Range drop

Hi. I am Ulf and new to this forum and electric cars. I live on a little Island called Bonaire in the Caribbean. Yes, hot and not all that suitable for led acid batteries.
I just bought, not one, but two Zenn cars and they were fantastic, for two weeks…
I bought 6 new 12V Vetus 100Ah marine batteries and I was a happy camper for about two weeks. After that the range dropped to less than 10 miles and the car(s) are more or less useless. The DeltaQ charger seems to work fine and set to the right algorithm. The batteries are tested and found “good” (since they are only 3 weeks old they should be).
I know the batteries i have installed are not THE best for the car, but is it a battery problem or could it be something else?

After the charger has fully charged your batteries. wait an hour for the surface charge to dissapate.

Check the voltage of each battery, it should be 12.65 volts or more.

Batteries can show full voltage but still be undercharged. The only way to be sure is to check specific gravity of the fluid (electrolyte)

Check each battery with a hydrometer. Specific gravity on new batteries fully charged should be 1260 to 1300.

If they are only in the 1200 to 1225 range your charger is not doing its job.
DeltaQ s have more than 1 setting. If it’s set on gel it wont fully charge flooded batteries.

Batteries not bubbling during bulk charge are an indicater of low charge rate.

Flooded batteries need to be charged at a rate fast enough to bubble and stir the fluid. If you continue to under charge sulfation develops rapidly and you rapidly lose range.

My Zivan finishes at about 95 volts and my range is good.

It’s sealed batteries. But I also have a feeling the charger not doing it’s job.
The whole deltaq charging process takes under 2 hours from empty to full. I have set the algorithm to what was recommended from Deltaq.