The batteries aren't able to provide the full (1.9-or-so) amperage, at
the voltage required by the VX-5. The voltage drops when you key up,
and the VX-5's safety disconnect shuts down the radio.

So wiring two of the batteries in parallel (for extra current) would be a
good fix until I have the funds for some better batteries?

That might help matters.

However, the low voltage that you are reading under light load
suggests to me that the battery you are using is either seriously
undercharged, or is dying (a shorted cell, or dried-out electrolyte).

First thing I'd do is check your battery-charging circuit. If you're
trying to charge them from a 12-volt regulated source, then this could
be the root of your problem... a "12-volt" gel cell won't accept a
useful amount of charge at this voltage. You'll need around 13.8
volts to slow/float-charge the battery, and perhaps 14.3 - 14.4 volts
(with an appropriate current limit) to fast-charge it.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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