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I have 24 VDC to 220 VAC Inverter and two 90 Ampere Batteries now I feel that backup is not what I need now I intend to connect these two existing batteries in parallel to give total of 12 volts and 180 or 170 amp and add a new single 150 Ampere battery.

Now Question is that what effect would be there on charging?

ups cuts of charging on 28vdc.

What I feel that if we have unbalanced battery ampere rating in parallel then first smaller one gets full and the overflow current helps the bigger battery to fill then .Am I wrong?

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Even parallel charging expensive batteries of the same type is unwise. If you are using deep-cycle batteries then they almost certainly justify using a charger that can manage each independently or separate chargers.

During UPS operation you want the combined energy of all batteries (of course). If these have all been floated and are fully charged then you can "with reasonable safety" combine them at this stage.

Best/nicest would be to have the ability to stagger discharge and apply each new battery as desired.

Either of the above schemes ideally requires the ability to zero or close to zero voltage drop connect the batteries to load. Traditionally this would be done with contactors but can be done with modern MOSFETS at low loss and quite reasonable price.

For example, this utter gem of modern technology Infineon N-Channel MOSFET IPB009N03L at $3.44 /1 in stock at digikey is rated at 30V, 180A, Rdson < 0.001 ohm [!!!!] 0.6k/W Tjc.

At 30A with a wisp of copper it would be warm.
At 100A it would need sensible heatsinking.
I do not know the sensible design current or the package (as opposed to the device ratings) but the package seems to be trying harder than some to deal with the current. A few of these or something similar could well allow you ease of charge-separately, "connect as required" operation.

enter image description here

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  • \$\begingroup\$ I would think that having batteries of similar chemistry connected rigidly in parallel "all the time" would be better than doing so "sometimes". If one battery is at 12.5 volts and another at 12.0 volts when they are connected in parallel, I would expect significant current (0.5v / (sum of their internal resistances) to flow. If the batteries are connected in parallel full-time, safe charging current would be limited to the worst-case value, but subject to that condition I'd think they should be happy and voltage differences should not develop. \$\endgroup\$ – supercat Jan 16 '12 at 16:07
  • \$\begingroup\$ @supercat - That's why I said "if all have been floated ..." ie if all are at full charge then they will be matched to better than 0.1V BUT if you have several batteries of different amp hour capacity and construction and age and you discharge them in parallel you cannot be sure how much of the discharge each will provide. If you then parallel charge them you MAY get proportionate charge current sharing but it's by no means certain. I do not have practical experience of this with multiple lead acid batteries but I'd wager* that experts will tell you not to do it. (* Peppercorn on demand) \$\endgroup\$ – Russell McMahon Jan 16 '12 at 18:16
  • \$\begingroup\$ If batteries are connected rigidly in parallel, there's no telling how charge or discharge currents might be divided among the cells, but I would expect that the charge state of all the batteries would always be such that the net current into or out of every battery, including both current into/out of the assembly and current from/to other batteries, would never exceed the maximum charging or discharging current to which the assembly as a whole was subjected. \$\endgroup\$ – supercat Jan 16 '12 at 19:01
  • \$\begingroup\$ If battery B1 has a lower internal resistance than B2, for example, B1 might initially receive a larger share of charging current than B2, but B1's share of the charging current would decrease as B1 pulls ahead and B2 falls behind. There would be no way that B1 could get so far ahead of B2 that connecting B1 to B2 would cause more current to flow between the cells than had been put into the assembly while it was charging. Note that depending upon the condition of the batteries, it may not be possible to fully charge all of them in the parallel configuration. \$\endgroup\$ – supercat Jan 16 '12 at 19:09
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If using a smart charger, having different rated batteries will mess with things. Typical for a smart charger is a higher voltage to start then it will lower it's output when the battery is at 85% capacity for the next 15%, then change into a trickle state for holding. It decides what to do by one of a few different methods. Current sensing and temperature are the two main ways it figures this out. If one battery has a bigger tank than another, the charger will maintain a higher voltage longer trying to top it up. This could cause over charging on a lower capacity battery in your bank and could be dangerous. Safest thing to do is keep all batteries at the same age and spec as the rest. When setting up a UPS system you want all the batteries to be of the same spec and age (the same batch is better) so you don't have any issues. If you need a UPS then you should be doing it right so it won't fail when you need it most.

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