Yep Ron, makes sense. Let me ask you this then, let's say we take the inverter out of the picture all together, and now all we have hooked up to the battery is the motors. Since the motors still run fine even if they are collapsing the battery depending on the load at the time, what harm effect will this have on the battery if any? I'm not sure if I stated it before, but after deactivating the motors, the voltage returns to it's normal level, and I've seen this happen with many personal/school projects in the past but thought it was normal.
OK, keep in mind what Nigel posted.
A battery, any battery is designed and built to deliver a specific amount of power over a given period of time. While delivering a current within its ratings its voltage should not drop below a certain level. While some drop is normal and to be expected a voltage drop exceeding set parameters is not acceptable. Under load for us to know what the battery is actually doing it becomes essential we measure the battery voltage at the battery terminals. Any point beyond those terminals the voltage we measure will be inclusive of the line drop. That is the point Nigel is trying to drive home.
Now I don't know your battery or batteries so we will use a rough example.
I have a couple of Werker 7.5 AH 12 volt batteries sitting here. They are part number WKA12-7.5F2. When I place these two batteries in series I basically have a 24 volt 7.5 amp hour battery.
I want to run my pump and my pump has a 24 volt DC motor and the name plate says 24 VDC 5 amps. OK, in theory my battery should run my pump. So I connect the pump and start it up. My pump should in theory run at least 1.5 hours. In real life it won't but right now matters not. I am measuring my battery voltage right at the battery terminals.
Before starting my pump I was reading 25.4 volts on my fully charged battery which is normal (see my last post) as between 25.2 and 25.6 is expected for my fully charged 24 volt SLA (Sealed Lead Acid) battery. Immediately after starting my pump my battery voltage drops to about 24.8 volts which is good and can be expected since my battery is under a load. Life is good and my pump is running. The Kingdom is happy as they all have fresh water to shower and wash dishes.
While my pump is running I watch my battery voltage slowly drop. Finally my battery voltage has dropped to 21 volts, my pump is still running but I shut it off. I shut it off because at 21 volts for all purposes my battery is dead and if I continue to run my pump I will destroy my battery. This may piss off the people of the Kingdom but screw them as my battery is important to me and I don't want to destroy it. Besides if they complain I'll raise their taxes!
Another possibility is when I start my pump I observe my battery voltage drops to below 21 volts. In this case my batteries have reached the end of their useful life and need replaced. I drag them to the Kingdom Recycle Center and use the tax increase to buy new batteries.
So some voltage drop is normal and to be expected. The trick is knowing when there is too much voltage drop. That tells us the batteries need replaced or they simply cannot supply the needed power because they are not rated for the load. In the latter case I need a larger battery. The voltage must be measured at the battery terminals. This precludes any I*R drop being measured if I measure voltage at the load. Measure at the source.
Ron