External Li-ion Battery for Topcon GPT-9003a

Edit: I'm looking for someone who has knowledge of the external power port for the GPT-9003a or someone who has access to the service manual. Here's why:

I've discovered that my GPT-9003a has a software switch that allows the user to specify Li-ion or 12v for the external battery. As is typical, my discovery was an accident.

I've been attempting to power my robot with an external battery. I say attempting, because I'd discovered that regardless of the size of the external battery, the battery indicator on the instrument would only show one bar (full=three bars). At one bar the low battery alarm sounds every few seconds and is really annoying. I was looking for a setting to disable the low battery alarm and instead found the battery type switch. Toggling the switch from the default Li-ion to 12v fixes the battery indicator/alarms issues I was having.

Now I have a new problem......I really want to run this instrument off an external Li-ion battery but I can't find any info to tell me what range of voltages the instrument can handle. The specification section of the manual lists the input voltage of the external port as DC12v with no elaboration and no mention of Li-ion. Based on how the instrument acted when set for Li-ion but attached to a lead-acid battery I can tell that it expects more voltage than a fully charged lead-acid can provide.

I've tested with both an 11.1v 3-cell Li-po pack and a 14.4v 4-cell Li-po pack.

The 11.1v pack puts out 12.6v when fully charged and can be taken to 9v without damaging the battery. The instrument powered up, showed two bars briefly then dropped to one bar. I set it to non-prism mode and had it measure continuously until it switched to the internal battery. When the instrument switched batteries the external pack was at 11.8v, leaving considerable capacity of the external battery unused.

The 14.4v pack puts out 16.8v when full and can be discharged to 12v. With this pack I initially connected it with only a half charge (14.4v) and ran the instrument until it switched batteries. It started at three bars for a few minutes, then two, then one, switching over when the pack reached 12v. I then fully charged the pack to 16.8v and ran the instrument for a couple of minutes with no issues. Then I chickened out.

Based on the hints from the battery indicator and the shutdown voltages, I believe that the 14.4v 4-cell pack is the answer. However, 16.8v from a full pack is a lot of voltage to be pushing through the instrument's internal power supply if it isn't designed for it.