Connecting batteries in series is only practical if the batteries are very similar. So if you know each of your pair of serial batteries (for instance the 2x 12V 55Ah) have the same capacity, you can do that. You might want to measure the available capacity of the batteries. You also must balance the loading process! Especially in solar / offgrid situations, where charging and discharging can switch often, any imbalance in the individual batteries will accumulate over time, leading to a serial pair where one is partially charged and one is discharged, leading to damage.

Now connecting batteries in parallel is a different beast altogether. Up to a point, especially regarding power drawn, they will balance themselves. If little power is drawn, internal resistance plays a lesser role to cell voltage and things will balance out.

Now you can reduce this somewhat by putting half the batteris in parallel and put these two grids in series, better averaging out the capacity imbalance. If you measure the capacities in advance, you can select the battery for the 12V grids so as to optimize the balance.

All that said: If you are hell bent on setting this up, I recommend leaving out the 7.5 Ah and 8 Ah batteries. They will cause more problems than they are worth, contributing only 15% anyway. Then make two grids of 2x 20Ah and 1x 55Ah each, put these in serial after you charged each battery with a reliable charger separately.

During use, monitor the batteries to make sure your use case works with the parallel setup. Switch off charging and discharging periphery and check if all batteries at rest have the same voltage after some use.

If not, set up a schedule to separately charge all batteries from time to time, depending on the imbalance in voltage.

The small batteries do have the same chemistry, but different internal resistance and possibly a slightly different discharge curve. Therefore you can expect them to be more imbalanced than the rest, leading to a more defensive discharging strategy (lower power, less total Ah). Therefore they are not worth using in this context. If they were SLA too, one might think about it, but still, its only 15% more capacity for 40% more connections and failure points.

Also, you have to check the individual battery currents. There is no guarantee that the current is balanced in any way. At any given point, it could be drawn from only one battery, balancing out charge imbalances in the 12V grids. This means, maximum current drawn is the max current of any given battery. Another important reason not to use the small batteries.

Connecting batteries in series is only practical if the batteries are very similar. So if you know each of your pair of serial batteries (for instance the 2x 12V 55Ah) have the same capacity, you can do that. You might want to measure the available capacity of the batteries. You also must balance the loading process! Especially in solar / offgrid situations, where charging and discharging can switch often, any imbalance in the individual batteries will accumulate over time, leading to a serial pair where one is partially charged and one is discharged, leading to damage.

You will have major damage, up to melting copper and fire, if you put batteries of different voltage in parallel!

You will have permanently damaged batteries if you put batteries of unequal capacity in series!

Now connecting batteries in parallel is a different beast altogether. Up to a point, especially regarding power drawn, they will balance themselves. If little power is drawn, internal resistance plays a lesser role to cell voltage and things will balance out.

Now you can reduce this somewhat by putting half the batteris in parallel and put these two grids in series, better averaging out the capacity imbalance. If you measure the capacities in advance, you can select the battery for the 12V grids so as to optimize the balance.

All that said: If you are hell bent on setting this up, I recommend leaving out the 7.5 Ah and 8 Ah batteries. They will cause more problems than they are worth, contributing only 15% anyway. Then make two grids of 2x 20Ah and 1x 55Ah each, put these in serial after you charged each battery with a reliable charger separately.

During use, monitor the batteries to make sure your use case works with the parallel setup. Switch off charging and discharging periphery and check if all batteries at rest have the same voltage after some use.

If not, set up a schedule to separately charge all batteries from time to time, depending on the imbalance in voltage.

The small batteries do have the same chemistry, but different internal resistance and possibly a slightly different discharge curve. Therefore you can expect them to be more imbalanced than the rest, leading to a more defensive discharging strategy (lower power, less total Ah). Therefore they are not worth using in this context. If they were SLA too, one might think about it, but still, its only 15% more capacity for 40% more connections and failure points.

Also, you have to check the individual battery currents. There is no guarantee that the current is balanced in any way. At any given point, it could be drawn from only one battery, balancing out charge imbalances in the 12V grids. This means, maximum current drawn is the max current of any given battery. Another important reason not to use the small batteries.

Connecting batteries in series is only practical if the batteries are very similar. So if you know each of your pair of serial batteries (for instance the 2x 12V 55Ah) have the same capacity, you can do that. You might want to measure the available capacity of the batteries. You also must balance the loading process! Especially in solar / offgrid situations, where charging and discharging can switch often, any imbalance in the individual batteries will accumulate over time, leading to a serial pair where one is partially charged and one is discharged, leading to damage.

Now connecting batteries in parallel is a different beast altogether. Up to a point, especially regarding power drawn, they will balance themselves. If little power is drawn, internal resistance plays a lesser role to cell voltage and things will balance out.

Now you can reduce this somewhat by putting half the batteris in parallel and put these two grids in series, better averaging out the capacity imbalance. If you measure the capacities in advance, you can select the battery for the 12V grids so as to optimize the balance.

All that said: If you are hell bent on setting this up, I recommend leaving out the 7.5 Ah and 8 Ah batteries. They will cause more problems than they are worth, contributing only 15% anyway. Then make two grids of 2x 20Ah and 1x 55Ah each, put these in serial after you charged each battery with a reliable charger separately. During use, monitor the batteries to make sure your use case works with the parallel setup. Switch off charging and discharging periphery and check if all batteries at rest have the same voltage after some use.

If not, set up a schedule to separately charge all batteries from time to time, depending on the imbalance in voltage.

The small batteries do have the same chemistry, but different internal resistance and possibly a slightly different discharge curve. Therefore you can expect them to be more imbalanced than the rest, leading to a more defensive discharging strategy (lower power, less total Ah). Therefore they are not worth using in this context. If they were SLA too, one might think about it, but still, its only 15% more capacity for 40% more connections and failure points.

Connecting batteries in series is only practical if the batteries are very similar. So if you know each of your pair of serial batteries (for instance the 2x 12V 55Ah) have the same capacity, you can do that. You might want to measure the available capacity of the batteries. You also must balance the loading process! Especially in solar / offgrid situations, where charging and discharging can switch often, any imbalance in the individual batteries will accumulate over time, leading to a serial pair where one is partially charged and one is discharged, leading to damage.

Now connecting batteries in parallel is a different beast altogether. Up to a point, especially regarding power drawn, they will balance themselves. If little power is drawn, internal resistance plays a lesser role to cell voltage and things will balance out.

Now you can reduce this somewhat by putting half the batteris in parallel and put these two grids in series, better averaging out the capacity imbalance. If you measure the capacities in advance, you can select the battery for the 12V grids so as to optimize the balance.

All that said: If you are hell bent on setting this up, I recommend leaving out the 7.5 Ah and 8 Ah batteries. They will cause more problems than they are worth, contributing only 15% anyway. Then make two grids of 2x 20Ah and 1x 55Ah each, put these in serial after you charged each battery with a reliable charger separately.

During use, monitor the batteries to make sure your use case works with the parallel setup. Switch off charging and discharging periphery and check if all batteries at rest have the same voltage after some use.

If not, set up a schedule to separately charge all batteries from time to time, depending on the imbalance in voltage.

The small batteries do have the same chemistry, but different internal resistance and possibly a slightly different discharge curve. Therefore you can expect them to be more imbalanced than the rest, leading to a more defensive discharging strategy (lower power, less total Ah). Therefore they are not worth using in this context. If they were SLA too, one might think about it, but still, its only 15% more capacity for 40% more connections and failure points.

Also, you have to check the individual battery currents. There is no guarantee that the current is balanced in any way. At any given point, it could be drawn from only one battery, balancing out charge imbalances in the 12V grids. This means, maximum current drawn is the max current of any given battery. Another important reason not to use the small batteries.