batteries are expensive per kWh of life, to be avoided.
The only way I see to make batteries competitive with utility rates is DIY LiFePO4 (or recycled). That can get down to $0.05/kWh, assuming they last the claimed number of cycles.
30 days/month divide by
48V
= 278 Ah/day.
How many days of power do you want to save (for a rainy day)?
Or, will you use most power as it is generated from PV?
In that case, you can get night time consumption under 100 Ah.
Are you on-grid or off grid?
Batteries are expensive per kWh of life, to be avoided.
The only way I see to make batteries competitive with utility rates is DIY LiFePO4 (or recycled). That can get down to $0.05/kWh, assuming they last the claimed number of cycles.
Swamprat
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I want to go off-grid and was thinking 3-4 rainy days.
we have fans to help move air around the wood stove.
Hedges
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With battery sized for several days of no sun, most of the time it might only cycle 15% of capacity.
I think that is the sort of use where forklift-type FLA can last 15 to 20 years.
I've read Rolls Surette is particularly good.
So that is worth considering.
The prices of LiFePO4 280 Ah cells for DIY are in the $80 to $120 range, which also makes them quite affordable.
Building them is a project in itself. Many people here do it.
It sounds like integrity of terminal connections is a key issue to address.
Some people have found sources of assembled packs, with busbars laser welded to join cells (what they were designed to have.)
One other alternative is to stick with a small battery, and use a generator for cloudy days.
I think that is the sort of use where forklift-type FLA can last 15 to 20 years.
I've read Rolls Surette is particularly good.
So that is worth considering.
The prices of LiFePO4 280 Ah cells for DIY are in the $80 to $120 range, which also makes them quite affordable.
Building them is a project in itself. Many people here do it.
It sounds like integrity of terminal connections is a key issue to address.
Some people have found sources of assembled packs, with busbars laser welded to join cells (what they were designed to have.)
One other alternative is to stick with a small battery, and use a generator for cloudy days.
Hedges
I See Electromagnetic Fields!
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If you were on-grid, we could use average sunshine year round. You can find insolation websites for your location, and some consider weather.
If 5 hours effective sun per day, 13.3 kWh/day / 5 hours = 2.7kW production.
2700W / 0.85 (for PTC vs. STC and other inefficiencies) = 3140W (STC) of panels.
To handle winter with shorter days, find the number of hours effective sun expected in the winter. That might be 2 hours, so 2.5x as much PV.
Or, maybe you need less power in the winter, more in the summer?
If 5 hours effective sun per day, 13.3 kWh/day / 5 hours = 2.7kW production.
2700W / 0.85 (for PTC vs. STC and other inefficiencies) = 3140W (STC) of panels.
To handle winter with shorter days, find the number of hours effective sun expected in the winter. That might be 2 hours, so 2.5x as much PV.
Or, maybe you need less power in the winter, more in the summer?
Jacob Rama
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is not recommended, they are better to be the same W and Ah
Short_Shot
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Unless you prefer redundancy and have space
Hedges
I See Electromagnetic Fields!
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Don't buy either PV panels or charge controller until you've selected both and designed the PV array series/parallel configuration.
Some choices won't work well together.
The PV panels in series add voltage together, which must meet SCC range for MPPT and remain within max limit open-circuit. But, the voltages on the panel data sheet aren't exactly the voltage needed for this calculation. There is an adjustment for temperature because voltage goes up in the cold, down in hot weather.
Post the specs and your proposed array configuration, and we can double-check.
Normally all panels in an array would be same model. However, what matters is that panels connected in series with each other be of similar current, and strings of panels connected in parallel with each other be of similar voltage. So, it can be OK to connect two "12V" panels in series, then connect one "24V" panel in parallel with them. Check using the Vmp and Voc ratings.
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