Safe Lowest batteryvolyage

I  have a Ewheel-38 scooter. I want to determine the max distance  I can travel on a level ground. Powered by a 24 AH 48V, Battery bank made up of four 12 V 24 AH Deep cycle lead acid Batteries, without damaging my batteries, What is the lowest lowest voltage that should not be operated under? I have installed a 0 to 60  Dc Volt digital and a 0 to 60 Dc Amp digital Amp meter that's providing constant and aquert  feedback during operation of the Scooter under different conditions. I appreciate any information anybody can provide. The goal is to reach the next town. I also installed a double pole double pole transfer switch enable too switch over to a second battery bank after draining the first bank. The second battery bank is in a small bicycle trailer I am pulling behind the scooter.

István

10.5V per 12V batt so 21V is DEAD flat, and very life shortening to go anywhere near that.

50% SoC is the recommended stopping point if you want decent lifespan

go a bit lower in an "emergency" occasionally

but a 10-15% average DoD difference can mean triple the cycles

50 percent would be the max you can drain for the pack to last, if you take it below that the pack starts losing capacity.

If both batteries are lead-acid, it works much better to put them in parallel vs. switching between them.

Also, the batteries will sag a lot under load, so you need to stop and let them bounce back before taking a voltage measurement.

fechter said:

If both batteries are lead-acid, it works much better to put them in parallel vs. switching between them.

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This is critical. Wire them in parallel before testing.

Thank you all for taking time to offer your advice. After the idea was born for this adventure I contacted an acquaintance who is a Electrical Engineer who advised not to parallel the two battery banks, based on that, how the  transfer switch between banks came to be used, which looked to be useful as the battery bank in the trailer gets depleted it could be disconnected and left to be charged and picked up on the return for completing the trip back home. As you travel your feedback from your battery bank is my digital voltmeter, so I was thinking that when the battery bank under load voltage declines there should be a safe low limit voltage that can be observed on the voltmeter. The scooter battery capacity indicator is a long way from the red, only halfway on the gage, my amp meter shows 8 Amp draw, digital voltmeter showing 48.3V one indicates lots of reserve the other says you're done. At this point I am definitely confused and not knowing the safe stay above the boundary or limits that could be observed on the voltmeter as you moving, This may look alimentary to those in the know, but here I am. 
Steve. 

István said:

After the idea was born for this adventure I contacted an acquaintance who is a Electrical Engineer who advised not to parallel the two battery banks

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Yeah, apparently everybody here has an EE friend that conveniently confirms their preexisting lay electrical theory

István said:

how the  transfer switch between banks came to be used, which looked to be useful as the battery bank in the trailer gets depleted it could be disconnected and left to be charged and picked up on the return for completing the trip back home

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So the trailer battery would be connected with a connector? Then it could be paralleled with a connector.

István said:

As you travel your feedback from your battery bank is my digital voltmeter, so I was thinking that when the battery bank under load voltage declines there should be a safe low limit voltage that can be observed on the voltmeter. The scooter battery capacity indicator is a long way from the red, only halfway on the gage, my amp meter shows 8 Amp draw, digital voltmeter showing 48.3V one indicates lots of reserve the other says you're done. At this point I am definitely confused and not knowing the safe stay above the boundary or limits that could be observed on the voltmeter as you moving, This may look alimentary to those in the know, but here I am. 

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If original, the scooter gauge is likely calibrated to discharge the batteries more deeply. This makes sense, since the range of a shitty lead-acid scooter discharging to only 50% would be terrible and nobody would buy one, or would buy it and ride it once and return it. Use your digital voltmeter.
If you parallel the batteries, they will sag less and you'll have a more accurate reading of the voltage.

Very few EEs actually know much about batteries

Thanks again. My understanding of the safe low limit voltage of the 48 V battery bank under load is 48.4 Volt.
By keeping with these guidelines I can establish my maximum travel range capability of my scooter so I have enough power for return home.
The attached picture shows some of the activities the scooter is used for. Such activity requires that I know what is in the tank at all times, so to speak.