Sounds like one of two possible problems:<div>
</div><div>A.    Your charger is not fully charging the battery.</div><div>
</div><div>B.    Something in the tank is draining the battery too quickly.</div><div>
</div><div>A.   Check the battery voltage about 15 min after fully charging it.  Your meter should show the voltage at about %10 over the listed voltage.   Example, with my 7.2 volt tank battery's, I show about 8.1 volts just after charging and about 6.8 volts when depleted.  A 12 volt car battery goes from about 13.75 volts to 10.8 volts.  If your battery never charges up above the voltage listed on the battery then it has a problem.  Put the meter on it while charging and you will read the target voltage.  If the voltage is low while charging then the charger has a problem.  Keep the battery cool while charging, it will accept more power that way.  Also slower rate charging will squeeze more power into the battery, ie. 1 amp hour charging will put more power in a battery than 4 amp hour charging.  Slower charging = more power.  If this is all OK then go to step B.</div><div>
</div><div>B.    If there is a short in the tank wiring it would drain the battery to fast.  This could cause the battery to heat up, check that.  Put the meter on the battery just before installing it in the tank, put it in and let it sit a while then check the voltage again, if it's the same then try one motor at a time checking between each motor and see if one of the motors produce a big voltage drop.  If so then check out that unit.</div><div>
</div><div>Without the tank/battery in my hands to look at, that's about all the help I can offer.   Good luck.</div>