Can you add 2-12 v 18 ah batteries in parallel to back up 2-12 volts 32 ah batteries?

One x 12 volt 9 ah. $34.

https://www.google.com/search?q=Energizer+ER+Pro+1000+UPS

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If the inverter is intended for 6 volt batteries, and I doubt it, then yes. However, it is common to connect two six volt batteries in series to get 12V, which most RV inverters use. I have four 6 volt, series parallel connected batteries in my home providing 830 AH to run the house during the peak periods of the day. I have never had a problem with this set up.

Connect the two extra 12 volt batteries in series to get 24 volts. Then connect this new 24 volt supply in parallel to existing 24 volt battery supply.

Here is the way to hook them up in series, or in parallel.
There are 3 batteries in top (series) pic, but a 4th would be added in same wiring manner. Disregard the letters "ve" in diagrams, but the +/- polarity is vitally important in hookup.
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Note that four 12v batteries hooked up in series will be 48 volts at the end terminals. (If all four are hooked up in "parallel" then the total would still be 12 volts).

If you want to supply a 12v inverter, you need to hook 12v batteries in PARALLEL.

Depends on distance between the batteries and the max load you may put on the batteries.Why not put one higher AH battery in instead of adding one?

Wire the batteries in parallel to double amp output.

With batteries, you can increase voltage or amperage by choosing to wire in series or in parallel.
For example D batteries are 1.5 volts.
Wire two D batteries in series, or stacked on top of each other like in flashlight, and you get 3 volts, but same amp output as one battery.
Wire two D batteries parallel, so the + terminals are connected together with one wire, and - terminals are connected together with another wire, and you get 1.5 volts, but twice the amps as one battery.
Test this yourself with two D batteries and a multimeter.

It depends on how you connect the batteries. If you connect the negative of battery one to the positive of battery 2, the negative of battery 2 to the positive of battery 3 the negative of battery 3 to the positive of battery 4 then the positive of battery 1 and the negative of battery 4 will give you 48v 7AH. (This is known as connecting in series.)

If you connect the four positives together. Connect the 4 negatives together it will give you 12v 28 AH. (This is known as parallel connection.)

If you connect the negative of battery 1 to the positive of battery 2. Connect the negative of battery 3 to the positive of battery 4. Connect the positive of battery 1 to the positive of battery 3 and the negative of battery 2 to the negative of battery 4. You'll get 24v 14AH from the connection of the positive of battery 1 and battery 3, and the negative of battery 2 to battery 4. (This is known as series parallel connection)

It depends on how you connect the batteries. If you connect the negative of battery one to the positive of battery 2, the negative of battery 2 to the positive of battery 3 the negative of battery 3 to the positive of battery 4 then the positive of battery 1 and the negative of battery 4 will give you 48v 7AH. (This is known as connecting in series.)

If you connect the four positives together. Connect the 4 negatives together it will give you 12v 28 AH. (This is known as parallel connection.)

If you connect the negative of battery 1 to the positive of battery 2. Connect the negative of battery 3 to the positive of battery 4. Connect the positive of battery 1 to the positive of battery 3 and the negative of battery 2 to the negative of battery 4. You'll get 24v 14AH from the connection of the positive of battery 1 and battery 3, and the negative of battery 2 to battery 4. (This is known as series parallel connection)

join them in parallel if there all 12 volt you could just charge the 1 battery with 12 volts and that would charge them all but all the other batterys have to be in series you cant do it if u have 2 24 volt batterys they all have to be 12 v

Hi. What size batteries you buy will be limited to the amount of room you have in the machine.
17 amp hour batteries are really a bit small for any mobility equipement. A lot of power chairs have 35 or 40 amp/hour batteries.
Buy the biggest that will fit into the available space.
Batteries are designed to perform a certain number of "charge cycles". If you run your batteries down by 75%, you will only get a small number of "charge cycles". If you only use 25% of the batteries capacity, you will get 100's of "charge cycles".
The best I have seen is 7 years out of a set of batteries. Short trips and recharging after each trip is the reason.
Good luck. Neil.